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The

Journal

Raptor Research

Volume 3!

June 1997

Number 2

Contents

Preface. Raptor Responses to Forest Management: A Holarctic

PERSPECTIVE. Gerald J. Niemi and JoAnn M. Hanowski 93

The Northern Goshawk ( Accipiter gentius atricapillus ): Is There Evidence

of a Population Decline? Patricia l. Kennedy 95

How, and Why, is the Goshawk {Accipiter gentius ) Affected by Modern

Forest Management in Fennoscandla? Per widen 107

Forest Management and Conservation of Boreal Owls in North America.

Gregory D. Hayward 114

Boreal owl Responses to Forest Management: A Review. Hard Hakkarainen,

Erkki Korpimaki, Vesa Koivunen and Sami Kurki 125

The Osprey (Pandion hauaetus) and Modern Forestry: A Review of

Population Trends and Their Causes in Europe. Pertti l. Sauroia 129

Osprey {Pandion hauaetus) Populations in Forested Areas of North
America: Changes, Their Causes and Management Recommendations. Peter
J. Ewins : 138

The Great Gray Owl ( Strix nebulosa ) in the Changing Forest Environment
OF NORTHERN Europe. Seppo Sulkava and Kauko Huh tala 151

Great Gray Owls {Strix nebulosa nebulosa) and Forest Management in
North America: A Review and Recommendations. James R. Duncan 160

Hawk Owls in Fennoscandia: Popuiation Fluctuations, Effects of Modern
Forestry, and Recommendations on Improving Foraging Habitats. Geir a.
Sonerud 167

The Long-eared Owl {Asio otus) and Forest Management: A Review of the
Literature. Denver w. Holt : 175

Northern Hawk Owls {Surnia ulula caparoch) and Forest Management in

NORTH America: A Review. Patricia a. Duncan and Wayne C. Harris 187

Concluding Remarks on Raptor Responses to Forest Management: A

HOLARCTIC Perspective. Gerald J. Niemi and JoAnn M, Hanowski 191

BOOK Reviews. Edited by Jeffrey S. Marks 197

The Raptor Research Foundation, Inc. gratefully acknowledges a grant and logistical support
provided by Boise State University to assist in the publication of the journal.

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THE JOURNAL OF RAPTOR RESEARCH

A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC.

Vol. 31 June 1997 No. 2

J. Raptor Res. 31(2):93-94
© 1997 The Raptor Research Foundation, Inc.

PREFACE

RAPTOR RESPONSES TO FOREST MANAGEMENT:
A HOLARCTIC PERSPECTIVE

Gerald J. Niemi and JoAnn M. Hanowski

Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Highway,

Duluth, MN 55811 U.S.A.

Forest raptors are one of the most sensitive
groups of vertebrates to forest management and
forest habitat change due to their position at the
top of the forest food chain, their relatively large
territories and home ranges, and their historical
persecution by man (Fuller 1996). The highly vis-
ible case of the Spotted Owl ( Strix occidentalis) con-
troversy in the northwestern U.S. exemplifies many
issues and conflicts between forest use and the
need for appropriate ecological management for
forest-dependent organisms (Yaffee 1994). Con-
flicts between forest resource use and the manage-
ment or preservation of forest areas can be mini-
mized with appropriate knowledge and under-
standing of how species respond to forest change
(e.g., through logging, natural disturbance or suc-
cession). With this increased understanding, we
can modify forest management to provide a sus-
tainable harvest, yet ensure that we protect biolog-
ical diversity and the fundamental processes of for-
est systems.

With this philosophical perspective, we em-
barked on organizing a symposium focused on se-
lected raptor species of northern temperate and
boreal forest habitats. The focus of the symposium
was to summarize our current understanding of
forest raptors with holarctic distributions — those
with distributions in the temperate and boreal
regions of North America and northern Europe.
The symposium focused on six species with holarc-
tic distributions: Osprey ( Pandion haliaetus ) , North-
ern Goshawk ( Accipter gentilis), Long-eared Owl
(Asia otus), Boreal/Tengmalm’s Owl (Aegolius fu-
nereus ), Northern Hawk Owl ( Surnia ulula ) and

Great Gray Owl ( Strix nebulosa ). For each of these
species, two individuals (one from North America
and one from northern Europe) were selected
based on recommendations from a variety of rap-
tor experts. Each of the individuals selected made
a presentation at the symposium and those papers
completing the peer-review process are included
here.

Symposium Organization and Questions
Posed to Authors

The focus of each paper was on forests, forest
management and how the ecology of each species
relates to these issues. Each author was asked to
address or consider the questions below. Because
solid quantitative information was lacking for many
questions, the presenters were invited to use edu-
cated guesses and common sense. Hence, if state-
ments in the papers are not supported with data
or references, then it is likely that the author did
not use such empirical information. This is highly
appropriate because in many cases a scientist has
worked a lifetime with a species and has accumu-
lated considerable knowledge on how a species
may respond to forest management.

Questions:

(1) Using the best available knowledge, what is the
present population trend of the species over the
past 10 yr, 25 yr, 50 yr and 100+ yr?

(2) What are the primary factors associated with
these trends? Factors such as food supply, habitat
availability, chemical effects, human persecution,
interspecific interactions and modern forestry
practices should be discussed in the context of
these trends.

Niemi and Hanowski

Vol. 31, No. 2

(3) If modern forestry is associated with these
trends, then how has the species been affected by
either past or current management practices?
Among additional factors to be considered were
riparian zone management and secondary effects
of logging on water quality (e.g., nonpoint source
pollution) .

(4) There are many ways that logging and forest
management can affect forest raptors. Among
these the following should be considered, (a) How
would the species be affected by cuts of different
sizes such as 1-3 ha cuts, 10-20 ha cuts, 20—100 ha
cuts or cuts greater than 100 ha? (b) How would
the species be affected by cuts of different shape?
Assume that shapes vary from the simplest shapes,
such as circular or square cuts, to those that are
infinitely complex with convoluted edges, (c) What
are the effects and what is the importance of leav-
ing live trees, dead trees, shrubs or patches of these
vegetational forms or different species of trees
(e.g., future snags) within cut areas? The responses
of forest raptors to these alternative ways to log
forests would be especially useful if considered in
the context of mitigation strategies that would im-
prove habitats and populations for the specific rap-
tor species.

(5) What is an ideal mix and spatial distribution
of forest cut sizes and shapes that would be both:
(a) highly beneficial to the species and (b) highly
detrimental to the species? For example, would
small cuts of 1-3 ha of circular or square shapes
with many dead trees remaining within the cuts be
beneficial or detrimental to the species in compar-
ison with large cuts of complex shapes with few
residuals? Alternatively, how should cuts be
grouped spatially within respective management
areas such as distributed randomly or connected
by corridors between uncut areas?

(6) Integrate the information available to the ex-

tent possible with specific management recommen-
dations. In addition, speculate on similarities and
differences in the species response to forest-man-
agement practices on the two continents. For in-
stance, forestry has occurred in northern Europe
for more than 100-300 yr, whereas forestry in
North America is generally less than 100-yr old.
Have there been any short-term evolutionary re-
sponses by the species to forest regeneration today
versus how forests have regenerated in the past
(e.g., forest regenerating following forest fire ver-
sus logged forests).

Acknowledgments

We thank the following organizations for their finan-
cial support of the symposium and this issue of the Jour-
nal of Raptor Research. US Bureau of Land Management;
USDA Forest Service including the North Central Forest
Experiment Station-St. Paul, Chequamegon National For-
est, Chippewa National Forest, Nicolet National Forest,
Ottawa National Forest and Superior National Forest;
Boise-Cascade, White Paper Division; Georgia-Pacific Cor-
poration; Lake Superior Paper Industries; National
Council of Stream and Air Improvement Inc.; Potiatch
Corporation, Northwest Paper Division; Minnesota State
Legislature (through the Minnesota Environment and
Natural Resources Trust Fund as recommended by the
Legislative Commission on Minnesota Resources); and
the University of Minnesota through its Natural Re-
sources Research Institute, Raptor Center, Department of
Biology and University College. We also thank Daniel E.
Varland of Rayonier for coordinating the manuscript re-
view and for his editorial assistance. This is contribution
number 206 of the Center for Water and the Environ-
ment, Natural Resources Research Institute, University of
Minnesota, Duluth.

Literature Cited

Fuller, M.R. 1996. Forest raptor population trends in
North America. Pages 167-208 in R.M. DeGraaf and
R.I. Miller [Eds.], Conservation of faunal diversity in
forested landscapes. Chapman-Hall, London, UK
Yaffee, S.L. 1994. The wisdom of the spotted owl: policy
lessons for a new century. Island Press, Santa Barbara,
CA U.S.A.

J. Raptor Res. 31(2):95— 106
© 1997 The Raptor Research Foundation, Inc.

THE NORTHERN GOSHAWK
(ACCIPITER GENTILIS ATRICAPILLUS) : IS THERE
EVIDENCE OF A POPULATION DECLINE?

Patricia L. Kennedy

Department of Fishery and Wildlife Biology and Graduate Degree Program in Ecology,

Colorado State University, Ft. Collins, CO 80523 U.S.A.

ABSTRACT. — I evaluate the claim that Northern Goshawk ( Accipiter gentilis atricapillus; hereafter referred
to as goshawk) populations are declining in North America based on a review of the published literature
and analyses of demographic data collected on two goshawk populations in New Mexico and Utah.
Evidence of a decline would include range contractions, temporal decreases in density, fecundity and/or
survival, and/or a negative rate of population change. The goshawk is a former Category 2 species as
determined by the U.S. Fish and Wildlife Service and two petitions have been submitted to list the
goshawk as threatened or endangered under the U.S. Endangered Species Act. The petitions claimed
that goshawks suffered significant declines in the U.S. because of logging practices and were threatened
with extinction as a result of overharvest. There is no evidence of range contractions in western North
America and the goshawk’s range appears to be expanding (perhaps due to reoccupancy of former
range) in the eastern U.S. The majority of data on abundance of breeding pairs indicate that goshawk
densities are highly variable spatially and temporally. There is some evidence to suggest that abundance
is correlated with food availability. One study claims that goshawk abundance has declined in the past
several decades in northern Arizona but the conclusions are based on an inadequate sampling design.
Fecundity fluctuates widely but there is no evidence of a negative trend. Fecundity is apparently influ-
enced by a combination of food availability and predation rates. Survival estimates are too limited to
analyze for temporal trends and, as a result of insufficient survival data, (X) have not been estimated
for any North American goshawk populations. I conclude there is no strong evidence to support the
contention that the goshawk is declining in the U.S. This result can be interpreted either that goshawk
populations are not declining or goshawk populations are declining but the declines have not been
detected with current sampling techniques (Type 2 error) . These two hypotheses cannot be rigorously
evaluated with existing published information and will probably not be evaluated in the future with
datasets from a single study area because of sampling limitations. To rigorously and objectively evaluate
the population trends of the North American goshawk in a timely and cost-effective manner, I recom-
mend a meta-analysis be conducted of all existing published and unpublished datasets.

Key Words: Accipiter gentilis atricapillus; Northern Goshawk, population status ; forest management, endan-

gered species listing.

El halcon norteno ( Accipiter gentilis atricapillus) : ^hay pruebas de una reduccion de poblacion?

Resumen. — Yo he evaluado la demanda que las poblaciones del {Accipiter gentilis atricapillus ; de aqui en
adelante referido como halcon) se estan reduciendo en norte america basado en un examen de liter-
atura publicada y un analisis de datos demograficos colectados de dos poblaciones de halcon en Nuevo
Mexico y Utah. Las pruebas en la reduccion deberian incluir contracciones del campo, reduccion
temporal en densidad, fecundo y/o supervivencia, y/o ritmo negativo de cambio en la poblacion. El
halcon fue marcado una especie de Categoria 2 como determinado por el U.S. Fish and Game Service
y dos demandas han estado entregadas para designar el halcon como amenazado o en peligro debajo
del U.S. Endangered Species Act. Las demandas susieren que los halcones sufrieron reducciones sig-
nificantes en los Estados Unidos por las reglas que dirigen la cortada de arboles y amenazado con
extincion por el resulto de cosechas muy numerosas. No hay ninguna prueba que de las contracciones
de campos en el oeste de norte america y el campo de halcones parece e star haciendo se mas amplio
(Tal vez por la ocupacion de nuevo de los campos antiguos) en el este de los Estados Unidos. La mayorfa
de datos sobre la abundancia de parejas de cria indica que la densidad de halcones varia mucho en su
espacial y su temporal. Hay pruebas que sugieren que la abundancia esta correlacionada con la dispon-
ibilidad de comida. Un estudio susiren que la abundancia se ha reducido en las pasados decadas en el

Kennedy

Vol. 31, No. 2

norte de Arizona pero las conclusiones estan basadas en un proyecto con insuficiente muestreo. Fluc-
tuaciones de fecundo varian mucho pero no hay ninguna prueba de una tendencia negativa. Fecundo
esta aparentemente influido por una combinacion de disponibilidad de comida y ritmos de cazar. Es-
timaciones de supervivencia son muy limitadas para analizar tendencias temporal y, el resultado de los
datos insuficiente de supervivencia, no han estado estimados para poblaciones de halcones en norte
america. Yo concluido que no hay pruebas fuertes para soportar el argumento que el halcon se esta
reduciendo en los Estados Unidos. Este resultado puede estar interpretado por un lado, que las pob-
laciones de halcon no estan reduciendose o por otro lado, que las poblaciones se estan reduciendo
pero las reducciones no han sido descubridas con la tecnica de muestreo usada hoy en dia (error Pipo
2). Estas dos hipotesis no pueden estar evaluadas con rigor con la informacion publica que existe y
probablemente no va estar evaluada en el futuro con los datos de un area singular de estudio por
limitaciones de muestreo. Para poder evaluar rigurosa y objetivamente las tendencias de poblaciones
del halcon de norte america en una manera oportuna y con un precio justo, yo recomiendoque una
meta-analisis sec condueida con todos los datos publicados y no-publicados que existen.

[Traduccion de Raul De La Garza, Jr.]

Because the Northern Goshawk (Accipiter gentilis
atricapillus; hereafter referred to as goshawk) often
nests (Siders and Kennedy 1996, Squires and Rug-
giero 1996) and hunts (Bright-Smith and Mannan
1994) in old-growth or mature forests, potential
conflicts between timber harvest and maintenance
of viable goshawk populations concerns various
publics (Hitt 1992, St. Clair 1992). These concerns
have resulted in two petitions to list the goshawk
as threatened or endangered in the southwestern
(Federal Register 1992a) and western U.S. (Fed-
eral Register 1992b, 1996a), and the classification
of the goshawk as a Category 2 species (Federal
Register 1992a) prior to the elimination of this cat-
egory by the USDI Fish and Wildlife Service (FWS)
in 1996 (Federal Register 1996b). In addition, it is
included on the Sensitive Species lists of several
USDA, Forest Service (USFS) regions (e.g., Pacific
Northwest, Southwest, Intermountain, Rocky
Mountain and Alaska) and is a Species of Special
Concern in several states (Wisconsin Bureau of En-
dangered Species 1995, K. Titus pers. comm.). The
goshawk has no federal or provincial protection in
Canada (World Wildlife Fund Canada Web Site,
http://www.wwfcanada.org/speclist.html) .

The listing petitions claimed that goshawks had
suffered significant declines because of logging
practices and that it was under threat of extinction
as a result of overharvest. In reviewing a listing pe-
tition, the FWS must determine if the petition pre-
sents substantial information to warrant a status re-
view. Both of the goshawk petitions were denied by
the FWS because the petitions could not document
that goshawk populations west of the 100th merid-
ian constitute a distinct population (Federal Reg-
ister 1996a). Only species, subspecies and, for ver-

tebrates, distinct populations are listable entities
under the Endangered Species Act (ESA) .

My goal in this paper is to evaluate the claim that
goshawk populations have suffered significant de-
clines in the western U.S. I address the following
question: is there demographic evidence that gos-
hawk populations are declining? The mark of a
species in trouble is not its population abundance
or geographic range size at one point in time, but
the rate of population decline or range contraction
(Caughley and Dunn 1995). A rare or uncommon
species can have a stable population or range size
(Gaston 1994). Conversely a species in decline can
seem relatively common until only a short time be-
fore it becomes rare (Caughley and Dunn 1995).
Evidence of a decline for both rare and common
species would include range contractions, tempo-
ral decreases in abundance, fecundity and/or sur-
vival and/ or a negative rate of population change
(\) (Caughley and Dunn 1995). In this paper I
evaluate these lines of evidence by reviewing the
available demographic data on goshawks through-
out its subspecific range. Although the listing pe-
titions pertain only to the western U.S., I did not
restrict my analyses to this region because it is not
recognized as a distinct population. Diagnosing
causes of decline is irrelevant if there is no evi-
dence that a decline has occurred.

Methods

This paper summarizes and evaluates the published de-
mographic literature on goshawks and presents results of
demographic analyses I have conducted on datasets from
New Mexico and Utah. The New Mexico population and
study area are described in detail in Siders and Kennedy
(1996). The Utah population is located in the Ashley Na-
tional Forest (ANF) in eastern Utah. During 1991-1995,
42 occupied nest sites were located on the ANF using

June 1997

Status of Northern Goshawk

survey methods recommended by Kennedy and Stahleck-
er (1993). The ANF is located in the Uinta Mountains
and contains approximately 340 000 ha of forested land.
The average annual precipitation is 70 cm (range 40-90
cm) , with roughly equal precipitation from winter snow*
fall (November-April) and summer rains (May— Octo-
ber). Lodgepole pine {Firms contort® ), spruce-fir ( Picea
engelmanni- Abies lasiocarpa) , mixed conifer (includes
lodgepole pine, Engelmann spruce and subalpine fir)
and ponderosa pine {Pinus ponderosa ) are the most prev-
alent forest communities present in the study area. Doug-
las fir ( Pseudotsuga menziesii), quaking aspen {Populus tre-
muloides), pinyon-juniper {Pinus edulis-Juniper osteosperma) ,
subalpine meadows, sagebrush grasslands and riparian
woodlands are also present.

To evaluate changes in ranges, I compared current dis-
tribution maps with historic maps and reviewed pub-
lished accounts of changes in the status of the goshawk
at the state and regional scale.

In this review I did not include the migration literature
which contains temporal data on counts of migrating gos-
hawks. These data were not included because the rela-
tionship between counts of migrants and population
abundance is unknown. I agree with Bednarz et al.
(1990) and Titus and Fuller (1990) who suggest that pop-
ulation fluctuations in this species may not be adequately
monitored by migration counts because goshawk migra-
tions are characterized by irruptive invasions which can
mask trends in abundance.

I also did not include results from non-peer-reviewed
literature because these datasets have not been subjected
to a rigorous scientific evaluation via peer review. Al-
though there is potentially valuable information in this
body of literature, the information hasn’t been sorted
through selectively to separate questionable from reliable
information (Bury and Corn 1995).

Results

Range Contractions. Range contractions may be
seen in a species’ distribution as a partial erosion
of the boundary or as a range fragmentation where
populations are removed from within the distri-
bution (Caughley and Dunn 1995). In range con-
traction, the agent of decline can often be identi-
fied by knowing something about those factors that
determine the boundary of the range (Caughley et
al. 1988).

Goshawks are holarctic in distribution, occupy-
ing a wide variety of boreal and montane forest
habitats throughout North America and northern
Mexico (Johnsgard 1990). I assume its breeding
range is discontinuous because there are no re-
cords of birds breeding in nonforested areas (e.g.,
prairie regions of Canada and the U.S.). However,
its winter range may not be discontinuous because
it is observed in nonforested habitats in the winter
(P. Kennedy unpubl. data, J. Squires pers. comm.).
The northern limit of its distribution is the bound-

ary of boreal forest and tundra habitats. The east-
ern and western boundaries are the Atlantic and
Pacific Oceans, respectively (Palmer 1988, Johns-
gard 1990). Factor (s) that limit the southern ex-
tent of the range are unknown.

In the eastern U.S., the goshawk may have been
more abundant before the extinction (early 1900s)
of the Passenger Pigeon {Ectopistes migratorkis; Bent
1937, Mengel 1965, Rrauning 1992) and before the
extensive deforestation of this region which
reached a peak at the end of the 19th century
(McGregor 1988, Foster 1992, Smith et al. 1993,
Pimm and Askins 1995). Since 1920, the amount
of forested habitat has been increasing throughout
the eastern U.S. resulting from the conversion of
primarily agricultural landscapes into landscapes
dominated by forest (Pimm and Askins 1995).
Since this time, there is evidence that eastern gos-
hawk populations may be expanding. Although lew
records exist before the 20th century, the goshawk
was considered a casual or accidental breeding spe-
cies in the northeast from the late 1800s into the
1950s (Forbush 1927, Bagg and Eliot 1937, Andrle
and Carroll 1988, Zeranski and Baptist 1990,
Brauning 1992). However, from the 1950s onward,
the species’ range appears to have expanded and
its numbers have increased in many northeastern
states (Bull 1974, 1976, Speiser and Bosakowski
1984, Leek 1984, Andrle and Carroll 1988). For
example, the first breeding record for Massachu-
setts was reported by Farley (1923) and there were
no reported nests in Connecticut at this time. By
1964 it was a casual nester in northwestern Con-
necticut and by 1978 at least 19 occupied nest sites
were located in this area (Zeranski and Baptist
1990). The first goshawk nest in New Jersey was
recorded in 1964 (Speiser and Bosakowski 1984)
and it was considered a rare summer resident in
New York until the 1950s. Forty-eight breeding sites
were located in New York between 1952 and the
early 1970s (Bull 1974) and 20 occupied sites were
recorded in the Highlands region of northern New
Jersey and southeastern New York by the mid-1980s
(Leek 1984, Speiser and Bosakowski 1987). In a
recent atlas of the breeding birds of New York, the
goshawk was recorded as a breeding bird in all but
11 counties (Andrle and Carroll 1988). Andrle and
Carroll (1988) compared Bull’s (1974) goshawk
distribution for New York with their atlas distribu-
tion for the state and concluded that the species
has expanded its breeding range in New York since
the early 1970s.

Kennedy

Vol. 31, No. 2

Table 1. Density of breeding goshawk populations from North America estimated from nest censuses.

Year 3

No.

100 km” 2

(A) b

Forest Type

Location

Source

1982-85

Ponderosa pine c

Arizona

Crocker-Bedford and Chaney (1988)

1974

3.6

Mixed conifer/
Ponderosa pine

Oregon

Reynolds and Wight (1978)

1971-72

7.5

Lodgepole pine d

Colorado

Shuster (1976)

1984-92

5.7-1 0.7 e

6-11

Mixed conifer

California

Woodbridge and Detrich (1994)

1990

10.0

Spruce

Yukon

Doyle and Smith (1994)

1992-93

4.6-6.2

4-8

Lodgepole pine

Oregon

DeStefano et al. (1994a)

1992-93

6.6-8. 8

6-8

Ponderosa pine/
Mixed conifer

Oregon

DeStefano et al. (1994a)

1992-93

2. 6-7.0

3-8

Mixed conifer

Oregon

DeStefano et al. (1994a)

1993

3.8-8.6 e

4-9

Mixed conifer/
Ponderosa pine

Oregon

DeStefano et al. (1994a)

a Time period in which study was conducted. If temporal variation in density is available, the range in annual estimates of density
and sample sizes are reported,
b N — Number of nests.

c Range of values for two different study areas in the same forest type.

Although these data suggest a range expansion
(or reoccupancy), this needs to be interpreted cau-
tiously. Increasing populations of goshawks report-
ed in the eastern U.S. could reflect an increased
search effort rather than a range expansion. An
inability to distinguish these two phenomena has
been documented for other poorly detectable rap-
tor species (Stahlecker and Duncan 1996).

Johnsgard (1990) suggested that range contrac-
tions might be occurring in the Pacific Northwest
and other parts of the west as a result of overhar-
vest of mature forests. However, the goshawk’s west-
ern distribution as described by Bent (1937) has
not changed (Palmer 1988, Johnsgard 1990). In
addition, there are no current reports of local pop-
ulation extirpation in any portion of the goshawk’s
geographic range.

Patterns of Abundance. Range size and abun-
dance are correlated variables. Within particular
taxa and geographical regions, species with large
ranges tend to have greater local abundances at
sites where they occur than do species that are
more restricted geographically (Gaston 1994, Law-
ton 1995). Based on these zoogeographic patterns,
the goshawk, which is widely distributed across
North America, is predicted to be more abundant
locally than comparably-sized forest-dwelling spe-
cies with more restricted ranges such as the Red-
shouldered Hawk ( Buteo lineatus ). Hejl et al. (1995)
recently classified the goshawk as a common breed-

er in the majority (6 of 8 types) of forest types in
the Rocky Mountains.

Breeding density has been estimated for several
North American populations of goshawks. Two
methods based on searches for occupied nests have
been used to estimate these densities: counts of
breeding pairs and distribution of nearest-neigh-
bor distances. Both methods are based on the un-
likely (and untested) assumption that all nests have
been located in the survey area (Gould and Fuller
1995). Comparability of these estimates also is
complicated by use of different survey techniques
among studies (Siders and Kennedy 1996).

Mean nearest-neighbor distances range from
3. 0-5.6 km [Oregon, 1974, N = 4, range = 2. 4-8. 4
(Reynolds and Wight 1978); California, 1984-1992,
N = 21, range = 1. 3-6.1 (Woodbridge and Detrich
1994); Arizona, 1992, N = 59, range = 2.4— 8,4
(Reynolds et al. 1994)]. Nest densities have been
estimated to range from 2.6-11 nests per 100 km -2
(Table 1). High densities of 10-11 nests per 100
km -2 have been recently reported in three study
areas: Arizona, California and the Yukon (Table 1).

In addition to the extensive spatial variation de-
scribed above, breeding densities can vary annual-
ly. Although densities did not vary during two years
in one study area in Colorado (Shuster 1976), in
three study areas in Oregon, densities varied from
33-270% during 2 yr (DeStefano et al. 1994a; Ta-
ble 1). The Bly study area censused by DeStefano

June 1997

Status of Northern Goshawk

et al. (1994a) in 1993 was the same study area cen-
sused by Reynolds and Wight (1978) in 1974. The
number of occupied nest sites located on this study
area (N = 4) did not change over the 21-yr period
and thus densities were equivalent (3.6 in 1974 and
3.8 in 1993; Table 1; variation due to slightly more
acreage censused in 1974).

Two studies have attempted to quantify popula-
tion trends in goshawk populations using data
from breeding populations (Grocker-Bedford
1990, Doyle and Smith 1994). Grocker-Bedford
(1990) was the first person to suggest in the sci-
entific literature that goshawk populations were de-
clining due to overharvest of their forested habitat.
This idea is important and it needed to be pub-
lished. However, his study does not do an adequate
job of rigorously evaluating this hypothesis. Crock-
er-Bedford claims that the goshawk population on
the North Kaibab Ranger District of the Kaibab Na-
tional Forest in Arizona declined from an estimat-
ed 260 nesting pairs to approximately 60 pairs by
1988. This estimated decline is not based on tem-
poral variation in densities. Rather, it is based on
a comparison of densities estimated during the
1985-87 breeding seasons between areas harvested
during two different time periods. He compared
densities from areas lightly harvested in the 1950s
and 1960s (controls) to areas that were more in-
tensively harvested from 1970-85 (treatments).
Crocker-Bedford estimated densities by censusing
the number of nest structures found per unit area
and multiplying the number of structures by the
ratio of nests to breeding pairs. He did not identify
how he differentiated nest structures of different
species such as Cooper’s Hawks ( Accipiter cooperii )
and Red-tailed Hawks (Buteo jamaicensis) that nest
in similar habitats and build similar structures
(Preston and Beane 1993, Siders and Kennedy
1996). Although his data suggest more nest struc-
tures can be found in lightly harvested areas as
compared to heavily harvested areas, his inference
from this dataset to estimating rate of population
change is unwarranted. The relationship between
number of nest structures and number of goshawk
breeding territories is unknown and the assump-
tion that spatial variation in nest structure density
reflects temporal variation in nest structure density
is not supported by any data and is probably un-
justified biologically.

Doyle and Smith (1994) examined variations in
an index of goshaw T k abundance (intensive surveys
of breeding pairs combined with year-round sight-

ings) from 1987-93 in the boreal forest in south-
west Yukon, Canada. Although these data were
not analyzed statistically, the abundance index
changed by more than a factor of four over a 2-yr
period. They also monitored hare abundance from
1987-93 and concluded that changes in goshawk
abundance probably resulted from cyclic changes
in hare densities. During periods of high hare den-
sity, goshawks were abundant on the study area all
years and hares accounted for over 55% of the to-
tal prey biomass. As hare populations declined,
goshawks became more nomadic and virtually dis-
appeared in the winter. They located 40 pr in a
400 km 2 area during 1990, a peak prey year. No
successful breeding was recorded in this same area
during 1992 w r hen hare numbers w T ere lowest.

Indirect evidence of a decline in abundance
might also be indicated by a loss of territories (de-
fined below) over time. However, evidence suggests
that more territories are being located annually as
search effort increases. For example, in the south-
western U S,, few locations of nesting goshawks
were known prior to 1990 and no systematic effort
was made to monitor known nest sites. After the
development of a standardized survey technique
by Kennedy and Stahlecker (1993), efforts by the
USFS to inventory proposed timber sale areas be-
gan on many of the national forests in this region.
Since 1991, the annual number of nesting loca-
tions discovered has risen steadily (Fletcher and
Sheppard 1992). In northcentral New Mexico, 39
goshawk territories were located during 1984—95
(Siders and Kennedy 1996). An average of 3.3 new
territories (SD — 4.9) have been located every year
since 1984 and only one territory has been aban-
doned since it was located. An average of eight new r
territories (SD = 5.1, N = 42) has been located
every year from 1991-95 in the Uinta Mountains,
Utah. Territory abandonments have not been doc-
umented in this study area. Rates of territory dis-
covery and abandonment are not available for oth-
er study areas with long-term (>5 yr) datasets.

Reproductive Patterns. Typically the reproduc-
tive patterns of raptors are subdivided into three
components, each of which is estimated separately:
occupancy rates, nest success and productivity. Ter-
minology defined by Postupalsky (1974), Steenhof
and Kochert (1982) and Woodbridge and Detrich
(1994) was used to define these components.

Occupancy rates. An occupied territory is defined
as a cluster of nest stands exhibiting regular use by
a minimum of one adult goshawk during the

100

Kennedy

Vol. 31, No. 2

breeding season. Occupancy rate is defined as the
proportion of known territories that are occupied.
Similar to many long-lived species (Newton 1979,
1991, Marti 1994), not all goshawks produce off-
spring annually. In three studies with a minimum
of four yr of data, average occupancy rates/ terri-
tory were remarkably similar: New Mexico —
74.4% (SD = 30.5%, N = 22); Utah = 74.7% (SD
= 28.7%, N = 26) and California = 74% (SD =
5.5%, N = 26, Woodbridge and Detrich 1994). The
sample sizes in each study were comparable and
the number of monitored territories increased
over time in each study. Territories with <4 yr of
data were not included in these statistics. The New
Mexico dataset included a maximum of 22 terri-
tories with 4—i 1 yr of occupancy data per nest. The
Utah dataset included a maximum of 26 territories
with 4—7 yr of occupancy data per nest and the
California dataset included a maximum of 26 ter-
ritories with 5-9 yr of data per nest (Woodbridge
and Detrich 1994).

Interstudy comparisons of occupancy rates need
to be done cautiously because occupancy rate is
probably positively correlated with the amount of
effort expended to determine territory status. Lev-
el of effort was comparable among the three stud-
ies where all territories were checked a minimum
of 2-3 times each year and most territories were
visited numerous times each season (B. Wood-
bridge pers. comm.). In New Mexico and Utah, an
area with a radius 0.7— 1.0 km (the postfledging
area as defined by Kennedy et al. 1994) surround-
ing the previously occupied nest was intensively
surveyed using broadcast vocalizations (Kennedy
and Stahlecker 1993) and visual searches of all in-
dividual trees. Woodbridge and Detrich (1994)
used the same searching methods but their search
area was larger, a 1.6 km radius surrounding the
previously occupied nests.

Doyle and Smith (1994) found that the number
of territorial pairs (range 0-8) of goshawks detect-
ed changed with hare densities. When hare densi-
ties were low, no goshawks were detected as breed-
ing birds. At maximum hare densities, eight terri-
torial pairs were recorded. The variation in occu-
pancy rates in other studies could be a function of
prey availability during the winter and courtship.

Nest success. I define nest success as the propor-
tion of occupied territories that produce at least
one young of bandable age. Average nest success
varies from 0.47-0.94 (Table 2). Annual variation
in nest success is high; in New Mexico over a 12-yr

period it varied from complete nesting failure to
100% success in two successive years (Fig. 1). In
Utah, over a 7-yr period it varied from 0.33-0.91
(Fig. 1 ) . To explore the possibility of a decline in
nesting success over time, I evaluated the temporal
variation in these nest-success estimates using lin-
ear regression (Regression Data Analysis Proce-
dure — Microsoft EXCEL Ver. 7.0 for Microsoft
Windows 95). There was no evidence of a negative
correlation between time and nest success in New
Mexico (R 2 — 0.20, P = 0.14, N = 12 yr) or Utah
(R 2 = 0.03, P = 0.694, N = 7 yr) (Fig. 1). It is
interesting to note that the temporal patterns in
nest success between 1990-95 are qualitatively sim-
ilar for both study areas.

In Arizona during 1991-92, Reynolds et al.
(1994) found that 3% (N - 3) of 98 nest attempts
did not lay eggs or failed in early incubation, 6%
( N = 6) of the clutches were lost later in incuba-
tion and 6% (N — 6) of the nests failed during the
nestling period. Possible causes of nest failure were
not discussed. In New Mexico, over a 12-yr period,
out of 122 nest attempts, 8% ( N — 10) failed dur-
ing incubation from predation and unknown caus-
es and 8% ( N =10) failed during the nestling pe-
riod from predation, disease, harvest by falconers
or inclement weather.

Productivity. I define productivity as the mean
number of bandable young produced per occu-
pied territory. Productivity of North American gos-
hawks ranges from 0.0-2. 8. The lowest estimate of
average productivity (0.0) are from the Yukon and
the highest average estimates (2.8) are from Ne-
vada and the Yukon (Table 2). In the Yukon, pro-
ductivity appeared to increase with hare abun-
dance (Doyle and Smith 1994). Pairs breeding at
the hare peak fledged 2.8 young per pair. In two
low-hare years they reported zero productivity.

To explore the possibility of a decline in pro-
ductivity over time, I evaluated the temporal vari-
ation in this variable for New Mexico and Utah
using linear regression (Regression Data Analysis
Procedure — Microsoft EXCEL Ver. 7.0 for Micro-
soft Windows 95). There was no evidence of a neg-
ative correlation between time and productivity in
New Mexico (R 2 = 0.05, P = 0.49, = 12 yr) and

Utah (R 2 = 0.07, P = 0.56, N = 7 yr) (Fig. 2).
Similar to nest success, the pattern in productivity
between the two study areas is qualitatively similar
during 1990-95.

Survival Patterns. Nestling survival. Nestling sur-
vival rates have been estimated in two studies in

June 1997

Status of Northern Goshawk

101

Table 2. Average nest success and productivity of goshawks in North America.

Location

Years

(No. Nests)

Nest SuccEss a
(SD)

Mean Productivity’
(SD)

Source

Arizona

1985-87 (19) c

NA d

2.1 (NA)

Crocker-Bedford (1990)

1985-87 (12)

0.5 (NA)

Arizona e

1991 (37)

0.94

2.0 (0.83)

Reynolds et al. (1994)

1992 (61)

0.83

1.7 (1.08)

California

1984-92 (28)

0.87 (NA)

1.93 (0-4) r

Woodbridge and Detrich (1994)

Nevada

1991 (14)

1.2 (NA)

Younk and Bechard (1994)

1992 (22)

2.8 (NA)

New Mexico

1984-95 (4-31)8

0.47 (0.34)

0.94 (0.64)

This study

E. Oregon

1969-74 (48)

0.94

1.7 (0.76)

Reynolds and Wight (1978)

E. Oregon

1992 (6-10) h

1. 0-2.2 (0.57-0.75)

DeStefano et al. (1994a)

1993 (3-7)*

0. 3-2.2 (0.72-1.08)

E. Oregon

1992 (12)

0.83

1.2 (NA)

Bull and Hohmann (1994)

Utah

1989-95 (3-42)8

0.59 (0.21)

1.22 (0.3)

This study

Yukon

1989 (3)

1.3 (0.88)

Doyle and Smith (1994)

1990 (8)

2.8 (0.57)

1991 (7)

1.3 (0.47)

1992 (1)

0.0

a Nest success is defined as the proportion of occupied territories that produce at least one young of bandable age. See text for
definition of territory.

b Productivity is the mean number of young of bandable age per occupied territory.
c Study included 19 control territories and 12 treatment territories. See text for more details.
d NA = not available.

e Same study area as Crocker-Bedford (1990).
f Range in one study area,
s Number of territories increased over time.
h Range from three study areas.

1 Range from five study areas.

YEAR

Figure 1. Temporal patterns in nest success of two gos-
hawk populations: northcentral New Mexico and eastern
Utah. Yearly sample sizes for New Mexico are 4, 3, 3, 3,
4, 2, 3, 18, 19, 24, 20 and 19 occupied territories, respec-
tively. Yearly sample sizes for Utah are 3, 2, 11, 27, 26, 22
and 25 occupied territories, respectively.

North America. Reynolds and Wight (1972) re-
ported a fledgling success rate (number of young
fledged/ number of young hatched) of 72% (28%
mortality rate) for 11 successful nests monitored
from 1969-74 in Oregon. This estimate is probably
underestimated because unsuccessful nests are not
included. In addition, this estimate was based on
data pooled over 1969-74 so temporal variation in
nestling mortality was not estimated. Ward and
Kennedy (1996) investigated the effect of food sup-
plementation on juvenile survival during 1992-93.
In 1992, survival of birds provided with supple-
mental food (treatment) averaged 80% (N — 15
nestlings) and was not significantly different from
the 100% survival rate of unfed (control) birds (N
= 16 nestlings). In 1993, treatment survival was sig-
nificantly higher (x = 90%, N = 10 nestlings) than
the survival of unfed birds (x = 37%, N — 8 nest-
lings). These data suggest that nesding mortality
can vary annually from 0—63%. No data are avail-
able to determine long-term temporal trends in
nestling mortality.

102

Kennedy

Vol. 31, No. 2

YEAR

Figure 2. Temporal patterns in productivity of two gos-
hawk populations: northcentral New Mexico and eastern
Utah. Yearly sample sizes for New Mexico are 4, 3, 3, 3,
4, 2, 3, 18, 19, 24, 20 and 19 occupied territories, respec-
tively. Yearly sample sizes for Utah are 3, 2, 11, 27, 26, 22
and 25 occupied territories, respectively.

Ward and Kennedy’s (1996) study suggest that
nestling survival rates are influenced by both food
availability and predation rates. They found that
no juveniles died of starvation and the majority
died of predation or disease. Based on behavioral
observations of the adults they suggest that food
limitation can result in higher predation rates on
nestlings because females must allocate more time
to foraging and less time to nest defense.

Juvenile survival. Using radio telemetry, Ward and
Kennedy (1996) estimated juvenile survival rates
from fledging until the juveniles were approxi-
mately 5.5 mo of age in 1992 (telemetry monitor-
ing ceased in mid-October) and from fledging un-
til the juveniles were approximately 7-mo old in
1993 (telemetry monitoring ceased at the end of
November). These survival rates include the fledg-
ing-dependency period (approximately 50 d) and
2.5-4 mo after independence (Ward 1994). During
1992-93, treatment survival was not significantly
higher than control survival; overall survival varied
from 67-100%. No estimates of annual juvenile
mortality are available for North America and tem-
poral trends in this parameter are unknown.

Adult survival. Adult survival estimates are avail-
able from two studies in North America: northern
California (DeStefano et al. 1994b) and northern
New Mexico (this study). Both studies estimated
survival using mark-recapture/resight methodolo-
gy (Lebreton et al. 1992, Gould and Fuller 1995).
Both studies used program RELEASE for data sum-

marization and goodness-of-fit tests (Burnham et
al. 1987). Goodness-of-fit tests examine the data
with a series of x 2 tests to determine if the data fit
the general capture-recapture model (Burnham et
al. 1987, DeStefano et al. 1994b). DeStefano et al.
(1994b) used program SURGE (Lebreton et al.
1992) to derive point estimates and variances of
survival. I used program MARK (G. White unpubl.
software) to develop the same estimates for the
New Mexico population. Both programs use Cor-
mack-Jolly-Seber models to estimate parameters. In
these models 4» = survival and p = probability of
resighting. MARK provides the same capabilities as
SURGE but it has an improved user-interface and
allows the user to test additional models not avail-
able in SURGE (G. White pers. comm.).

DeStefano et al. (1994b) examined eight models
and I examined 12 models, where cf> and p are as-
sumed to vary among years (<j) t , p t ) and between
sexes ((J) s , p s ) and all possible interactions of sex
and time were evaluated. Both studies used Akai-
ke’s Information Criteria (AIC) to select the model
that best fit the data with the fewest number of
parameters and was still biologically reasonable
(Lebreton et al. 1992). AIC is a quantitative meth-
od of selecting the “best” data-driven model
among a set of competing models. AIC selects a
model that balances bias and variance tradeoffs
(Lebreton et al. 1992).

The New Mexico estimates were based on cap-
ture-recapture/resighting histories on 45 adult
breeding goshawks that were trapped and banded
from 1984—95 (Kennedy et al. 1994). This dataset
fit the general capture-recapture model (x 2 — 3.69,
P = 0.72). The two models [(j> , p s+t ) (cj) , with
the lowest AIC values (Table 3) indicated there was
no evidence that survival varied by sex or time and
recapture probabilities varied by sex (higher for
females because they were sighted more frequendy
at the nest) and year (increased efficiency of re-
sighting with time). It is likely that survival does
vary by sex and year and my inability to detect this
variation is a result of small sample sizes and low
resighting probabilities. Annual adult survival in
this study area during this time period is estimated
to be 0.86 ± 0.09. Because recapture probabilities
varied by sex and year and sample sizes were small,
the precision in this estimate is low: 95% Cl =
0.60-0.96. In addition, these estimates may be low
because some marked birds may have emigrated
from the study area.

DeStefano et al. (1994b) estimated survival with

June 1997

Status of Northern Goshawk

103

Table 3. Capture-recapture models used to estimate sur-
vival of adult, breeding northern goshawks in north-cen-
tral New Mexico, 1984-95.

Model

No. Pa-
rameters

Deviance

AIO

General model

(<k> p.) h

55.346

137.001

Time-specific models

(4u fit )

48.748

136.403

Sex-specific models

(4*s> fa)

48.707

134.362

(4>., A)

50.709

134.364

Time- and sex-s

pecific models

(<A, A+t)

40.283

129.937

(4**> Ps*t)

35.669

131.324

(<f> s , A+t)

40.278

131.932

(4> t . A+t)

38.359

132.014

(4> s , A)

43.152

132.807

(4\, A*t)

33.854

133.509

(*A+t> A+t)

37.861

133.516

(<A* t > A)

37.593

137.247

(4>s*t> Ps*t )

32.084

133.739

a AIC = Akaike’s Information Criteria (AIC = [2 X No. Param-
eters] + Deviance).

b <)> = survival rate and p = recapture probability.

a larger dataset (N — 95) over a comparable time
period (1983-92). In their analysis, the model ((J) st ,
p) had the lowest AIC value indicating that survival
varied among years and by sex. Female survival was
estimated to vary annually from 0.35-0.93. Male
survival was estimated to vary from 0.20-0.94. How-
ever, the overall fit of the California data to the
model was inadequate so their survival estimates
must be interpreted cautiously. As the authors in-
dicated, this lack of fit is probably a function of
three factors: sample size, high rates of breeding
dispersal resulting in an underestimation of surviv-
al and methodological constraints (only resighted
birds at successful nests) .

Although the results of these two studies provide
imprecise point estimates of goshawk survival in
North America, they are not adequate to evaluate
temporal trends in survival. As noted by DeStefano
et al. (1994b), temporal trends in goshawk survival
can only be estimated with capture-recapture tech-
niques if the estimates are based on large numbers
of marked birds (>100), high resighting rates and
at least five yr of data. This will require large study
areas and large field crews. In addition, this tech-

nique is not appropriate if breeding dispersal out-
side of the study area is common.

Rate of Population Change. Because of the
aforementioned insufficient survival information,
rates of population change (X) are not available for
any North American goshawk population.

Conclusions

Based on an analysis of nesting records, there is
no evidence of range contractions in western
North America and the goshawk’s range appears
to be expanding (or reoccupied) in the eastern
U.S. Populations may have been lost in the west as
a result of deforestation but these losses have not
been recorded in the published literature. A de-
tailed analysis of 20th century deforestation and
reforestation rates throughout North America
would provide additional indirect information on
potential temporal changes in the goshawk’s
range.

The majority of data on abundance of breeding
pairs indicate that goshawk densities are highly
variable spatially and temporally. There is some ev-
idence to suggest that abundance is correlated with
food availability. Breeding densities in one study
area in Oregon were estimated during 1971 and
1993, and these two estimates were identical.
Crocker-Bedford (1990) has claimed that goshawk
abundance has declined in the past several decades
in northern Arizona but his conclusions are sus-
pect for reasons detailed earlier in this paper.

No declines in fecundity have been recorded
and fecundity fluctuates widely. Results from sev-
eral studies indicate that fecundity is influenced by
a combination of food availability and predation
rates. Survival data are too limited to analyze for
temporal trends and as a result of insufficient sur-
vival data, X has not been estimated for any North
American goshawk population,

I conclude there is no evidence to support the
hypothesis that goshawk populations are declining.
This result can be interpreted in two ways: (1) gos-
hawk populations are not declining; or (2) gos-
hawk populations are declining but the declines
have not been detected with current sampling
techniques (Type 2 error). If the first interpreta-
tion is correct then goshawk populations are not
declining and thus, it should not be listed as threat-
ened under the ESA. The lack of demographic ev-
idence to support a decline corroborates the re-
sults of the FWS analyses of both listing petitions
(insufficient evidence to support a status review).

104

Kennedy

Vol. 31, No. 2

These results also suggest that the national con-
cern for goshawk populations may not be driven
by concerns for goshawk viability but is motivated
by concerns of overharvest of old-growth forests.
Although the concerns about overharvest of for-
ested communities is certainly justifiable, listing a
species for which there is no evidence of a popu-
lation decline would be a misuse of that legislation
and could greatly erode the credibility of the ESA.
In addition, it would impact the recovery process
of truly threatened and endangered (T&E) species
by diverting the limited resources available for
T&E species conservation to goshawk recovery.

Alternatively, it is possible the goshawk is declin-
ing and the decline is going undetected because
of the paucity of data on temporal trends in mor-
tality and abundance. Typical of many raptor stud-
ies, goshawk research has focused on quantifying
trends in reproduction, not mortality or abun-
dance. This is because reproductive data are easier
and less expensive to collect than abundance or
mortality data. Obtaining estimates of abundance
and mortality with reasonable levels of precision
requires large sample sizes of goshawks and long-
term sampling (>5 yr). Unbiased estimates of gos-
hawk abundance also require use of randomized
or stratified study designs where all forested com-
munities (not just old-growth forests) are surveyed
for goshawk presence (Siders and Kennedy 1996,
Squires and Ruggiero 1996).

Because of the low detectability of the goshawk
and the resulting analysis problems associated with
limited sample sizes, it is unlikely that data collect-
ed by any single investigator will be sufficient to
determine whether or not goshawk populations
are declining. It is clear that the information cur-
rently available to the agencies concerning gos-
hawk population trends and demographic param-
eters is insufficient to diagnose population de-
clines. However, l think goshawk population trends
could be diagnosed with a meta-analysis of all ex-
isting datasets.

Meta-analysis is a method of integrating statisti-
cal results from independent studies. It provides
both a rigorous, quantitative analysis of cumulative
evidence and a practical method of systematically
and objectively developing and examining a large
dataset based on pooled observations (VanderWerf
1992). Meta-analysis is frequently used in the bio-
medical field (Mann 1990) but rarely has it been
applied in ecology and conservation biology (Jar-

vinen 1991, VanderWerf 1992, Burnham et al.
1996, Forsman et al. 1996).

If goshawk researchers are willing to collaborate,
a meta-analysis could be conducted to evaluate the
existing demographic datasets on the goshawk.
The main objective of this meta-analysis would be
to conduct a rigorous and objective analysis of the
empirical data available on the North American
populations of this species to determine the pop-
ulation trends of the North American goshawk.
The results of this analysis would provide an objec-
tive analysis of existing information for federal and
state agencies involved with goshawk management
and listing decisions and identify future goshawk
research needs, if the aforementioned questions
cannot be answered definitively with existing da-
tasets.

Acknowledgments

I would like to thank Gerald Niemi for organizing this
symposium and patiently waiting for the first draft of this
manuscript. Sarah Dewey, Kathy Paulin and other Ashley
National Forest personnel collected the Utah data. Nu-
merous people helped find, capture and band goshawks
in New Mexico. However, a special thanks goes to Tylan
Dean, Vicki Dreitz, Joan Morrison, Missy Siders, David
Sinton and Joni Ward who performed the majority of the
fieldwork over the past several years. Gary White analyzed
the New Mexico resighting data in program MARK and
Ken Wilson assisted with the analysis interpretation.
USDA, Santa Fe National Forest assisted with funding for
this analysis. I thank Mike Britten, Mike Collopy, Steve
DeStefano, Sarah Dewey, Steve Spangle, Kim Titus, Clay-
ton White and one anonymous referee for their careful
and thoughtful critiques of this manuscript. Finally I
would like to thank Skip Ambrose and Michelle James
for answering my numerous questions about the Endan-
gered Species Act and Bob Rosenfield for his encourage-
ment and constructive comments.

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J. Raptor Res. 31 (2):107-113
© 1997 The Raptor Research Foundation, Inc.

HOW, AND WHY, IS THE GOSHAWK ( ACCIPITER GENT I LIS)
AFFECTED BY MODERN FOREST MANAGEMENT IN

FENNOSCANDIA?

Per Widen

University of Karlstad, S-651 88 Karlstad, Sweden

Abstract. — The Northern Goshawk ( Acdpiter gentilis) is a common raptor in the boreal forest of Fen-
noscandia (Norway, Sweden and Finland), with a present breeding density of about 3 pr/100 km 2 of
forest area. Several independent studies show that goshawk populations in Fennoscandia have declined
by 50-60% from the 1950s to the 1980s. This decline coincides in time with an intensification of forest
management, which has changed the forest landscape. Among other effects, forests are more frag-
mented and the proportion of old forest is decreasing. Neither pesticide use nor persecution can explain
the goshawk decline. However, changes in habitat and prey populations are both important factors that
are affected by forestry. Goshawks need only a small patch of suitable habitat for nesting, but for their
foraging home ranges cover 2000-6000 ha, and in boreal forest areas they prefer large patches of mature
forest. I suggest that changes in the boreal forest landscape have resulted in a deterioration of goshawk
hunting ranges, making it more difficult for them to secure adequate food for breeding. This factor is
more important than a shortage of suitable nest sites. Declining prey densities (e.g., grouse) may be
associated with forestry and is also an important factor that may affect goshawk numbers.

Key Words: Accipiter gentilis; Northern Goshawk, forest management, home range, breeding, habitat selection',

Fennoscandia', Sweden', Norway, Finland.

^Como, y Porque, esta el Accipiter gentilis afectado por la administracion forestal moderna en Fenno-
scandia?

Resumen. — El Accipiter gentilis norteno es un rapaz comun en el bosque boreal de Fennoscandia (No-
ruega, Suecia y Finlandia) con una densidad de cria presente como 3 pr/100 km 2 de area bosque,
varios estudios independientes ensenan poblaciones de Accipiter gentilis en Fennoscandia aun reducido
por uno 50-60% de los 1950s a los 1980s. Esta reduction coincide con el tiempo de intensification de
administracion de bosque, que ha cambiado el paisaje del bosque. Entre otros efectos, bosques estan
mas fragmentados y la proportion de bosques viejos se esta reduciendo. Ni uso de pesticida ni perse-
cution puede explicar reduccion del Accipiter gentilis. Sin embargo, cambios en el habitat y poblaciones
de cazar son las dos importantes factores que son afectados por forestales. Accipiter gentilis necesitan no
mas una parcela chiquita de habitat conveniente para hacer nidos, pero sus forrajes naturales cubren
2000-6000 ha, y en areas de bosque boreal ellos prefieren parcela grandes de bosque maduro. Yo,
propongo que cambios en el paisaje de bosques boreal han resultado en un empeoramiento en campos
de cazar del Accipiter gentilis, haciendo mas dificil para ellos a proveer suficiente comida para cria. Este
factor es mas importante que una falta de nidos conveniente. La reduccion de densidad de cazados,
(por ejemplo, urogallo) puede ser asociada con forestales y es tambien un factor importante que puede
afectar la cantidad de Accipiter gentilis.

[Traduction de Raul De La Garza, Jr.]

The forests of Fennoscandia (Norway, Sweden,
and Finland) have been used by man for a very
long period of time. However, in the 1950s, a major
change occurred in forest-management practices,
including intensified methods based on clear-cut-
ting, replanting and thinning. This practice grad-
ually replaced the traditional way of harvesting for-
est by selective cutting. In Sweden, 58% of the land

area is productive forest which is very intensively
managed. About 40% of this area has been clear-
cut since 1950 and is now covered by forest estab-
lished according to modern methods (Anonymous
1989).

As a result of this intensive management, the bo-
real forest landscape of Fennoscandia is now a
highly fragmented patchwork of clear-cuts and for-

107

108

Widen

Table 1. Population studies showing goshawk density
changes in boreal forests of Fennoscandia.

Study Area

Period

Change in
Breeding
Pairs

Density

Change

(pr/100

km 2 )

Central Norway 3

1964-93

8 — > 0

5.7 -> 0

Southern Norway b

1950-84

13 — » 5

7.2 -> 2.7

Southern Norway

1950-85

35 -> 20

9^3

Southern Norway^

1985-88

20 ^ 26

3 -> 4

North-central Sweden' 1

1950-76

12 -» 5

2.4 — > 1

Central Sweden 0

1950-70

10 -» 5

2 -> 1

Central Sweden 1

1960-80

35 -» 15

35 -> 15

Southern Finland^

1974-81

25 — > 10

5 — > 2

Southern Finland 11

1977-84

16 — > 10

5.3 -> 3.3

a T 0 mmeraas 1993.

b Hansen 1985, Frydenlund Steen 1989.
c Selas unpubl. data.
d Carelius (1978).
e Bylin (1975).

1 Lind (in Nilsson, 1981).
g Wikman and Linden (1981).
h Forsman and Ehrnsten (1985).

est stands in different successional stages. Less than
5% of the Swedish forests are primeval, as com-
pared to 22% and 60% of the forests in the U.S.
and Canada, respectively (Olsson 1992).

The Northern Goshawk (Accipiter gentilis) occurs
in forested areas throughout the Holarctic region
(Brown and Amadon 1968), and is one of the
more numerous birds of prey in Fennoscandia.
The object of this paper is to review available in-
formation about goshawk population status and
trends in the Fennoscandian countries and to dis-
cuss possible effects of modern forest management
on those trends.

Population Status

In Norway, the goshawk population was estimat-
ed to be 2700 breeding pairs (Bergo 1992). This is
equivalent to 0.8 breeding pr/100 km 2 of land area
and 3.1 pr/100 km 2 of forest area.

The Swedish goshawk population was estimated
at 10 000 breeding pairs by Svensson (1979), based
on a nationwide bird censusing program. However,
Nilsson (1981) suggested that there were only 6000
breeding pairs after analyzing a number of differ-
ent local studies. Marcstrom and Kenward (1981),
based on capture-recapture estimate of ringed
(banded) birds, calculated that the number of gos-
hawk pairs older than two yr was between 3500 and

Vol. 31, No. 2

Figure 1. Map showing the location of goshawk popu-
lation studies cited in the text. 1. T0mmeraas (1993), 2.
Hansen (1985), Frydenlund Steen (1989), 3. Selas (pers.
comm.), 4. Carelius (1978), 5. Bylin (1975), 6. Lind (in
Nilsson 1981), 7. Wikman and Linden (1981) and 8.
Forsman and Ehrnsten (1985).

13 600. I judge the two latter estimates to be the
most reliable and conclude that the Swedish gos-
hawk population is about 7000 breeding pairs,
which is equivalent to a breeding density of 1.9
pr/100 km 2 of land area and 2.9 pr/100 km 2 of
forest area.

The goshawk population in Finland was estimat-
ed at about 6000 breeding pairs by Saurola
(1985a), implying a breeding density of 2.0 pr/100
km 2 of land area and 3.0 pr/100 km 2 of forest area.

Thus, although the density per land area differs
between the Fennoscandian countries, the density
per forest area is virtually the same, about 3 breed-
ing pr/100 km 2 of forest.

Population Trends

The best way to study long-term population
changes in raptors is to monitor a breeding pop-
ulation of a given area for a long period of time.
By examining trends in individual populations, we
should be able to make conclusions regarding
trends over larger areas. In Fennoscandia, a num-
ber of such local, long-term goshawk population
studies have been reported, and they are summa-
rized in Table 1. The locations of the studies are
shown in Fig. 1. A paired t-test between early and
late years show a statistically significant decline ((
= 3.474, df = 8, P = 0.0084).

Thus, it is well documented from a number of

June 1997

Goshawks and Forestry in Fennoscandia

109

different, independent studies in all three Fenno-
scandian countries that goshawk populations have
decreased from around 1950 to around 1980. In
most studies, the decrease has been 50—60%. After
that period, the pattern is less clear since most
studies have not continued. However, the nation-
wide raptor monitoring scheme in Finland indi-
cates stable populations after 1982, when the pro-
gram started (Haapala et al. 1994), and Selas (pers.
comm.) reports a temporary, slight increase in one
area of Norway.

Why Has the Goshawk Declined?

To determine the reasons for such a dramatic
decline, we must look at all possible environmental
factors, not only forest management. The factors
most often associated with declining raptor popu-
lations are pesticides, persecution, declining prey
populations and habitat degradation or loss (New-
ton 1979).

Pesticides. Most adult goshawks in the boreal
forests of Fennoscandia are not migratory and re-
main in or close to the boreal forest throughout
the year. Further, their most important prey species
are also sedentary. Thus, they do not directly pick
up contaminants from other regions, probably
making them less vulnerable than other raptor spe-
cies to pesticide contamination. However, juveniles
and some adult females move south and winter in
farmland areas (Widen 1985), where there is more
prey, but also generally more pesticide use than in
forested habitats.

Mercury, In Sweden, alkyl-mercury was used for
seed dressing in agriculture from the 1940s until
1966, when it was prohibited. This use caused wide-
spread contamination of the terrestrial fauna, and
as a result many terrestrial bird species were seri-
ously affected (Berg et al. 1966, Borg et al. 1969,
Jensen et al. 1972, Westermark et al. 1975, Johnels
et al. 1979). Increased levels of mercury were
found in breeding female goshawks’ feathers in the
period 1940—65, but decreased to background lev-
els rapidly after alkyl-mercury was banned in 1966
(Johnels et al. 1979).

Organochlorines. A common way of assessing the
impact of organochlorines on raptor populations
is by measuring the eggshell thickness. Newton
(1979) concluded that whenever a population
showed more than 16-18% shell-thinning over sev-
eral years, it declined. Nygard (1991) reported a
6.6% decrease in eggshell thickness in eggs from
goshaw r ks in Norway after 1947, a result that sug-

gests organochlorines have not been an important
factor in their population decline.

Thus, although pesticide use has been reported
as the cause of declines in goshawk populations in
other parts of Europe (Bijlsma 1991), I conclude
that there is no evidence that this has been the
case in the boreal forest region. When the use of
persistent pesticides stopped in the early 1970s,
positive goshawk population trends were reported
throughout Europe (Bijlsma 1991). In the boreal
forest region, this has not occurred and goshawks
did not recover when the pesticide situation im-
proved. In fact, several population studies show
that goshawks declined even after mercury levels
dropped. This can be compared to the Sparrow-
hawk ( Accipiter nisus), which decreased drastically
in Sweden from the 1950s, but recovered markedly
when organochlorines were prohibited in the
1970s (Wallin 1984).

Persecution, Goshawks have always been perse-
cuted in Europe, especially in farmland areas by
hunters wanting to protect small game species
from predation. In Fennoscandia, this has mainly
affected wintering juvenile goshawks. Locally, per-
secution also affected adult breeding birds since
some hunters specialized in finding and destroying
breeding goshawks. However, during the period of
goshawk decline between 1950-80, legal protection
has improved and there has been a gradual chang-
ing opinion favoring raptors, leading to reduced
pressure of persecution. Accordingly, Saurola
(1985b) reports a 50% decrease in goshawk per-
secution between 1960-80. Thus, persecution is
not likely to be the major reason for goshawk de-
cline in Fennoscandia.

Prey Populations. The goshawk feeds on a wide
variety of prey species, but in the boreal forests of
Fennoscandia different grouse species are the most
important prey (Hoglund 1964, Sulkava 1964,
Tornberg and Sulkava 1990), although in winter
squirrels ( Sciurus vulgaris) may also be a major prey
item (Widen 1987). It is well documented that gos-
hawks respond numerically and functionally to
short-term fluctuations in grouse populations (Lin-
den and Wikman 1980, 1983) and they are likely
to respond also to long-term population changes.

Selas (pers. comm.) suggested that the goshawk
decline was caused by a decline in forest grouse,
due to a long-term increase in the number of gen-
eralist predators such as red foxes Vulpes vulpes
(Storaas and Wegge 1985, Storaas 1993). The in-
crease in goshawk numbers from 1985 in his area

110

Widen

Vol. 31, No. 2

Birds/sq.km

Capercaillie
Black Grouse
Hazel Grouse

Figure 2. Population trends in forest grouse: Capercaillie ( Tetrao urogallus). Black Grouse ( Tetrao tetrix) and Hazel
Grouse ( Bonasa bonasia ) in southern Finland (Finnish Game and Fisheries Res. Inst, unpubl. data).

was explained as a temporary reversal of this pro-
cess, when the red fox drastically decreased due to
sarcoptic mange, resulting in an increase in grouse
numbers. However, the red fox is now recovering
and there will probably not be any long-term effect
on prey numbers. Thus, Selas explains the goshawk
population changes as long-term numerical re-
sponses to changing prey populations.

Wikman and Linden (1981) found that the gos-
hawk decline coincided with a general decline in
grouse numbers in the same area, but concluded
that the rather moderate grouse fluctuations could
not explain the 60% decline in the goshawk pop-
ulation. Grouse populations were very low in 1976-
77 when the goshawk decline started. Although
grouse numbers increased for a number of years
after 1977, goshawks failed to respond numerically
and still remain at a low population level. More
recent grouse data (Finnish Game and Fisheries
Research Institute unpubl. data) show a continuing
downward trend in numbers of Capercaillie ( Tetrao
urogallus ), Black Grouse ( Tetrao tetrix ) and Willow
Grouse ( Bonasa bonasia) in Finland (Fig. 2).

Thus, there is some indication that decline in
prey populations, mainly grouse, is a factor involved
in the long-term decline of Fennoscandian goshawk
populations. Unfortunately, for most of the studies
reporting goshawk decline, there are no good cor-
responding data on grouse populations, so it is un-

clear how general this explanation is. Further, the
fact that the goshawks in southern Finland failed to
respond to increasing grouse populations in the late
1970s and the early 1980s, indicates that the rela-
tionship between grouse and goshawks is not always
a simple numerical response.

Habitat Degradation or Loss. Goshawk habitat
can mean different things. Quite often it refers to
nesting habitat (e.g., the site where the bird builds
its nest and breeds). Less often it refers to the rest
of the bird’s living space (e.g., the home range that
is used to find the food necessary for survival and
raising of young). Here, I will cover both aspects,
since both of them are important for goshawk sur-
vival.

Nesting habitat. Several reports (Carelius 1978,
Forsman and Ehrnsten 1985, Hansen 1985, Fry-
denlund Steen 1989, Tommeraas 1993) have indi-
cated that goshawk population declines were main-
ly or partially due to the loss of nest sites during
modern forest management.

The breeding habitat of Northern Goshawks has
been described by several authors (Dietzen 1978,
Reynolds 1983, Reynolds and Meslow 1984, Link
1986), and generally it is found that goshawks do
not select nest sites randomly. Since nest sites are
relatively easy to find and describe, there is a ten-
dency to emphasize the importance of that part of
the bird’s environment, as compared to the much

June 1997

Goshawks and Forestry in Fennosgandia

111

larger home range. Goshawks need only a small
patch of suitable habitat for nesting and successful
goshawk breedings have been reported in forest
patches as small as 0.4 ha (Lindell 1984). One gos-
hawk nest was recorded in an isolated willow tree
( Salix alaxensis ) in Alaska, 145 km north of the tree
line on the tundra (Swem 1992).

The reported habitat requirements may not be
specific, observed relationships may not be causal
and, if they are, they may not represent major re-
straints (Kenward and Widen 1989). Although lack
of nest sites may become a problem on a local
scale, it seems unlikely that it should become lim-
iting on a larger scale for goshawk populations in
boreal forests, even in strongly impacted systems.
For example, the study area in central Sweden
where Widen (1989) studied goshawk hunting hab-
itats was an area with very intensive forest manage-
ment, yet the proportion of mature forest suitable
for nesting goshawks was about 24%.

I conclude that nesting habitat availability is not
likely to be a major factor behind the recorded
decline in goshawk numbers.

Hunting habitat. Goshawks move over large areas
when hunting, and in Sweden home range sizes
are between 2000—6000 ha (Kenward 1982, Widen
1989). Important clues regarding hunting habitat
requirements might be found by taking a closer
look at how the goshawk uses this landscape, es-
pecially where and how it hunts.

Widen (1989) studied goshawk hunting habitat
with radio telemetry in continuous coniferous for-
est in the boreal forest region of Sweden (Sjors
1965), at Grimso Wildlife Research Station. Of the
area, 74% was conifer-dominated forests. Bogs and
fens comprised 18% of the area and only 3% was
farmland. Of the six different habitat classes, gos-
hawks strongly preferred mature forest. Some
hawks were monitored more intensively in order
to record their predation, and results showed that
most kills were made in mature forest, strongly in-
dicating that this was the most important hunting
habitat. It was also clear that goshawks preferred
large patches of mature forest, although the pref-
erence for large patches was evident in the mature
forest only. It was also in the large patches that
most kills were made.

The goshawk hunts by making short flights be-
tween perches, where it stays for longer periods
and from which nearly all attacks are made (Ken-
ward 1982, Widen 1984). With such a hunting
technique, it is obvious that hunting success de-

pends not only on prey density, but also on differ-
ent habitat features that determine its ability to
hunt. This may be a major factor behind their pref-
erence for hunting in mature forest. It is important
for the hawk to reach perches undetected by prey
and to remain undetected. At the same time, hab-
itat must be open enough for the goshawk to ma-
neuver and attack. The mature forest is the best
compromise; prey in more open or denser habitat
is less accessible. For example, goshawks avoided
young forest, despite the fact that this habitat was
preferred by one important prey species, the Black
Grouse (Kolstad et al. 1985).

Due to forestry, the proportion of old, mature
forest in Sweden has decreased (Svensson 1980)
and the forest is being fragmented into smaller
patches. Obviously, both trends may negatively af-
fect goshawks in boreal Fennoscandia.

Kenward (1982) studied goshawk habitat utiliza-
tion in three areas with mixed farmland/woodland
in central Sweden, containing 41-61% woodland. In
all areas, he found preference for forest as com-
pared to farmland, although he did not separate
successional forest stages. He also found a prefer-
ence for forest edge. The forest patches he studied
were surrounded by farmland where prey occurred,
predominantly Ring-necked Pheasants ( Phasianus
colchicus ) and European hares ( Lepus europaeus).
Goshawks used the forest edge as cover where they
perched and from where attacks were launched.

In the Widen (1989) study area, the forest patch-
es were mainly surrounded by forests in other suc-
cessional stages (e.g., younger than the preferred
mature forest). These younger stages offered no
good hunting habitats for the hawks and thus the
edges were not preferred.

Discussion

Effects of Forest Management. Available data
show that Fennoscandian goshawk populations
have declined by 50-60% from the 1950s to the
1980s, and I have concluded that neither pesticides
nor persecution can explain this decline, but that
changes in prey populations and habitats are im-
portant factors. Further, it is striking that the de-
cline coincided in time with forest-management in-
tensification. Thus, w 7 e are left with the conclusion
that forest management, acting in different ways,
is a prime factor behind the goshawk decline. I
suggest that large-scale changes in the boreal forest
landscape, caused by modern forest management,
has resulted in a deterioration of goshawk hunting

112

Widen

Vol. 31, No. 2

range quality, and that this, although difficult to
measure, is more important than nest-site avail-
ability. When discussing habitat suitability, it is im-
portant to include prey accessibility and density in
addition to nest-site availability. The goshawk re-
quires prey that it is able to catch. To discover im-
portant hunting habitat requirements, one needs
to know where raptors hunt and their hunting suc-
cess in each place (Kenward and Widen 1989).

In Denmark, which is south of the boreal forest,
the population trend seems to have been different.
Here, the goshawk has increased from the 1960s
until the beginning of the 1980s (j 0 rgensen 1989).
This was explained as a result of decreased pres-
sure from persecution and pesticides.

My conclusion that hunting habitats are more
crucial than nesting habitats for goshawks in the
modern forest landscape does not indicate that
availability of good nesting habitat can be complete-
ly rejected as a possible limiting factor for goshawk
populations. Since goshawks are territorial, with a
regular spacing of nests (Widen 1985), they cannot
breed close together, and therefore it is important
how the patches of good nesting habitat are spaced.

Further, there must be more suitable habitat
patches than are currently needed. New individ-
uals recruited into the population must be able to
find unoccupied sites. In order for the whole pop-
ulation to survive, sites that are temporarily unoc-
cupied must be available for colonization by new
breeders. A site that has become temporarily un-
occupied is a potential resource for new breeders,
which are recruited into the population. If unoc-
cupied sites are destroyed because there are no
longer any hawks there, we lose that possibility. A
raptor population may go extinct because of a lack
of nest sites, even if we never destroy a single oc-
cupied nest site, but destroy those that are tem-
porarily unoccupied.

Declining prey populations can be an important
factor, but the relationship between prey decline
and forestry needs to be explained. Selas (pers.
comm.) explains grouse number declines as an ef-
fect of increasing numbers of red fox, but does not
explain why foxes have become more common.
Forsman and Ehrnsten (1985) argue that the gos-
hawk decline is due to modern forestry, affecting
the goshawk in two different ways. Birds of optimal
prey size (e.g„, grouse) are becoming rarer and are
being replaced with smaller birds. Second, good
nesting habitat (e.g., mature forests) is becoming
rarer. Wikman and Linden (1981) argue that lack

of nesting habitat is not a problem, but that habitat
destruction may act indirectly by depleting habitat
for prey animals.

A general discussion about grouse populations
in Fennoscandia is beyond the scope of this paper,
but considering the effects that forestry has had on
the forest landscape, it would be surprising if
grouse have not been affected.

The goshawk problem in boreal forests cannot be
solved by creating protected areas; they need areas
too large to be effectively protected that way. We
must concentrate on determining what is important
for goshawks and use that knowledge to direct for-
estry practices that establish adequate protection.

Recommendations. First, when mature forest is
fragmented by clear-cutting, the fragments should
be as large as possible. It is generally better to
make one large clear-cut than several small ones.
Second, nest sites must be protected, even if they
are unoccupied. A surplus of well-spaced patches
of good nesting habitat is needed. Third, there
must be enough forest with old-forest qualities in
the landscape. Research is needed to determine
how much is enough. Fourth, we need more re-
search on the goshawk’s hunting technique and
hunting success in different habitats.

Acknowledgments

I am grateful to Vidar Selas who allowed me to use his
unpublished manuscript on a goshawk population in
south Norway, to Marcus Wikman who gave me access to
unpublished data from the Finnish Game and Fisheries
Research Institute and to three referees for improving
the manuscript.

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Received 2 November 1995; accepted 6 March 1997

J. Raptor Res. 31 (2) :1 14-124
© 1997 The Raptor Research Foundation, Inc.

FOREST MANAGEMENT AND CONSERVATION OF
BOREAL OWLS IN NORTH AMERICA

Gregory D. Hayward

Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071 U.S.A.

Abstract. — Boreal Owls ( Aegolius funereus) in North America occur throughout the boreal forests of
Canada and Alaska and in subalpine forests of the Rocky Mountains north of central New Mexico. A
recent assessment of Boreal Owl conservation status in the western mountains of North America sug-
gested that Boreal Owls were not in immediate peril. However, in the long-term and in selected local
areas, Boreal Owls likely face conservation problems. This conclusion reflects the hypothesized response
of Boreal Owls to the type and pattern of forest harvest that occurred in the past and may occur in the
future. Over the last 40 yr, a majority of timber harvest occurred as clear-cutting that removed the older,
more diverse forest stands. Forest structure influences the availability of suitable cavities, the quality of
roost sites, the foraging movements of individual owls and prey availability. Components of mature and
older forests are especially important to Boreal Owl habitat quality; the owls nest in large tree cavities
and prey populations are most abundant in older forest stands. Clear-cut sites will remain unsuitable
for roosting or foraging for a century or more and new nest trees will not develop in some situations
for two centuries or longer. Timber harvest which maintains components of mature forest well dispersed
across the landscape may be compatible with conservation of Boreal Owls. In particular, forest manage-
ment must consider the consequences of management decisions across broad spatial scales and over a
long-term horizon. Metapopulation modeling and experimentation through adaptive management will
be necessary to develop timber harvest practices compatible with conservation of Boreal Owls.

Key Words: forest management, Boreal Owl, Aegolius funereus; woodpeckers, small mammals, adaptive management.

Administracion Forestales y Conservacion de Buhos Boreal en Norte America

Resumen. — El Buho Boreal Aegolius funereus en norte america ocurre en todas partes de bosques boreal
en Canada y Alaska y en bosques sub-alpino en las montahas Rocosas norte de centro Nuevo Mexico.

Una evaluacion reciente del estado de conservacion del Buho Boreal en las montahas del oeste en norte
america sugiera que el Buho Boreal no esta en peligro inmediato. Sin embargo, en la larga duracion, y
en areas seleccionadas en el local, Buho Boreal pueden encontrarse con problemas de conservacion. Esta
conclusion reflecta la respuesta hipotesisada del buho boreal para el tipo y ejemplo de cosechas de bosque
que ocurrio en el pasado y puede ocurrir en el futuro. En los ultimos 40 anos una mayorfa de cosechas
de madera ocurrio en el corte-completo que quito areas de bosque maduros y de mas diversidad. La
estructura de bosque influencia la disponibilidad de parcelas suficiente, la calidad de perchas, los movi-
mientos de forraje de buhos solitarios, y la disponibilidad de cazar. Componente de bosques maduros y
viejos son especialmente importante al habitat del Buho Boreal: Los buhos hacen nidos en cavidades
grandes de los arboles y poblaciones de cazar son mas abundante en parcelas de bosque viejas. Sitios
cortados-completo se quedaran inconveniente para perchas o forraje para un siglo o mas y arboles con
nidos nuevos no se desarrollan en unos situaciones por dos siglos o mas. Cosecha de maderas que man-
tienen componente de bosques maduros bien dispersos a traves el paisaje puede estar compatible con la
conservacion de Buhos Boreal. En particular la administracion de bosques necesita considerar las conse-
cuencias de las decisiones que hace a traves de la escala de espacio amplio y sobre suficiente tiempo con
perspectiva. Modelos de meta-poblacion y experimentacion a traves de administracion adoptivo va ser
necesario para desarrollar costumbres compatible de cosechas de madera con conservacion de Buhos
Boreal.

[Traduccion de Raul De La Garza, Jr.]

The North American distribution for Boreal coasts in the boreal forests of Alaska and Canada

Owls ( Aegolius funereus) forms a relatively continu- (Godfrey 1986). South of the continuous transcon-

ous band extending from the Pacific to Atlantic tinental band, disjunct populations occur in the

114

June 1997

Forest Management and Boreal Owls

115

Rocky Mountains extending from Canada to north-
ern New Mexico (Palmer and Ryder 1984, Hay-
ward et al. 1987, Whelton 1989, Stahlecker and Ra-
winski 1990). Throughout this broad distribution
the owl occurs in a variety of boreal and subalpine
forests: conifer and mixed forests in Canada (Bon-
drup-Nielsen 1978), transition forests in Minnesota
(Lane 1988) and subalpine forests in the Rockies
(Hayward et al. 1993). Boreal Owl populations are
intimately linked to the composition, structure and
dynamics of these forests (Hayward and Hayward
1993, Hayward and Verner 1994). Therefore, the
distribution and abundance of Boreal Owls may be
strongly influenced by forest management practices.

How do populations of Boreal Owls respond to
alternative approaches in forest management? In
this paper I provide a perspective on the potential
impacts of forest management on the owl. Forest
management represents the human activity most
likely to influence the long-term distribution and
abundance of Boreal Owls. Among Holarctic rap-
tors, Boreal Owls, at least in the North American
Rockies, may represent the species whose ecology
is most universally tied to the forest system. An un-
derstanding of the potential response of Boreal
Owls to various changes in forest structure and dy-
namics is a critical step in designing management.

In the U.S., management of Boreal Owls has be-
come an important task on public lands. Four Na-
tional Forest Regions and the Superior National
Forest which represent most of the species’ range
south of Canada have designated the Boreal Owl
as a “sensitive species.” Within the National Forest
System, sensitive species are plants and animals
whose population viability is identified as a con-
cern by a Regional Forester. Sensitive species re-
quire special management and programs are un-
derway to develop management plans for Boreal
Owls (J, Friedlander pers. comm.).

Unfortunately, the knowledge needed to devel-
op a sound management strategy may be lacking
(Hayward 1994a). To date, only four major pub-
lished investigations from North America provide
the ecological basis for management planning
(Bondrup-Nielsen 1978, Palmer 1986, Hayward et
al. 1992, Hayward et al. 1993). None of these in-
vestigations represent experimental approaches to
ecological questions, none of these was designed
to directly address forest management issues and
all extended for 4 yr or less — a temporal scale in-
sufficient to address important issues in forest
management or the ecology of a long-lived verte-

brate. The Boreal Owl in North America repre-
sents a classic example of uncertainty in wildlife
management.

Over 14 yr ago, Romesburg (1981) admonished
wildlife managers for the development of manage-
ment plans built upon unreliable knowledge. Man-
agement built on poor science leads to a loss of
credibility and poor resource management. Cur-
rent understanding of Boreal Owl ecology and bi-
ology is poor. Management built on this founda-
tion alone will invite criticism and loss of credibil-
ity. Recently though, Murphy and Noon (1991) dis-
cussed an approach to deal with the inherent
uncertainty associated with management of a forest
raptor, the Spotted Owl (Strix occidentals). They ad-
vocate applying the hypothetico-deductive ap-
proach to management. Through a rigorous as-
sessment of the assumptions that form the basis of
management, they reduce the uncertainty cloud-
ing an evaluation of the efficacy of various man-
agement options. Walters’ (1986) adaptive man-
agement concepts are another attempt to deal with
the uncertainty that accompanies wildlife manage-
ment.

My perspectives on forest management for Bo-
real Owls is guided by a philosophy that combines
the concepts of the hypothetico-deductive method
and Walters’ adaptive management to develop
management in the face of poor knowledge.
Therefore the statements I make regarding the po-
tential response of Boreal Owls to forest manage-
ment, must be regarded as hypotheses. I would ad-
vocate the testing of these hypotheses through
multi-scale experiments in the spirit of adaptive
management.

To provide a perspective on forest management
and Boreal Owls, I will review the conservation sta-
tus of Boreal Owls in North America including a
discussion of trends in forest management, exam-
ine our understanding of the ecology of Boreal
Owls as it relates to the owl’s potential response to
forest management, present some hypotheses con-
cerning how different forest management ap-
proaches may influence Boreal Owls on different
geographic and temporal scales and provide some
ideas concerning strategies to approach forest
management for Boreal Owls.

The perspective I present is biased by the geo-
graphic limits of my field experience with Boreal
Owls — I have worked in the Rocky Mountains.
More important, the literature on Boreal Ow4 ecol-
ogy in North America is limited. Literature from

116

Hayward

Vol. 31, No. 2

Europe significantly broadens our understanding
of the species. However, the ecology of Boreal Owls
differs geographically within Europe (Korpimaki
1986) and within North America (Hayward et al.
1993). I suspect that the response of Boreal Owls
to forest management differs between the Old and
New Worlds and geographically within both.

Although our understanding of Boreal Owl ecol-
ogy in North America is limited to three forest sys-
tems (one in each of northcentral Canada, central
Idaho and northern Colorado), the Boreal Owl ap-
pears to occupy a variety of forest types. These for-
ests range from deciduous and mixed forests to
subalpine conifer forests (Meehan and Ritchie
1982, Palmer 1986, Lane 1988). The dynamics of
these forests differ substantially due to differing
patterns of forest growth and different disturbance
regimes (Knight 1994). Likewise, Boreal Owl pop-
ulation dynamics, relationships with primary cavity
nesters and relationships with prey populations dif-
fer among these forest types (Hayward 1994b).
Therefore, the response of the owl to alternative
forest management patterns almost certainly dif-
fers geographically. Any forest management
scheme must be cognizant of the differences
among the forest systems.

Status of Boreal Owls in North America

Trends in population abundance or trends in
habitat conditions are often used to assess status
(Anderson 1991). In 1994, the U.S. Forest Service
published an assessment of Boreal Owl status (Hay-
ward and Verner 1994). That document concluded
that Boreal Owls were not in immediate peril
throughout their range but that over the long-term
and in local areas over the short-term, Boreal Owls
likely face significant conservation problems in the
absence of conservation planning. To reach this
conclusion the assessment examined evidence con-
cerning trends in the distribution and abundance
of the owl and the habitat relationships of the owl.

Distribution and Abundance of Boreal Owls. Lit-
tle evidence exists to assess changes in the distri-
bution of Boreal Owls in North America. Prior to
1979 the owl was not recognized as a breeding bird
south of Canada (Eckert and Savaloja 1979). Since
then numerous published reports have extended
the recognized range of Boreal Owls in North
America (Palmer and Ryder 1984, Hayward et al.
1987, Whelton 1989). Today, evidence exists for
breeding populations throughout the Rocky
Mountains south to southwestern Colorado and

northern New Mexico (Stahlecker and Rawinski
1990, Stahlecker and Duncan 1996). Do these re-
cords indicate an extension of the species range?

I suggest that the actual distribution of Boreal
Owls has not changed recently, but our knowledge
of distribution has increased because of an in-
crease in survey effort. Historical records indicate
that Boreal Owls were recorded in the western
United States but not recognized as breeding. A
closer look at the literature indicates that Boreal
Owls w T ere documented in Colorado for nearly 100
yr (Ryder et al. 1987). Despite the occurrence of
Boreal Owls in the western U.S., checklists and
field guides did not list the species even after
breeding populations were documented in 1983.
Biologists in Europe also located new populations
of Boreal Owls during the past three decades and
attributed these to increased interest in the species
(Cramp 1977).

Direct evidence concerning trends in Boreal
Owl abundance is completely lacking for North
America. Breeding populations of Boreal Owls
were only recently documented throughout most
of the species’ range in the U.S. Studies in North
America generally have not focused on demogra-
phy, precluding any assessments of trend in the
near future. I am aware of only two populations
(one in Idaho and one in Montana) that have been
sampled using methods that will facilitate rigorous
assessment of trends within the next 5 yr (Hayward
et al. 1992). The prospects for assessing trends in
the near future appear bleak.

Abundance and Distribution of Important Hab-
itats. Information on trends in condition of forest
habitats used by Boreal Owls offers an indirect
method to infer population trends. Gathering and
summarizing the necessary information at a broad
geographic scale is not feasible for this paper. Fur-
thermore, most statistics on timber harvest do not
include the information necessary to evaluate the
pattern in distribution and abundance of impor-
tant forest types. For instance, stand-replacement
harvests (clear-cuts) create stands without habitat
value for Boreal Owls for a century or more, while
partial cutting may leave stands with high habitat
value if dominant trees are not removed. An ob-
jective evaluation of habitat trends relies not only
on knowledge concerning recent timber harvest
but knowledge on succession of lands that experi-
enced large disturbance events 100-150 yr ago.

Maybe more important than the problems with
describing impacts from past harvest are the diffi-

June 1997

Forest Management and Boreal Owls

117

culties in predicting future harvest. As the avail-
ability of timber has declined on lower elevation
forest lands in western North America, focus is
shifting to high elevation spruce-fir forests used by
Boreal Owls. Furthermore, the rules regulating
timber harvest in the U.S. have changed recently
regarding salvage after fire (U.S. Public Law 104-
19). The consequences of these changes are diffi-
cult to predict. As they might say in a prospectus,
the extent of future harvest and therefore impact
on Boreal Owl habitat may not be related to past
trends.

Summary. There is little direct evidence con-
cerning trends in North American Boreal Owl pop-
ulations. In a Boreal Owl conservation assessment
(Hayward 1994c), evaluation of habitat use pat-
terns, life history and trends in habitat condition
were used to infer owl trends.

Habitat Relationships of Boreal Owls

I review the habitat relationships of Boreal Owls.
My goal is to establish the relationship between the
owl and the forest to form hypotheses concerning
the potential response of Boreal Owls to forest
management.

Habitat relationships of Boreal Owls and habitat
relationships of principal prey species will, in large
part, dictate the potential response of Boreal Owls
to timber management. The realized impact of for-
est management in a particular situation will be
determined by the interaction of habitat relation-
ships of the owl and prey populations mediated by
those factors currently limiting population growth.
Nesting habitat conditions (especially cavity avail-
ability), prey availability (winter and summer) and
microclimatic conditions related to owl thermoreg-
ulation likely limit the distribution and abundance
of Boreal Owls in different populations (Hayward
1994b). Management that focuses on these limit-
ing factors, after examining evidence suggesting
which factor may be most critical in a particular
setting, will most effectively target management ac-
tions.

As I have emphasized, the ecology of Boreal
Owls varies geographically. For instance, daily and
annual movement patterns, relationship with prin-
cipal prey populations, population stability and
limiting factors vary from the boreal forests of Can-
ada to southern New Mexico (Hayward et al.
1993). Despite this variation, Boreal Owls are forest
owls throughout their range and their ecology is
linked to forest habitats with particular structural

characteristics. I also consider nesting, roosting
and foraging habitat separately because each of
these may be limiting in different management set-
tings. I will review the evidence describing the link
between forest conditions and Boreal Owl popu-
lations. In my review I move from fine scale habitat
characteristics to more broad scale relationships.

Fine Scale Habitat Relationships. Nesting habitat.
The requirement for a large tree cavity constrains
the range of sites used by Boreal Owl for nesting
habitat. As secondary cavity nesters, boreals are in-
timately linked with the organisms and processes
associated with formation of large tree cavities. An
envirogram (Andrewartha and Birch 1984) empha-
sizes the linkage between forest structural condi-
tions, primary cavity nester populations (wood-
peckers), forest insects and pathogens (Fig. 1).
The elements of the centrum relate directly to the
owl while the web depicts components of the sys-
tem important to maintaining the centrum. Ele-
ments of this envirogram are forest characteristics
associated mainly with the presence/absence of
suitable nesting cavities.

Beyond cavity availability, observations in the
Rocky Mountains suggest that forest structural
characteristics are important in nest-site selection.
In Idaho, comparisons of forest structure at nest
sites and random sites indicated use of stands with
mature and older forest structure. Forest structure
at nest sites differed from the random sample (101
sites) of available forest. Used sites occurred in
more complex forest, with higher basal area, more
large trees and less understory development than
available sites (Hayward et al. 1993). Also in Idaho,
a small nest-box experiment evaluated whether
choice of nest sites is driven solely by cavity avail-
ability or if forest structure per se is important
when a range of alternatives are available (Hay-
ward et al. 1993). In this experiment nest boxes
were hung in three forest types that differed sig-
nificantly in structural characteristics. Owls used
boxes in two forest types with complex structure
(e.g., multiple canopy layers, many tree size class-
es) but did not use boxes in the forest type with a
more simple structure (e.g., single canopy layer,
more uniform tree diameters). Based on our ob-
servations I hypothesize that forest structure is im-
portant in an indirect way. Owls first search for nest
sites in forests of a particular structure because the
probability of finding cavities is highest in those
types. So selection of old forest for nesting may be

118

Hayward

Vol. 31, No. 2

WEB

CENTRUM

MATES

mature forest .

water

weather .

mature forest .

water

insects and other ,
foods

large live trees

primary cavity nesting r snags with cavities
birds

large snags or
trees with rot

topography and micrq- ^ature aspen stand
site conditions

fire or other

major disturbance

water

Lstand with moderate
to large trees

mature and older .
forest stands

insects and other .
foods

large live trees

. large trees with

cavities (esp. aspen)

site with structure

associated with
high probability of
haviing suitable cavities

forest rodent ,
populations

primary cavity
nesting birds

dispersed large snags
or trees with rot

water .

mature forest

landscape of suitable .
foraging habitat

.landscape with
dispersed suitable
nesting habitat

nest site:
large tree cavity
(suitable nest; as a
token)

nest site:

forest stand (recognized as
potential breeding habitat; as
a token)

intact population

Figure 1. Envirogram (Andrewartha and Birch 1984) illustrating the relationship between Boreal Owls and specific
components of the forest system. This portion of a larger envirogram (Hayward 1994b) focuses on Boreal Owl nesting
ecology.

based more on efficiency in finding a cavity than
increased survival after locating a nest.

The same studies in Idaho suggest that patch
size may not be an important characteristic of nest
stands. Nest stands ranged in size from 0.8 to 14.6
ha and averaged 7.6 ha.

Roosting habitat. Patterns of roosting habitat use
also suggest these owls choose forests with partic-
ular structural features during certain times of the
year. In Idaho, forest structure at summer roost
sites differed substantially from paired random
sites. Roost sites had higher canopy cover, basal
area, and maybe most important, were significantly
cooler microsites (Hotelling’s T 2 , P< 0.001) (Hay-
ward et al. 1993). In summer, and particularly in
the southern portion of their range, Boreal Owls
find roost sites to minimize heat stress. We wit-
nessed owls gular fluttering and other behaviors
associated with heat stress when the temperature
was as mild as 18°C. I hypothesize that the eleva-
tional distribution of Boreal Owls in the Rockies
may be determined, in part, by summer tempera-

tures and the availability of cool microsites for
roosting. Forest structure, then, may influence the
distribution of Boreal Owls through an interaction
with limitation by heat stress.

Foraging habitat. A variety of evidence suggests
that Boreal Owls in the Rockies forage principally
in mature and older forest, especially spruce-fir for-
ests (Hayward 1987). These observations are cor-
roborated by evidence that red-backed voles ( Cleth-
rionomys gapperi) represent a dominant prey for Bo-
real Owls throughout their range in North Amer-
ica (Bondrup-Nielsen 1978, Palmer 1986, Hayward
and Garton 1988, Hayward et al. 1993). Red-
backed voles are principally forest voles (Hayward
and Hayward 1995). Our studies of small mammals
in Idaho found redbacks were up to nine times
more abundant in mature spruce-fir forest than
other forest habitats (Hayward et al. 1993). The
argument for the importance of mature forest for
foraging stems also from observations of snow
characteristics in openings, young forest and ma-
ture forests. Snow crusting is significantly reduced

June 1997

Forest Management and Boreal Owls

119

in mature forests facilitating access to small mam-
mals during critical winter periods (Sonerud 1986,
Sonerud et al. 1986). In Idaho, mortality and sig-
nificant movement events most often occurred
during warm winter periods when snow crusting
became severe.

An envirogram further emphasizes the link be-
tween Boreal Owl foraging habitat and particular
features of the forest, especially features linked
with mature forests (see Hayward 1994b). The en-
virogram illustrates the indirect tie between Boreal
Owl fitness and abundance of lichen, fungi and
Vaccinium ground cover — all of which can be influ-
enced by various forest management practices.

The evidence regarding habitat use for nesting,
roosting and foraging in the Rockies suggests that
at a fine scale, Boreal Owls rely on particular char-
acteristics of mature and older forests. This rela-
tionship suggests that forest management at the
level of stands will likely influence abundance of
Boreal Owls.

Landscape Scale Habitat Relationships. Analysis
of patterns of Boreal Owl abundance in relation to
landscape patterns is not available for North Amer-
ica. Indirect evidence from Europe and North
America suggests that Boreal Owls differentiate
among forest habitats at the landscape scale. Our
observations of owls in Idaho suggest that land-
scapes dominated by mature spruce-fir forest or
those with mature spruce-fir juxtaposed with ma-
ture larch ( Larix sp.), ponderosa pine {Pinus pon-
derosa ) or aspen ( Populus tremuloides) sites will have
the greatest abundance of boreals (Hayward et al.
1992, 1993). In other words, an interspersion of
forests that generally support high density of cavi-
ties in mature spruce-fir forest will provide quality
habitat.

More direct evidence from Europe supports the
notion that landscape scale forest cover influences
Boreal Owl density and productivity. As the pro-
portion of Scotch pine ( Pinus sylvestris ) forest de-
creased and the proportion of Norway spruce for-
est ( Picea abies) and agricultural land increased,
quality of territories (those with more frequent
nesting) increased (Korpimaki 1988). The conclu-
sion that territories with spruce forest and agricul-
tural land (in small patches) were the highest qual-
ity habitat was corroborated by evidence on breed-
ing frequency and clutch sizes.

Regional Scale Habitat Relationships. At very
broad geographic scales, distribution patterns of
Boreal Owls may also have important implications

Figure 2. Pattern of potential Boreal Owl habitat in Ida-
ho suggesting the distribution of a portion of the meta-
population extending along the Rocky Mountains. Poten-
tial habitat is defined as forested sites in the subalpine-
fir zone throughout the state and Douglas-fir woodland
in southeastern Idaho. Other montane forests are not
considered potential habitat (adapted with permission of
Wildl. Monogr. from Hayward et al. 1993).

for management. In portions of the boreal forest,
distributions of Boreal Owls may be quite contin-
uous. Along the southern and northern borders of
the boreal forest and in the Rockies, the owl may
occur in an interesting geographic pattern which
likely results in a strong metapopulation structure
(Hayward et al. 1993). In Idaho, patches of suitable
habitat occur throughout the mountainous land-
scapes in a wide range of patch sizes (Fig. 2). As-
suming that subpopulations of owls occupy habitat
as hypothesized in Figure 2, the metapopulation
structure of the owl in the region is a complex mix
of subpopulations. Because of this structure, man-
agement of forest at the scale of individual national
forests may have important implications for neigh-
boring national forests over a broad geographic re-
gion.

Hypotheses: Boreal Owl Response to
Forest Management

Stand-Replacement Harvest. The importance of
mature forest to Boreal Owls for nesting, roosting
and foraging suggests that the short-term impact
of stand-replacement harvest (clear-cut) will be
negative. Open habitats as well as young, even-age
forests provide few resources for Boreal Owls. Fur-

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Hayward

Vol. 31, No. 2

thermore, these habitats generally do not enhance
habitat for woodpeckers or small mammals. Large
dear-cuts appear to provide no resource values for
Boreal Owls except along edges where owls may
capture prey (Hayward 1994b). However, impacts
will depend upon the size and spacing of cuts and
the forest type being harvested. Furthermore,
long-term impacts may not parallel short term re-
sponse.

I hypothesize that small, patch clear-cuts imple-
mented with long rotations may not negatively im-
pact Boreal Owl habitat over the short- or long-
term. Boreal Owls generally attack prey within 30
m of a perch (Hayward et al. 1993), so most of a
1-3 ha patch cut will be accessible for foraging.
Furthermore, in small patch cuts, ground cover,
which could reduce prey availability, often does not
change significantly from that found under the for-
est, snow crusting affects only a small proportion
of a small forest opening and small patch cuts em-
ulate, to some extent, the landscape structure of
mature spruce-fir forests (Knight 1994). In cases
where small patch cutting is employed, I hypothe-
size that potential negative impacts will be reduced
if the patch cutting is concentrated in a portion of
each watershed rather than dispersed throughout
entire watersheds and mature forest remains in the
matrix between cuts.

Larger clear-cuts in conifer forest most often will
reduce habitat quality for 100 to 200 yr. However,
clear-cutting of aspen may be important in main-
taining the long-term availability of cavities in some
systems. In many forest systems aspen is a pioneer
species that is lost through succession (DeByle and
Winokur 1985), Restoration of aspen forests
through silviculture may be an important manage-
ment tool to maintain Boreal Owl habitat in forest
systems where aspen provides a majority of the
nesting habitat. Through coordinated timber har-
vest, large aspen which provide cavities for nesting
may be maintained over the long-term, at the land-
scape scale, despite loss from individual stands. Fo-
cus on aspen management may even be more im-
portant in systems where aspen occupies a small
proportion (<1%) of the landscape and occurs in
small patches associated with particular microsites.

The shape of clear-cuts will likely influence both
the short- and long-term impact on Boreal Owls.
Although no direct evidence is available, I hypoth-
esize that more complex shaped cutting units, es-
pecially those with stringers of forest extending
into cutting units in upland areas, riparian buffers

and patches of forest remaining within the cut
unit, will have fewer negative impacts than large
rectangular or circular cuts. This hypothesis stems
from the pattern of habitat use by Boreal Owl prey
species (Williams 1955, Merritt 1981, Wells-Gosling
and Heaney 1984) and observations that Boreal
Owls will nest in small patches of forest (G. Hay-
ward unpubl. data).

Based on the same arguments, sloppy clear-cuts
(clear-cuts with residual standing dead and live
trees, especially aspen and patchy slash), and cuts
that retain standing and downed wood on the site,
will have fewer negative impacts, especially over the
long-term. The mitigating qualities of retaining
patches of live trees and shrubs, snags and woody
debris arise from several factors. These elements
will accelerate the rate at which the future stand
attains mature and older forest characteristics
(Knight 1994). In particular, recovery of fungi and
lichen populations may be accelerated by mainte-
nance of residuals (Ure and Maser 1982, Flansen
et al. 1991).

Partial Cutting and Uneven-Age Management.

Discussion of sloppy clear-cuts or irregular shelter-
wood prescriptions leads logically to discussion of
partial cutting and uneven-age regeneration pre-
scriptions. I hypothesize that group selection (har-
vest of small groups of trees in an uneven-age
stand, maintaining the uneven-age properties) may
not significantly reduce Boreal Owl habitat quality
in many situations if, over the long-term, mature
and old forest qualities are maintained and tree
species composition does not exclude important
cavity trees. Timber harvest prescriptions such as
group selection and single tree selection (harvest
of individual trees from an uneven-age stand in a
pattern that maintains the size structure of the
original stand) that retain forest structure, are
compatible with developing owl nesting habitat.
Thinning from below (harvest which removes in-
dividuals smaller than the dominant size class) and
single tree selection that reduces competition
among dominant trees and increases tree growth,
could accelerate the process of developing suitable
nest structures. While clear-cutting eliminates red-
backed voles in Rocky Mountain forests (Campbell
and Clark 1980, Scrivner and Smith 1984, Ramirez
and Hornocker 1981), preliminary results of an ex-
periment examining clear-cuts and group selection
harvests indicate that red-backed voles remain
abundant in partial cut stands when many large

June 1997

Forest Management and Boreal. Owes

121

trees are retained and ground disturbance is min-
imal (G. Hayward unpubl. data).

Broad Scale Predictions, Predicting the response
of Boreal Owls to differing landscape scale patterns
is more difficult. The lack of information on pat-
terns of Boreal Owl abundance at the landscape
and broader scales precludes extensive predictions
at broad scales. I would argue that a primary focus
of adaptive management approaches should be at
this scale.

The issue of fragmentation seems to dominate
much of the discussion of landscape scale impacts,
so preliminary predictions regarding fragmenta-
tion may be useful in stimulating inquiry. In refer-
ring to potential response to fragmentation, I ex-
plicitly separate the influence of habitat loss from
the influence of increased landscape heterogene-
ity. Fragmentation effects result from the process
of changing the characteristics of the landscape
mosaic and must be considered after eliminating
the direct influence of reducing habitat area.

The high mobility and the extensive areas used
on a daily basis by Boreal Owls suggests they may
react to fragmentation differently from passerines.
For instance, timber harvest of 30% of a basin
through clear-cutting mature lodgepole pine (Fi-
rms contort a ) in 1-5 ha patches dispersed through-
out the area may not significantly reduce habitat
quality if the remaining forest is dominated by ma-
ture and older spruce-fir forest. The forests used
by Boreal Owls exhibit a patchy mosaic under nat-
ural disturbance (Knight 1994). In a natural forest
mosaic, owls move between distant patches on a
daily basis (Hayward et al. 1993). This hypothesis
assumes that timber harvest would not significantly
reduce small mammal populations in the unhar-
vested stands.

Aside from fragmentation, it is important to con-
sider the impact of harvest schemes that target dif-
ferent forest types: aspen, lodgepole pine or old
spruce-fir forests. I hypothesize that the negative
impacts of any stand replacement harvest scheme
will be decreased if stands of mature and older
spruce-fir or aspen forest remain dispersed
throughout the landscape.

Predicting the consequences of management at
the broadest spatial scales is challenging. Conser-
vation strategies at the regional scale should focus
on maintaining the continuity of Boreal Owl me-
tapopulations. This involves identifying subpopu-
lations and landscapes that likely play key roles in
the persistence of owls within the region and

neighboring regions. These subpopulations would
receive special attention to assure that manage-
ment actions either favored the owl or did not neg-
atively impact the subpopulation. Spatial modeling
and good information on dispersal will be neces-
sary to make sound management predictions at
this scale.

Strategies to Approach Forest Management for
Boreal Owls

I began this discussion by emphasizing the ex-
tent of uncertainty in our understanding of Boreal
Owls and noted the substantial geographic varia-
tion in Boreal Owl ecology across North America.
In combination, these factors produce a discour-
aging management environment where predic-
tions must be made tentatively. Therefore, the re-
sponse of Boreal Owls to forest management was
framed as a series of hypotheses to be tested and
likely only testable through adaptive management.
Despite the degree of uncertainty and the extent
of geographic variation, I believe some general
points can be made concerning approaches to for-
est management and planning for Boreal Owls.

Limiting Factors. Site-specific forest manage-
ment for Boreal Owls must consider the factors
most likely limiting the population in a particular
setting. Thermal stress likely limits the elevation
distribution of Boreal Owls in the central and
southern Rocky Mountains. Therefore, availability
of cool microsites, which often occur in mature
and older forests, may be important in many
regions.

The availability of nest cavities and prey likely
limit populations of Boreal Owls in different situ-
ations. In regions with few or no Pileated Wood-
pecker (Dryocopus pileatus) or Northern Flicker ( Co -
laptes auratus ) cavities, nest-site availability will limit
Boreal Owl abundance. Even within the geograph-
ic range of Pileated Woodpeckers, the absence of
these woodpeckers at higher elevations may limit
Boreal Owl abundance (Hayward et al. 1993). If
cavity availability limits Boreal Owl populations,
management of primary cavity excavators as well as
the forest processes that support large snags will
influence Boreal Owls.

In some forests, cavities are abundant and prey
availability may play a strong role in Boreal Owl
population dynamics. It is unclear whether abso-
lute abundance or variation in prey populations is
more important in owl population regulation.
However, small mammal populations appear to be

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Hayward

Vol. 31, No. 2

linked to forest conditions (Hayward and Hayward
1995) and forest management will influence the
abundance of potential prey, and in turn, affect
owl population persistence. Forest structure will
also influence the availability of prey by changing
owl access to prey. For instance, forests with dense
ground cover or a high density of small trees will
reduce the efficiency of foraging Boreal Owls. Fur-
thermore, forest structure affects snow conditions
which influence prey availability (Sonerud 1986).

Cavity availability and prey availability likely in-
teract to influence Boreal Owl population growth.
Tree cavities occur nonrandomly across the land-
scape as do small mammal populations. The spatial
arrangement of cavities and prey (relative to one
another) are important in determining Boreal Owl
abundance. The conservation status of Boreal Owls
will be intimately tied to the interaction of these
resources.

While cavities and prey likely limit Boreal Owl
populations in most landscapes, predation and
competition may influence populations in certain
circumstances. In local situations, mustelids de-
stroy a high proportion of owl nests in some years
(Sonerud 1985). The influence of these losses on
population abundance is unknown. Evidence also
indicates that interactions with other owls may in-
fluence the distribution of Boreal Owls suggesting
that competition may be an important limiting fac-
tor in some situations (Hakkarainen and Korpi-
maki 1996).

Boreal Owl Management Within Ecosystem Man-
agement. In western North America the ecology of
Boreal Owls is linked with many characteristics of
mature and older spruce-fir forests (Hayward
1994b). Management which facilitates the long-
term maintenance of a landscape with significant
representation of mature and older forest habitat
will provide quality Boreal Owl habitat. Therefore,
management schemes which promote the process-
es that maintain productive spruce-fir forests, and
management which facilitates the stand dynamics
necessary to produce old spruce-fir forest, will pro-
vide the habitat characteristics necessary for Boreal
Owis. As indicated earlier, this is not incompatible
with timber harvest.

Most applications of ecosystem management
strive to manage systems to emulate natural distur-
bance patterns and processes. As reviewed by-
Knight (1994), spruce-fir forests experience a va-
riety of disturbance agents that act at scales rang-
ing from single trees to hundreds of hectares. De-

velopment of old forest conditions following stand
replacement disturbance proceeds slowly; succes-
sion to mature forest conditions takes >150 yr.
However, old forest stands represent a mosaic re-
sulting from the frequent action of small scale dis-
turbance. Partial cutting emulates (to some extent)
insect mortality and windthrowr, two common dis-
turbances integral to the formation of old spruce-
fir forest structure. Alexander (1987:59) indicated
that “uneven-aged cutting methods — individual
tree and group selection — have seldom been used
in spruce-fir forests, they appear to simulate the
natural dynamics of these forests.” Therefore, care-
ful harvest of trees from spruce-fir forest may not
be incompatible with maintaining important ele-
ments of old forest and habitat characteristics
linked with Boreal Owls.

The paucity of information available on the re-
sponse of Boreal Owls to specific forest manage-
ment actions presents an obstacle to the formula-
tion of management within an ecosystem frame-
work. A strong conservation strategy for Boreal
Owls cannot be produced without new knowledge
on Boreal Owl ecology. Management based on cur-
rent knowledge must contend with uncertainty and
be devised specifically to deal with this uncertainty.
Adaptive management (Walters 1986), then, must
be built into any approach to manage the species,
particularly an ecosystem management strategy.

Conclusions

Based on my review- of the habitat relationships
of Boreal Owls and management considerations, I
offer the following conclusions: (1) Maintaining Bo-
real Owls on a local scale is not incompatible with
timber harvest but is incompatible with extensive,
stand replacement silviculture implemented over
entire watersheds, employing large cutting units;
(2) Forests with high habitat value for Boreal Owls
develop through long successional trajectories.
Therefore forest management must consider long-
term forest patterns on broad spatial scales; (3)
The hypothesized metapopulation structure of Bo-
real Owls in North America suggests that forest
management must be coordinated at a regional
scale; (4) Adaptive management which links man-
agers and research ecologists is necessary to pro-
duce the knowledge needed to understand the re-
sponse of Boreal Owls to alternative management
approaches at a variety of spatial scales; (5) As a
top carnivore that preys upon the dominant small
mammal species in subalpine forests and nests in

June 1997

Forest Management and Boreal Owls

123

large tree cavities, the Boreal Owl integrates into
its ecology many aspects of forest dynamics. As
such, the owl may represent a good model to aid
in developing ecosystem management; (6) At all
spatial scales, an eye to restoration management
must be taken in landscapes that have experienced
intensive harvest in the past. Restoration may be
particularly appropriate in aspen forests of the
Rocky Mountains.

Forest management which sustains mature sub-
alpine and boreal forests likely will conserve Boreal
Owls. Such management, however, must consider
(among other things) the successional dynamics of
spruce-fir forests including the detritus food chain,
the consequences of various disturbances and the
long-term (post-glacial) trends in these forests.
Management must focus as much on the long-term
condition of the plant communities used by Boreal
Owls as on the population dynamics of the owl.

Acknowledgments

I sincerely thank Gerald Niemi and JoAnn Hanowski
for organizing the symposium that led to the develop-
ment of this paper. Erin O’Doherty, Edward (Oz) Garton,
Ronald Ryder, Gerald Niemi and Daniel Varland re-
viewed an earlier draft of this paper; their comments
were very helpful. The ideas presented here would not
be possible without the dedicated fieldwork of many per-
sons including folks who have aided in my field studies.
Discussions with Pat Hayward, Floyd Gordon, Edward
(Oz) Garton and Joan Friedlander have been influential
in development of my philosophy of wildlife manage-
ment in forest systems. Development of this paper was
supported in part by U.S.F.S., Intermountain Forest and
Range Experiment Station, RWU 4201.

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, P.H. Hayward and E.O. Garton. 1993. Ecology

of Boreal Owls in the northern Rocky Mountains,
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, P.H. Hayward, E.O. Garton and R. Escano.

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and J. Verner [Eds.]. 1994. Flammulated, Bore-
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KorpimAki, E. 1986. Gradients in population fluctuations
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Received 2 November 1995; accepted 26 February 1997

J. Raptor Res. 31 (2): 125-1 28
© 1997 The Raptor Research Foundation, Inc.

BOREAL OWL RESPONSES TO FOREST
MANAGEMENT: A REVIEW

I Iarri Hakkarainen, Erkki Korpimaki, Vesa Koivunen and Sami Kurki

Laboratory of Ecological Zoology, Department of Biology, University of Turku, FIN-20014 Turku, Finland

Abstract. — Modern forestry during the last decades has strongly increased fragmentation of forest
habitats. This may result in harmful effects on raptor species which are strictly dependent on boreal
forests, such as the vole-eating Boreal Owl ( Aegolius funereus). The long-term data from Finland shows
that in extensive forest areas, fledgling production of Boreal Owls is higher on intensively clear-cut
territories than on less clear-cut territories. Breeding frequency, clutch size and laying date, however,
have not been shown to be related to the proportion of clear-cut areas within a territory. Snap-trapping
data suggests that large clear-cut areas sustain more Microtus voles than small clear-cut areas. The in-
creased number of saplings and clear-cut areas during the last two or three decades has created new
suitable grass habitats for Microtus voles, and simultaneously new hunting habitats for Boreal Owls. There
is some experimental evidence that the presence of the Ural Owl (Strix uralensis) decreases the breeding
density of Boreal Owls within 2 km of Ural Owl nests. Therefore, forest fragmentation does not seem
to harm Boreal Owls at the present day scale, but a lack of nest holes has to be compensated for by
setting nest boxes far (>2 km) from medium-sized and large raptors that can prey upon the Boreal
Owl. In the long-term, however, establishment of snags and patches of mature forests with large trees,
dense enough to satisfy the ecology of the hole-nesting Black Woodpecker ( Dryocopus martins), will
provide a natural way to establish new nesting cavities for Boreal Owls.

Key Words: Aegolius funereus; Strix uralensis; dear-cuttings ; modern forestry, vole density.

Respuesta del Buho Boreal a la Administracion Forestal: Un Reviso

Resumen. — El forestal moderno durante los ultimos decadas ha aumentado con frecuencia la fragmen-
tacion de habitat de bosque. Esto puede resultar en efectos danosos en especie de rapaces que estan
estrictamente dependiente en bosques boreal, como el Buho Boreal ( Aegolius funereus) que come rato-
nes. La informacion de Finlandia ensena que larga duracion en areas de bosques enormes, la produc-
cion de pajaritos de buhos es mas alto en territorios cortados-completo con intensidad que en territorios
menos cortados-completo. La frecuencia de cria, tamano de nidada, y la fecha de poner, no han ensen-
ado estar relacionado a la proporcion de areas cortadas-completo entre el territorio. Informacion de
trampas sugiere que areas grandes que estan cortadas-completo sostienen mas ratones, y simultanea-
mente habitat nuevo para cazar para los buhos. Hay un poco de pruebas experimental que la presencia
de Buho Ural (Strix uralensis) reduce la densidad de cria del Buho Boreal dentro de 2 km del nido del
Buho Ural. Por lo tanto, la fragmentacion del bosque no parece ha eerie daho al Buho Boreal en la
escala presente, pero la falta de nidos de agujero necesita que estar compensado con poniendo nidos
de agujero lejos (>2 km) de rapaces medianos y grandes que pueden cazar a los buhos boreal. En la
larga duracion el establecimiento de tocones y parcelas de bosque maduros con arboles grandes, de
suficiente densidad para satisfacer la ecologia de los nidos de agujero de el Carpintero Negro ( Dryocopus
martius), va proporcionar una manera natural para establecer cavidades de nidos nuevos para el Buho
Boreal.

[Traduccion de Raul De La Garza, Jr.]

During the last decades, modern forestry has
had a strong and perceivable impact on boreal for-
est ecosystems, both in Palearctic and Nearctic
regions. At the landscape level, there is a lack of
large pristine forests (Ohmann et al. 1988), while
remaining mature forest patches have become in-
ternally more homogeneous and more isolated

from larger forest complexes (Hansson 1992). Rap-
tors living in forest habitats are generally consid-
ered to be one of the most sensitive groups of ver-
tebrates to forest management and habitat change
(Newton 1979, Forsman et al. 1984, Carey et al.
1992). This is at least in part because raptors in-
habit large territories (Newton 1979) where as top

125

126

Hakkarainen et al.

Vol. 31, No. 2

Table 1. Annual breeding percentage of nest boxes, laying date (1 = 1 April), clutch size and fledgling production
in sparsely and widely clear-cut territories of Boreal Owls in the Kauhava region, western Finland (ca 63°N, 23°E) .
Statistical tests were performed by Student’s #-test and Mann-Whitney Latest (two-tailed). N = number of territories.

Proportion of Clear-cut Areas within Territory

Low a

HlGH b

Test Value

(± SD)

Breeding percentage

(9)

(15)

U= 139.0

0.22

Laying date

1.41

(19.44)

1.10

(21.98)

T = 0.04

0.97

Clutch size

5.43

(0.88)

5.20

(1.26)

T = 0.54

0.59

No. of fledglings

2.45

(1.26)

3.55

(1.39)

T = 2.06

0.05

a 18% (SD = 7%, range = 10—30%) of total area within 1.5 km of nest was clear-cut.
b 49% (SD = 11%, range = 35—70%) of total area within 1.5 km of nest was clear-cut.

carnivores capture prey which is scarce and diffi-
cult to catch (Temeles 1985). Therefore, they ex-
pend considerable energy in each feeding event,
especially if prey is sparsely and patchily distributed
within the territory. In addition, due to forest har-
vesting, there often is a lack of suitable nesting
places, such as natural cavities and large nesting
trees for many raptor species.

The Boreal Owl ( Aegolius funereus) is a small noc-
turnal hole-nesting raptor which commonly breeds
in coniferous forests in northern Europe (Mikkola
1983). Microtus voles (field vole, Microtus agrestis\
sibling vole, M. rossiaemeridionalis; and bank vole,
Clethrionomys glareolus ) are the main prey of this
species (Korpimaki 1988). Field and sibling voles
inhabit fields as well as clear-cut areas, whereas the
bank vole inhabits mainly forest habitats (Hansson
1978). In poor vole years alternative food sources
have to be used, such as shrews ( Sorex spp.) and
small passerine birds (Korpimaki 1988). Males are
resident after the first breeding attempt, while fe-
males disperse widely (up to 500 km) between suc-
cessive breeding attempts (Korpimaki et al. 1987).

In this review, we focus on how clear-cut areas in
Boreal Owl territories affect reproductive output
and breeding frequency of this species. We also dis-
cuss how clear-cut areas affect the main prey den-
sities of Boreal Owls. Finally, we identify how inter-
specific interactions have to be considered when
setting new nest boxes for owl species that suffer
from the lack of natural cavities. This review is
based on recent investigations (Hakkarainen and
Korpimaki 1996) and on snap-trapping data which
are now examined especially from the perspective
of forest management.

The Effects of Clear-cut Areas on Boreal Owls

The long-term study (1981-95) conducted in the
Kauhava region of western Finland made it possi-
ble to evaluate the effects of clear-cut areas on the
Boreal Owl. These areas comprise clear-cut areas
with 0.2-1. 5 m high saplings (<10-yr old) covering
about one-third of the forests in our study area.
Boreal Owls breeding in areas that are primarily
forested with a mean of 18% (SD = 7%, range 10-
30%) (herein referred to as sparsely clear-cut) of
the total forest area clear-cut within 1.5 km of nests
produced about one fledgling less than those in
areas with a mean of 49% (SD = 11%, range 35-
70%) of the area clear-cut (herein referred to as
widely clear-cut) (Table 1). Most of the territories
and areas sampled within sparsely clear-cut areas
were small cuts of <10 ha with most areas between
1-5 ha. In contrast, in the territories sampled with-
in the widely clear-cut areas, most were relatively
large cuts of up to 200 ha. In addition, territories
within the widely clear-cut areas exhibited relatively
high fledgling production (x = 3.6) for Boreal
Owls (Korpimaki and Hakkarainen 1991). Terri-
tories in both clear-cut areas were occupied with
equal frequency in different vole years (Table 2),
indicating that Boreal Owls breed successfully in
the neighborhood of large clear-cuts also in low
vole years. Clutch size, breeding frequency and lay-
ing date, however, were not affected by the pro-
portion of clear-cut areas within a territory (Table
1). Therefore, forest management does not seem
to harm Boreal Owls at present day scales, if no
more than half of the total forest area is clear-cut
at long intervals enough (>60 yr). In contrast, the
positive effects of clear-cut areas on fledgling pro-

June 1997

Boreal Owl and Forest Management

127

Table 2. The number of Boreal Owl nests in proportion
of landscape with clear-cuts of low and high percentages
(see Table 1 ) , in different phases of the vole cycle in the
Kauhava region, western Finland (ca. 63°N, 23°E).

Proportion of Clear-cut
Areas within Territory

Phase of Vole Cycle

Low

High

Low

Increase

Peak

Total

duction suggest that this species may achieve ben-
eficial fitness from clear-cut areas because, for Bo-
real Owls, lifetime reproductive success (LRS) is
dependent on the success of males in rearing
young to the fledgling state (Korpimaki 1992). To-
day, LRS is the best known estimate of fitness (Clut-
ton-Brock 1988, Newton 1989).

What would be the reason for the higher fledg-
ling production for Boreal Owls in areas with high-
er level of clear-cuts within territories? The in-
creased number of saplings and clear-cut areas dur-
ing the last two or three decades (Jarvinen et al.
1977) has created new suitable grass habitats for
field voles (Henttonen 1989), which is the pre-
ferred prey of Boreal Owls (Korpimaki 1988, Koi-
vunen et al. 1996). Snap-trapping in the peak vole
year of 1994 in western Finland also suggested that
large clear-cut areas sustain dense field vole pop-
ulations. Similar results have also been found in
Sweden (Hansson 1994). Because of intensive
growth of hay species in new clear-cut areas, hay-
eating field voles may colonize them successfully
for about 10 yr (Hansson 1978). In constrast, small
clear-cuts (ca. 1-3 ha) may not achieve such high
densities of field voles, especially if small clear-cuts
are isolated from source habitats, such as large
fields and large clear-cuts. This may explain why
fledgling production of Boreal Owls may increase
with the increasing amount of clear-cut area within
territories, especially if saplings are tall enough (ca.
2 m) for perch hunting by Boreal Owls (Bye et al.
1992). Densities of many bird species are also
found to peak at forest edges (Helle 1984, Hansson
1983), especially Chaffinch ( Fringilla coelebs ) den-
sities (Hansson 1994). This species is the most im-
portant bird prey of Boreal Owls on our study site
(Korpimaki 1981, 1988). Therefore, the edges of

forests and clear-cuts may increase the amount of
alternative prey of Boreal Owls in poor vole years.

Prey abundance and fledgling production ap-
pear to increase with forest fragmentation. How-
ever, clear-cutting also decreases the number of
suitable natural cavities for Boreal Owls. Large
trees and aspen groves with suitable nesting cavities
for the Black Woodpeckers ( Dryocopus martins ) are
decreasing due to logging. There is a need to pro-
tect these suitable nesting sites in forest landscapes.
Alternatively, nest boxes can be provided for Bo-
real Owls to compensate for the lack of natural
cavities.

Establishing Nest-box Locations for Boreal Owls

Interspecific competition is expected to reduce
the fitness of individuals (Roughgarden 1979).
Therefore, coexisting large owl species may reduce
the breeding success of smaller owl species, includ-
ing preying upon these owls (Mikkola 1983, Hak-
karainen and Korpimaki 1996). At our study site,
the Ural Owl ( Strix uralensis) is a large owl species
that is probably most harmful to the Boreal Owl.
Nest-box experiments, along with long-term obser-
vational data (Hakkarainen and Korpimaki 1996)
revealed that Boreal Owls avoid breeding within 2
km of Ural Owl nests. When nesting <2 km from
Ural Owls, breeding was delayed substantially when
compared with breeding >4.5 km away. Further-
more, when in the neighborhood of Ural Owl
nests, male Boreal Owls were younger and paired
more often with short-winged females. Most breed-
ing near Ural Owls failed during the courtship pe-
riod (Hakkarainen and Korpimaki 1996). This sug-
gests that inexperienced male Boreal Owls are
forced to establish their territories in the vicinity
of Ural Owls where they pair with less experienced
females. These findings suggest that nest boxes for
Boreal Owls should be set >2 km from the medi-
um-sized and large raptors that may have adverse
effects on Boreal Owls.

In conclusion, moderate forestry may not harm
Boreal Owls at the present day scale if suitable nest
holes are available. A lack of nest holes can be
compensated for by erecting nest boxes, but boxes
should be set far from threatening allospecifics. In
the long-term, however, the establishment of snags
and patches of old mature forests with large trees,
dense enough for hole-nesting Black Woodpeck-
ers, will provide a natural way to establish new nest-
ing cavities for Boreal Owls.

128

Hakkarainen et al.

Vol. 31, No. 2

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Carey, A.B., S.P. Horton and B.L. Biswell. 1992. North-
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Clutton-Brock, T.H. 1988. Reproductive success. Univ.
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Forsman, E.D., E.C. Meslow and H.M. Wight. 1984.
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Hakkarainen, H. and E. Korpimaki. 1996. Competitive
and predatory interactions among raptors: an obser-
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1142.

Hansson, L. 1978. Small mammal abundance in relation
to environmental variables in three Swedish forest
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. 1992. Landscape ecology of boreal forests.

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. 1994. Vertebrate distributions relative to clear-

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Helle, P. 1984. Effects of habitat area on breeding bird
communities in northern Finland. Ann. Zool. Fenn. 21:
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Henttonen, H. 1989. Metsien rakenteen muutoksen
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Jarvinen, O., K. Kuusela and R.A. VaisAnen. 1977. Met-
sien rakenteen muutoksen vaikutus pesimalinnus-
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fects of modern forestry on the numbers of breeding

birds in Finland in 1945-1975). Silva Fenn. 11:284—
291.

Koivunen, V., E. Korpimaki, H. Hakkarainen and K.
Norrdahl. 1996. Prey choice of Tengmalm’s Owl
{Aegolius funereus ) : preference of substandard individ-
uals? Can.]. Zool. 74:816-823.

Korpimaki, E. 1981. On the ecology and biology of Teng-
malm’s Owl {Aegolius funereus) in Southern Ostroboth-
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. 1988. Diet of breeding Tengmalm’s Owls Aego-
lius funereus'. long-term changes and year to year vari-
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30.

. 1992. Fluctuating food abundance determines

the lifetime reproductive success of male Tengmalm’s
Owls . J. Anim. Ecol. 61:103-111.

and H. Hakkarainen. 1991. Fluctuating food

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Newton, I. 1979. Population ecology of raptors. T. &
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Ohmann,J.L., P.L. Carleson and A.L. Oakley. 1988. Sta-
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Received 2 November 1995; accepted 6 March 1997

J. Raptor Res. 31 (2): 129-137
© 1997 The Raptor Research Foundation, Inc.

THE OSPREY ( PANDION HALIAETUS) AND MODERN
FORESTRY: A REVIEW OF POPULATION TRENDS AND

THEIR CAUSES IN EUROPE

Pertti L. Saurola

Zoological Museum, Ringing Centre, RO. Box 17, FIN-00014, University of Helsinki, Finland

ABSTRACT. — Nearly all European Osprey ( Pandion haliaetus) populations have had a similar fate during
the 20th century. In the first two decades, if not earlier, dramatic decreases and even extirpations of
many local populations occurred due to heavy persecution. There was then a recovery period until the
second decrease from the 1950s to the mid-1970s, caused by DDT and other contaminants. Since then,
populations have been recovering. The annual rates of population increase have varied from about 1 %
in Fennoscandia to about 10% in Scotland during the last 20 years. At present, 90% of all European
Ospreys breed in Finland, Sweden and Russia. The nesting habitats vary widely from steep cliffs in the
Mediterranean to closed climax coniferous forests, open peat bogs and large clear-cut areas in northern
Europe. In some areas (e.g., Finland), cutting of old, flat-topped potential new nests by intensive com-
mercial forestry has been the most important national threat for the local Osprey population during
the last three decades. As early as the late 1960s dedicated bird banders started to construct artificial
nests for Ospreys to compensate for the losses caused by one-track forestry. In 1995, 46% of all occupied
Finnish Osprey nests ( N =951) were artificial. Also, clear-cuts around nesting trees are harmful because
nests become more exposed to storms, predation by Eagle Owls ( Bubo bubo) and disturbances. In Finland
and some other countries, new guidelines for foresters also account for the welfare of the Osprey.
However, the principles and practices are still quite far from each other.

Key Words: Osprey, Europe, forestry, Pandion haliaetus; artificial nest.

El Pandion haliaetus y forestal moderno: un reviso de las tendencias de poblacion y sus causas en Europa

Resumen. — Casi todas las poblaciones de Pandion haliaetus europeo han tenido destino similar durante
el siglo viente. En las primeras dos decadas, si no mas temprano, aumentos dramaticos y tambien el
desarrollo de muchas poblaciones local ocurrieron a causa de alta persecucion. Luego hubo un tiempo
de recuperacion hasta la siguiente reduccion de 1950s hasta el medio de 1970s, causado por DDT y
otros contaminantes. Desde esos tiempos, poblaciones han estado recuperando. Los ritmos anual de
aumento poblacion han variado de 1% en Fennoscandia a casi 10% en Escocia durante los ultimo viente
anos. A1 presente, 90% de todo los Pandion haliaetus europeos se crfan en Finlandia, Suecia y Rusia. Los
habitats de nidos varia muy diferente de precipicio abrupto en el mediterraneo a bosque de conifero
cerrado y climax, turbera abierto, y areas grandes cortadas en el norte de Europa, en unas areas (e.g.,
Finlandia), de potencia de nuevos nidos por intensidad comercial de forestales ha tenido lo mas im-
portante peligro nacional para la poblacion local de los Pandion haliaetus durante las tres decadas pa-
sadas. Tan temprano como los ultimos anos de los 1960s marcadores de pajaros a empezaron construir
nidos artificiales para el Pandion haliaetus para compensar la perdicion causada por fores-
tales con solo una meta. En 1995, 46% de todos los nidos de Pandion haliaetus ocupados en Finlandia
( N = 951) eran artificial. Tambien, areas cortadas alrededor de arboles con nido eran peligrosos porque
los nidos estaban mas desprotegidos a tormentas, en peligro de buho aguila Bubo bubo, y disturbios. En
Finlandia y otros pafses, nuevas reglas por guardabosques tambien cuenta por el bienestar de Pandion
haliaetus. Sin embargo, los principios y costumbres estan todavla muy lejos de cada uno.

[Traduccion de Raul De La Garza, Jr.]

The Osprey ( Pandion haliaetus), the emblem of
the former International Council for Bird Preser-
vation (ICBP) , is a suitable species as a flagship for
bird protection. It is cosmopolitan and around the

world has suffered heavily from several human im-
pacts; persecution, pesticides, acid rain, distur-
bances, fishery practices and modern forestry
(Saurola & Koivu 1987). However, it is now recov-

129

130

Saurola

Vol. 31, No. 2

Table 1. Present population estimates (breeding pairs) and trends of European Ospreys.

Estimate

Trend 3

Reference

Norway

200

Fremming 1988, O. Steen pers. comm. 1996

Sweden

3200

Risberg 1990

Finland

1200

P. Saurola unpubl. data

Denmark

3-5

M. Grell pers. comm. 1996

Estonia

30-35

E. Tammur pers. comm. 1996

Latvia

120

M. Kreilis pers. comm. 1996

Lithuania

25-30

B, Sablevicius pers. comm. 1996

Scodand

99-105

R. Dennis pers. comm. 1996

Germany

290

Schmidt 1996

Poland

50-60

T. Mizera pers. comm. 1996

Belarus

120-180

A. Tishechkin pers. comm. 1996

European Russia

2500-4000

0(±)

V. Galushin pers. comm. 1996

Ukraine

1-5

—

Tucker and Heath 1994

Moldova

0-3

—

Tucker and Heath 1994

Bulgaria

3-6

—

Tucker and Heath 1994

France

— mainland

Y. Tariel pers. comm. 1996

— Corsica

Y. Tariel pers. comm. 1996

Spain

— mainland

C. Viada pers. comm. 1996

— Balearic Islands

C. Viada pers. comm. 1996

— Canary Islands 1,

13-15

C. Viada pers. comm. 1996

Portugal

—

L. Palma pers. comm. 1996

a Symbols: + = increasing, — = decreasing, 0 = stable, ± = in some parts of area increasing and in other parts decreasing.
b Canary Islands belong administratively to Spain but not geographically to Europe.

ering almost everywhere in its range, as a result of
successful protection efforts. In many areas the Os-
prey has been classified as a species for which fur-
ther monitoring and support is still necessary.

Here, I give a short review of the present distri-
bution, population estimates, production and pop-
ulation trends of Ospreys in Europe. In addition,
I describe the significance of human factors, es-
pecially modern forestry, to the welfare of Euro-
pean Ospreys. The meyority of these data come
from Finland where a nationwide monitoring pro-
gram Project Pandion was started in 1971 and con-
tinues today (Saurola 1995a).

European Ospreys

Historical Records. Bijleveld (1974) has collect-
ed historical records on all European birds of prey.
During the 19th century, Ospreys were breeding
throughout Europe. Due to heavy persecution, lo-
cal populations decreased rapidly and, in many
countries, they were extirpated. The last known
breeding in former Czechoslovakia was recorded
in about the 1850s, in Switzerland in 1911, in Great

Britain and Denmark in 1916, in Austria in the
1930s, in the former West Germany in 1933 and in
Italy in 1956 (Bijleveld 1974).

In the beginning of this century, the Osprey was
a rare bird everywhere in Fennoscandia (Finland,
Sweden and Norway) . After legal protection in the
1920s in Finland and Sweden, populations slowly
recovered until a new decrease occurred in the late
1950s and 1960s (Saurola 1986). This decrease was
mainly due to toxic chemicals.

Present Distribution and Status. The present dis-
tribution of the European Osprey population ex-
tends from northern Norway and Finland to south-
ern Portugal, the Balearic Islands and Corsica and
from Scotland to the eastern border of the Euro-
pean part of Russia (Table 1). The total European
population is estimated at 7000—9000 breeding
pairs; about 50% of the population breeds in Swe-
den and Finland, 35-40% in Russia, 8% in eastern
Germany, Poland, Belorus, Estonia, Latvia and
Lithuania, 3% in Norway and Scotland and less
than 1% in southern Europe (Table 1).

The accuracy of these population estimates var-

June 1997

Ospreys in European Forests

131

Table 2. Average breeding output in some local Osprey populations in Europe.

Country

Period

Young/

Occupied

NEST a

Young/

Active

Nest 3

Young/

Successful

Nest 3

Reference

Finland

(1971-95)

1.46

1.91

2.17

Saurola this study

Sweden

(1971-93)

1.59

Odsjo pers. comm. 1996

Germany

(1972-93)

Meyburg et al. 1996

— trees

1.32

1.47

2.08

— pylons

1.65

1.81

2.22

Scotland

(1954r-94)

1.29

Dennis 1995

Poland

(1976-92)

1.34

1.81

Mizera 1995

a See Postupalsky (1977) for definitions.

ies greatly from country to country, although most
were provided by Osprey specialists from each
country. For example, in Scotland (Dennis 1995)
and Finland (Saurola 1995a) all known occupied
territories have been checked annually for more
than 20 yr. In contrast, the estimate for European
Russia (V. Galushin pers. comm.) is based on ex-
trapolation of information from a handful of large
study areas, but still small if compared with the
huge area for which the estimate is given.

Productivity and Population Trends. At the mo-
ment, all local Osprey populations breeding in
northern and central Europe seem to be either sta-
ble or increasing (Table 1) and the average breed-
ing output is good (Table 2). Definitions are ac-
cording to Postupalsky (1977). Also, the remnant
populations in Corsica, mainland France (Tariel
pers. comm.) and the Balearic Islands (Viada pers.
comm.) are now increasing, but in Portugal only
one breeding pair remains (Palma pers. comm.)
and in mainland Spain there are no breeding Os-

year

Figure 1. Average annual breeding success of Finnish
Ospreys in 1971-95 (see Figure 2 for sample sizes and
Postupalsky (1977) for definitions).

preys (Viada pers. comm.). The real situation in
southeastern Europe, in Ukraine, Moldova and
Bulgaria is poorly known; however, all population
trends from this area are negative (Tucker 8c
Heath 1994).

In Finland, Project Pandion was started in 1971
(Saurola 1980) and since then almost all known
occupied territories have been checked by bird
banders (ringers) every year. These data indicate
that breeding success has increased significantly
since the start of the project. During the 1970s,
Finnish Ospreys raised on average 1.38 young/oc-
cupied nest/year, but during the 1980s and 1990s,
the corresponding figures have been 1.47 and 1.61.
The trend for these three decades is similar in pro-
duction per occupied (1.81, 1.96 and 2.03) and per
successful nests (2.09, 2.21 and 2.23; Fig. 1).

According to all data from Project Pandion , the
Finnish Osprey population remained stable
through the 1970s and then increased during the
1980s and 1990s (Table 3, Fig. 2). A part of this
increase, especially in sparsely inhabited northern
Finland, may be only a result of increasing survey
coverage. In Hame, southern Finland, where my
intensive study area is located and where few, if
any, nests are not known by Project Pandion, the in-
crease rate in 1972-1995 has been 0.7% per year.
This is considerably less than the 2% per year cal-
culated from all data for the whole country (Table
3) . My estimate for the real growth rate of the total
Finnish Osprey population during the last 25 yr is
between 1% and 1.5% per yr.

Swedish Ospreys have been monitored at six
study areas located in southern and central Swe-
den. These areas have been carefully checked in
1971—73 and after that every 5th yr in 1978, 1983,
1988 and 1993 (Odsjd pers. comm.). The average

132

Saurola

Vol. 31, No. 2

Table 3. Mean annual rate of population increase of the Osprey in some European study areas.

Finland (active nests)

— all known
— Hame

Sweden (active nests)
— 6 study areas
Germany (pairs)

— Mecklenburg
— Brandenburg

Scotland (pairs)

— all known

Change

Period

N ! a

iv 2 b

PER YEAR C

Reference

1972-95

465

736

2.0%

Saurola, this study

1972-95

110

0.7%

Saurola, this study

1972-93

113

0.7%

Odsjo 1982 and pers. comm. 1996

1980-93

3.3%

Meyburg et al. 1996

1980-92

120

8.5%

Meyburg et al. 1996

1977-95

9.3%

Dennis 1987 and pers. comm, 1996

- 1 Ni = number of active nests or pairs in the first year of study period.
b N 2 = number of active nests or pairs in the last year of study period.

c Mean increase per year ( p ) was calculated from the formula: N 2 = iV^l + jb/lOO)*, where t = elapsed time in years.

annual increase during the last 20-25 yr within
these study areas has been 0.7%, which is the same
as in Hame, but much lower than in Germany and
Scotland (Table 3) . So far, no clear explanation has
been proposed for these geographic differences in
rates of population increase (Saurola 1990, 1995a).

Migration and Wintering Areas. European Os-
preys migrate to the tropics (Osterlof 1977, Dennis
1991, Saurola 1994), except for the Mediterranean
populations, which remain in the Mediterranean
(Thibault et al. 1987). The main wintering area is
the Sahel-zone between latitudes 5— 15°N. Band re-
coveries revealed longitudinal differences in win-
tering areas of the local populations from different
parts of the breeding range: Scottish Ospreys win-

year

Figure 2. Total number of known occupied (squares),
breeding (triangles) and successful (dots) Osprey nests
in 1971-199.5 in Finland (see Postupalsky (1977) for def-
initions) .

ter along the west coast of Africa (Dennis 1991),
Swedish birds mainly in inland waters of west Africa
and the Finnish ones still further east, in west and
central Africa (Osterlof 1977, Saurola 1994). So far,
only four banded European Ospreys have been re-
covered from South Africa, about 10 000 km from
their natal area, all of Finnish origin.

In the late 1970s, a detailed study on the winter
ecology of European Ospreys was made in Sene-
gambia (Prevost 1982).

Nesting Habitats and Nest Sites. The Osprey eats
live fish almost exclusively (e.g., Hakkinen 1977,
1978, Saurola 8c Koivu 1987) and for this reason
its distribution is always restricted by the distribu-
tion of favorable fishing waters. In ideal conditions
the nest is located just at the shoreline. However,
in areas disturbed by human activities, the distance
from the nest to the fishing grounds may be several
km.

In addition to sufficient food resources, the most
important prerequisite of a good nest site is a sta-
ble and exposed base to support the nest. Because
the Osprey nest has to be exposed to all directions,
it is nearly always at the very top of the tree and
no branches reach the upper edge of the nest.
There are few exceptions from this general rule.
The nesting habitat and the base of the nest can
be varied if the two main requirements are filled.
More than 95% of European Ospreys breed in for-
ested habitats (coniferous forests or on peat bogs).
The cliff-nesting birds in Corsica (Terasse and Ter-

June 1997

Ospreys in European Forests

133

asse 1977), the Balearic Islands (Gonzalez et al.
1992) and in Portugal (L. Palma pers. comm.), and
pairs nesting on power line pylons in the middle
of open fields in Germany (Moll 1962) are the only
exceptions to this general pattern. The succession-
al stage, structure and openness of the forests
around the nest varies from closed climax conif-
erous or mixed forests to dear-cuts where the nest
tree is the only one left. One of the favorite natural
sites is a small islet in a lake covered by big trees.

The most common nesting tree species both in
European forests and peat bogs is the Scotch pine
(Pinus silvestris) . For example, this tree species
hosts 88% of natural nests in Finland. In this spe-
cies the structure of the flat top of an old tree pro-
vides a stable base for the huge stick nest of the
Osprey. Norwegian spruce ( Picea abies ) is the next
most commonly utilized tree species (3% in Fin-
land), and broad-leaved trees (e.g. Belula, Populus,
Alnus , Quercus ) are rarely used as nesting trees by
European Ospreys (only 1% in Finland). A total of
7% of natural Osprey nests are on dead trees in
Finland. Norwegian spruce is suitable for the Os-
prey only if the top has been broken some meters
from the tip, so that the branches are thick enough
to carry the heavy nest. In Scotland, about one-
quarter of the nests are now on an introduced spe-
cies, Douglas fir ( Pseudotsuga menziesii ) with broken
tops (Dennis pers. comm.).

Human Impact

Persecution. Birds of prey were heavily persecut-
ed throughout Europe as early as the 17th century.
This persecution intensified during the 18th cen-
tury and peaked in the 19th and early 20th cen-
turies (Bijleveld 1974). For more than 200 yr, mil-
lions of birds of prey were killed because they were
considered harmful pests. During World Wars I
and II, hunters were allowed to shoot each other,
so killing of birds of prey decreased. Immediately
after World War II, intensive persecution resumed.
For example, in autumn 1953 at least 93 Ospreys
were killed at three fishponds in Lower Saxony (Bi-
jleveld 1974).

The Osprey has been legally protected since
1926 in Finland (Saurola & Koivu 1987) and since
the late 1920s in Sweden (Osterlof 1973). In many
other European countries full legal protection was
given to the Osprey less than 40 yr ago, for exam-
ple, in Denmark in the 1950s, Poland in 1952,
United Kingdom in 1954, former East Germany in
1954, Norway in 1962, France in 1964, former

USSR in 1964 (enforced in 1974), Spain in 1966,
former West Germany in 1968 and Italy in 1971
(Bijleveld 1974).

Legal protection does not necessarily mean that
killing ceases. Saurola (1985a, 1994) attempted to
assess changes in persecution of Fennoscandian
Ospreys in Europe and Africa by calculating area-
specific persecution indices from band recoveries.
This analysis, which might be biased by changes in
reporting rates, suggested that persecution de-
creased in Italy, France and in the former USSR in
the 1970s after changes in legislation. In contrast,
killing of Ospreys in Africa has remained the same
during the last 30 yr.

In addition to being killed as a competitive con-
sumer of fish, European Ospreys have suffered
from illegal egg and skin collecting. In Scotland,
egg robbing still continues, perhaps at least partly
as a challenging game against police and conser-
vation authorities. For example, in 1988 and 1989,
1 1 and 9 out of 49 nests were robbed in Scotland,
respectively (Dennis 1991).

Pesticides, In the late 1940s and 1950s, when
persecution increased again after World War II,
DDT and other environmental contaminants ap-
peared as a new threat to the future of the Osprey
and other birds of prey all over the world (Poole
1989). Odsjo (1982) found that eggshell thickness
of unhatched Swedish Osprey eggs was 11% lower
than that of shells collected before DDT was first
in use in 1947. In nests where all eggs were broken,
eggshell thickness was 20% lower than in pre-DDT
eggs, and, as expected, breeding success had de-
creased from the pre-DDT level.

In Finland, during Project Pandion, all addled
eggs have been collected and DDT, DDD, DDE and
total PCB concentrations have been analyzed but
not yet published. Preliminary results show that
DDT concentrations in Finnish Osprey eggs have
decreased significantly during the last 20 yr. More-
over, even in the early 1970s the concentrations
were much lower than in Swedish eggs (Odsjo
1982).

Acidification of Lakes. Eriksson et al. (1983) and
Eriksson (1986) suggested that reduced breeding
success of the Osprey in southwestern Sweden was
due to higher nestling mortality caused by reduced
foraging success in acidified lakes. They predicted
that a population decrease would occur as a con-
sequence of more widespread acidification. So far,
no further evidence of the negative effects of acid-

134

Saurola

Vol. 31, No. 2

ification on European Osprey populations has
been published.

Fishing and Fish Farms. Of Finnish band recov-
eries (returns) of dead Ospreys in 1950—1987, 29%
were found dead with no more information; while
of the remainder, 53% were shot or otherwise
killed intentionally, 25% were entangled in a fish-
ing net and 10% were hit by overhead wires (Sau-
rola & Koivu 1987). Although the distribution of
causes of death assessed from ring recoveries is bi-
ased, it clearly demonstrates that fishing is an im-
portant factor. In Finland, the most dangerous pe-
riod for Ospreys is early spring when most of the
fishing grounds are still covered by ice. At this time
Ospreys are caught in nets in small areas of open
and shallow water exploited both by Ospreys and
by fishermen.

In Finland, at commercial fish farms growing
North American rainbow trout ( Oncorhynchus my-
kiss ), Ospreys have been killed both by illegal
shooting or by poorly placed strings or nets set to
protect trout. At the moment, most Finnish fish
farms are safe for Ospreys because the state pays
compensation to the owners from damages caused
by Ospreys.

Illegal shooting of Ospreys at fish farms is still a
problem at least in Poland (Mizera 1995) and
probably in other countries in eastern Europe.

Land Use and Disturbances. In Finland, about
15% of the present nest sites of the Osprey are
close to the shoreline ( Project Pandion ) . The main
reason for this unexpectedly low proportion is land
use because the dream of every Finn is to have a
summerhouse by a lake or in the Baltic archipela-
go. Hence, there is little shoreline left for Ospreys.
In many cases the historic nest sites have been
abandoned and Ospreys have moved to the middle
of forests, often several kilometers away from their
historic nest sites.

After the persecution and pesticide eras in the
1980s, human disturbances (fishing, canoeing, sail-
ing and bathing) became the major threats to the
species in Swedish lake areas, where many Ospreys
still bred close to the shore (Odsjo & Sondell
1986).

Ospre\s and Modern Forestry

Modern forestry may have four kinds of negative
effects on the welfare of the Osprey: cutting of oc-
cupied nest trees, cutting of potential alternative
nest trees, cutting of trees from the protection
zone around the nest and noise disturbance from

forestry activities in the neighborhood of the nest
during the breeding season.

Cutting of Occupied and Potential Alternative
Nest Trees. The Osprey is fully protected by na-
tional laws in those European countries which have
breeding Ospreys (Bijleveld 1974). Consequently,
the occupied nest trees should be protected during
the breeding season throughout Europe. In con-
trast, during the nonbreeding season the nests and
nesting trees are not protected in all European
countries. Hence, in some countries the nest tree
can legally be cut after the breeding season, even
though this nest tree would likely be used again
the following summer if left intact.

The same Osprey nest may be in use for decades
(Saurola 8c Koivu 1987) and for this reason it is
crucial to protect the nest tree all year. However,
the protection of an occupied nest tree is not
enough because of the evolution of the top of an
Osprey nest tree. The Osprey brings new sticks to
the nest every year, the nest grows higher and high-
er, and finally falls down. After this the top of the
tree usually is not of sufficient quality to serve as a
base for the nest. Thus, within each territory, a suf-
ficient number of old, flat-topped nest trees should
be saved as alternative nest trees for the future.

Cutting of Trees from the Protection Zone
Around the Nest. If all trees around the nest tree
are removed, the probability of a breeding failure
increases for several reasons. First, a solitary tree is
much more exposed to damage caused by storms.
Second, a tall tree in a clear-cut is an ideal hunting
perch for the Eagle Owl ( Bubo bubo), which is main-
ly an open-land hunter (Mikkola 1983). Thus, the
probability is high that a hunting Eagle Owl will
locate and kill an incubating or brooding Osprey
or the entire brood. Moreover, the fledged young
are especially vulnerable because they use their
nest as a perch for eating for 4 wk after fledging
(Saurola 8c Koivu 1987). The noisy begging of the
young at sunset from the middle of a clear-cut is
like a dinner bell for an Eagle Owl starting to hunt.
In Finland, where the Eagle Owl population has
been increasing rapidly during the last decades
(Saurola 1985b, 1995b) and where many of the Os-
prey nest sites have been classified as dear-cuts or
other types of open forests (22% in 1995, Project
Pandion ), more and more Osprey nests have been
predated by Eagle Owls. Third, it is clear that the
disturbance zone of many activities (e.g., forestry,
recreation, sports) around the nest is wider in
open clear-cuts than in closed forests.

June 1997

Ospreys in European Forests

135

Forestry Activities Near the Nest During Breed-
ing. According to the 26-yr data from Project Pan-
dion, inappropriate timing of forestry work in the
neighborhood of the nest has caused several breed-
ing failures in Finland. These failures have been
demonstrated as results of construction of logging
roads, digging ditches, harvesting, improving of
young stands and planting seedlings.

A Promising Example for a Better Future:
Guidelines by the Finnish Forest and Park Service.
Finnish Forest and Park Service (1994) has recent-
ly published the new guidelines for all activities
near the Osprey nests for land owned by the gov-
ernment. The main points of these guidelines are
that the nest tree is protected all year under the
Nature Conservation Act, a protective tree stand
(density 200 stems/ha) must be left around the
nest for a radius of approximately 50 m, a bog sur-
rounding a clump of trees in which there is an
Osprey nest must be left in a natural state, any for-
estry activities must be avoided close to the nest in
the period 15 April-31 July, old Scotch pines and
saw timber trees must be left in clumps for future
development into ideal nest sites and paths and
hiking routes must not be established within about
500 m from the nest. Almost identical advice has
been given by the Forestry Center Tapio (1994) for
the management of Osprey nest sites on private
land.

These guidelines for state-owned and private
lands are sufficient for the protection of Finnish
Ospreys. In practice, these guidelines, especially on
private lands, are only recommendations and
therefore not always followed by foresters. For ex-
ample, clear-cuts still occur around nest trees and
seedlings are planted close to active nests during
sensitive periods in the breeding season.

In some countries the guidelines are even more
strict than in Finland. For example, in Poland no
trees are allowed to be cut within 200 m from the
nest and during the breeding season (1 February-
31 July), all forestry activities are forbidden within
500 m from the nest (Mizera 1995). In many other
European countries, guidelines for forestry near
nest sites of endangered or rare birds, such as ea-
gles and Ospreys, are under changes or in prepa-
ration. For example, in the eastern states of Ger-
many, old, and often very strict, regulations are no
longer officially enforced but are still in practice
because new ones are not yet available (D. Schmidt
pers. comm.). Hence, it is difficult to make an
overall European summary of this subject.

Artificial Nests. Construction of artificial nests
has been the only possible direct measure to com-
pensate for the effects of one-track commercial for-
estry. In Finland, the first artificial nests for Os-
preys were constructed in 1965 (Saurola 1978). In
1995, 45.8% of all occupied nests in Finland were
artificial ( Project Pandion). In my intensive study
area in southern Hame, the percentage of artificial
nests was as high as 90% of 79 occupied nests. I
have estimated that in this area which, without in-
tensive modern forestry, would be an ideal natural
area for the Osprey, the population would be less
than 50% of the present level without artificial
nests (in total 160 artificial nests are available for
the Osprey in this area) .

Meyburg et al. (1996) have demonstrated that,
in Germany, breeding output was clearly higher in
artificial nests on power line pylons than in natural
tree nests within the same area (Table 2) . However,
in Finland, no difference in breeding success be-
tween artificial and natural nests was detected
(Saurola 1990). This perhaps unexpected result
was probably because most of the unstable natural
nests were replaced by artificial nests. Therefore,
artificial nests were not compared with normal nat-
ural nests but with high quality natural nests.

In Europe, artificial nests have been constructed
during the 1980s and 1990s in almost all countries
wdth breeding Ospreys and in most cases with good
success. For example, soon after artificial nests
were provided in southern Norway, Ospreys started
to expand their range westward back to their historic
breeding sites, where the number of suitable nest
trees had greatly decreased because of forestry
(Steen 1993). In Sweden, artificial nests have been
constructed to move Ospreys from disturbed areas
to undisturbed areas with good results (Hallberg
et al. 1983).

Concluding Remarks

During the last 10 yr, local Osprey populations
in northern and central Europe have been stable
or are still recovering from the effects of persecu-
tion and organochlorine pesticides. These two
threats are currently not major problems in Eu-
rope but still may be so in Africa.

In contrast, both past and present effects of
modern forestry may be an important negative fac-
tor for the Osprey. In addition to the lack of suit-
able nest trees in some areas, many breeding fail-
ures are due to modern forestry, either direcdy
(forestry activities near the nest during the breed-

136

Saurola

Vol. 31, No. 2

ing season) or indirectly (nests on the open clear-
cuts are more exposed to storms, Eagle Owls and
disturbances) .

Official silvicultural guidelines are important for
the protection of traditional nest sites of Ospreys
in commercially treated forests. Instructions for
management of Ospreys have been provided in
some countries for foresters. In some others, such
as the former socialist countries in eastern Europe,
the new guidelines are under preparation.

Construction of artificial nests has been an ef-
fective tool to compensate for some of the effects
of modern forestry. However, the extensive protec-
tion of natural nest trees and their surroundings
should always be the primary long-term goal. Con-
struction of artificial nests should be used as the
last and temporary measure to save or reintroduce
local populations, but never as an excuse to destroy
natural breeding sites.

Acknowledgments

The invitation from Gerald Niemi made this contri-
bution possible. Unpublished information on present sta-
tus of the Osprey in Europe is based on personal com-
munications with Roy Dennis, Vladimir Galushin, Mikael
Grell, Maris Kreilis, Tadeuz Mizera, Tjelvar Odsjo, Luis
Palma, Bronius Sablevicius, Odd Frydenlund Steen, Ei-
nar Tammur, Yvan Tariel, Alexey Tishechkin and Carlota
Viada. Finnish data is from Project Pandion, a monumental
voluntary work by Finnish bird ringers. Previous draft of
this paper was reviewed by Peter Evins, Charles Henny,
Mikael Kilpi and Daniel Varland. I am very grateful to all
these people for their valuable help.

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. 1978. Diet of the Osprey Pandion haliaetus in

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J. Raptor Res. 31 (2) : 138-1 50
© 1997 The Raptor Research Foundation, Inc.

OSPREY ( PANDION HALIAETUS) POPULATIONS IN FORESTED
AREAS OF NORTH AMERICA: CHANGES, THEIR CAUSES AND

MANAGEMENT RECOMMENDATIONS

Peter J. Ewins 1

Canadian Wildlife Service, Environment Canada, 4905 Dufferin Street, Downsview, Ontario M3H 5T4 Canada

Abstract. — Prior to European settlement of North America, Ospreys ( Pandion haliaetus) bred throughout
much of the continent in tall trees near productive shallow-water freshwater bodies. Ospreys need exposed
locations to build their large nests, often in dead tops of older trees or snags in beaver swamps. Historical
nest sites are poorly documented, but timber extraction and shoreline development have undoubtedly
removed many preferred nest trees, likely causing population declines. Widespread use of persistent or-
ganochlorine pesticides after 1945 caused dramatic declines of breeding ospreys. Since bans on these
toxins were imposed in the 1970s, most populations have increased at average rates up to 15% per year.
Ospreys have adapted well to nesting on a wide range of artificial substrates, and in some areas up to 70%
of nests are now on such structures. In many areas nowadays, up to 80% of tree nests occur within 500
m of open water. It is difficult to know what this figure was historically since more recent forest manage-
ment often retains trees in shoreline buffer zones primarily for recreational and landscape reasons. Other
important factors currendy affecting breeding ospreys are: nest predation from raccoons ( Procyon lotor)
and Great Horned Owls ( Bubo virginianus) , degradation and loss of foraging areas, human disturbance
and Bald Eagle ( Haliaeetus leucocephalus) population increases. Forestry guidelines protecting Osprey habitat
vary considerably among regions. Maintaining nonintervention buffer zones around Osprey nest trees
results in substantial lost profit for foresters, yet the ecological basis for such zones is often unclear.
Systematic studies of breeding Ospreys in relation to different forestry practices, and associated activities,
are needed to provide more consistent, realistic and integrated conservation advice to resource managers.

Key Words: forestry, North America; Osprey ; nest trees; Pandion haliaetus.

Poblaciones de Pandion haliaetus en areas de bosque en Norte America: cambios, sus causas y recomen-
daciones de administracion

Resumen. — Antes de la colonization de Norte America, Pandion haliaetus se criaban por mucho del con-
tinente, en arboles de grande altura cerca de aguas de pesca productivas. Pandion haliaetus necesitan
lugares desabrigados para construir sus nidos grandes. Con frecuencia en la copa de arboles maduros o
tambien tocones en pantanos de castor. Sitios de nido historicamente estan documentados de ser pobres,
por extracciones de madera y el desarrolla a la orilla del agua, sin duda han quitado muchos arboles de
nido preferidos, probablemente causando reduction de la poblacion. Usos amplios de pesticidas de or-
ganoclorados (OC) despues de 1945 causo reducciones dramaticas en la cria de Pandion haliaetus. Prohi-
bition de estos toxicos fueron imponados en los 1970s, y desde entonces la mayoria de poblacion a subido
a ritmo regular hasta 15% por ano. Pandion haliaetus se han adaptado bien haciendo nidos que abarcan
un campo amplio de soportes artificial, y en unos areas hasta 70% de nidos estan en tal estructuras. En
muchas areas hoy hasta 80% de los nidos en arbol ocurren dentro de 500 m al agua fibre. Es dificil saber
que fue la cantidad historicamente; mas reciente administracion de bosque muchas veces retiene arboles
dentro la orilla del agua en zonas de espacio primeramente para razones recreational y aesthetico, a un
extenso grande que en bosques mas lejos de la agua. Otros factores importantes actualmente afectando
los Pandion haliaetus de cria son: depredador de nido, (la mayoria de mapaches, Procyon lotor, y buhos, Bubo
verginianus) , degradation y la perdition de areas de forraje, molestia humana, y aumento en poblacion
de aguilas Haliaeetus leucocphalus. Reglas del forestal protegiendo los Pandion haliaetus varia considerable-
mente entre regiones. Manteniendo zonas de no-intervencion de espacio alrededor de nidos de Pandion
haliaetus resulta en suficiente ingresos perdidos para la industria de madera, y la razon ecologica para tal
zonas es muchas veces poco claro. Estudios sistematicos de Pandion haliaetus de cria en relation a diferente

1 Present address: World Wildlife Fund, 90 Eglinton Avenue East, Suite 504, Toronto, Ontario M4P 277 Canada,

138

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Ospreys in North American Forests

139

costumbres del forestal, y actividades asociadas, son necesarias para proveer mas consistenes realistarias y
consejos de conservation integrada para la administration de recurso.

[Traduction de Raul De La Garza, Jr.]

Prior to the colonization of North America by
Europeans, Ospreys ( Pandion haliaetus) bred in trees
throughout most of North America, though a few
pairs nested on cliffs or on the ground on small
islands (Poole 1989). The major changes in land-
use patterns (notably forest clearance for agricul-
ture and residential and industrial development)
which have occurred since European setdement
(Lawrie and Rahrer 1973, Caldwell 1978, Sly 1991)
have undoubtedly affected the breeding distribu-
tion of Ospreys, but many other factors have also
impacted these populations. In this paper, I review
the documented population changes, highlighting
cases for which the causes are reasonably well estab-
lished. I then focus on Osprey nesting require-
ments, especially in relation to forestry practices
and current timber management guidelines for Os-
preys. I also suggest some key studies which should
be done to better evaluate the sensitivity of Ospreys
to different timber management regimes.

Background

Ospreys are large (1.5-2 kg) raptors which eat
almost exclusively fish, which they catch in water
usually up to 2 m deep by diving in feet-first, either
from a shoreline perch or from a hover or stoop
from up to 40 m above the water (Poole 1989).
They are monogamous, breed first when 3-4 yr
old, have an 85-90% adult annual survival rate
and can live for up to 25 yr. They have relatively
long wings for their body mass and so are rather
poor at maneuvering among trees. For this reason
they require very open sites in which to nest so that
birds can readily fly to and from the nest in any
wind direction without getting tangled in branches.
They build large stick nests which are added to each
year. Thus, Ospreys favor nesting at the top of old,
large trees, live or dead, with adequate strong sup-
port branches at the top and clear air space around
the nest. Nest sites surrounded by water are usually
preferred, since mammalian predators are thus de-
terred. Most Ospreys which breed in North America
winter in northern South America or the Caribbean
basin, but there are resident populations in Florida
and Californ ia/Baja California (Poole and Agler
1987, Poole 1989, Ewins and Houston 1993).

Despite major reductions in both the extent and

age of forests in North America over the past two
centuries (Lawrie and Rahrer 1973, Caldwell 1978,
Holla and Knowles 1988, Sly 1991), Ospreys persist
as a relatively widespread and highly visible breed-
ing species near to many waterways. Unlike some
other raptor species. Ospreys have in many cases
adapted remarkably well to living in close proximity
to humans, and will nest readily and very success-
fully on artificial nest structures, especially when
there is a tradition of this habit in an area (Postu-
palsky 1978, Ewins 1994, 1996). It has been estimat-
ed for the mid-1980s that North America supported
about 18000—20 000 pairs of breeding Ospreys or
about 57-84% of the world population and that
about two-thirds of these bred in Canada and Alaska
(Poole 1989). Although Ospreys do breed in loose
colonies in some areas, particularly near to rich
food supplies in marine estuaries (Greene et al.
1983, Hagan 1986), the bulk of these birds breed as
scattered, isolated pairs in relatively remote forests
close to fishing areas in the numerous rivers and
lakes of northern North America.

Population Changes and Associated Factors

Ospreys have been relatively well studied over the
past 30 yr in North America (Henny 1977, Poole
1989) and many factors are now known, or are sus-
pected, to have influenced their populations since
European settlement of the continent (Table 1),

Historical Populations (>100 years ago). Unlike
Bald Eagle ( Haliaeetus leucocephalus ) nests, Osprey
nests were seldom noted by early naturalists in
North America, so it is often difficult to assess cur-
rent occupancy of nesting areas occupied in the last
century. However, given what we now know of the
Osprey’s nesting requirements, it is likely, given the
massive reductions to the extent and mean age of
forests, that prime nesting trees are very scarce in
many former breeding areas. Impressions noted by
Victorian naturalists lead us to suspect very large
declines in some areas. For example, Reardslee and
Mitchell (1965) cite a visit by the naturalist De Witt
Clinton to the Niagara River in 1820: “In various
places I have seen bald eagle, grey eagle and osprey
falco haliaetus. . . . the immense quantities of fish
which collect below the falls of Niagara. . . . draw
together these birds, and I have never seen so many

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Vol. 31, No. 2

Table 1. Main factors affecting North American Osprey populations.

Nest-site availability

— timber extraction
— shoreline development
— fur trade (beaver populations)

— water level changes/ reservoir creation
— artificial nest structures

Food availability

— loss of foraging habitat to: — agriculture

— shoreline development

— nutrient changes

— fish removal (chemical, over-fishing)

— exotic species effects
— lake acidification

Human activities

— egg collecting, taxidermy
— persecution
— disturbance at nest

— environmental legislation and societal attitudes

Toxic chemicals

— persistent organochlorine pollutants
— heavy metals (mercury)

Competition

— Bald Eagles ( Haliaeetus leucocephalus)
— intra-specific

Predators

— raccoon ( Procyon lotor )

— Great Horned Owl ( Bubo virginianus)

Weather

— wind storms (nest loss)

— cold and wet (chick starvation/hypothermia)

Wintering and Staging

— habitat loss

areas

— hunting/persecution
— mercury (gold mining)

— organochlorine pesticides

as appear to occupy this region.” Today, eagles and
Ospreys are rare sights along the entire Niagara Riv-
er, even though there are still huge quantities of fish
available below the falls, supporting very large con-
centrations of foraging gulls and fish-eating ducks
in autumn. Very little undisturbed nesting, perching
or roosting habitat now exists along the river banks,
due to recreational access and residential develop-
ment.

Along the Oregon-California border, a huge col-
ony of 250-300 pairs of Osprey was recorded at Tule
Lake in 1899 (Bailey 1902). The birds bred in two
groves of large ponderosa pine ( Pinus ponderosa )
and junipers (Juniperus occidentals ) 6-10 km from
the shallow, highly productive lake, because these
were apparently the nearest stands of suitable nest
trees to the lake (Henny 1988). So, even 100 yr ago
it appears that the availability of preferred nest trees
was influencing Osprey nesting distribution. After
1906, Tule Lake was drained to provide irrigation
and new, fertile agricultural land; the area now sup-
ports a range of cash crops but only about 12 pairs
of Ospreys (Henny 1988).

There are reasonable historical population esti-
mates for Ospreys in six areas and biologists have
been able to suggest factors associated strongly with
the population change over the period (Table 2).
In four of these cases, large declines were associated
with combinations of factors such as persecution,
egg/skin collection, wetland drainage for agricul-
ture, loss of nesting trees to forestry or shoreline
development and toxic effects of organochlorine
pesticides. The provision of artificial nesting struc-
tures seems to have offset the effects of other factors
and maintained reasonably stable populations in
parts of Maryland and Ontario (Reese 1969, Ewins
1996).

Changes Since the 1930s. The simple chemical
process of adding a chlorine atom to a benzene
molecule probably had a greater effect on Osprey
populations than all other factors combined. From
the mid-1940s to the early-1970s, organochlorine
pesticides were used widely and effectively in North
America and these molecules proved to be extreme-
ly persistent environmental contaminants. Ospreys,
like other raptors at the top of food webs, bioaccu-

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Ospre\s in North American Forests

141

Table 2. Historical records of Osprey population changes, and factors implicated by authors.

Area

Numbers /Year

Factors Implicated

Gardiner’s I., NY

300 prs./ 1850s
300 prs./ 1940
100 prs./1960
31 prs./1975

Protection from persecution
Organochlorine pesticides

Seven Mile Beach, NJ

100 prs./1884
<25 prs./ 1890
30 prs./ 1927

Egg collecting and shooting

Queen Annes Co., MD

32 prs./1892
31 prs./1968

Forestry, artificial nest structures

Georgian Bay, Lake Huron, ONT

“generally distributed” /1890s

0/1940s-72

43 prs./ 1993

Forestry, shoreline development, organo-
chlorine pesticides, artificial nest
structures

Tule Lake/Klamath, OR

250-300 prs./ 1899
ca. 12 prs./1976

Drainage, agriculture

Eagle Lake, CA

>2 prs./ 1905
30-35+ prs./1925
23 prs./1975

Water level changes providing snags

mulated substantial concentrations of these lipo-
philic compounds from their diet. Most notable was
DDT and its more stable metabolite DDE, which
impaired shell gland function and led to severe
thinning of eggshells and resultant reproductive fail-
ures as the eggs broke during incubation (Ames
1966, Cooke 1973). The cyclodiene dieldrin was also
highly toxic and may well have increased mortality

Table 3. Population trends for North American Os-
preys since the 1930s. Means expressed as percentage
change per annum.

Location

Period

Mean % per
Annum

Migration look-outs

Northeast U.S.A.

1972-87

+ 8.9

Hawk Mt., PA

1934-86

+0.1

Duluth, MN

1974-89

+5.5

Grimsby, ONT

1975-90

+ 6.8

Western U.S.A.

1983-91

+ 7.0

Breeding areas

Wisconsin

1974-90

+8.9

Northeast U.S.A.

1975-87

+ 10.0

Upper NY

1976-90

+ 6.8

St. Marys R., MI

1975-93

+ 15.4

Michigan

1976-89

+6.0

L. Huron, ONT

1975-94

+ 13.2

Oregon

1976-93

+ 10.5

California

1981-93

+9.4

rates of Ospreys. By the 1960s, naturalists noticed
numerous empty Osprey nests, broken eggs, large
population decreases (Ames and Mersereau 1964,
Ames 1966, Petersen 1969) and even local extirpa-
tions (Ewins et al. 1996). By the early to mid-1970s
the use of organochlorine pesticides and polychlor-
inated biphenyls (PCBs) had been banned through-
out North America.

Increases in breeding Osprey populations were
noted in most parts of North America from the mid-
1970s (Table 3, Fig. 1), associated with declining or-
ganochlorine contaminant residue levels in eggs
and increases in eggshell thickness towards pre-DDT
values (Henny et al. 1977, Spitzer et al. 1978, Wie-
meyer et al. 1987, 1988, Ewins et al. 1996). The
mean rates of population recovery across the con-
tinent (6-15% per annum) have been remarkably
similar in different areas (Table 3) , suggesting that
the organochlorine pesticide effects were wide-
spread and relatively uniform. The long-term mon-
itoring at Hawk Mountain migration station in
Pennsylvania started just before the introduction of
these pesticides, so the 50-yr population trend in-
cludes many of the pesticide-use years. Extensive
and intensive state- and province-wide Osprey sur-
veys over the past 20 yr have shown similar recovery
trends in reproductive output.

Some Cause-effect Examples. The availability of
suitable nest sites appears frequently to limit local
breeding populations. The creation of reservoirs for

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Vol. 31, No. 2

400

300 -

4*:

t 1 1 — i 1 — i — i — i — i — i — i 1 1 1 — i 1 — i — i — i — r

72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93

year

Figure 1. Changes in breeding populations of Ospreys since early 1970s in Wisconsin and Georgian Bay (Lakes
Huron and Ontario), at artificial nest-platforms (stippled) and other (solid shading) sites. Most “other” sites were
in trees. Wisconsin data are from Gieck et al. (1992).

hydroelectric power generation, irrigation of agri-
cultural land and raising of water levels for naviga-
tion or other purposes, has often provided quality
nest sites for Ospreys by flooding trees. At Eagle
Lake, California, population increases earlier this
century were attributed to reused water levels pro-
viding prime nest sites, but subsequent steady decay
of these flooded trees had reduced the nesting pop-
ulation by the 1970s (Table 2). Similar phenomena
have been observed in the Great Lakes basin, at
Ogoki Reservoir (Postupalsky 1971) and in the Ka-
wartha Lakes and in Montana (Mace et al. 1987).

Human attitudes towards raptors and general en-
vironmental issues have changed markedly in recent

decades. Protective legislation is now available for
many habitats and species and people often want to
take positive actions to assist with restoration of de-
graded ecosystems. In many areas, particularly those
close to centers of human population, customized
artificial nest structures are occupied readily by Os-
preys (Poole 1989, Gieck et al. 1992, Ewins 1994,
1996, Ewins et al. 1995), and these initiatives have
greatly assisted population recoveries post-DDT. In
some areas, up to 70% of occupied Osprey nests
now occur on artificial support structures. For ex-
ample, in Wisconsin and Ontario, much of the re-
cent population increase has been due to increases
in the number of artificial sites available and not

June 1997

Ospreys in North American Forests

143

the number of tree sites occupied (Fig, 1), Hydro
poles, high-voltage transmission towers, navigation
aids and a wide range of other structures are also
used by Ospreys, enabling diem to reoccupy areas
in which preferred large trees and snags are in short
supply close to good foraging areas (Reese 1970,
Westall 1983, Poole 1985, Martin et al. 1986).

Reductions in fish populations and their preda-
tors in northern, base-poor lakes and rivers have
been associated with acidification from precipitation
(Aimer et al. 1974, Mason and Seip 1985, Bevanger
and Albu 1986, Schindler et al. 1989, Gill 1993).
Although there are few North American Osprey
lake acidification studies, reduced productivity and
breeding population density of Swedish Ospreys has
been noted in lakes experiencing high degrees of
acidification (Eriksson et al. 1983, Eriksson 1986).
An increased availability of naturally-occurring met-
als (such as mercury and aluminum) in highly acid-
ified lakes may also prove to be a significant toxi-
cological factor for Ospreys (Nyholm 1981, Poole
1989, Gill 1993, Scheuhammer and Blancher 1994).

The influence of human disturbance of Ospreys
at their nest seems to vary according to whether the
birds are already used to human presence or not,
whether the disturbance is regular from the onset
of the nesting season or if it commences during a
sensitive stage such as the incubation or small chick
stage (Swenson 1979, Poole 1981, Van Daele and
Van Daele 1982, Levenson and Koplin 1984). In
many areas nowadays Ospreys nest very successfully
within 100 m of cottages, roads, railways, boating
channels etc., and it is likely that birds recruiting to
such sites have been raised in similar situations.
Contrastingly, reduced breeding success is often ex-
perienced by birds disturbed after nesting has be-
gun, particularly in remote areas or where little or
no human disturbance has occurred earlier in the
season (Swenson 1979, Levenson and Koplin 1984,
Poole 1989). There is little evidence that propeller
or jet fixed-wing or rotor-winged aircraft flying low
over Osprey nests, even in remote areas, cause
marked reductions in breeding output or site oc-
cupation in subsequent years (Carrier and Melquist
1976).

Bald Eagles are generally more sensitive to hu-
man disturbance than are Ospreys, particularly in
the early spring, but in more remote areas Ospreys
are excluded from suitable nest trees and foraging
areas by the eagles (Ogden 1975, Gerard et al.
1976). As Bald Eagles continue to slowly recover
from the effects of DDT and other organochlorine

contaminants, they are likely to move into former
nesting areas which already support Ospreys, which
will result in local declines in Osprey numbers or
shifts to suboptimal nesting locations, further away
from the foraging areas. In various parts of the
Great Lakes basin, this phenomenon is already well-
established.

Trees and Ospreys

Ospreys will nest in a wide range of tree species,
heights and ages. I agree with Henny (1986) that
historically most Ospreys probably nested . in
the tops of snags or trees with dead tops, although
live trees were also used.” The most important nest-
site selection criteria today seem to be: clear aerial
access to the nest, at least one strong side branch
to support the heavy nest, proximity to water/inac-
cessibility to mammalian predators, avoidance of
Bald Eagle territories and nearby elevated perch. Is-
lands are particularly attractive to Ospreys, largely
due to proximity to foraging areas and reduced
mammal populations, and at least 50% of the
world’s ospreys are thought to breed on islands
(Poole 1989). Of equal importance as the charac-
teristics of occupied nest trees is the surrounding
stand. In forests, Ospreys usually build their nests
above the surrounding canopy, whether it is 15 m
or 50 m above ground level. As a result, they tend
to select older trees and often dead trees or those
with dead, flat or blown-out tops. For example, of
85 occupied nest trees I documented in various
parts of Ontario between 1990-95, 80% were coni-
fers (mosdy white pine, Firms strobus ) and 20% were
deciduous species (mostly white birch Betula spp.,
with some poplars Populus spp.). Live trees support-
ed 47% of the nests, flat-topped or dead-topped co-
nifers supported 12% and the remaining 41% of
nests were in totally dead trees, often in swamps cre-
ated originally by beaver activity.

Some species have more open, irregular crown
architecture than o fliers, making them more suited
to Osprey nests. For example, in Minnesota’s Su-
perior National Forest, 77% of 301 Osprey nests
over 31 yr were in super canopy white pine, even
though this species represented less than 0.5% of
trees with dbh >10 cm (Rogers and Lindquist
1993). In Oregon’s Deschutes National Forest, large
ponderosa pine are the preferred nest tree (90% of
nestings), with mean tree height 35 m, mean dbh
95 cm and 30% of nests are in live trees, 21% of
dead-topped trees and 49% on dead snags (Gerdes
pers. comm.). Dead-topped tall trees are often more

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common in areas infected with insect pests, blister
rusts (Eckstein pers, comm.) or in areas with heavy
winter icing or wind storms which snap off the grow-
ing top.

As timber has been removed from North Ameri-
can forests, so the mean age of forests has declined.
The mean height of trees available to nesting Os-
preys has decreased, as presumably has the
number of trees with sufficiently strong side branch-
es at the top to support Osprey nests over a number
of years. The lowest rates of nest collapse from blow-
down or branch breakage (% nests lost per year)
are in the north, likely reflecting tree age/suitabil-
ity: northwestern Ontario 5% (Grier et al. 1977),
southcentral Ontario 12% (Ewins 1996), Montana
10-15% (Grover 1983), New York 30-40% (Poole
1984), Maryland 10% (Reese 1977), Florida 50-
70% (Poole 1984), California 30% and Mexico 18-
44% (Airola and Shubert 1981). Various studies
have found that reproductive output from tree
nests is often lower than at nests on artificial plat-
forms (Reese 1977, Postupalsky 1978, Van Daele
and Van Daele 1982, Westall 1983, Poole 1989), but
when hydro poles are considered separately from
customized nest platforms, these differences are
less obvious, especially in areas with large trees
(Ewins 1996, Henny and Kaiser 1996). In general,
with populations still increasing at rates averaging
up to 15% per annum, there is little evidence for
population-level impairment of reproduction due
to shortage of quality nest support structures.

Surveys of Osprey breeding distribution over the
past 25 yr have usually found most tree nests to be
close to water. For example, 55% of nests in north-
ern California were within 1 km of water and all
were within 10 km (Garber 1972), and in Oregon
83% of 78 nests in 1978 were within 1 km and all
were within 2 km of water (Henny et al. 1978). In
the Deschutes National Forest in Oregon, success-
ful nests in large ponderosa pines >200 yr old in
1970-71 were not significantly closer to water than
unsuccessful nests (x = 1.2, SD = 1.6 km, range =
0. 1-4.8 km for successful nests; x — 1.6, SD = 1.4
km, range = 0. 1-4.8 for unsuccessful nests; Lind
1976). In Ontario during the 1990s, 93% of 179
tree nests were within 500 m of water; the median
distance to water for tree nests was 10 m, but only
4 m for nests on artificial platforms (Fig. 2).

To what extent does this evidence indicate that
Ospreys prefer to nest close to water? Clearly, Os-
preys will seek to minimize energy expenditure
wherever possible and nest close to food resources.

Figure 2. Frequency distribution for 304 occupied Os-
prey nests in Ontario of distance from open water (river,
or lake >2 ha), 1990-95.

But, this must be balanced against nest stability and
risk of predation. Selective and clear-cut logging in
many areas has usually been more intensive further
away from water courses, due in part to landscape
considerations and the need to provide visual
screens for recreational boaters and canoeists.
Thus, the availability of potentially suitable large
trees for nesting Ospreys may not be comparable
at differing distances from the foraging areas. Per-
haps there were, historically, many more suitable
large trees for ospreys further away from the water.
A clue is provided from studies along the Atlantic
coast. There, some Ospreys regularly nest in trees
14 km or more from the main fishing areas due to
a shortage of suitable nest sites (Greene et al. 1983,
Hagan 1986). In New Brunswick, Ospreys com-
monly nest on hydro poles, and for 151 nests in
1993, the median distance to water was 1.0 km, but
45% of nests occurred from 1-5 km from water
(Stocek pers. comm.). This suggests that provided
suitable tall nest support structures are available, a
greater proportion of Ospreys will breed further
from the foraging areas than is found in areas
where suitable tree sites are in short supply. Thus,
I suspect that, historically, considerable numbers of
Ospreys bred well away from the water in most of
North America and especially in areas where Bald
Eagles occupied the prime super canopy or large
snag nesting trees in water’s-edge territories (Os-
preys generally avoid nesting near Bald Eagles) .

Nesting immediately over water, such as on a
stump, flooded tree or navigation aid, presumably
reduces the risk of predation at the nest, and so
would be expected in preference to tree nesting
on land. There is little firm evidence for this, but

June 1997

Ospreys in North American Forests

145

on Chesapeake Bay, navigation aids, duck blinds
and nesting platforms over water seem to have
been occupied even though large trees were ap-
parently still available along the river banks (Reese
1969). However, predation by raccoons ( Procyon lo-
tor) and Great Horned Owls ( Bubo virginianus )
does occur at over-water nests (Poole 1989) so no
generalizations can yet be made about the selective
advantage to nesting in these different situations.

There is remarkably little published or unpub-
lished information on Osprey nest trees and repro-
ductive outcomes in areas subjected to timber ex-
traction. Collating data from many individuals
across North America, I reached the conclusions
that there is an urgent need for a systematic field
study and that no firm generalizations can be
made. In some cases Ospreys continued to nest
successfully in an isolated tall tree or snag left
after clear-cutting, in others Ospreys have actually
moved to an isolated tree within a clear-cut and in
other cases Ospreys have abandoned a nest during
logging activities, or road construction, or have
abandoned the entire area after a nest tree was
removed and no suitable alternatives seemed to be
available nearby.

In California, 15 Osprey nests within 500 m of
logging roads suffered significantly reduced breed-
ing output if logging traffic use of roads com-
menced once Ospreys had already initiated breed-
ing (Levenson and Koplin 1984). This study re-
jected the conclusion of Melo (1975), which was
based upon a single nest observation, that logging
activities could safely continue during the Osprey
breeding season to within 30-35 m of the nest.

It is likely, though not quantified, that the large
changes in beaver ( Castor canadensis) populations
over the past two centuries have greatly reduced
the availability of snag trees over water, often pre-
ferred by nesting Ospreys. Intensive trapping led
to severe depletion of beaver populations at vari-
ous stages and regions over the past two centuries
(Newman 1985, Dunstone 1993). In many north-
ern parts of North America, Ospreys breed in snags
in swamps formed as a result of beaver activity.
Thus, this human trapping pressure almost certain-
ly greatly reduced the number of suitable snags
available for nesting Ospreys.

Current Forest-Management Guidelines for Ospreys

With populations of Osprey and other raptor
species at record low levels in many areas during
the 1960s, due largely to the effects of organochlo-

rine pesticide accumulation, considerable atten-
tion turned towards restoration measures. Restric-
tions on the use of DDT and dieldrin were finally
introduced in the early 1970s and many agencies
then focused on habitat management for Ospreys.
The first Osprey Management Area was designated
at the Crane Prairie Reservoir in Oregon’s Des-
chutes National Forest (Roberts 1969) and the man-
agement plan formed the basis for subsequent for-
est-management guidelines and recovery plans for
Ospreys across the continent. These management
guidelines vary considerably among areas, most no-
tably in the distances they recommend for the var-
ious types of buffer or exclusion zones, but also in
the suite of exclusions and various proactive con-
servation measures (Roberts 1969, Kahl 1972, Gar-
ber et al. 1974, Penak 1983, Gieck 1986, Henny
1986, Nova Scotia Dept. Lands and Forests 1987,
U.S. Forest Service 1974, 1991).

Nesting Habitat. Absolute buffer zone — within a
40-200 m radius of an occupied nest tree, access
is restricted year-round and limited to activities
benefiting the nest site (e.g., nest support modifi-
cation, collection of scientific data, tree safety
pruning) .

Seasonal buffer zone — within 100—800 m radius of
an occupied nest, or up to 600 m beyond the pe-
riphery of the absolute buffer zone and for the
duration of the breeding season (usually 1 April to
31 August), certain activities are restricted or
banned. These include logging, road or pipeline
construction, mining, peat extraction and some
forms of recreation. Outside this breeding period,
recreational activities and controlled tree harvest-
ing and planting is permitted. Within clear-cuts,
some small- and medium-sized trees should be re-
tained in clumps, as well as some large snags. Some
plans advise retaining >4 flat-topped tall dominant
trees or snags, or all snags >36 cm dbh (U.S. For-
est Service 1974), or even to remove the tops from
some live large trees to create more suitable nest-
ing trees.

Riparian /lacustrine buffer zone — for distances of
70-350 m back from the water’s edge, the guide-
lines vary from no cutting, to retention of up to 5
snags and 5 clumps of tall trees, or the preserva-
tion of clumps of large living or dead trees, or >10
trees/ha. In addition, Kahl (1972) and Garber et
al. (1974) recommend retention of all broken-top
and other suitable nest trees up to 3.5 km beyond
this 350 m buffer zone.

Foraging Habitat. Restrictions apply to develop-

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Vol. 31, No. 2

ment which could degrade shallow-water fish hab-
itat. Recommended bans on the use of chemical
control of undesirable, nongame fish species. Wa-
ter levels should be maintained so as to allow Os-
preys access to fish.

General Guidelines. Early consultation with the
area wildlife biologist is stressed by most plans and
their approval required prior to any timber sale.
Suitable training for field foresters in wildlife iden-
tification and the forest-management guidelines is
recommended. Protective measures may be lifted
after prolonged inactivity of a nest tree. The need
for ongoing monitoring is stressed. Some plans
recommend “guarding against the effects of pes-
ticide sprays” (Penak 1983), which presumably re-
fers to the persistent, yet highly effective and toxic
organochlorine pesticides. Proactive measures are
stressed especially by the early management plans
in California, where dead and live trees were mod-
ified to provide stable nest supports for Ospreys,
wherever human safety was not compromised.

Thus, there is wide variation in forest-manage-
ment guidelines for Ospreys across North America.
To a large extent this reflects the uncertainty in
the response of Ospreys at the population level to
different types of forest management and our lack
of understanding of basic components of Osprey
ecology 7 in forested areas. This has naturally led to
some confusion and questioning among resource
managers. For example, Ontario’s new Forest Plan-
ning Manual has 39 complex guidelines of this
type, which many foresters find much too complex
and ignore (Euler pers. comm.). Hence, although
the Ontario guidelines for forest management in
the vicinity of Osprey nests are relatively stringent
(Penak 1983), we find that adherence is largely de-
pendent upon the inclinations of individuals on
the ground, both foresters and local wildlife biol-
ogists.

An interesting economic concern has been
posed by Opper (1988). Based on 1980s mean tim-
ber values and forest yield parameters in Ontario
pine forests, he has calculated that adherence to
the provincial forest-management guidelines near
an Osprey nest “preempts about 465 units of
wood, which would produce approximately 280

tons of pulp thus costing about $GAN

168,000 to protect a single Osprey nest.” While ac-
cepting the principle of integrated and sustainable
forest and wildlife resource management, Opper
understandably questions “. . . the scientific or bi-
ological rationale upon which (such) wildlife pre-

scriptions are made.” These questions highlight an
urgent need for sound biological data to justify
particular management guidelines for Ospreys,
since we currently have only scattered and anec-
dotal evidence. This suggests that Ospreys at either
the individual or population level exhibit variable
tolerance to forest-management activities and as-
sociated disturbance of different types.

The protection of mature, over-mature and
deadwood timber in riparian zones, or as isolated
trees or small clumps in clear-cuts clearly provides
Ospreys and Bald Eagles with suitable nest trees.
But one might expect elevated nest predation rates
in such strips/corridors, due to predators mov-
ing along such corridors or between clumps of
trees. In clear-cut areas, retention of isolated tall
trees or snags generally increases the exposure to
wind and the likelihood of trees blowing down.
Thus, it is important to retain a number of alter-
native sites/clumps and ideally to conserve some
younger trees which would, in time, replace the
suitable nest trees at the time of timber extraction.

Finally, we should remember that the decisions
regarding conservation of trees for nesting Ospreys
must be made in an ecosystem context. Many other
important components of the forest wildlife com-
munity will benefit by retaining groups of larger
and dead trees in any clear-cut areas and these de-
cisions should clearly be made on an ecosystemic
and long-term basis, not just for one species of rap-
tor on a short-term basis.

Recommendations

There is a clear need for a systematic study of
nesting Ospreys in relation to different forest-man-
agement activities. This need is as much from a
forestry-economic perspective as from an Osprey-
conservation angle. Such a study could well be
done cooperatively with the forest industry and a
suitable student/university/conservation or gov-
ernment agency. The ultimate objective would be
to provide an objective assessment of the responses
of nesting Ospreys to factors associated wath timber
extraction, both at an individual and population
level and over the course of a few years (not just
1-2 yr). For example, the study should compare
breeding productivity and nest occupancy rates
over 3-5 yr at sites with different intensities and
types of human disturbance, with different sizes
and ages of clear-cuts, at various distances from wa-
ter and with different numbers of alternative nest
trees within the vicinity. Nest predation rates and

June 1997

Ospreys in North American Forests

147

tree stability in narrow riparian corridors and at
isolated clumps of trees should also be assessed.
Such a study would require large study areas, but
foresters are operating at this scale already. The
benefits to the industry would be substantial if it
were found that Osprey populations can generally
adapt well to logging activities.

The final recommendations in relation to Os-
preys should then be integrated with the results
from any studies of other wildlife species with spe-
cific niche requirements (e.g., other raptors, cavity-
nesting birds, mammals), to produce general
guidelines for sustainable forestry which would ac-
commodate the needs of a wide range of wildlife
species and not just one or two top predators. If in
doubt, I would strongly recommend erring on the
cautious side by retaining more dead snags, larger
clumps of dominant trees and broader noninter-
vention riparian strips.

General Conclusions

North American Osprey breeding populations in
the 1980s-1990s appear relatively healthy and are
still increasing in most areas, following the dra-
matic declines caused by the effects of organochlo-
rine contaminants during the 1950s-1970s. Many
factors are known or suspected to impact Osprey
populations and their breeding productivity and
the relative importance of these varies considerably
across the continent. Prior to European coloniza-
tion of North America, most Ospreys probably
bred at the top of large trees, but as forests were
cleared and the mean age of forests declined con-
siderably, Ospreys have adapted to nesting on ar-
tificial structures, often over water. In many areas,
they have also habituated to nesting in close prox-
imity to humans. Artificial nesting structures are
not a viable long-term alternative to natural, tall
tree support structures for Osprey nests.

Many of the present guidelines for forest man-
agement in the vicinity of Osprey nests stem from
advice used 15-25 yr ago, when Ospreys were clas-
sified as threatened or endangered in many parts
of the range. In light of the dramatic population
recovery since the mid-1970s, a review of these
guidelines is appropriate. Forest-management
plans should ensure, in an ecosystemic context,
that sufficient large live trees and standing dead-
wood snags are retained after timber extraction to
provide nesting Ospreys with a number of alter-
native nest trees close to shallow-water foraging ar-
eas. In relation to nest-site requirements, Ospreys

are relatively adaptable, compared to many other
raptors and other animals of North American for-
ests. The precise nature and extent of this adapt-
ability needs to be confirmed and properly quan-
tified for Ospreys nesting in commercial forests
and more consistent guidelines adopted across the
continent once this type of study has been com-
pleted. Current guidelines for Ospreys in some for-
est-management plans may be difficult to justify
based on the needs of Ospreys, at least at the pop-
ulation level. However, retaining large residuals of
mature trees benefits many other species in forest
ecosystems, not only Ospreys, so we clearly need to
adopt an ecosystem approach. Forest-management
guidelines for wildlife should not restrict timber
harvesting or recreational use of forests unneces-
sarily, yet they should ensure that the needs of the
most sensitive components of the forest ecosystem
are provided for, even if they are not so highly vis-
ible or adaptive as a top predatory species like the
Osprey.

Acknowledgments

I am grateful to many people for providing details of
unpublished observations and work, notably: Ron Eck-
stein, Jules Evens, Dave Euler, Mike Gerdes, John Hagan,
Chuck Henny, John Mathisen, Bruce Rantor, Rudy Sto-
cek and Rob Swainson. The paper also benefitted from
useful discussions with Pertti Saurola, Dan Welsh, Dan
Varland, Sergej Postupalsky and Steve Zender. I thank
Gerald Niemi for assisting with my participation in this
symposium. The Canadian Wildlife Service, Environment
Canada’s Great Lakes 2000 Cleanup Fund and the On-
tario Ministry of Natural Resources provided funding for
much of my work on Ospreys in the Great Lakes basin.

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Received 2 November 1995; accepted 6 March 1997

J. Raptor Res. 31 (2) :151-159
© 1997 The Raptor Research Foundation, Inc.

THE GREAT GRAY OWL (STRIX NEBULOSA) IN THE CHANGING
FOREST ENVIRONMENT OF NORTHERN EUROPE

Seppo Sulkava

Planeetankatu 2 D 24, FIN-02210 Espoo, Finland

Kauko Huhtala

Tehtaanpuistokatu 3, FIN-85310 Sievi as., Finland

Abstract. — The Great Gray Owl ( Strix nebulosa) breeds in northern Europe mostly in older coniferous
forests. Nest sites are usually in twig nests of large hawks, sometimes on stumps, and occasionally on
the ground. The availability and the quality of tree nest sites is generally lower in managed forests;
however, use of artificial nests has partly compensated for declines in natural nest sites. The Great Gray
Owl has increased in abundance in northern Europe over the last 30 yr. It was absent almost entirely
from Finland from 1940-60, having been numerous there, especially from 1880-1910. It feeds mainly
on field voles ( Microtus agrestis), which are abundant in fields and grassy areas following forest clear-
cuts. The area of clear-cuts has increased since 1950, providing more open hunting habitat and vole
resources. Also, protection of all owls and increasing positive attitude toward birds of prey has coincided
with the Great Gray Owl increases since the late 1960s. Forest management practices that may benefit
the Great Gray Owl include shape of cuts which should be irregular and not broader than 400 m. Perch
trees left in cut areas would expand the hunting area from the forest edge.

Key Words: Strix nebulosa; Great Gray Owl; trends in distribution; breeding, diet, effects of forestry.

El Gran Buho Gris Strix nebulosa en cambios del ambiente en los bosques de Norte Europa

Resumen. — El Gran Buho Gris Strix nebulosa se cria en el norte de Europa mas frecuente en bosques coniferos
maduros. mas viejos. Sitios de nidos estan muchas veces en nidos de ramita de halcones grandes, a veces en
tocones, y de vez en cuando en el terreno. La disponibilidad y la calidad de arboles con nido es generalmente
mas bajo en bosques manejados. Sin embargo, uso de nidos artificiales ha compensado un poco
en la reduction de nidos natural. El Gran Buho Gris ha aumentado en abundancia en el norte de Europa
sobre los ultimos 30 anos. Estuvo ausente casi completamente en Finlandia de 1940-1960, viendo sido nu-
meroso alii, especialmente de 1880-1910. Se alimenta principalmente con ratones de labor Microtus agrestis,
que son abundante en los labores y areas pastosas despues de cortes-completos de bosques. El area de cortes-
completos ha aumentado desde 1950, proporcionando mas habitat abierto para cazar y recursos de Microtus
agrestis. Tambien, protection de todos los buhos y aumentando el actitud positivo para los ave de rapina ha
coincidido con el aumento del Gran Buho Gris desde el fin de los 1960s. Costumbres del administration de
bosques que puede dar beneficio al Gran Buho Gris incluye la forma de los cortes que deben ser irregular
y no mas amplio que 400 m. Arboles de percha dejados pueden aumentar areas de cazar de la orilla del
bosque.

[Traduction de Raul De La Garza, Jr.]

A number of raptor populations have declined
during the past 50 yr (e.g., the Peregrine Falcon,
Falco peregrinus, and the Osprey, Pandion haliaetus) ,
largely due to pesticides. Many forest species, such
as nonmigratory owls, have not experienced simi-
lar declines, but have been affected by rapid
changes in forest structure caused by forestry prac-
tices.

Public attitudes toward raptors, especially owls,
have improved in recent times. Killing by hunters

is now rare in the northern countries of Europe.
In addition, ornithologists have prepared many
nest boxes and other nesting structures for several
species of owls. In Finland during 1994, 22 691 owl
nest boxes were checked for occupancy (Saurola
1995).

The purpose of this paper is to review existing
knowledge on the distribution and ecology of the
Great Gray Owl in northern Europe and to discuss
possible effects of forestry. The main topics cov-

151

152

SULKAVA AND HUHTALA

Vol. 31, No. 2

ered are: changes in distribution, population size,
causes of mortality, nest sites, diet and the possible
effects of forestry.

Nesting and Feeding Ecology

Nesting Habitats. The nesting-habitat require-
ments of the Great Gray Owl are fairly flexible, but
most nests are found in older spruce-dominated
coniferous or mixed forests. Great Gray Owls do
not build their own nest structures. Nest location
is, therefore, determined by nest-site requirements
of large hawks, which build twig nests. Older for-
ests may be preferred because the Northern Gos-
hawk (Accipiter gentilis), the main nest-site produc-
er, prefers this nesting habitat. The Great Gray Owl
also nests in pure deciduous and pine forests. Nests
in structures other than twig nests have in a few
cases been in open habitats (e.g., almost open
clear-cut area or open field) .

Nest location is often near an opening such as a
natural open bog, a clear-cut area or a small field
(Pulliainen* and Loisa 1977, Mikkola 1983, Hilden
and Solonen 1987). When nests are 50-100 m
from forest edges, the Great Gray Owl often perch-
es at the forest edge. The most commonly used
perch trees are often near the edge. Nests located
m more exposed environments have shelter avail-
able nearby and young leave the nest relatively ear-
ly to avoid direct sunlight (Helo 1984).

Nest Sites. Forest-management practices have
decreased natural nest sites available for many
large forest owls, including large stumps and holes,
the former of which are used by the Great Gray
Owl (Table 1). Natural twig nest sites used by the
Great Gray Owl are mainly old goshawk or buzzard
( Buteo spp.) nests, both occupied and abandoned.
Nests on top of stumps (1-5 m in height) have
been found in Finland, Sweden (Stefansson 1979)
and Alaska (Osborne 1987). The Great Gray Owl
also occupies artificial nest structures, either open
boxes or platforms (Table 1). Use of artificial nests
indicates a lack of natural nest sites, as in the case
of the Kemi-Tornio area, but also indicates an abil-
ity to use a wide variety of nest sites and structures.

Stump nests are more common in southern ar-
eas (Hilden and Solonen 1987, Osborne 1987).
This is observed to some extent in Finland, but
more clearly in the U.S., where only stump nests
were used in most southerly areas (Osborne 1987).
Imprinting on nest structures by young (Hilden
and Solonen 1987) and lack of large stumps in
northern areas (Osborne 1987) are possible expla-

Table 1. Nest-site distribution (%) of the Great Gray
Owl in three areas: all of Finland, western Finland and
northeast of the northern Bothnian Bay (Kemi-Tornio
area) .

Nest Sites

All of
Fin-
land 3

West-

ern

Fin-

land 1 ’

Kemi-

Tornio

Area c

Hawk twig nest

72.7

88.9

42.6

Corvid nest

4.8

1.0

3.8

Other twig nest

4.4

—

0.8

Man-made twig nest

3.6

2.0

37.2

Man-made platform or open box

—

3.0

9.3

Stump

10.8

2.0

5.4

Ground

2.4

2.0

0.8

Cliff, stone, ant hill

0.8

1.0

—

Barn roof

0.4

—

Number of nestings

249

129

a Hilden and Solonen 1987.

b K. Huhtala unpubl. data, not included in “All of Finland.”
c Liehu et al. 1995.

nations. In addition, because owls avoid long pe-
riods of exposure to sunlight (Osborne 1987),
nests on low stumps may offer more shade, which
is likely more important in southern areas. Many
stump nests in Finland have been found in rela-
tively warm springs (Mikkola 1983), but correla-
tions with temperature need further investigation.
Ground nests, which have been recorded several
times in Finland (Table 1), may be a response to
forests with no twig or stump nest sites or they may
be a response to microclimatic factors.

Nesting Density. Nests were usually some dis-
tance apart. Saurola (1985) estimated a mean of 1
pair/ 100 km 2 for Finland. In several cases, howev-
er, two or more nests have been reported only 100-
400 m apart (Mikkola 1981, 1983). Some group
nestings are likely due to local abundance of field
voles. In some other cases, old “goshawk forests”
with several good alternative twig nests were the
reason why two nests were situated only 100-200
m from each other. In some cases, male Great Gray
Owls have been polygamous with the second fe-
male laying about 1 wk later than the first. Also,
two pairs have nested only 100-300 m apart (Hog-
lund and Lansgren 1968, Mikkola 1983) to use gos-
hawk nests which were situated in groups in older
forests.

June 1997

Great Gray Owl in Northern Europe

153

Table 2. Composition of Great Gray Owl diet in northern Europe (%).

Sweden

1955 - 64 3

Northern

Europe

1955 - 74 b

Western

Finland

1966 - 89 c

Western

Finland

1973 d

Kola

PENINSUIA e

Field vole, Microtus agrestis

54.3

66.2

73.3

42.6

1.5

M. oeconomus and M. spp.

17.6

7.3

—

Bank vole, Clethrionomys glareolus

7.7

10.3

9.6

11.0

0.8

Grey-sided vole, C. rufocanus

1.5

2.9

—

93.8

Clethrionomys spp.

8.4

3.2

—

Water vole, Arvicola terrestris

1.5

1.7

4.3

1.0

—

Wood lemming, Myopus schisticolor

2.8

1.8

1.3

—

Common shrew, Sorex araneus

2.3

2.8

7.4

36.3

—

Soricidae spp.

2.5

1.7

2.2

2.3

0.8

Birds, Aves

0.8

1.0

0.6

0.8

—

Frogs, Amphibia

0.2

0.5

0.1

1.0

—

Other animals

—

0.4

2.1

5.0

3.1

No. items identified

1977

5177

4858

830

130

a Hoglund and Lansgren 1968.
b Mikkola 1983.
c K. Huhtala unpubl. data.
d K. Huhtala unpubl. data.
e From Mikkola 1983.

Hunting Habitats and Diet in Northern Europe.

There are numerous observations of Great Gray
Owls hunting in open habitats in Finland, but
quantitative data are lacking. They may fly over
open terrain like Short-eared Owls ( Asio flammeus) ,
but most observations have been of perching birds
on trees, bushes or telephone poles at or near the
edges of forests (Wahlstedt 1969, Mikkola 1981,
1983). In winter, hunting dive pits in the snow are
usually <50 m from the forest edge.

The diet of the Great Gray Owl in northern Eu-
rope is mainly Microtus voles (M. agrestis, M. oecono-
mus in Lapland; Table 2) . This specialization is sur-
prising because of the owl’s size and because there
are also other numerous small mammals ( Cleth-
nonomys voles and Sorex shrews) available in its en-
vironment. Field voles ( Microtus spp.) comprise an
average of 72—74% of the diet, while 8—10% consists
of bank voles ( Clethrionomys spp.) and 5-10% of
shrews. In most years other prey, such as birds, frogs
and larger voles ( Arvicola spp.) are only occasionally
found in the diet. The diet of the Great Gray Owl
suggests that it hunts mainly in grassy areas (fields,
meadows, open bogs and clear-cut areas) , where Mi-
crotus species are found. Although there are numer-
ous shrews available in north European grasslands,
they seem to be avoided.

A comparison with the diet of Tengmalm’s Owl
{Aegolius funereus) in the same area in western Fin-

land (Table 3) indicates that other small mammals
as well as field voles are available in the same lo-
cality. Tengmalm’s Owl hunts mainly in forests, but
also at forest edges and in grasslands (Korpimaki
1981, 1988). However, it preys on field voles much
less and feeds more on bank voles (37-46%) and
shrews (15-24%). This confirms that the Great
Gray Owl hunts mostly in open habitats and pre-
fers Microtus.

The preference for field voles in open habitats
may be partly due to the large size of the Great
Gray Owl, which may make it difficult for the spe-
cies to hunt in the dense forests of central Finland.
This notion is supported by results of Pulliainen
and Loisa (1977) in northeastern Finnish Lapland,
where most old forests are rather open. There, Mi-
crotus and Clethrionomys voles are represented in the
diet of the Great Gray Owl in the same percentages
as are found from small mammal captures.

High abundance of Microtus voles in the species’
diet may be overemphasized, because data are pri-
marily based on its nesting diet and the species
usually nests only in good Microtus years. Three
samples of food from poorer Microtus years when
the Great Gray Owl nested (Table 2) indicate that
it is also capable of capturing other prey. Field
voles, however, were still the main prey even in
these exceptional samples from 1973-77. All three
pairs nesting in central Ostrobothnia in 1973 fed

154

SULKAVA AND HUHTALA

Vol. 31, No. 2

Table 3. Composition of Great Gray Owl and Tengmalm’s Owl diet (%) in central Ostrobothnia (western Finland)
in 1966, 1977 and 1989 (K. Huhtala unpubl. data).

Great Gray Owl

Tengmalm’s Owl

1966

1974

1989

1966

1974

1989

Field vole, Microtus agrestis

83.0

76.7

25.8

28.7

39.2

Bank vole, Clethrionomys glareolus

9.2

12.2

7.6

45.8

40.4

37.2

Wood lemming, Myopus schisticolor

—

1.2

2.3

—

6.7

Shrews, Soricidae

6.0

5.4

5.0

23.7

23.3

15.3

Birds, Aves

1.1

0.9

0.4

2.8

7.2

1.5

Other animals

—

1.4

0.4

1.9

0.4

0.2

No. items identified

283

1064

931

528

460

406

largely on shrews (33—41% of the diet compared
to 1-6% of the diet in other years). Whereas the
diet of great grays usually consists of only 0-2%
frogs, 12% of the diet of a pair nesting on the is-
land of Hailuoto in the Bothnian Bay in 1977 con-
sisted of frogs. Similarly, the great gray diet consists
of only 1-3% water voles but, in western Finland
in 1977 it contained 8—15% water voles. More wa-
ter voles were available, because Eagle Owls ( Bubo
bubo ) also consumed more water voles in 1977 than
normal.

Distribution and Population Trends in This Century

In this century, the Great Gray Owl has nested
in most of Finland, northern Sweden and occa-
sionally in the far north of Norway (Mikkola 1983).
It breeds rarely throughout Russian Karelia and is
included in the Karelian Red Book on endangered
animals (Shehter 1985). The most southwesterly

Years (by decade)

Figure 1. Number of Great Gray Owl nests found in
Finland at 10-yr intervals in 1880-1960 and at 5-yr inter-
vals in 1960-1995. Before 1940 data are from clutches in
Finnish egg collections.

breeding localities in Europe are in Belorus and
Poland (Mikkola 1983).

The nesting population in northern Europe has
changed considerably in the past 100 yr (Fig. 1).
One hundred years ago (1880-1910), there were
several good nesting years and Finnish egg collec-
tions alone contain more than 10 clutches from
several years. At that time, the population bred in
the northernmost coniferous forests, mainly in
northern Finland and in northeastern Norway
(Fig. 2).

Number of nests

Figure 2. Distribution and relative density of Great Gray
Owl nests in Finland and northern Sweden in 1955—1974
(adapted from Mikkola 1983). Squares show the main
breeding areas in northern Finland and Norway in 1880-
1910.

June 1997

Great Gray Owl in Northern Europe

155

Years

Figure 3. Number of Great Gray Owl nests found in
Finland in 1955—1994, yearly (lower line) and at 4-yr pe-
riods (upper line) .

From 1910-1930, few nests were reported in Fin-
land. Some nests were again seen during the 1930s,
especially in the far north of northeastern Finnish
Lapland in 1938 (Haartman et al. 1963-72, Fig. 1).
Breeding Great Gray Owls were then almost absent
from Finland for about 10 yr from 1940—1950. A
few nests were found in the 1950s farther south, in
central Finland (Merikallio 1958, Mikkola and Sul-
kava 1969, Hilden and Helo 1981). Few nests were
found during this period in Sweden (Curry-Lin-
dahl 1961).

There have been several winter invasions of
Great Gray Owls in south and central Finland,
south of the normal breeding area. However, only
occasional nestings occurred in spring following
invasions. Invasion years were 1895-1896, 1907-
1908, 1911-1913, 1935, 1949 and 1955 (Merikallio
1958, Haartman et al. 1963-1972).

The number of nests found has increased in Fin-
land since the 1960s (Fig. 3). Regular breeding was
reestablished in Finland in 1966—1967, and since
then Great Gray Owls have been nesting in the two
best years of every 4-yr vole cycle (Fig. 3) . In Swe-
den, nesting was rare until 1973 and has increased
since then (Fig. 4).

There has been a steady increase in number of
nests found in Finland over the last 25 yr, especially
in the last 10 yr (Fig. 3). There was a slight reduc-
tion in number of nests in the 1980s compared to
the number found afterward. The 4-yr vole cycle,
regular since the 1950s, became irregular in the
early 1980s (Henttonen et al. 1987). Consequendy,
the field vole peaks were not high enough to allow
all Great Gray Owls to breed.

The reported increase in the number of nests

Years

Figure 4. Number of Great Gray Owl nests recorded
yearly in Sweden in 1955-1989, according to Mikkola
(1983), Stefansson (1983) and Niemi (1989).

found in northern Europe in 1960-1994 was par-
tially due to the increase in nest searching for all
owls for ringing (banding) and monitoring (Sau-
rola 1986) and for building of artificial nests (Ta-
ble 1). No increase in the Finnish Great Gray Owl
population was determined from 1966—1984 by
Saurola (1985), but since then the numbers have
been higher (Fig. 3). An increase in the Great Gray
Owl population is also mentioned in other studies
(Mikkola 1983, Helo 1984, Solonen 1986).

The breeding area of the Great Gray Owl after
1960 has been concentrated in central and south-
eastern Finland, 300-500 km south of the main
area of breeding before 1940 (Mikkola and Sulkava
1969, Hilden and Helo 1981, Fig. 2). The main
breeding area now is in the Oulu district and since
1980 also in the Kemi-Tornio area (Solonen 1986).
Because extensive cutting in Lapland began 20—40
yr later, this move from Lapland to central Finland
was not caused by forestry. After this range shift,
clutch size was smaller (x = 4.30, SE = 0.10, N =
70) than in 1880-1910 (x = 4.63, SE = 0.12, N =
80; t = 2.16, P < 0.05). However, this potential
decrease in offspring production has not affected
an increase in the population in recent times.

Effects of Forestry Practices

Effects of forest-management practices on owls
vary, depending on the practices and the needs of
the owl. Forest owls may lose nesting habitats and
nest sites, but hunting for prey may be easier in
cut areas. No numerical data are available on the
effects of forestry on Great Gray Owls or their pop-
ulation in Finland. However, possible influences
may be determined from indirect data on food

156

SULKAVA AND HUHTALA

VOL. 31, NO. 2

1000 ha

160 -

120 -

ivale St other

Forest industries

Sfafe

40 -

Year

Figure 5. Area of clear-cut forest seeded or planted in Finland in 1950-1993 (total and by forest ownership category)
(from Aarne 1994).

habits, changes in the environment, and from mis-
cellaneous direct observations.

Forestry Practices in Finland, 1950-1990. The
“modern and efficient” logging of forests began
in northern Europe about 1950. Until that time,
timber was harvested mosdy by thinning the for-
ests. Since then, forests have changed rapidly in
many ways that may affect the Great Gray Owl.
First, the area of old forests has rapidly decreased
resulting in a decrease in large trees, in which di-
urnal birds of prey build their twig nests. Second,
dead and broken trees have been removed result-
ing in a reduction in stump nest sites. Third, clear-
cut areas have increased (Fig. 5) resulting in an
increase in field vole production and an increase
in food. Fourth, the area of young, dense forests
has increased, decreasing the area available for
hunting or nesting. Fifth, the area of drained wet
bogs has increased, leading to a decrease in the
area available for field vole production. Sixth,
roads are being built in remote forest areas leading
to increased disturbance (Fig. 7).

Nesting Habitat Availability. Large areas of clear-
cutting (several km 2 at a time) indicate that the
Great Gray Owl has lost considerable breeding
habitat, which may reduce local breeding popula-
tions. It may be argued that, because of the species’
flexibility in nest habitat use, it will have adequate
forests for breeding in northern Europe in the fu-

ture. The availability of nesting habitat may not be
a limiting factor, although it is not known how
large a forest a nesting pair of Great Gray Owls
needs between cut areas.

Decline in Availability and Quality of Nesting
Sites. The Great Gray Owl nests in a variety of
structures (Table 1), but large twig nests and
stumps are the main natural sites used. The num-
bers of these nest sites have decreased because of
forestry practices for several reasons. First, the area
of older forests that contain hawk nests has de-
creased because of clear-cutting. Second, the num-
ber of stump nest sites in forests has decreased be-
cause dead and broken trees have been removed
in the course of forest management. Third, the
number of alternative nests per hawk pair is prob-
ably smaller in young forests. Young trees have
weaker branches and, consequently, nests collapse
more often. In addition, poor quality of twig nests
in young forests may lead to nest failures, but the
extent of such losses is unknown.

The number of twig nest sites currently available
seems to be sufficient in Finland. Solonen (1986)
estimated that there were about 50 000 twig nests
available in Finland in 1986, and even if 50% of
these were outside the normal breeding area of the
Great Gray Owl, the number would still probably
suffice for the estimated 500-1500 Great Gray Owl
pairs in the country. In addition, populations of

June 1997

Great Gray Owl in Northern Europe

157

large hawks, goshawks, Honey-buzzards ( Pernis api-
vorus ) and buzzards ( Buteo spp.) have been stable
in Finland in recent times (Saurola 1985, Haapala
et al. 1995). These species will likely produce
enough new twig nests for Great Gray Owl use in
the future, too, despite the fact that hawk nests are
sometimes destroyed by the Great Gray Owl when
it digs out the nest bowl. In the first digging phase
in early spring, the owl often digs a 10-15 cm deep
bowl. This perhaps dries the nest material; the final
bowl for the eggs is only about 6 cm deep. Rapid
wear of hawk nests used by Great Gray Owls has
also been observed by Stefansson (1979) in Swe-
den.

The readiness of the Great Gray Owl to use var-
ious nest structures points to a lack of suitable nest-
ing sites. Man-made nests are often used in North
America (Nero 1980, 1982, Bohm 1985, Bull et al.
1987) and, in Finland, open box nests have been
used in Kainuu (Helo 1984) and central Ostro-
bothnia. Almost half the Great Gray Owls reported
in the Kemi-Tornio area in the 1990s occupied ar-
tificial nests composed of twigs (Liehu et al. 1995,
Table 1).

Increase in Hunting Habitat Availability. To find

enough Microtus voles, the Great Gray Owl needs
open habitats with grasses, herbs and sedges. These
habitats include hay fields (cultivated, temporarily
unused or abandoned), open wet bogs, bog mar-
gins or clear-cut forest areas. The area in hay fields
has increased, especially those unused or aban-
doned. Only a small proportion of abandoned
fields have been planted with trees. The number
of clear-cut areas of different ages has increased
substantially since the 1950s (Fig. 5). The vegeta-
tion on most of these areas will support field vole
populations. The common practice of plowing cut
areas has increased the growth of grasses. The
number of wet sedge bogs has decreased due to
extensive draining for forestry or peat production
(Aarne 1994). Drained areas are only partly suit-
able for field voles.

Clear-cutting is obviously the main factor which
has increased hunting habitat availability to the
Great Gray Owl in forest areas since the 1950s. Be-
fore 1950, timber was removed by selective harvest,
but between 1950-1994, clear-cutting was the pri-
mary logging practice. Only in the last few years
has “continuous cultivation” (selective harvest) of
the forest been allowed again; clear-cutting is still
the main practice. Before the early 1990s, clear-cut
areas were mostly carefully cleared of all timber,

O Clear-felled area
□ Great Gray Owl nests

Figure 6. Total clear-cut area (in 1000 ha) in Finland
from 1970—1993 (from Aarne 1994) and Great Gray Owl
nests recorded at 4-yr intervals from 1970-1994.

including dead and young trees, and the land was
often burned, ditched or plowed before it was
seeded or planted (Fig. 5).

During the expansion of the Great Gray Owl
population in Finland since the 1960s, clear-cut ar-
eas have provided more field voles for raptors than
in earlier periods (Teivainen 1979). In state forests
in northern Finland and in forests of timber com-
panies in central Finland, the clearings have often
been too large (more than 20 ha) and too open
to be used as hunting grounds. The owls probably
hunted only at the edges of these large openings.

The future may be better because new rules for
forest management and cutting practices have
been prepared and partly introduced in 1994—
1995. Cut areas will be smaller in size (mostly not
more than 5-10 ha) and groups of both live and
dead trees will be left in cut areas. Also, forest strips
will be left along lake shores and stream valleys.

Prey Availability. The Great Gray Owl requires a
high density of voles (mostly field voles) to breed,
and therefore generally breeds only in increasing
and peak years of the northern vole population
cycles which are more pronounced than in more
southern areas of Europe (Hansson and Hentto-
nen 1985). In the northernmost breeding areas of
northern Europe (Fig. 2), M. oeconomus and even
C. rufocanus may produce enough food for breed-
ing (Table 1) because these species are often the
most abundant voles in open habitats.

The increased vole resources due to clear-cuts
(Teivainen 1979) have obviously been important to
support the increase in Great Gray Owls in north-

158

SULKAVA AND HUHTALA

Vol. 31, No. 2

Kilometres

5000-1-

Figure 7. Permanent forest roads (in km) completed in Finland from 1950-1993 (total and by forest ownership
category, from Aarne 1994).

ern Europe. In Germany, the number of breeding
Tengmalm’s Owls increased after extensive clear-
cutting and decreased again with reforestation.
This was due to the greater abundance of rodents
in recently cut areas (Mebs 1987). The breeding
population of Great Gray Owls seems to have
grown more than what would be supported by only
an increase in area of recent clear-cuts in Finland
(Fig. 6). Since about 1980, the total area of clear-
cuts has decreased slightly, but the number of nests
of the Great Gray Owl identified has increased.

Size, Shape and Distribution of Harvested Areas.
Our knowledge of the breeding and hunting re-
quirements of the Great Gray Owl is still inade-
quate for precise recommendations on forestry
practices. Its hunting habits in large openings and
the amount of forest necessary for breeding are
not sufficiently well known. The following descrip-
tions of beneficial and detrimental forestry practic-
es are therefore only approximate: (1) Most cut-
tings should be restricted to areas of 20 ha in size;
cuts of 2-5 ha are probably the best size. Cut areas
larger than 20 ha should be irregular in shape, not
broader than 400-500 m, and with convoluted
edges to give shelter when hunting at the edge. All
cuts should have groups of trees left for perching.
(2) Forest strips (corridors), 50-100 m wide,
should be left between the cut areas for moving
and sheltered resting places, and there should be

some larger forest areas, 5-10 ha in size, between
the cut areas to provide nesting habitat.

Most detrimental for the Great Gray Owl would
be large-scale clear-cuts of more than 100 ha that
are circular or square in form, especially if they are
totally treeless and with only small patches of forest
remaining between them. In this situation, the spe-
cies would not have appropriate food resources,
nesting habitats or resting and perching places.

In practice, fairly diverse cut sizes are acceptable
by the Great Gray Owl as long as small groups of
live and dead trees are left and forest corridors
are left along shores and streams, and between
large cuts.

Acknowledgments

The authors are most grateful to the reviewers, T. Dun-
can, G. Niemi, D. Varland and an anonymous referee,
who patiently gave numerous constructive comments and
advice for revising and improving the content and the
language of the manuscript.

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Received 2 November 1995; accepted 6 March 1997

J Raptor Res. 31 (2): 160-1 66
© 1997 The Raptor Research Foundation, Inc.

GREAT GRAY OWLS (STRIX NEBULOSA NEBULOSA) AND FOREST
MANAGEMENT IN NORTH AMERICA: A REVIEW AND

RECOMMENDATIONS

James R. Duncan

Box 253, Balmoral, Manitoba, Canada ROC OHO

Abstract. — Great Gray Owl ( Strix nebulosa nebulosa ) populations in North America have likely been
stable over the past 10-100+ yr. Local populations fluctuate in response to food supply and/or nest-
site availability. Breeding Great Gray Owls require preexisting nest structures in forest stands that are
adjacent to open foraging habitat, preferably with hunting perches. Current forestry practices have the
potential to affect about 75% of the Great Gray Owls’ breeding range in North America. Intensive
timber management typically removes large diameter and deformed nest trees, leaning trees used by
juveniles for roosting before they can fly and stands with dense canopy closure used by juveniles and
adults for cover and protection. Modified forest management can, however, create new foraging habitat
by opening up large, dense forest stands. Specific recommended guidelines include restriction of harvest
unit size (<5-10 ha), but within a mosaic of multi-sized units, retention of forest stands within 300 m
of known or potential nest trees/sites, provision of hunting perches in cut-over areas, ensuring irregu-
larly shaped harvest units and maintenance of forested travel corridors between cut-over areas. Because
Great Gray Owls can breed on home ranges up to 800 km apart in successive years, integration of local
management regimes at a landscape scale is recommended. Ideally, spatio-temporal patterns of natural
disturbance (e.g., fire) should be emulated in a management plan to sustain the region’s natural bio-
logical diversity, including Great Gray Owls when appropriate.

Key Words: Strix nebulosa; North America ; forest management; habitat use. Great Gray Owl.

El Gran Buho Gris Strix nebulosa nebulosa y administracion forestal

Resumen. — Las poblaciones del buho Strix nebulosa nebulosa en norte america han estado estable con
seguridad los pasados 10 a 100+ anos. Poblaciones locales fluctuan en reaccion a suministros de comida
y/o la disponibilidad a sitios de nido. Buhos en cria necesitan estructuras de nidos hechos en bosque
que estan pegados al habitat de forraje libre preferible con perchas de cazar. Costumbres actualmente
de forestales tienen la potencia para afectar casi 75% de el campo de cria de el buho en norte america.
La administracion de madera con intensidad tipicamente quita arboles con diametros grandes y arboles
deformados con nido, arboles inclinados usados por juveniles para percha antes que pueden volar, areas
con densidad cerrada usada por juveniles y adultos para cubrirse y proteccion. Administracion de bosque
modificados pueden, sin embargo, inventar nuevo habitat de forraje con abriendo grandesy densas
areas de bosque. Recomendaciones especificos de las reglas incluyen: (1) restricciones de cosecha
de cierta altura (<5-10 ha), pero dentro de un mosaico de varios tamanos, (2) retencion de areas de
bosque conocido o en potencia de arboles/ sitios con nido dentro de 300m, (3) provision de perchas
de cazar en areas cortadas, sugiriendo conjuntos de cosecha con formas irreguladas, y (4) manteni-
miento de corredores de viaje en areas de bosque en parcelas cortadas. Porque buhos se pueden criar
en campos naturales hasta 800 km aparte en anos seguidos, integracion de administracion local con
una escala de paisaje es recomendado. Idealmente, que muestras de spacio-temporal de disturbios
naturales (e.g., lumbre) debe ser emulado en el proyecto de la administracion para sostener las regiones,
diversidad natural y biologica, incluyendo buhos cuando es oportunado.

[Traduccion de Raul De La Garza, Jr.]

In November 1995, a symposium on Holarctic
raptor responses to forest management was held in
Duluth, MN U.S.A. Presenters were asked to review
information on a species according to guidelines

provided and in response to specific questions.
This paper is a review of such information for the
Great Gray Owl ( Strix nebulosa ) in North America.
The Great Gray Owl is unevenly distributed

160

June 1997

Great Gray Owls and Forest Management

161

across the Flolarctic over 30 million km 2 of boreal
forests in Eurasia and North America (Clark et al.
1987). Throughout its range it occupies forest hab-
itat; however, it also successfully breeds north to
within the transition zone between the boreal for-
est and the treeless tundra (Lang et al. 1991). In
western North America, it extends its range south
by occupying montane forests in the Rocky Moun-
tains, the Cascade Range and the Sierra Nevada
Range (Duncan and Hayward 1994). It is the larg-
est northern forest owl, although not the heaviest.
Its yellow eyes, set in a facial disk with nearly con-
centric gray and white rings, are framed by a large
round head that lacks ear tufts. Two subspecies are
currently recognized: S.n. nebulosa in North Amer-
ica and S.n. lapponica in Eurasia (Bull and Duncan
1993).

Great Gray Owls in the boreal forest region of
North America have greater diet similarity (% sim-
ilarity) to populations in Eurasia (x - 95.3%, SD
— 1.46, N — 3, range = 94—96.9%) than to those
in the southwestern U.S. that occur in montane
forests (x — 49.8%, SD = 13.9, N= 9, range ■ 39-
69.4, Duncan 1992). Great Gray Owls from both
continents exhibit similar plasticity in selection of
nest sites, although ground nesting is more fre-
quently reported from northern Europe (Mikkola
1983). They appear to use similar foraging habitat
in Eurasia and North America (Mikkola 1983, Dun-
can and Hayward 1994). Therefore, one would ex-
pect S.n. nebulosa and S.n. lapponica populations to
respond similarly to forest changes that alter the
availability of nest sites and/or foraging habitat.
Eurasian Great Gray Owls are paler with more ver-
tical barring, perhaps relating to habitat differ-
ences (Oeming 1955, Mikkola 1983); they also ap-
pear to be more aggressive toward humans at nest
sites than North American Great Gray Owls (Nero
1980, Mikkola 1983).

Considerable attention has recently been direct-
ed toward evaluating the status of North American
Great Gray Owl populations (Winter 1986, Hay-
ward 1994). As with any species, this invariably re-
quires an assessment of the habitats that are
thought or known to sustain populations. Current
forestry practices have the potential of affecting
about three-quarters of the Great Gray Owls’
breeding range in North America (Bull and Dun-
can 1993). McCallum (1994) provides a critical re-
view of the complexities involved in determining
owl-habitat relationships and the importance in
distinguishing between habitat requirements, pref-

erence and use. To best evaluate the relationship
between Great Gray Owl populations and forest
management, one would ideally have knowledge of
the species’ habitat requirements and/or prefer-
ence. Since these do not currently exist, except for
a few habitat preference studies (e.g., Servos
1986), the following review of Great Gray Owls and
forest management is based primarily on nesting
and foraging habitat use data.

Population Trends

Typically considered rare, the Great Gray Owl
occurs at low densities within its range (Nero 1980,
Bull and Duncan 1993). Nero estimated a conti-
nent-wide population of 5000-50 000 owls in North
America; up to 25 000 breeding pairs have been
estimated for Canada (Kirk and Hyslop pers.
comm.). However, Great Gray Owls are easily over-
looked and are probably more common. I estimate
the current North American population of Great
Gray Owls to be 20 000-70 000 breeding pairs. The
range of this estimate reflects the dependence of
this species on prey, primarily microtines (e.g., Mi-
crotus spp.), that exhibit unstable population fluc-
tuations over 3-5-yr periods (Duncan 1992).

The degree to which local prey population fluc-
tuations are synchronous over the Great Gray
Owl’s North American range is unknown. Extrinsic
factors such as severe temperatures with little or
no snow cover, occasionally synchronize (reduce)
microtine populations over large geographic areas.
More commonly, other factors (both intrinsic and
extrinsic) appear to disrupt such synchronizing ef-
fects (Pruitt 1968, Lidicker 1988).

Great Gray Owls are most often seen in winter,
and in more heavily settled areas along the south-
ern portions of their breeding range. Irregular,
large-scale, continental movements of bird popula-
tions are called irruptions (Collins 1980). The
Great Gray Owl is one of several irruptive species
of owls in North America. When large numbers of
birds appear locally, it is commonly referred to as
an influx or invasion. The fluctuations in winter
occurrence of Great Gray Owls in these areas (Fig.
1) suggest that large-scale irruptions occur less fre-
quently than local invasions. Large-scale irruptions
are thought to be the product of one or more years
of high Great Gray Owl reproductive success fol-
lowed by a widespread decrease in prey availability
on the breeding range. The recent apparent in-
crease in Great Gray Owl winter occurrences is

162

Duncan

Vol. 31, No. 2

1890 1900 1910 1920 1930 1940 1950 1960 1970

Year

Figure 1. North American Great Gray Owl winter irruptions based on specimens and sight records, 1890-1976. Data
from southern Canada and northern U.S.A. Modified from Collins (1980).

likely due to increased observer effort and in-
creased access to winter habitat (Collins 1980).

The overall North American Great Gray Owl
long-term (>10 yr) population trend is unknown.
There are no long-term, rigorous or standardized
Great Gray Owl breeding population trend data on
a range-wide, regional or local scale. There is cir-
cumstantial evidence that some local and/or re-
gional populations have either remained stable, in-
creased or decreased over periods of <10 yr (Fyfe
1976, Collins 1980, Nero 1980, Nero et al. 1984,
Winter 1986, Bryan and Forsman 1987, Franklin
1988, Collins and Wendt 1989, Bull and Henjum
1990, Duncan 1992). Ongoing local surveys are un-
derway at a number of locations in Canada and the
U.S. Nonetheless, it is useful to speculate on the
relationship between forestry and local Great Gray
Owl population trends.

Reliable population distribution data may only
be available after widespread and standardized
monitoring programs have been operating for sev-
eral years. While classic “playback” survey tech-
niques (Smith et al. 1987) can increase the num-
ber of Great Gray Owls detected by up to 40%, it
is unlikely that they can yield data that identifies
statistically significant population trends.

Primary Factors Associated with Trends

North American Great Gray Owl populations are
relatively unaffected by human persecution or di-
rect chemical effects (Nero 1980, Winter 1986,
Hayward 1994), notwithstanding occasionally high

local human-caused mortality (Nero and Copland
1981). The availability of nest sites and suitable for-
aging habitat are considered the most important
factors limiting Great Gray Owl populations (Dun-
can and Hayward 1994). I will address these first,
but some discussion is warranted on the short-
term (3-5 yr) influence of prey availability, which
can profoundly affect conclusions drawn from
short-term (<5 yr) local Great Gray Owl surveys.

Diet. Local Great Gray Owl breeding densities
fluctuate considerably, primarily due to the insta-
bility of microtine prey populations (Henttonen
1986, Duncan 1992). Individual radio-marked owls
in Manitoba and northern Minnesota have dis-
persed up to 684 km between breeding home
ranges in response to prey population crashes (x =
328.8, SD = 184.9, N = 27, range = 41-684 km);
those marked birds that did not disperse ( N = 11)
did not survive (Duncan 1992).

In contrast, breeding Great Gray Owl popula-
tions in montane regions of the western U.S. are
thought to be relatively stable (Winter 1986, Bull
and Henjum 1990, Bull and Duncan 1993). In
northeastern Oregon, the maximum distance trav-
elled from nest sites by adult radio-marked Great
Gray Owls averaged 13.4 km (N = 23, range = 2.4-
43.2 km; Bull and Henjum 1990). Bull and Hen-
jum (1990) speculate that in years when prey pop-
ulations are low, Great Gray Owls in their study
area remain as nonbreeding residents. In Califor-
nia, Winter (1986) suggested that under similar cir-

June 1997

Great Gray Owls and Forest Management

163

cumstances, Great Gray Owls possibly sustain them-
selves on pocket gophers ( Thomomys spp.), as well
as other prey species. Pocket gopher populations
are relatively stable and noncyclical (Chase et at
1982, Teipner et al. 1983).

Because breeding populations of Great Gray
Owls in different parts of their North American
range exhibit variable breeding dispersal strategies,
owl surveys undertaken to determine distribution
or local population trend data need to be con-
ducted for at least as long as one prey population
cycle to ensure reliable results. Therefore, ideally,
one should concurrently monitor prey populations
and Great Gray Owl diet. Great Gray Owls may or
may not be present during years of low prey pop-
ulations; if present at such times, they are less likely
to respond to conspecific call playback used in sur-
veys (Smith et al. 1987).

Nest-site and Foraging Habitat Availability.
These factors are significantly affected by natural
forest disturbances such as disease outbreaks, suc-
cession and the effects of fire and wind (Larsen
1980, Habeck 1994). The temporal and spatial
scale of the impact of these ecological processes,
and the relative stability of prey biomass productiv-
ity, have had a strong influence on the evolution
of Great Gray Owl life history traits, such as breed-
ing dispersal and post-fledging nest-site fidelity
(Duncan 1992). Consequently, forest management
does affect Great Gray Owl populations by altering
natural disturbance regimes and by the application
of management protocols. Anecdotal observations
and current knowledge of Great Gray Owl ecology
permit some speculation about how forest man-
agement likely impacts Great Gray Owl popula-
tions.

Nest sites. Great Gray Owls use preexisting struc-
tures for nesting, including deserted or vacant stick
nests of some Buteo hawks, Northern Goshawks ( Ac-
cipiter gentilis) and larger corvids (Duncan and Hay-
ward 1994). They will also nest on a variety of ar-
tificial structures, in natural depressions in broken-
topped snags or stumps, and, rarely, on the
ground, on rock cliffs, or on top of haystacks (Mik-
kola 1983, Duncan and Hayward 1994). Nest-struc-
ture type or nest-tree species appears to be less im-
portant than nest-site habitat characteristics and
the availability of foraging habitat (Duncan and
Hayward 1994). Forest tree pathogens (e.g,, ants
and fungi) and fire can weaken trees, possibly re-
sulting in tree death and subsequent snag or stump
formation. More directly, dwarf mistletoe ( Arceu -

thobium spp.) causes exaggerated branch configu-
rations which are conducive to nesting and/ or that
promote nest-building activity by raptors and corv-
ids (Bull and Henjum 1990). Nest-site availability
generally increases with forest stand age (Duncan
and Hayward 1994).

Tree pathogen outbreaks and other nest-site cre-
ating disturbances tend to have a clumped spatial
and temporal distribution. The territories of the
Northern Goshawk and Broad-winged Flawk ( Buteo
platypterus ) often hold several stick nests (Palmer
1988, Goodrich et al. 1996). If nest sites are limit-
ing and have a clumped distribution, perhaps it is
no coincidence that Great Gray Owls have nested
in what has been described as loose colonies
(Wahlstedt 1976, Bull and Duncan 1993), a trait
that undoubtedly also relates to their specialized
diet (Mikkola 1983).

Nest-site availability appears to be important
enough to Great Gray Owls that they have evolved
the ability to relocate and use nest sites hundreds
of kilometers apart over 2 or more yr (Duncan
1992). Therefore, forest management activities
that reduce the number of nest sites (e.g., fire sup-
pression, disease control and shorter rotation pe-
riods) have the potential to reduce Great Gray Owl
breeding densities. Mitigation by installing artifi-
cial nest structures is impractical at larger spatial
scales, but works well locally (Bull and Henjum
1990).

Foraging habitat. Voles and/or pocket gophers
dominate the diet of Great Gray Owls (Duncan
and Hayward 1994). Microtine voles generally oc-
cupy moist grass/sedge openings and open forests
with herbaceous ground cover. Meadows consid-
ered in good ecological condition for voles, and
hence Great Gray Owls, are dominated by a variety
of climax perennial grasses, sedges, and forbes.
Factors that reduce vole abundance (e.g., moder-
ate to heavy grazing) decrease the suitability of for-
aging areas for Great Gray Owls (Winter 1986).

Great Gray Owl foraging habitat includes bogs,
fens, muskeg, peatland, natural meadows, open
forests and selective and clear-cut logged areas
(Nero 1980, Mikkola 1983, Servos 1986, Winter
1986). Dense coniferous stands (e.g., jack pine, Pi-
nus banksiana and black spruce, Picea mariana),
open areas with few or no trees and habitats with
dense shrub layers are avoided by hunting Great
Gray Owls (Servos 1986).

Great Gray Owls hunt primarily from perches,
listening for prey and watching the ground intent-

164

Duncan

Vol. 31, No. 2

ly. When prey is detected the owl usually stoops
only a short distance, generally no more than 50
m. Bull and Henjum (1990) recorded an average
perch to prey distance of 10.5 m.

Bull and Duncan (1993) reported that Great
Gray Owls also forage in open forests. In northeast
Oregon, males foraged in stands with 11-59% can-
opy closure (Bull and Henjum 1990). These stands
had meadowlike grass-dominated ground cover.
Open tamarack ( Larix laricina ) stands with dense
sphagnum/sedge/grass understory are often used
by foraging Great Gray Owls in Manitoba.

While hunting, Great Gray Owls perch at varying
heights, but usually 3-5 m above the ground, in
both live trees and snags adjacent to or within
open grassy areas (Duncan and Hayward 1994).
Perch heights for male Great Gray Owls averaged
5.5 m in Oregon (Bull and Henjum 1990). In Cal-
ifornia, perch heights varied from 0-12.2 m above
the ground (x = 3.3, SD — 2.3, N — 143; Winter
1986). Great Gray Owls rarely hunt while perched
on the ground or while flying (Bull and Duncan
1993).

Successful Great Gray Owl reproduction de-
pends on the availability of suitable foraging hab-
itat within 1-3 km of nest sites (Bull and Henjum
1990, Duncan and Hayward 1994), Such habitat
can be ephemeral over shorter periods (e.g., post-
fire or post-cutting early succession habitat) or rel-
atively permanent (e.g., sedge meadows, peatland
and muskeg). Burned or cut-over areas can pro-
vide foraging opportunities for Great Gray Owls for
20 yr or more, depending on the rate of succes-
sion or on post-harvest management practices.
Kirkland (1977) and Parker (1989) reported that
meadow vole populations increase 3-18 yr after
clear-cutting forests.

Great Gray Owl population declines from ances-
tral levels have been reported in California (Win-
ter 1986). These were attributed to habitat
changes, e.g., fire suppression and overharvesting
of forests. Paradoxically, cleat-cuts can create Great
Gray Owl foraging habitat in dense forests in pre-
viously unoccupied areas.

Forest Management Recommendations

Throughout its North American range, the
Great Gray Owl thrives in a variety of habitats
(Duncan and Hayward 1994). It is adapted to cap-
turing prey in permanent open habitats and in ear-
ly forest successional stages (Nero 1980). Older
and mature forest habitats adjacent to foraging ar-

eas provide suitable nest structures. Therefore,
Great Gray Owl populations can likely persist with
some amount of forest cutting. The following rec-
ommendations are based on what is generally
known about Great Gray Owl ecology and not on
specific responses of owls to measured habitat
changes. New information should alter these spe-
cific conservation strategies through an adaptive
management approach. The large lifetime home
ranges of Great Gray Owls, e.g., in Manitoba and
Minnesota (Duncan 1992), suggest that a coordi-
nated landscape-level perspective to management
is needed to maintain viable populations. With this
in mind, I suggest the following management rec-
ommendations.

Occurrence Data. The occurrence of Great Gray
Owls is poorly documented in many parts of its
range (Duncan and Hayward 1994). A review of
historic site-specific occurrence information (e.g.,
literature, specimen data and personal communi-
cations) is an appropriate first step. It should be
determined if Great Gray Owls currently occur in
the management area because pre- and post-har-
vest occurrence information can be used to adapt
harvest guidelines accordingly. Secondly, the pres-
ence or absence of Great Gray Owls may influence
the degree to which forest resources are managed.
Assuming that forest resources are to be managed
for this species (e.g., the management area is with-
in suitable habitat and is within or adjacent to its
expected North American range) then one must
decide to implement a landscape-level or a specific
management regime only at sites where Great Gray
Owls are known to occur. The efficiency of various
survey techniques have not been rigorously tested
and survey methods may not be practical over
large expanses and in remote areas. Therefore, in
the absence of Great Gray Owl occurrence data,
an appropriate landscape-level management rec-
ommendation would be the retention of a 300 m
buffer area around natural openings such as mead-
ows or fens (Winter 1986, Bouchart 1991).

Clear-cut Size. Clear-cuts up to 10 ha in size are
probably ideal for Great Gray Owls, but these
should occur within a mosaic of multi-sized units
across a landscape. Great Gray Owls will use larger
clear-cuts, but typically they catch prey within 50 m
of hunting perches. While they hunt from isolated
perches in open areas farther than 50 m from edg-
es, in these situations they are more vulnerable to
avian predators such as Northern Goshawks and

June 1997

Great Gray Owls and Forest Management

165

Great Horned Owls ( Bubo virginianus) (Duncan
1987).

Clear-cut Shape. Because Great Gray Owls fre-
quently hunt from forest edges, irregular cut
shapes with convoluted or scalloped-shaped edges
would reduce mortality from avian predators. This
design will therefore also increase access to newly
created open foraging habitat. Because they catch
prey within 50 m of hunting perches, larger cuts
should therefore be elongated so that the maxi-
mum distance across the cut is <100 m.

Nest-site Availability. Timber management has
reduced nesting opportunities for all forest rap-
tors, including Great Gray Owis (Habeck 1994).
Therefore, the impact of management practices on
nest-site availability needs to be assessed.

Some types of nest structures (e.g., mistletoe
brooms and snags) used by Great Gray Owls are
either directly or indirectly created by tree patho-
gens (e.g., insects and fungi). These pathogens can
cause significant financial losses on commercial
forest land. Likewise, fire-killed trees can provide
Great Gray Owl nest sites (e.g., snags), but fire also
destroys valuable timber. Great Gray Owls often
nest in stick nests built by large corvids and diur-
nal raptors. These sites occur more frequently in
older forest stands with larger trees. Shorter rota-
tion periods or selective removal of large-diameter
trees has reduced nest-site availability. Forest
pathogen control, fire suppression and shorter ro-
tation periods are economically important forest
management practices that impact Great Gray Owl
nesting opportunities. The provision of artificial
nest structures, while locally effective, is labor in-
tensive and costly (Bohm 1985). Their use may still
be justified in certain situations.

Influence of Residuals and Optimal Mix, Includ-
ing Dispersal Corridors. Leaving residuals in cut-
overs (e.g., live trees and dead snags) provide im-
portant hunting perches. In Manitoba, the smallest
Great Gray Owl nest stand was 4 ha ( N =18, me-
dian = 232 ha) and there were at least 69 ha (N
= 15, median = 136 ha) of foraging habitat within
1 km of nest sites. Bull and Henjum (1990) re-
ported that 52-99% of the area within 500 m of
nest sites in Oregon was forested. While Great Gray
Owls have successfully nested on the edge of for-
aging habitat, the distance to the nearest opening
averaged 256 m in Manitoba (Bouchart 1991) and
143 m in Idaho and Wyoming (Franklin 1988).
Therefore, retention of forest stands within 300 m
of known or potential Great Gray Owl nest sites is

recommended as a minimum guideline (see also
Winter 1986, Bouchart 1991). The provision or re-
tention of leaning trees used by juveniles for roost-
ing before they can fly and stands with dense can-
opy closure (>60%) for cover and protection
(from heat stress and predators) of adults and ju-
veniles is also thought to be critical (Duncan and
Hayward 1994). Maintenance of forested travel cor-
ridors between nesting habitat is considered nec-
essary to minimize predation of dispersing adults
and juveniles.

Because Great Gray Owl breeding dispersal can
be significant (e.g., up to 800 km, Duncan 1992),
a coordination of local management regimes at a
landscape scale is recommended. Ideally, spatio-
temporal patterns of natural disturbance (e.g.,
fire) should be emulated in management plans to
sustain a region’s naturally occurring biological di-
versity, including Great Gray Owls when appropri-
ate.

Acknowledgments

I thank Gerald Niemi and others involved with orga-
nizing and hosting the November 1995 Symposium on
Raptor Responses to Forest Management: A Holarctic
Perspective, for the opportunity to write this paper. The
Symposium was an event that I will fondly remember I
also thank Evelyn Bull, Patricia Duncan, James Habeck,
Seppo Sulkava and Daniel Varland for their review of ear-
lier drafts of this manuscript.

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Received 2 November 1995; accepted 28 February 1997

f. Raptor Res. 31 (2) si-67— 174
© 1997 The Raptor Research Foundation, Inc.

HAWK OWLS IN FENNOSCANDIA: POPULATION FLUCTUATIONS,
EFFECTS OF MODERN FORESTRY, AND RECOMMENDATIONS

ON IMPROVING FORAGING HABITATS

Geir A. Sonerud

Department of Biology and Nature Conservation , Agricultural University of Norway,

RO. Box 5014, N-1432 As, Norway

Abstract. — Hawk Owls ( Surnia ulula) are diurnal raptors found across the northern hemisphere in
open boreal forest habitats. In Fennoscandia (Finland, Sweden and Norway), their diet consists mainly
of microtine rodents. The population densities of these microtines usually peak every 3-4 yr and fluc-
tuations are geographically asynchronous. Hawk Owls respond by concentrating and breeding where
and when microtine abundance is high. The resulting strong temporal and spatial short-term variation
in Hawk Owl density in Fennoscandia makes any long-term population trends difficult to detect. Hawk
Owls locate their prey visually from elevated perches and need ample space for long-range scanning
and attack. Therefore, harvesting old forest by clear-cutting is believed to benefit the Hawk Owl. How-
ever, this may depend on the values of several variables, such as clear-cut size and shape, height of trees
in clear-cut edges, density and height of residual trees left in clear-cuts after logging, type and extent
of ground cover and prey species composition and abundance in clear-cuts compared to old forest.

Based on data on the Hawk Owl’s attack range, recommendations are made for spacing of residual trees
as hunting perches. No single habitat in Fennoscandian forests seems to be superior for foraging Hawk
Owls throughout the year and the effects of modern forestry on Hawk Owls, although probably positive,
at present are difficult to predict.

Key Words: Foraging, forestry, habitat, Hawk Owl, management, Surnia ulula.

Buho Halcon en Fennoscandia: fluctuaciones de poblaciones, efectos de forestal moderno, y recomen-
daciones en mejorando habitat de forraje

Resumen. — El Buho Halcon Surnia ulula son rapaces del dia encontrados a traves del hemisferio norteno
en habitat abierto en bosques boreal. En Fennoscandia (Finland, Sweden y Norway), su dieta consiste
por mayor de “roedor microtine.” La poblacion den si d ad de estos “microtines” normalmente llega su mas
alto cada 3-4 anos, y fluctuaciones son geograficamente sincronizados. Buho Halcon responde en concen-
traciones y criando donde y cuando abundancia de “micro tine” esta alto. El resultado del temporal fuerte
y variation especie de corto tiempo en la densidad del Buho Halcon en Fennoscandia hace cualquier
tendencia de duration larga de poblacion dificil a descubrir. Buho Halcon localiza su cazado visualmente en
percha elevada, y necesitan espacio amplio para visualizar de larga distancia y ataque. Por eso, cosechas de
bosques viejos con cortadas-completas es crefdo dar beneficios al Buho Halcon. Sin embargo, esto puede
depender en el valor de varios variables, como el tamano de corto-completo y forma, altura de arbol en
orillas de corte-completo, densidad y altura de arboles residuales dejados en areas de cortes completos
despues de la cosecha, tipo y extenso de terreno, composition y abundancia de especie de cazar en cortes
completos comparado a bosques viejos. Basado en los datos del alcance de ataque del Buho Halcon, reco-
mendaciones estan hechas para el espacio de arboles residuales como perchas de cazar. Ningun habitat
singular en bosque de Fennoscandia, parece ser superior para el forraje del Buho Halcon a traves de todo
el ano, y los efectos de forestales moderno en los Buho Halcon, aunque problamente positivo, en el presente
es dificil pronosticar.

[Traduction de Raul De La Garza, Jr.]

Hawk Owls ( Surnia ulula) are medium-sized
(body mass about 0.3 kg) diurnal raptors which
range across the northern hemisphere in open bo-
real forest habitats (Glutz von Blotzheim and

Bauer 1980, Cramp 1985, Norberg 1987). In the
western Palearctic region their diet consists almost
exclusively of small mammals, mainly microtine ro-
dents (Mikkola 1983, Cramp 1985, Hogstad 1986,

167

168

SONERUD

VOL. 31, No. 2

Sonerud 1986), which they locate visually from el-
evated perches (Sonerud 1980, 1992),

In Fennoscandia (Finland, Sweden and Nor-
way), the original role of fire and storms as the
major disturbance agents in the boreal forest (see
Zackrisson 1977) has been gradually superseded by
humans during recent centuries. During the past
30—40 yr, modern forestry has transformed most of
the semipristine and continuous forest, shaped
by selective harvesting of the largest trees, into
a mosaic of patches interspersed with clear-cuts
and plantations (see Rolstad and Wegge 1989).
This transformation may affect Hawk Owls
in three major ways. First, Hawk Owls mostly use
cavities or broken tops of snags for nesting (Cramp

1985) and may therefore experience more limited
breeding opportunities in modern forests with few-
er old trees as nesting sites. Second, Hawk Owls
locate prey by sight (Norberg 1977, 1978, Sonerud

1986) and find suitable conditions in open habitats
created by clear-cutting, where long-range scan-
ning for and attack of ground-dwelling small mam-
mals is enhanced. However, Hawk Owls also re-
quire elevated perches from which they search for
prey (Sonerud 1980) and such perches are often
lacking in clear-cuts. Third, clear-cuts support
greater populations of voles than are found in old
forest, especially populations of the Microtus voles
(Henttonen et al. 1977, Larsson and Hansson
1977, Hansson 1978), although the availability of
these voles may be low for much of the year (So-
nerud 1980, 1986, Nybo and Sonerud 1990, Jacob-
sen and Sonerud 1993).

In this paper I will describe short- and long-term
population fluctuations of Hawk Owls in Fenno-
scandia by reviewing the literature and presenting
personal data, identify factors associated with these
fluctuations, evaluate whether modern forestry,
which emphasizes clear-cut harvesting, creates a
habitat where Hawk Owls hunt more efficiently
compared to old forest and suggest strategies that
would improve clear-cuts as foraging habitat for
Hawk Owls.

Ml 1 HODS

In a 50 km 2 (4 km X 12.5 km) area in the boreal forest
at 550-750 m elevation in southeastern Norway (6I°N,
U°E) the annual number of breeding Hawk Owls were
recorded during 1971-95. Each year, all nest boxes and
known natural cavities were checked at least once to re-
veal nesting attempts by Hawk Owls and Tengmalm’s
Owls (Aegolius funereus funereus). Further information on

the area and the nest-visit procedure is given by Sonerud
(1985, 1986).

Information on microtine abundance was obtained
from observations made when checking nest boxes and
from snap-trapping in a permanently established trap
line system at the southern end (60°56'N, 11°08'E) of the
Hawk Owl nesting area since 1977. This system covers
about 40 ha in a clear-cut and the surrounding old forest
stands, as described by Sonerud (1986, 1988). During
spring (mid- or late-May) in 1977-78 and 1981-95, sum-
mer (late July or early August) in 1977 and 1981-90 and
fall (late September or early October) in 1977 and 1981-
95, about 300 traps (type “Rapp”) baited with stained
cocoa fat were put out 5 m apart in seven separate lines
and checked on each of the following four days (Sonerud
1986, 1988).

In the same clear-cut as microtines were snap-trapped,
I also recorded Hawk Owl foraging behavior during
1976-92. The clear-cut covers 20 ha (800 m X 250 m)
and contains an average mix of the dominant boreal for-
est types in Fennoscandia. Before I started recording
Hawk Owl foraging behavior, I experimentally modified
the clear-cut. First, I removed all trees and snags remain-
ing after logging, except 10 mature Scotch pines ( Pinus
sylvestris) . I then divided the clear-cut into eight squares,
and within four of these a total of 178 artificial poles with
a top-mounted perch were erected. The poles were of
three different heights, providing perches 1.5 m (61
poles), 3.0 m (59 poles) and 6.0 m (58 poles) above
ground. Within each square, the poles were spaced 20 m
apart in a grid. The positions of the different heights
were assigned randomly. On the borders between the
squares another 23 poles with a top-mounted nest box
for owls with a perch height of 4.5 m were erected 40 m
apart and 14 m from the nearest adjacent pole. The pines
varied in height from 13-20 m, and were within two of
the squares containing poles. Thus a total of 211 perches
with heights from 1.5-20 m were provided on approxi-
mately half the area of the clear-cut.

In this clear-cut, I recorded the foraging behavior of
10 Hawk Owls (Table 1). These were the only ones avail-
able for observation, as Haw k Owls are not resident over
a longer time in any part of their range. The behavior of
the owls was observed from a permanent blind on a hill
in the middle of the clear-cut using 7X or 10X binoculars
and a 25-40 X spotting scope and recorded by the focal-
animal method (Altmann 1974). The owls were observed
at all times of the day and as long as they remained for-
aging within the clear-cut. In most cases only one owl was
foraging in the clear-cut at a time. The exceptions were
nesting pairs. If both mates were available for observa-
tion, I always observed the male (Sonerud 1992).

The Hawk Owls always searched for prey by perching
Hovering was employed only as an interruption of an
attack upon prey and generally for a few seconds only.
The owls were never observed to attack prey while flying
between perches. They abandoned perches either to
move to another perch or to attack prey. I only recorded
perch records of the latter type, which were made when
the ground was snow free and for which the perch height
and the corresponding horizontal attack distance were
known. This sample included 246 attacks (Table 1).

The heights of perches other than poles were estimat-

June 1997

Forestry and Hawk Owl Foraging

169

Table 1. Individual characteristics, sample period and
sample size used in this study, of 10 Hawk Owls observed
in an experimentally modified clear-cut. Attack records
included were those made on snow-free ground and for
which both the horizontal attack distance and the height
of the perch from which the attack was launched were
known.

Ind.

Sex

Provid-

ing

Mate or
Nest-
lings

Sampling

Period

Num-
ber OF
At-
tack
Rec-
ords

Male

28 Sept.-3 Oct. 1976

Unknown

Yes

26-28 June 1977

Unknown

1 Sept. 1977

Male

Yes

26 April-11 June 1981

Unknown

1-30 Oct. 1983

Male

Yes

20 April-3 June 1984

Female

Yes

7-22 June 1984

Female

26 Aug.-6 Nov. 1984

Male

Yes

14 May-16 June 1985

Unknown

29 Sept.-1 Oct. 1992

ed by using a clinometer. Horizontal attack distances
were either paced or calculated from the map of the
grids, while those outside the grids were paced. Some of
the long attack distances (>50 m) were measured on a
specially made aerial orthophoto of the study area (area-
correct scale 1:5000). Flight time elapsed from launching
an attack to capturing (or missing) prey was recorded by
using a stopwatch. Horizontal attack distances that I was
unable to measure, but for which the corresponding
flight time had been recorded, were estimated from the
perching height and the real attack distance; the latter
was in these cases estimated from a linear equation be-
tween flight time and real attack distance calculated for
each individual.

Population Trends

Hawk Owls in the western Palearctic are thought
to fluctuate in synchrony with their microtine prey
(Cramp 1985). In my study area Hawk Owls were
found nesting in only seven of the 25 yr from
1971-1995 (Fig. 1). These 7 yr were all within the
12-yr period 1977-88. All nestings occurred when
microtine density was increasing or in peak years,
although Hawk Owls were not present in all such
years (Fig. 1). Hawk Owls were observed outside
the nesting season here in an additional 5 yr (1976,
1982, 1983, 1989 and 1992).

In an area in western Finland (63°N, 23°E),
Hawk Owls nested in only 5 of the 14 yr from
1979-90 and only when microtines were abundant.

Year

Figure 1. The number of recorded Hawk Owl nests and the population density of microtines in a 50 km 2 study area
in the northern boreal zone in southeastern Norway during 1971-95. The microtine density is scored as low, increas-
ing and high.

170

SONERUD

VOL. 31, No. 2

Hawk Owls held winter territories here in an ad-
ditional 3 yr (Korpimaki 1994).

In Fennoscandia, population densities of micro-
tines fluctuate widely, usually with peaks every 3-4
yr (Fig. 1, Hansson and Henttonen 1985). These
microtine population fluctuations are geographi-
cally asynchronous (Hagen 1956, Myrberget 1965,
Myllymaki et al. 1977, Christiansen 1983), even
over a scale of less than 100 km (Steen et al. 1996).
Because Hawk Owls seem to depend on high mi-
crotine densities for nesting (Fig. 1, Korpimaki
1994), the large temporal and spatial variations in
microtine densities in Fennoscandia probably se-
lect for Hawk Owls with high capabilities of track-
ing microtine populations. Although the nomadic
life of Hawk Owls is accepted as fact (Mikkola
1983, Cramp 1985), documentation is limited to
records of invasions (Hagen 1956, Ryrkjedal and
Langhelle 1986) and scattered ringing (banding)
recoveries. Hawk Ow T ls ringed as nestlings in Fen-
noscandia have dispersed up to 1900 km in all di-
rections, including east into Russia (Glutz von
Blotzheim and Bauer 1980, Cramp 1985, Sonerud
1994). Recoveries of Hawk Owls ringed as breeding
adults include examples of both males and females
residing in an area from one nesting season to the
next when microtine abundance remained high
and of both sexes leaving when microtine popula-
tions declined. There are no examples of Hawk
Owls residing in an area from one microtine peak
to the next (Sonerud 1994).

In Fennoscandia the number of Hawk Owls at
any time is determined largely by the arrival of no-
madic owls from Russia and the level of local re-
productive output. In this century, large popula-
tions were noted in the fall of 1912, 1950 and 1983
(Hagen 1952, 1956, Edberg 1955, Byrkjedal and
Langhelle 1986, Risberg 1990). The relative roles
of immigration and local reproduction will vary be-
tween Hawk Owl population peaks because these
factors are determined by the current microtine
rodent population phase in Russia and Fennoscan-
dia, respectively. The unprecedented population
peak in 1983 seemed to mostly include owls from
outside Fennoscandia (Mikkola 1983, Byrkjedal
and Langhelle 1986). The dispute about the origin
of the Hawk Owls which made up the large pop-
ulation in the southern parts of Fennoscandia in
fall 1950 (Edberg 1955, Hagen 1956), however, il-
lustrates the difficulty in determining the relative
importance of immigration and local reproduction

for the current size of the Fennoscandian Hawk
Owl population.

The number of breeding Hawk Owls in Fenno-
scandia seems to have peaked in years when immi-
gration from the east during the preceding fall oc-
curred when the local microtine population was in-
creasing toward a peak. The highest recorded
number of breeding Hawk Owls in the central and
southern parts of Norway and Sweden occurred in
the microtine peak years 1984 and 1985, after the
unprecedented immigration of Hawk Owls in fall
1983 (Byrkjedal and Langhelle 1986, Risberg
1990). This influx occurred at a time when the lo-
cal microtine populations were increasing (Sone-
rud 1988, Lindstrdm and Hornfeldt 1994). Simi-
larly, a rather large influx of Hawk Owls into Fin-
land from Russia in fall 1957 coincided with a mi-
crotine peak in Finland and many owls stayed to
breed there in 1958 (Mikkola 1983).

The total number of Hawk Owls present in Fen-
noscandia at any time may vary over two orders of
magnitude, from a few hundred up to tens of
thousands. The temporal and spatial variation in
Hawk Owl population density in Fennoscandia
makes an estimate of the total population difficult.
The numbers of Hawk Owls during peak years have
been estimated at 3600 pairs in Finland (Merikallio
1958, cited in Cramp 1985), 10 000 pairs in Sweden
(Ulfstrand and Hogstedt 1976) and 10 000 pairs in
Norway (Sonerud 1994).

Information on long-term population changes
of Hawk Owls is sparse, especially from the north-
ern parts of Fennoscandia where Hawk Owls occur
more commonly and regularly than in the south-
ern parts (Haftorn 1971, Hyytia et al. 1983, Risberg
1990), but where few people live. In southern and
central parts of Norway and Sweden the Hawk Owl
has been more common in the last 25 yr, especially
during the period 1975-1989, than in the preced-
ing 25 yr (Risberg 1990, Sonerud 1994). In Fin-
land, the Hawk Owl was thought to have declined
from the 19th century until the 1950s (Merikallio
1958, cited in Cramp 1985). In southern Norway,
Hawk Owls were less frequent during 1914-1948
than during 1880-1913; in fact none were record-
ed nesting here between 1913 and 1949 (Hagen
1952). In conclusion, the Hawk Owl seems to have
been more common in Fennoscandia during the
first and last quarters of the past 100 yr than in the
intervening time.

June 1997

Forestry and Hawk Owl Foraging

171

Factors Associated with Fluctuations in
Hawk Owl Numbers

One explanation for the decline in the number
of breeding Hawk Owls in Finland from the 19th
century until the 1950s was thought to be human
persecution (Merikallio 1958, cited in Cramp
1985). Hawk Owls are easy targets because they are
diurnal, use exposed perches and are relatively
tame. Earlier in this century Hawk Owls were often
shot in years of population peaks in Norway (Ha-
gen 1952). Nowadays, Hawk Owls are protected by
law in Fennoscandia and fewer are killed.

Hawk Owls usually nest on top of broken trees,
in cavities made by the Black Woodpecker ( Dryoco -
pus martins ) , in nest boxes and sometimes in nests
made by corvids (Cramp 1985, Sonerud 1985).
Since modern forests contain fewer trees old
enough to provide cavities suitable as nesting sites
for Hawk Owls, the breeding opportunities of
Hawk Owls might locally be limited where the for-
est has been intensively managed. One reason for
the decline in the number of breeding Hawk Owls
in Finland from the 19th century until the 1950s
was thought to be the disappearance of hollow
trees (Merikallio 1958, cited in Cramp 1985).

Two factors appear to explain why Hawk Owls
have been more common in the central and south-
ern parts of Norway and Sweden during the past
25 yr. The first is the occurrence of a moderate
invasion in fall 1975 and a large invasion in fall
1983 (Risberg 1990). The second factor is probably
the opening of the forest by clear-cutting that has
taken place the past 30-40 yr. This has made larger
areas of the forest suitable for the Hawk Owl (Nor-
berg 1987). In addition, clear-cuts support greater
populations of microtines than does old forest
(Henttonen et al. 1977, Larsson and Hansson
1977, Hansson 1978).

How Should Clear-cuts be Designed to
Suit Hawk Owls?

Harvesting old forest by clear-cutting may ben-
efit the Hawk Owl by creating habitats that are
more profitable for hunting than the natural or
the selectively cut old forest. Due to its depen-
dence on vision for locating prey, the Hawk Owl is
capable of using open forest with ample space for
long range scanning of and attack at ground-dwell-
ing small mammals (Norberg 1987). However, due
to the Hawk Owl’s dependence on elevated perch-
es from which it searches for prey (Sonerud 1980),
clear-cuts may not offer suitable types and densities

of hunting perches. In the experimentally modi-
fied clear-cut (see Methods), the four blocks pro-
vided with perches were utilized more for hunting
by Hawk Owls, Common Buzzards ( Buteo buteo)
and European Kestrels ( Falco tinnunculus) than the
four blocks without perches (Sonerud 1980). In
Sweden, clear-cuts experimentally provided with
perches were utilized more by Common Buzzards
than clear-cuts without perches (Widen 1994). By
knowing the size of the Hawk Owl’s search range
from hunting perches, it may be possible to cal-
culate an optimal interperch distance and to de-
sign clear-cuts where Hawk Owls can forage effec-
tively.

Size of Clear-cuts. To know how wide clear-cuts
lacking residual trees may be without being inac-
cessible to Hawk Owls, I estimated how far into a
clear-cut a Hawk Owl’s search range extends from
a perch in the forest edge. An appropriate estimate
of the search range may be the distance containing
all recorded attack distances or all but the most
extreme ones (90th percentile). Use of the latter
is justified because very long attack distances may
be traveled under special conditions, and because
successful attacks tended to be made at shorter dis-
tances than unsuccessful ones when the ground
was snow free (Sonerud 1992). From perches >9
m above the ground, as typically found at the edges
of clear-cuts, the estimated search range of the
Hawk Owls in the experimental clear-cut would be
about 70 m if taken as the 90th percentile and 110
m if taken as the longest of the recorded attack
distances from such perches on snow-free ground
(N = 29). Thus, even if clear-cuts are 140 m wide,
Hawk Owls hunting from perches in the remaining
forest seem able to utilize all the area for hunting
in the snow-free season.

Shapes of Clear-cuts. The results above suggest
that square-shaped clear-cuts up to about 2 ha in
size may be accessible for Hawk Owls from perches
at the forest edge. Rectangular clear-cuts of all sizes
may be accessible from the edge provided the
short side does not exceed 140 m. If no residual
trees remain in a clear-cut after logging so that
Hawk Owls are left only with the perches made up
by the forest edge, larger parts of the clear-cut will
be available to Hawk Owls as the edge-area ratio
becomes larger. Thus, complex clear-cuts with con-
voluted edges may be more beneficial to Hawk
Owls than simple clear-cuts with linear edges.

Residual Trees. The results above suggest that if
clear-cuts are made wider than 140 m and no re-

172

SONERUD

VOL. 31, No. 2

Figure 2. Search range of Hawk Owls in relation to
perch height, estimated as the 90th percentile and the
longest of the recorded horizontal attack distances, re-
spectively.

Figure 3. Maximum acceptable distance between resid-
ual trees, left as hunting perches for Hawk Owls in clear-
cuts, as a function of their height. The values are taken
as the search range values given by the regression line in
Fig. 2 multiplied with V2.

sidual trees are left, the central areas >70 m from
the edge may be inaccessible to Hawk Owls. There-
fore, stumps, snags and other trees of little eco-
nomic value should be left as hunting perches for
Hawk Owls. If such residuals are provided, there
may be no upper size limit on clear-cuts that may
be utilized by Hawk Owls, unless the Hawk Owl’s
vulnerability to other raptors increases in large
clear-cuts. Most documented Hawk Owl kills in
Fennoscandia are made by Eagle Owls ( Bubo bubo),
which also hunt in open habitats (Mikkola 1983).

An important issue is to estimate how far apart
the residual trees in a clear-cut may be spaced with-
out making some of the clear-cut inaccessible to
Hawk Owls. If we assume that the search area from
a single perch is confined within a circle around
that perch (Andersson 1981) and that all parts of
the clear-cut are to be covered by the search areas
from the perches there, then the longest accept-
able interperch distance for perches of a certain
height is the estimated search range from that
height multiplied with \ / 2.

When the ground was snow free, the horizontal
attack distance of the Hawk Owls foraging in the
experimental clear-cut increased with the height of
the perch from which the attack was launched (So-
nerud 1992). I therefore estimated the search
range, both as the 90th percentile and as the max-
imum of the recorded horizontal attack distances
for different perch heights (Fig. 2) . Perches of sim-
ilar height were combined in some cases to obtain
sufficiendy large samples. These estimates of the
search range increase with increasing perch height
(Fig. 2).

To obtain a simplified picture of the Hawk Owl’s
search range as a function of perch height, I re-
gressed the search range estimates for each group
of perch heights on the average perch height of
the actual group (Fig. 2). Thereafter, I calculated
the longest acceptable interperch distances by mul-
tiplying the search range values as given by the re-
gression line with V2 (Fig. 3). If the search range
is taken as the 90th percentile of the recorded at-
tack distances, the maximum acceptable inter-
perch distance increases from about 30 m for re-
siduals ^3 m high to about 130 m for residuals
^15 m high (Fig. 3). Thus, an array of residuals
should be left with a density varying from >14 per
ha for the shortest ones to ^0.6 per ha for the
tallest ones. If the search range from a certain
perch height is taken as the longest recorded at-
tack distance from that height, the maximum in-
terperch distance increases from about 50 m for
residuals ^3 m high to about 140 m for residuals
>15 m high (Fig. 3). Thus, an array of residuals
should be left with density varying from >5 per ha
for the shortest ones to ^0.5 per ha for the tallest
ones.

Because these estimates are based on data from
one clear-cut only, forest managers should use flex-
ible strategies for the spacing of residuals. As a rule
of thumb, I would suggest that in parts of clear-
cuts >100 m from the forest edge residuals should
include about 1 per ha for tall (>15 m) residuals,
and about 10 per ha for short (<3 m) residuals.

Effect of Seasonal Change in Vegetation Height.
When I estimated the Hawk Owl’s search range

June 1997

Forestry and Hawk Owl Foraging

173

above, I disregarded any seasonal change in the
cover for voles provided by the field vegetation.
However, a Hawk Owl’s view of voles moving along
the ground would be expected to be more ob-
structed in late summer and fall than in spring just
after the snow has disappeared and before the veg-
etation has leafed out (Jacobsen and Sonerud
1993). Thus, the maximum acceptable interperch
distance, as estimated above based on data from
the whole snow-free season, may be too long in late
summer and fall.

Effect of Snow Cover. Hawk Owls live in areas
where the ground may be snow covered for half the
year or more. Because they rely on sight to locate
prey, Hawk Owls nearly always attack prey exposed
on top of the snow (Sonerud 1986, Nybo and So-
nerud 1990). Voles are more visible when moving
on top of the snow than when moving in the vege-
tation. In fact, the Hawk Owl’s search range is lon-
ger for prey moving on top of the snow than for
prey moving on snow-free ground (Sonerud 1980,
1992). Thus, the maximum acceptable interperch
distance, as estimated above based on data from the
snow-free season, would allow Hawk Owls access to
all parts of a clear-cut in winter as well.

Optimal Habitat of Hawk Owls in Modern Forests

Hawk Owls usually switch their diet from mostly
bank voles ( Clethrionomys glareolus) to mostly Micro-
tus voles as snow disappears in spring, prob-
ably because bank voles move on top of the snow
cover more frequently than do Microtus voles (So-
nerud 1986, Nybo and Sonerud 1990, Jacobsen
and Sonerud 1993). In Fennoscandia, Microtus
voles are almost exclusively found in clear-cuts,
whereas Clethrionomys voles, especially the bank
vole, occur in a wide range of habitats but more
commonly in forest than in clear-cuts, especially
during spring (Henttonen et al. 1977, Larsson and
Hansson 1977, Hansson 1978, Sonerud 1986).
Thus, variation in diet suggests that Hawk Owls
switch major hunting habitat from forests to clear-
cuts as snow disappears and that Hawk Owls may
find best foraging opportunities in forest habitats
during the winter, provided the forest is open
enough to allow the Hawk Owl to take full advan-
tage of its long search range on snow.

In Fennoscandia, the overall density of voles is
usually higher in clear-cuts than in forest, primarily
because Microtus voles reach higher densities than
Clethrionomys voles (Henttonen 1989). Except just
after snow melt and before the vegetation leafs out,

vegetation cover is on average more luxurious and
offers more protection for voles in clear-cuts than
in forest (Sonerud et al. 1986). Hence, for Hawk
Owls prey availability is greater in clear-cuts than
in forest only from just after snow melt until the
new vegetation leafs out; thereafter the relative
prey availability in clear-cuts compared to forest de-
clines (Jacobsen and Sonerud 1993). When snow
starts accumulating, prey availability soon becomes
higher in forest than in clear-cuts. Thus, the rela-
tive prey availability in clear-cuts compared to for-
est is lowest when snow covers the ground, highest
just after snow melt and declines gradually
throughout the snow-free season.

Because neither forest nor clear-cuts can serve
as the best foraging habitat for Hawk Owls
throughout the year in Fennoscandia, the optimal
forest landscape for Hawk Owls would be a mix of
old forest and clear-cuts. The area ratio between
these two main habitat types that maximizes the
Hawk Owl’s intake rate cannot be determined at
present because the change in relative prey avail-
ability in each habitat throughout the year is not
sufficiently known. However, if clear-cut areas too
large to be covered by the Hawk Owl’s attack range
from the forest edge are left with a sufficiently
dense array of snags, stumps and other residuals
after logging, and if trees with potential nesting
cavities for Hawk Owls are left after logging, the
species may benefit from modern forestry.

The Hawk Owl is unlikely to show any evolution-
ary response to modern forestry or other logging
operations by man in Fennoscandia because any
genetic effect is probably swamped by the long dis-
persal undertaken by young and old birds of both
sexes. Therefore, Hawk Owls in Fennoscandia are
mainly adapted to the forest habitats in Siberia,
which probably have been shaped mainly by fires
until recently.

Acknowledgments

I gratefully acknowledge information on Hawk Owl
nests from K. Prestrud, R. Solheim and H. Strom, and
helpful comments on the manuscript by J.B. Buchanan,
S. Dale, P. Duncan, G.J. Niemi, D. Varland and an anon-
ymous reviewer.

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Received 2 November 1995; accepted 27 February 1997

THE LONG-EARED OWL ( ASIO OTUS) AND FOREST
MANAGEMENT: A REVIEW OF THE LITERATURE

Denver W. Holt

Owl Research Institute, RO. Box 8335, Missoula, MT 59807 U.S.A.

Abstract. — In North America, 13 of 20 breeding season studies reporting on Long-eared Owl ( Asio
otus) reproduction were conducted in open country habitats, four in woodland or edge habitats and
three in predominantly woodland habitat. Sixteen of 22 nonbreeding season studies that reported com-
munal roost sites were located in forest/ edge habitats, five reported locations in open space and one
was found within forest habitat. There is currently little data to indicate either a negative or positive
effect of forest-management practices on this species. Although there appears to be some evidence of
population declines in specific geographic areas, these impacts have been attributed to loss of riparian
vegetation, conversion of foraging areas to agricultural fields and reforestation of open habitats. The
Long-eared Owl’s ecomorphology is suggestive of a species that inhabits open country. Additionally, its
primary food is small mammals (e.g., microtine and heteromyid rodents) which inhabit open country.
Should the Long-eared Owl be considered a forest owl? Research data would suggest no; however, studies
from extensive deciduous and coniferous woodlands are needed.

Key Words: Long-eared Owl ; forestry, habitat, diet, ecomorphology, Asio otus.

El buho ( Asio otus) y administration forestal: un reviso de la literatura

Resumen. — En norte america, 13 de 20 estudios de tiempos de cria reportadas en el buho Asio otus
fueron evaluados en habitat del campo amplio, cuatro en bosques o orillas de habitat, y tres en mayoria
de habitat de bosque. Dieciseis de 22 estudios en tiempos sin cria que reportaron sitios de percha
comunal fueron localizadas en bosque/habitat de orilla, cinco lugares reportados en espacio abierto, y
uno fue encontrado dentro de un habitat de bosque. Actualmente poca information indica si los afectos
de la administration de bosques son negativo o positivo en el especie. Aunque parece que un poco de
pruebas con reduction de poblaciones en areas especificas geograficamente, estos impactos estan atri-
buido a la falta de vegetation cerca de los rios, conversion de areas de forraje a parcela agricolas, y
repoblacion forestal de habitat abiertos. La eco-morfologia del buho evoca una especie que ocupa el
campo abierto. Tambien, su comida principipal es mamiferos pequenos (i.e. microtine y roedor het-
eronmyid) que ocupan campos abiertos. ^Debe ser el buho considerado un buho del bosque? Infor-
mation investigada sugieren que no, sin embargo, estudios de bosque conifero y de hoja caduca extensa
es necesaria.

[Traduction de Raul De La Garza, Jr.]

The Long-eared Owl ( Asio otus) is a widely dis-
tributed Holarctic species, with six recognized sub-
species (Cramp 1985). In the Northern Hemi-
sphere, it ranges from approximately 30-65° lati-
tude, with isolated populations occurring in North
and East Africa, the Azores and Canary Islands
(Mikkola 1983, Marks et al. 1994). Some aspects of
Long-eared Owl natural history have been well
studied in the U.S. and some European countries,
but most studies have been short in duration, av-
eraging about two seasons.

In North America, two subspecies are currently
recognized (A. o. wilsonianus and A. o. tuftsi; see
Marks et al. 1994 for further discussion). The

Long-eared Owl has been considered an open
country species, inhabiting areas such as grass-
lands, shrubsteppe, marshes and woodland patches
near open areas. Most studies seem to support this.
To my knowledge, there have been no attempts to
evaluate the affects of forestry practices on this spe-
cies. Herein, I review the literature and use some
inferences from my ongoing 10 yrs of study to ad-
dress some of the questions concerning the im-
pacts of forest management on Long-eared Owls.

Population Trends

Few data exist for population trends of Long-
eared Owls in North America over the past 10, 25,

175

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Vol. 31, No. 2

Table 1 . Status of the Long-eared Owl in North America.

Province/Region or State

Status

Population Trend " 1

CANADA (Fyfe 1976)

British Columbia

Low

Unknown

Maritime

Low/Medium

Fluctuating

Northwest Territory/Yukon

Unknown

Ontario/ Quebec

Low/Medium

Fluctuating

Prairie

Low/Medium

Fluctuating

NORTHEASTERN UNITED STATES (Melvin et al.

1989)

Connecticut

Special Concern

Delaware

Unknown

Massachusetts

Special Concern

Maryland

Decreased

Maine

Unknown

New Hampshire

Special Concern

New Jersey

Unknown

New York

Unknown

Pennsylvania

Decreased

Rhode Island

Special Concern

Vermont

Special Concern

MIDWEST (Petersen 1991)

Illinois

Endangered

Unknown

Indiana

Uncommon

Declining

Iowa

Threatened

Unknown

Kansas

Uncommon

Stable

Michigan

Special Concern

Unknown

Minnesota

Regular

Unknown

Missouri

Special Concern

Unknown

Nebraska

Unknown

North Dakota

Special Concern

Unknown

Ohio

Unknown

South Dakota

Rare

Declining

Wisconsin

Special Concern

Unknown

WEST (Marti and Marks 1989)

California

Special Concern

Declining

Colorado

Common

Stable

Idaho

Common

Unknown

Montana

Special Concern

Unknown

Nevada

Common

Stable

Oregon

Common

Stable

Utah

Common

Unknown

Washington

Unknown

Wyoming

Common

Unknown

a Trend data not known for northeastern U.S.

50 or 100 yrs, but there are some regional data.
The Breeding Bird Survey (BBS) does not include
the Long-eared Owl in its data set from 1966-89.
For inclusion, a species must have been detected
on >10 BBS routes in a physiographic region; 25
or more detections in the three biomes (Eastern,
Central, Western); 35 or more detections in Can-

ada; or 50 detections in the U.S. and Canada
(Droege pers. comm.).

In Canada, Fyfe (1976) reported population
trends and relative abundance of raptors for prov-
inces or specific geographic areas (Table 1 ) . There
were no data to support these designations. Also
in Canada, Christmas Bird Count (CBC) results

June 1997

Long-eared Owl Conservation

177

62 64 66 68 70 72 74 76 78 80 82 84 86 88

YEARS

Figure 1. Summary of winter counts of Long-eared Owls from Christmas Bird Counts in the northeastern U.S.,
1963-87 (after Melvin et al. 1989).

showed a significant decline in Long-eared Owl
numbers, but these data should be interpreted cau-
tiously (Kirk et al. 1994).

In the northeastern U.S., Melvin et al. (1989)
reported that the Long-eared Owl was listed as a
species of special concern in all the New England
states except Maine and decreasing in Maryland
and Pennsylvania (Table 1). Within the northeast-

ern states, Melvin et al. (1989) concluded that no
clear population trend could be detected for Long-
eared Owls, although numbers seemed to fluctuate
about every three to six yr (Fig. 1). In New Jersey,
Bosakowski et al. (1989, 1989a) analyzed 31 yr
(1956-86) of Long-eared Owl Christmas Count
Data reporting one or more Long-eared Owls and
concluded that the species was declining (Fig. 2).

D Owls/1 ,000 party hrs
° Total Owls

Year

Figure 2. Long-eared Owls reported on New Jersey Christmas Bird Counts (dotted line) and per 1000 party hours
(solid line). Regression line (dashed), Y = — 0.70x + 25.5, P < 0.0001, r = 0.67 for party hours is significant.
Regression line, Y = — 0.629x + 31.9, P = 0.005, for total owls had a lower correlation (r = 0.50) (after Bosakowski
et al. 1989a).

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Vol. 31, No. 2

In the midwestern U.S., Petersen (1991) report-
ed that Long-eared Owls have declined in Indiana
and South Dakota, are stable in Kansas and are of
unknown status elsewhere (Table 1). This was
based on state and regional birding publications
and raptor survey forms. In Minnesota, however,
Evans (in Marks et al. 1994) noted a decline in mi-
grant Long-eared Owls in his study area from 1976-
93 (Fig. 3).

In the western U.S., White (1994) reported the
Long-eared Owl as stable, but with some local
losses in the far west. He did not report how these
species designations were assigned, Marti and Marks
(1989) reported a Long-eared Owl population de-
cline in California and a stable or unknown popu-
lation status in the rest of the west (Table 1). In
coastal southern California, Bloom (1994) has
shown the Long-eared Owl to have been extirpated
in some areas, with small remnant populations still
occurring inland. The number of historic nesting
areas have declined by 55% (Bloom 1994). In
Montana, Long-eared Owls were listed as a species
of special concern (Marti and Marks 1989). I have
shown yearly fluctuations in numbers during CBC
counts and breeding seasons (Figs. 4 and 5) with
a consistent research effort in the same areas. In
Mexico, the status of the Long-eared Owl has not
been reported (Enriquez-Rocha et al. 1993).

In summary, realistic population trends for
North American Long-eared Owls are difficult to
determine. The use of CBC data to determine avi-
an population trends has been controversial, but
Root (1988) has presented some of the strengths
and weaknesses to this approach.

Population demographics for Long-eared Owls
are uncertain because of the paucity of data on
mortality, emigration, immigration, migration and
other factors. Because Long-eared Owls are highly
migratory in some areas, nocturnal, difficult to lo-
cate and appear to show food-based nomadism, it
is very difficult to determine their status. For ex-
ample, five notable recoveries of banded owls in
Mexico >800 km from banding sites illustrate the
Long-eared Owls’ high degree of mobility. These
long distance recoveries include: one owl banded
in Saskatchewan, Canada and recovered 4000 km
away in Oaxaca, Mexico; one owl banded in Mon-
tana and recovered 3200 km away in Guanajuato,
Mexico; and one owl banded in Minnesota and re-
covered 3100 km away in Puebla, Mexico. Long-
term breeding season studies in Montana show lit-
tle site fidelity by Long-eared Owls. Of 77 breeding

pairs intensively monitored for 9 consecutive yr,
only 11 males and two females have returned to
the same breeding site more than once. Addition-
ally, no mate fidelity has been recorded. These
data buttress the argument for highly migratory
and nomadic tendencies in Long-eared Owls.

Primary Factors Responsible For Long-eared Owl
Population Trends

In most cases, there were insufficient data to
convincingly conclude which factors influence
population trends. Population declines have been
attributed to habitat alteration, forest succession,
urbanization, competition with Great Horned Owls
(Bubo virginianus ) , loss of habitat for prey species,
rodenticides (Bosakowski et al. 1989a), shooting
and habitat loss (Marks et al. 1994) and loss of
riparian habitats and grasslands (Bloom 1994).

Some forestry practices are also thought to have
affected Long-eared Owls. In New Jersey, Bosa-
kowski et al. (1989a) suggested that forest removal
and thinning affected wintering Long-eared Owls
and caused them to abandon the area.

On the contrary, many of the nonbreeding and
breeding season studies from the eastern U.S. were
located at roost sites in plantations of exotic coni-
fers or other man-made habitats such as cemeteries
(Tables 1 and 2). In the western U.S., shelter-belts
planted for wind and snow breaks, as well as cover
and food for wildlife have allowed Long-eared
Owls new winter and breeding sites. In other in-
stances, Long-eared Owls have been radiotracked
(Ulmschneider 1990) and found to be using forest
clear-cuts as foraging areas.

Affects of Past and Present Forest Management
Practices on Long-eared Owls

There is insufficient information to conclude
that forest management has affected Long-eared
Owl populations. There is some data from New Jer-
sey, Minnesota and California that show declines.
In New Jersey and California, habitat loss or
change appears to have affected Long-eared Owls.
Bosakowski et al. (1989a) theorized that Long-
eared Owls in New Jersey were probably rare
breeders prior to European settlement. After the
clearing of forests in the 18th and 19th centuries,
Long-eared Owl populations increased and ex-
panded in range. When forests were reestablished
in the 20th century, Long-eared Owl numbers de-
clined (Bosakowski et al. 1989b). In Minnesota, no
explanation for the apparent decline has been given.

June 1997

Long-eared Owl Conservation

179

Years

Figure 3. Long-eared Owls caught per 1000 net hours in the fall at Duluth, Minnesota (D. L. Evans, in Marks et al.
1994). Regression line indicates a downward trend (Y = 16.25 — 0.59x).

Many studies, however, indicate that exotic and do-
mestic conifer plantations, wind-rows and shelter-
belts planted near or within open areas provide ad-
ditional nesting and winter roosting habitats that are
beneficial to Long-eared Owls.

How Would Long-eared Owls Be Affected By Size,
Shape, and Residuals of Forest Cuts?

This is unknown, but a few studies have data
which may be relevant. Craig et al. (1988) reported

that two pairs of radio-tagged Long-eared Owls in
Idaho avoided scattered areas of juniper (Juniperus
spp.) trees within open sagebrush shrubsteppe
habitats. The owls generally foraged 1—3 km from
their nests, with males using about 240-325 ha and
females using about 235—425 ha during nightly for-
ays. Also in southwestern Idaho, Hilliard et al.
(1982) reported that one radio-tagged Long-eared
Owl (sex unknown) foraged over 70 ha during
three consecutive nights in winter, and a second

Year

Figure 4. Long-eared Owls recorded on Christmas Bird Counts in western Montana, 1986-95 (D. Holt unpubl.
data).

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Vol. 31, No. 2

Year

Figure 5. Breeding pairs of Long-eared Owls observed in western Montana, 1987-95 (D. Holt unpubl. data). There
were no nests found in 1995.

Long-eared Owl (male) foraged over 190-220 ha
each night for five nights in spring. Ulmschneider
(1990) reported that seven of 13 radio-tagged
Long-eared Owls traveled 73-97 km, and one owl
moved 125 km from a shrubsteppe sagebrush
breeding area to forested mountains. All the owls
were at first in open country and heavily logged
areas, four later moved into forest habitat with
small openings where three stayed within 1 km of
an active logging site and the fourth stayed near a
1-yr-old clear-cut. The three owls near the active
logging site stayed for several weeks. She felt the
owls had chosen the active logging sites and re-
cently logged sites over older ones.

In Montana, Long-eared Owls nesting in steep
mountain hillsides of second growth Douglas-fir
( Pseudotsuga menziesii ) forests and mixed pondero-
sa pine ( Pinus ponderosa) forests adjacent to open
lands foraged at dusk in nearby dear-cuts and
grasslands, respectively (Holt and Hillis 1987).
These observations suggest that certain logging
practices may benefit Long-eared Owls.

Is the Long-eared Owl a Forest Species?

I reviewed studies from across North America
and tried to address information on habitat asso-
ciations, diets and ecomorphology of Long-eared
Owls. I separated diet into breeding and nonbreed-
ing seasons. Habitat was separated into grassland,
edge and forest. For ecomorphology (the relation-
ship between an animal’s ecology and morpholo-

gy), I found only two studies pertaining to Long-
eared Owls (Poole 1938, Mueller 1986), but then
incorporated that into literature direcdy related to
ecomorphology of birds in general and owls in par-
ticular.

Habitat Associations. Of 20 studies providing
breeding habitat information, only three (Craig-
head and Craighead 1956, Bull et al. 1989, Bloom
1994) reported that the Long-eared Owl was asso-
ciated with forest habitat and only Bull et al.
(1989) defined the breeding habitat as extensive
forest. Four other studies (Wilson 1938, Armstrong
1958, Reynolds 1970, Enriquez-Rocha et al. 1993)
described Long-eared Owls as associated with for-
est or edge, while the remaining 13 studies report-
ed Long-eared Owls to be associated with open
habitats (Table 2). Seventeen breeding season
studies were conducted in the western Great
Plains, Great Basin, Rocky Mountains, West Coast
and Mexico; six of these were from Idaho. In gen-
eral, these studies suggest that Long-eared Owls
primarily breed in open spaces (but see Peck and
James 1983, mjohnsgard 1988). Other good an-
ecdotal information refers to Long-eared Owls
heard calling from extensive forest stands (Hay-
ward and Garton 1988).

Of 22 studies providing nonbreeding season in-
formation, 15 reported that edge habitats were oc-
cupied and five reported open habitats occupied
(Birkenholz 1958, Bosakowski 1984, Marti et al.

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Long-eared Owl Conservation

181

Table 2. Breeding and non-breeding season habitat associations for Long-eared Owls in North America.

Habitat

Location

Source

BREEDING SEASON
Forest

Michigan

Wilson (1938)

Michigan

Craighead and Craighead (1956)

Michigan

Armstrong (1958)

Oregon

Reynolds (1970)

Oregon

Bull et al. (1989)

Mexico

Enriquez-Rocha et al. (1993)

California

Bloom (1994)

Edge

Michigan

Wilson (1938)

Michigan

Armstrong (1958)

Oregon

Reynolds (1970)

Mexico

Enriquez-Rocha et al. (1993)

Open

Nevada

Johnson (1954)

Arizona

Stophlet (1959)

Colorado

Marti (1969)

Washington

Knight and Erickson (1977)

Idaho

Craig and Trost (1979)

Idaho

Marks and Yensen (1980)

South Dakota

Paulson and Sieg (1984)

Idaho

Thurow and White (1984)

Idaho

Marks (1986)

Idaho

Craig et al. (1988)

Idaho

Ulmschneider (1990)

Manitoba, Canada

Sullivan (1992)

Montana

Holt (unpubl. data)

NON-BREEDING SEASON

Forest

Michigan

Armstrong (1958)

Mexico

Enriquez-Rocha et al. (1993)

Edge

Illinois

Cahn and Kemp (1930)

Wisconsin

Errington (1932)

Michigan

Spiker (1933)

Ohio

Randle and Austing (1952)

Kansas

Rainey and Robinson (1954)

Michigan

Craighead and Craighead (1956)

Michigan

Stapp (1956)

Michigan

Getz (1961)

Iowa

Weller and Fredrickson (1963)

New York

Lindberg (1978)

Iowa

Voight and Glenn-Lewin (1978)

Pennsylvania

Smith (1981)

Massachusetts

Andrews (1982)

Massachusetts

Holt and Childs (1991)

Connecticut

Smith and Devine (1993)

Mexico

Enriquez-Rocha et al. (1993)

Open

Illinois

Birkenholz (1958)

New Jersey

Bosakowski (1984)

New Mexico

Marti et al. (1986)

Washington

Denny (1991)

Montana

Holt (unpubl. data)

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Vol. 31, No. 2

Figure 6. Geographic distribution of breeding (•) and non-breeding ( ♦ ) season studies in North America.

1986, Denny 1991). One each reported forest
(Armstrong 1958) or forest and edge (Enriquez-
Rocha et al. 1993) (Table 2). In contrast to the
western breeding studies, 18 nonbreeding studies
were conducted in the midwest and northeast, ex-
cept for four: one in New Mexico (Marti et al.
1986), Washington (Denny 1991), Montana (D.
Holt unpubl. data) and Mexico (Enriquez-Rocha
et al. 1993). The geographic distribution of these
studies (Fig. 6) is almost nonoverlapping.

Diet. Of 21 nonbreeding season studies repre-
senting 45 671 prey, 17 studies reported a Microtus
vole to dominate the long-ear diet. These voles are
open country inhabitants (Table 3). The remain-
ing prey species also inhabit open country. Results
were similar for the breeding season but with slight
differences in prey composition. Of 14 studies rep-
resenting 13858 prey, all except Bull et al. (1989)
reported an open country prey species (Table 4).
Bull et al. (1989) reported that Long-eared Owls
nested in extensive stands of Grand Fir ( Abies gran-
dis) and that the prey species comprising the ma-
jority of the diet was the northern pocket gopher
( Thomomys talpoides). This species is primarily an
open country inhabitant, but also occurs within
openings in closed canopy forests and may move
into recent dear-cuts (Ingles 1967).

These data are further supported by Marti’s
(1976) extensive review of the feeding ecology of

Long-eared Owls. He included data from North
America, several European countries and Iraq. He
concluded that Long-eared Owls feed on small ro-
dents found in open country with Microtus voles
eaten most frequently, followed by Peromyscus mice
and Heteromyid rodents.

Ecomorphology. I reviewed the literature to de-
termine if the Long-eared Owl’s morphology was
consistent with adaptive radiation for particular
habitats. Bird groups in general have similar flight
morphology, as do birds living in similar habitats.
For example, open country bird species like the
Long-eared Owl and Snowy Owl ( Nyctea scandiaca)
have more pointed wings for better agility than for-
ests owls. Forest owls such as the Great Gray Owl
( Strix nebulosa ) and Boreal Owl ( Aegolius funereus)
which live in dense vegetation, have short broad
wings and a large wing area which aid in maneu-
verability (see Rayner 1988).

Owls generally exhibit low wing loading and low
aspect ratio and are among birds with the lowest
wing loading (Norberg 1987). Relative wing load-
ing is defined as the owls’ body mass divided by
the wing area or; Mg/S (mass M multiplied by the
acceleration of gravity g, divided by wing area S),
and aspect ratio is defined as wingspan divided by
mean chord length, or &/ S (wingspan b squared,
divided by wing area S, or wingspan divided by
mean wing chord) (Norberg and Norberg 1986).

June 1997

Long-eared Owl Conservation

183

Table 3. Non-breeding season diet of Long-eared Owls in North America.

Habitat

Dominant Group

Location

Source

Edge

Peromyscus

1198

Illinois

Cahn and Kemp (1930)

Microtus

210

Iowa

Errington (1933)

Microtus

1261

Ohio

Randle and Austing (1952)

Peromyscus

249

Indiana

George (1954)

Microtus / Sigmodon

1087

Kansas

Rainey and Robinson (1954)

Microtus

952

Wisconsin

Craighead and Craighead (1956)

Microtus

1000

Michigan

Stapp (1956)

Microtus

2995

Michigan

Armstrong (1958)

Microtus

2328

Illinois

Birkenholz (1958)

Microtus

126

Iowa

Weller and Fredrickson (1963)

Microtus

301

New York

Lindberg (1978)

Microtus

2112

Iowa

Voight and Glenn-Lewin (1978)

Microtus

915

Massachusetts

Holt and Childs (1991)

Open

Microtus

3272

Wisconsin

Errington (1932)

Microtus

199

Michigan

Spiker (1933)

Perognathus

2821

New Mexico

Marti et al. (1986)

Microtus

18 956

Montana

Holt (unpubl. data)

Unknown

Microtus

108

Iowa

Scott (1948)

Microtus

1494

Pennsylvania

in Latham (1950)

Microtus

2495

Nebraska

in Latham (1950)

Microtus

1622

Pennsylvania

Smith (1984)

Among owls, forest species tend to have much low-
er aspect ratios than open country species (Nor-
berg 1987), because foraging within vegetation is
favored by those species with short wings, large
wing area and low wing loading. This enables these
species to have slow, maneuverable flight (Norberg
1987). Contrasting this are open country migratory
species such as the Long-eared Owl, Snowy Owl
and Short-eared Owl ( Asio flammeus) which have

Table 4. Breeding season diet of Long-eared Owls in North America.

Habitat

Dominant Group

Location

Source

Forest

Thomomys

1123

Oregon

Bull et al. (1989)

Edge

Microtus

1935

Michigan

Wilson (1938)

Microtus

274

Michigan

Armstrong (1958)

Microtus

153

Oregon

Reynolds (1970)

Open

Microtus

114

Nevada

Johnson (1954)

Microtus

129

Wyoming

Craighead and Craighead (1956)

Perognathus

315

Arizona

Stophlet (1959)

Peromyscus

993

Colorado

Marti (1969)

Perognathus

171

Washington

Knight and Erickson (1977)

Peromyscus

346

Idaho

Marks and Yensen (1980)

Peromyscus/ Dipodomys

4208

Idaho

Marks (1984)

Peromyscus

1000

Idaho

Thurow and White (1984)

Perognathus/ Peromyscus

3977

Idaho

Craig et al. (1985)

Microtus

3020

Montana

Holt (unpubl. data)

long wings for sustained flight, yet have relatively
low wing loading (Norberg 1990).

Conclusion

Is the Long-eared Owl a forest species? These
data suggest that the Long-eared Owl may not be
a forest species; however, more forest studies are
needed. Long-eared Owls obviously depend on
trees and shrubs for nesting and roosting. In large

184

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Vol. 31, No. 2

open grasslands or shrubsteppe habitat, Long-
eared Owls nest and roost in predominately shrub-
like vegetation. In smaller openings in forests and
along forest edges adjacent to open areas, Long-
eared Owls use trees (often conifers) to nest and
roost. Data from this review emphasize that per-
haps too much forest may cause Long-eared Owls
to leave an area (Bosakowski et al. 1989b), while
patches of open areas within or near forest edges
may benefit them. Unfortunately, forest age and
stand structure requirements are not known for
this species. Thus, the impacts of forest manage-
ment cannot be ascertained at this time. Addition-
ally, forest managers may need to define what a
forest owl species is. Perhaps the Long-eared Owl
can best be defined as an edge species, when found
in or near forest habitats. It may be presumptuous
at this point in time to suggest forest-management
guidelines regarding the Long-eared Owl, particu-
larly since no conclusive data exist pertaining to
effects of present or past forestry practices.

Given the recent interest of metapopulation
analysis (Levins 1969), which includes the core-sat-
ellite (Boorman and Levitt 1973) and sink and
source (Pulliam 1988) models, forest-management
considerations must include results of long-term
studies from several geographic areas. Within these
studies, comparative data of the Long-eared Owl
natural history is essential for these models to be
useful. Specifically, the following data are needed:
Long-eared Owl residency, mating system, repro-
ductive success and home ranges; Long-eared Owl
prey species populations, how these effect owl res-
idency, density and home range and how prey spe-
cies are affected by forest practices; quantitative
measures of nest sites; vegetative cover for adult
and nestling owl roosting areas; and seasonal use
of habitats because different habitats may be im-
portant at particular times of the year and avoided
at other times.

Therefore, forestry practices may have to be stag-
gered over space and time and perhaps from a few
to hundreds of kilometers of habitats must be man-
aged simultaneously or alternately to cover the
Long-eared Owls’ migratory or nomadic tenden-
cies. The use of artificial nest sites as a manage-
ment tool must be carefully considered before im-
plementation — what is the biological justification
for their use? Consideration of how forestry prac-
tices affect the interspecific relationships between
Long-eared Owls and invader species must also be
taken into account.

To adequately address the questions concerning
impacts of forestry, research needs to cover longer
periods of time and must also research the species
year-round. Although many short-term studies pro-
vide useful information, they simply cannot pro-
vide enough data to answer questions such as those
addressed herein. Given that Long-eared Owls are
migratory and nomadic, and often dependent on
small mammal cycles three to four yr long (e.g.,
voles), studies should at the least cover this dura-
tion, and preferably several cycles.

Ackn o wledgments

I thank Gerald Niemi for the invitation to this sympo-
sium. This manuscript benefited from comments by Stacy
Drasen, Tom Bosakowski, Dan Varland and Gerald Nie-
mi.

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Received 2 November 1995; accepted 6 March 1997

NORTHERN HAWK OWLS (SURNIA ULULA CAPAROCH) AND
FOREST MANAGEMENT IN NORTH AMERICA: A REVIEW

Patricia A. Duncan

Box 253, Balmoral, Manitoba, Canada ROC OHO

Wayne C. Harris

Saskatchewan Environment and Resource Management, 350 Cheadle St. W., Swift Current,

Saskatchewan, Canada S9H 4G3

Abstract. — Northern Hawk Owl ( Surnia ulula caparoch ) populations in North America likely have been
stable over the past 10-100+ yr. Population trends are impossible to quantify due to this species’ remote
breeding range, low breeding densities and erratic distribution and numbers during winter irruptions
in inhabited areas. Mortality due to incidental trapping and shooting is unknown, but its diurnal habits
and lack of fear of humans make it vulnerable to persecution. More than 50% of the hawk owl’s breeding
range occurs in northern forests that are currently noncommercial. Until recently, the majority of the
hawk owl’s breeding range was unaffected by forestry practices. In the last 20 yr, forestry activities have
expanded in commercial northern forests. Modification of clear-cut logging practices have the potential
to enhance hawk owl habitat. Variable-sized cuts of <100 ha, interspersed with forest patches and stag-
gered over time, are thought to be optimal. If cuts contain suitable numbers of stumps, snags and trees
for hunting perches and nest sites, they will offer year-round habitat. Other factors, such as cut shape
and juxtaposition, are probably less important to this striking sentinel of our northernmost forests.

Key Words: Surnia ulula; North America ; forest management, habitat use, Northern Hawk Owl.

El Buho Halcon del norte Surnia ulula caparoch y administracion forestal en Norte America: un reviso

Resumen. — Poblaciones de Buho Halcon Surnia ulula caparoch en norte america ha estado estable en
los ultimos 10 a 100+ anos. Tendencias de poblacion estan imposible para cuantificar por los campos
remotos de cria de la especie, densidad baja de crfa, y distribucion y cantidad variable durante irrup-
ciones del invierno en areas inhabitadas. Mortalidad a causa de trampas y disparos es desconocido, pero
sus costumbre de volar en el dia y falta de tener miedo a gente lo hace vulnerable a persecution. Mas
de 50% de los campos de cria del Buho Halcon ocurren en bosques en el norte que estan actualmente
no-comercial. Hasta recientemente, la mayorfa de campos de cria del Buho Halcon estaban sin afecta-
cion por costumbre de los forestales. En los ultimos 20 anos actividades forestales han expansionado
en bosque comerciales en el norte. Modificaciones en costumbre de corta-completa tienen la potencia
para aumentar habitat de Buho Halcon. Cortadas variables de <100 ha, introducidas en parcelas de
bosque escalonado con tiempo, es pensado ser optimo. Si cortadas contienen cantidad conveniente de
tocones, ramas sueltas y arboles con perchas de cazar y sitios de nido, pueden ofrecer un habitat por
todo el ano. Otros factores, con forma de cortar y yuxtaposicion, es probable menos importante para
este centinela de los bosques mas nortenos.

[Traduction de Raul De La Garza, Jr.]

There are very few published papers on the ecol-
ogy of the Northern Hawk Owl ( Surnia ulula ) in
North America; this is in contrast to Europe where
the majority of studies have been done (Clark et
al. 1987). The intent of this paper is to review the
literature regarding the effects of forestry on hawk
owls in North America and to make management
guidelines to maintain hawk owl populations. Un-
fortunately, the effects of forestry on the hawk owl

are poorly understood and there are virtually no
published reports on this subject. With this in
mind, the management guidelines we have pre-
sented are hypothetical and based on the limited
information that is available and from our own ex-
periences in the field with this enigmatic owl. We
have focused on the species in North America only
and, by doing so, have pointed out the serious lack
of information on hawk owls in the New World.

187

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Duncan and Harris

Vol. 31, No. 2

Year

Figure 1. Number of Northern Hawk Owls banded dur-
ing winter in North America, 1956—1992 (N = 363). Un-
published data from the Bird Banding Office, Canadian
Wildlife Service, Ottawa, Canada.

Estimated Population Size and Trends

Accounts of Northern Hawk Owl population size
and trends in North America are largely anecdotal.
Trends are difficult to impossible to assess due to
the remote breeding range, rare winter irruptions
and low densities (Newton 1976). Fyfe (1976) re-
ported that in the maritime provinces, the popu-
lation trend was unknown and relative abundance
was rare. For Ontario and southern Quebec, the
trend was fluctuating, with relative abundance rare
to low. The prairie provinces and British Columbia
also reported fluctuating populations, with low to
medium relative abundance. Analysis of Breeding
Bird Survey data showed a nonsignificant decrease
for Labrador and the central prairie provinces
from 1966-77 to 1978-83 (Collins and Wendt
1989). The number of hawk owls banded between
1956—92 (Fig. 1) reflects low owl numbers, al-
though banding effort was variable and not stan-
dardized (Canadian Wildlife Service data). Irrup-
tion years presumably relate to regional variation
in food availability and hawk owl reproduction.
Based on the hawk owl’s North American breeding
range (American Ornithologists’ Union 1983), we
hypothesize that its overall North American popu-
lation has remained stable at 10 000-50000 pairs
over the past 10—100+ yr, with local or regional
populations showing fluctuations in owl numbers.

Habitat Needs

Of the three major habitat regions (Rowe 1972)
within the boreal forest, the northernmost Forest
and Barren Region is likely where the majority of
hawk owls occur and breed. This region is char-

acterized by open, stunted forests interspersed with
bogs and muskeg (Rowe 1972). Typical breeding
habitat of the hawk owl is described as open to
moderately dense coniferous or mixed coniferous-
deciduous forests bordering marshes or other
open areas, including those cleared by logging
(American Ornithologists’ Union 1983). In moun-
tainous areas, its range extends to timberline as
high as 2650 m elevation (Campbell et al. 1990).
Open hunting areas such as muskegs, dry ridges,
burn areas, clearings and swampy valleys or mead-
ows with suitable perches also characterize breed-
ing habitat. Areas with stumps, snags or dead trees
with bare branches serving as hunting perches are
favored; impenetrable spruce-fir forests are avoid-
ed (Henderson 1919, Smith 1970, Kertell 1986,
Lane and Duncan 1987). Nest sites are located in
cavities in decayed trees, open decayed hollows
where tops have broken off (Lane and Duncan
1987) and rarely in stick nests or on cliffs (Bent
1938).

We believe forest regions south of its breeding
range are important for the Northern Hawk Owl’s
survival during southward irruptions and for oc-
casional breeding. The winter range of the hawk
owl is more extensive than its breeding range
(American Ornithologists’ Union 1983). In winter,
hawk owls may be found in wooded farmlands,
open areas of parkland and prairie regions where
haystacks, posts, trees or bushes are used as perch-
es (Jones 1987). It also hunts in old burns, cut-
overs and riparian areas surrounded by agricultur-
al land, old hay fields, open spruce forests and
along roadsides with large rights-of-way (Lane and
Duncan 1987, Rohner et al. 1995). Second growth
woodlands and lake shores are used in British Co-
lumbia (Campbell et al. 1990).

Factors Associated with Trends

Long-term North American Population Trends.

More than half of the breeding range of the hawk
owl is currently noncommercial forest (Godfrey
1986). Therefore, one would expect that hawk owl
populations have remained stable over the last 10—
100+ yr.

Northern forests are subject to natural fires
which usually are left unchecked if far from human
settlements. Burns may benefit hawk owls because
they are known to hunt and nest in old burns
(Mindell 1983). In some habitats (e.g., deep sphag-
num moss), small mammalian prey species fre-
quently survive forest fires and populations recover

June 1997

Hawk Owls and Forestry

189

quickly (Kelsall et al. 1977). Fire suppression near
populated areas or in areas with merchantable tim-
ber reduces the availability of suitable nest sites.

Hawk owls occur throughout the year in the
commercial forest region of Canada (numbers and
distribution varies year to year). During the past 20
yr, clear-cut harvesting in boreal forests has in-
creased. In the short-term, hawk owls may be neg-
atively affected by large cuts (>100 ha), where no
perches remain within the cut, or later, by regen-
erating dense forests (Sonerud pers. comm.). The
current trend in forestry is for more numerous,
smaller clear-cuts. Given its preference for open ar-
eas for hunting and breeding, habitat has likely im-
proved for the hawk owl as long as suitable hunting
perches are available. However, expansion of cur-
rent practices over many years may reduce habitat
quality, (e.g., fewer late successional forests that
provide nest sites) .

Short-term Population and Local Fluctuation
Trends. Northern Hawk Owl numbers fluctuate lo-
cally (Kertell 1986, Lane and Duncan 1987, Ro li-
ner et al. 1995). When prey populations crash,
hawk owls may be forced to leave breeding areas
and wander in search of food. Irruptions have
been well documented in North America (Thomp-
son 1891, Barrows 1912, Roberts 1932, Bernard
and Klugow 1963, Green 1963, Lane and Duncan
1987, Speirs 1985).

Other Factors Associated with Population
Trends. Reports of hawk owls in northern areas are
infrequent and incidental; during southern irrup-
tions the hawk owl is vulnerable to human-induced
mortality (K. McKeever pers. comm.). Predators in-
clude the Great Horned Owl ( Bubo virginianus ) ,
Northern Goshawk {Accipiter gentilis) , marten ( Mar-
ies americana), fisher (Martes pennanti) and weasels
( Mustela spp.).

Effects of Forestry on Hawk Owl Habitat

Primary Effects. Logging practices have the po-
tential to enhance hawk owl habitat. Because hawk
owls prefer open habitat, cut-overs with perches at-
tract them. Furthermore, if cut-overs contain
enough stumps and trees for nest structures, they
offer year-round habitat.

Secondary Effects. The secondary effects of for-
est harvesting include their impacts on prey pop-
ulations such as meadow voles {Microtus pennsylvan-
icus ). Meadow vole populations increase 3-18 yr
after clear-cutting forests (Kirkland 1977, Parker
1989). In Saskatchewan, deer mice ( Peromyscus

maniculatus ) are more abundant after clear-cutting
than after fire.

Hypothetical Specific Forestry Effects. Cut size.

There are no published studies on the influence
of cut size on hawk owl habitat use. We hypothesize
that suitable cuts should be <100 ha in size, inter-
spersed with forest stands and staggered over time.
Forest stands will provide hunting perches and nest
sites, as w 7 ell as cover.

Cut shape. We suspect that edge irregularity in-
creases the availability of perches and provides cov-
er as well as access to open foraging habitat.

Residuals. It is important to leave residuals such
as live trees and dead snags for hunting perches
and nest sites. Thus, small residual stands within
cuts would be beneficial. Without these, the use of
cut-over areas by hawk owls is limited to cut edges.

Phenology. We have observed hawk owls in Sas-
katchewan and Manitoba in 5-10 ha cuts from 3-
10-yr-old. Bortolotti (pers. comm.) observed owls
hunting in cuts 8-10-yr-old in Saskatchewan. In
Ontario, Russell (pers. comm.) found dispropor-
tionate use of cuts that were 1 1— 15-yr-old. The time
lag in cut use by hunting hawk owls is likely a factor
of prey availability. For example, Kirkland (1977)
and Parker (1989) reported that meadow vole
numbers increased 3-18 yr after harvest. Dense re-
generation growth after approximately 20 yr would
limit prey availability.

Conclusions

North American boreal forests are, for the most
part, being harvested for the first time and are not
intensively managed. Most regenerating cut-overs
contain a variety of tree species. A noteworthy
trend in North America is toward increasingly in-
tensive forest management, resulting in reduced
rotation ages and more homogeneous forest stands
(Environment Canada data). It is expected that
such a trend may negatively impact hawk owl pop-
ulations, if no perches are left within these cut ar-
eas. Furthermore, as the demand for wood increas-
es, forests in remote areas will be used. This wall
only emphasize the need to better integrate timber
and wildlife management objectives into regional
forest use plans.

We presume hawk owl populations to be stable
in North America due to its remote range, about
half of which is currently noncommercial forest.
The species appears to use a variety of cut sizes and
shapes, provided that hunting perches and breed-
ing sites are retained. We emphasize the need for

190

Duncan and Harris

Vol. 31, No. 2

further research on this boreal forest owl in North
America. A lack of information on the ecology of
the hawk owl and its responses to forestry practices
precludes us from recommending definitive forest
management guidelines or from forecasting its fu-
ture status.

Acknowledgments

James R. Duncan, Jeff S. Marks, Robert W. Nero,
Dwight G. Smith and an anonymous person provided
valuable comments that improved the manuscript.

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Received 2 November 1995; accepted 5 March 1997

CONCLUDING REMARKS ON RAPTOR RESPONSES TO
FOREST MANAGEMENT: A HOLARCTIC PERSPECTIVE

Gerald J. Niemi and JoAnn M. Hanowski

Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Highway,

Duluth, MN 55811 U.S.A.

Recent and Historical Trends

Participants were asked to provide perspectives
on the current (past 30 yr) and historical (past
100+ yr) population trend for each species (Table
1). Both Pertti Saurola and Peter Ewins present
strong evidence that Osprey ( Pandion haliaetus )
populations have increased in both North America
and Europe since the 1960s when populations of
many fish-eating birds declined due to the inges-
tion of pesticides such as DDT. Historically, how-
ever, Osprey populations varied considerably over
the past 100+ yr on both continents.

Per Widen has shown that the Northern Gos-
hawk ( Accipter gentilis ) has likely declined during
recent years in Fennoscandia, possibly due to frag-
mentation of forests and reductions in total
amounts of mature forest and associated prey pop-
ulations such as grouse. In North America, Patricia
Kennedy found no evidence for a decline in this
species based on its range, population demograph-
ics (density, fecundity and survival) and population
trends. She suggested that a more detailed meta-
analysis is required to further address this ques-
tion. The historical trend for this species is un-
known on either continent, although she speculat-
ed that the Northern Goshawk may have been
more abundant in the eastern U.S. prior to the
extinction of the Passenger Pigeon ( Ectopistes mig-
ratorius ) and the deforestation in this region at the
end of the 19th century.

Like most of the raptor species included here,
there is little information on Long-eared Owl ( Asio
otus) trends for North America. Based on admit-
tedly sparse data, Denver Holt hints at a possible
recent decline in the species in some parts of
North America. Nothing is known about historical
population trends for this species. No paper was
included for this species from Europe.

Neither Greg Hayward nor Harri Hakkarainen
were willing to speculate as to whether there were
recent or historical population trends for the Bo-
real Owl (Aegolius funereus) in North America and

Europe. Hayward stated that although the Boreal
Owl was not known as a breeding bird in the lower
48 U.S. until the early 1970s, the increased obser-
vations over the past 20 yr is likely due to increased
search efforts rather than a population increase. In
Fennoscandia, especially in Finland, an increase in
nest boxes for owls (22 691 nest boxes for owls
checked in 1994) has likely increased populations
in many areas and also our understanding of this
species’ biology.

Geir Sonerud presents data showing an apparent
recent increase in Northern Hawk Owl ( Surnia ulu-
la) populations in northern Europe during the last
part of this century. Over the past 90 yr, the pop-
ulation was high in the early part of the century,
followed by a decline, and then a recent increase.
In North America, Patricia Duncan and Wayne
Harris speculate that the population appears to be
relatively stable but that it fluctuates in response to
available food supplies.

There is relatively strong evidence for an in-
crease in the Great Gray Owl (Strix nebulosa) pop-
ulation in northern Europe over the past 30 yr, but
Seppo Sulkava and Kauko Huhtala present evi-
dence that the long-term trend is highly variable.
They suggest that the recent increase is due to a
combination of factors including reduced killing of
owls by humans, increased availability of artificial
nest sites (hundreds of twig nests and nest plat-
forms), but warn that although regional Great
Gray Owl populations have been relatively stable
both recently and historically, local populations
fluctuate widely with available food supply.

Forest Management

Stand Size and Shape. In general, we know very
little on how species might respond to variations
in the size and shape of logged stands (Table 1 ) .
The Osprey is likely not affected directly by stand
size and shape. However, availability of suitable
nest trees, effects of logging on aquatic systems and
fish supply and populations of major nest preda-
tors (e.g., Eagle Owl, Bubo bubo, in Europe or Great

191

192

Niemi and Hanowski

Vol. 31, No. 2

Table 1. Summarization of trends and possible responses to forest management of six forest raptors in Europe and
North America.

Forest Management

Species

Continent

Trend

Stand Size

Stand Shape

Residuals

Recent

Long-Term

Small

Medium

Large

Simple

Complex

Osprey

Europe

Increase

Variable

Neutral

Essential

N America

Increase

Variable

Neutral

Essential

Northern

Europe

Decline

Unknown

Negative

Unknown

Negligible

Goshawk

N America

No evidence

Possible de-

Unknown 3

cline in

—

Eastern US

Long-eared

N America

Possible

Unknown

Positive

Unknown

Negative

Positive

Negligible

Owl

decrease

Boreal Owl

Europe

Unknown

Positive?

Neutral

Positive

Essential

N America

Unknown

Positive

Essential

Northern

Europe

Possible

Postive

Unknown

Negative

Neutral

Positive

Hawk Owl

increase

decrease

N America

Stable

Positive

Unknown

Negative

Neutral

Positive

Great Gray

Europe

Increase

Variable

Positive

Unknown

Negative

Positive

Owl

N America

Stable

Positive

Unknown

Negative

Positive

A All factors were not examined for this species because of the focus on the species demography.

Horned Owl, Bubo virginianus in North America)
may indirectly affect Osprey populations. Perth
Saurola points out that young Osprey vocalizing for
food in a single tree in the middle of a clear-cut is
a dinner bell to an Eagle Owl. Although more is
known about the ecology of Ospreys relative to the
other five species included here, Peter Ewins
points out that remarkably little is known about
Osprey nesting ecology relative to timber extrac-
tion. He concluded based on his review that there
is a need for a systematic field study and no firm
generalizations can be made. His discussion on the
discrepancy between protection standards for nest-
ing Ospreys and the potential costs associated with
that protection are thoughtful. The recently devel-
oped guidelines on Osprey nests presented by Pert-
ti Saurola provides a step toward improving this
situation.

Although his conclusions are based on admitted-
ly sparse data, Per Widen finds that the Northern
Goshawk has declined in Fennoscandia due to the
loss of mature forests and consequent reductions
m available foraging areas and food resources.
Northern Goshawks forage primarily on grouse
(many species of which are also declining in Fen-
noscandia) , squirrels and lagomorphs; the former
two of which are found primarily in mature forests.
Per Widen emphasizes that the species primarily
forages in mature and older forests with open un-

derstories where it makes short flights between
perches. The species seldom uses recently cut areas
for foraging presumably because of the dense un-
derstories where prey is hard to detect. He also
suggests that the Northern Goshawk prefers larger
tracts of forest for foraging and, hence, is further
affected by fragmentation of forested areas. There-
fore, logging of forests, especially clear-cuts that re-
duce foraging area and fragment large blocks of
mature forest, appears to be contributing to de-
clines of the Northern Goshawk in Fennoscandia.

Based on a variety of evidence for Northern Gos-
hawks across North America, Patricia Kennedy con-
cludes that there is no strong evidence to support
the contention that goshawk populations are de-
clining. She emphasizes two possible conclusions
based on her analysis: (1) either the goshawk is not
declining or (2) current sampling techniques are
insufficient to detect population trends. Reynolds
et al. (1992) provide comprehensive guidance on
forest management for the Northern Goshawk in
the southwestern U.S. In addition, Kenward (1996)
points out additional complexities in understand-
ing Northern Goshawk ecology, especially differ-
ences that may be operating in North America and
Europe. He indicates that further study is needed
on interspecific interactions, winter diet and life
history information between fledgling and breed-
ing periods. Clearly, additional data are needed on

June 1997

Raptors and Forestry

193

responses of this species to forest management
practices that will help us understand how to main-
tain adequate populations of Northern Goshawks,
yet provide sustainable and ecologically sound har-
vest levels. These studies, however, will not be easy,
must be long-term, and will not be cheap because
the Northern Goshawk has relatively low popula-
tion levels, a large home range and a food base
that varies substantially. Moreover, despite some of
the differences that exist between North America
and Fennoscandia (e.g., available food supply),
there appear to be many opportunities to better
our knowledge on how Northern Goshawks react
to variations in forest management by additional
comparisons and coordination of studies on the
two continents.

Again using a limited amount of published in-
formation on the ecology of Long-eared Owls,
Denver Holt suggests that forest management mea-
sures producing relatively small and open cut areas
in which owls can forage juxtaposed with forested
areas with nest sites provide ideal habitat. Hence,
this species may be negatively affected by large
cuts, unless the shape is relatively complex to pro-
vide access to forested areas. There is some ques-
tion on the extent to which the Long-eared Owl
uses contiguous forested areas because data from
these areas are limited. Forest management that
provides habitat for prey, plus roost and nest-site
cover for Long-eared Owls will be most beneficial.

Summaries by Greg Hayward and Harri Hakka-
rainen for Boreal Owls in Fennoscandia and North
America are enigmatic. Harri Hakkarainen and his
colleagues show that, in Fennoscandia, nesting suc-
cess is highest in landscapes with relatively large
proportions of recently clear-cut areas (e.g., 35-
70%) compared with landscapes with small pro-
portions of clear-cut area (10-30%). However, in
their studies, nest boxes were provided presumably
due to the lack of natural nest cavities. In contrast,
Greg Hayward states that clear-cutting creates
stands without habitat value for Boreal Owls for a
century or more. Harri Hakkarainen reasons that
clear-cut areas in Finland create suitable habitat for
held voles ( Microtus spp.), the primary prey for the
Boreal Owi in this region. Those factors (stand and
landscape characteristics) that contribute to high
vole densities appear to be most critical for suc-
cessful nesting of the Boreal Owl. In contrast with
these data, Sonerud (1986) and Jacobsen and So-
nerud (1993) emphasize the considerable variation
in prey availability and foraging habitat for the Bo-

real Owl throughout its annual cycle. For instance,
Microtus voles may not be available in some clear-
cuts in winter w r hen the snow has a hard crust or
in summer when the vegetation is too thick. Dur-
ing these times, mature forests provide the best
cover and available prey populations.

Greg Hayward’s data for the Rocky Mountain re-
gion suggest that Boreal Owls primarily forage in
mature and older spruce-fir forests in the western
U.S. In these forests, the red-backed Able ( Cleth -
rionomys gapped ) is the dominant prey. Similar to
Sonerud (1986), he suggests that there is less snow
crusting in mature and older forests relative to
openings and young forests and, therefore, less
prey is available in openings and young forests dur-
ing winter months. He emphasizes that the ecology
of this species appears to vary considerably geo-
graphically, such as northern and southern popu-
lations of the Boreal Ow T l in North America (Hay-
ward and Aferner 1994).

It is obvious that studies from northern Europe
and western North America may not be compara-
ble, although greater quantification of nesting and
foraging habitat, landscape context of nesting hab-
itat and improved understanding of the food base
for the Boreal Owl on both continents w r ould aid
comparisons. In northern Europe, nest boxes,
hunting perches and adequate food have allowed
Boreal Owls to nest near clear-cuts. However, pro-
viding nest boxes over large geographic areas is a
daunting task and likely not an economically viable
means to manage a species. The work by Hakka-
rainen and Korpimaki (1996) also illustrates the
influential role of interspecific interactions with
other owl species, Boreal Owl distribution and re-
production. Data like these are not available for
North America and nest-box studies are likely the
only way to address these questions. Although pa-
pers on the Boreal Owl from the two continents
may be enigmatic, they are fascinating in terms of
providing insights on complexities involved in
studies for just one species in regard to forest man-
agement issues.

Based on limited knowledge on Northern Hawk
Owl ecology, Patricia Duncan, Wayne Harris and
Geir Sonerud conclude that this species likely ben-
efits from relatively small and complex cut sizes in
forests. Key issues for this species are hunting
perch availability, nest trees and cover for protec-
tion within a logged landscape. Geir Sonerud de-
scribes a relatively intense, albeit with limited spa-
tial replication, study of foraging by Northern

194

Niemi and Hanowski

Vol. 31, No. 2

Hawk Owls. He emphasizes that hunting perches
within logged areas are required. If no live or dead
residuals are left in clear-cuts, the only hunting
perches that allow this species to use these areas
for foraging are trees remaining along the edges.
The species can tolerate larger clear-cut areas if the
shape is convoluted providing edges or if many
suitable hunting perches are left distributed within
the cut areas allowing access to most of the clear-
cut area. In addition, suitable areas for cover and
nesting are also required.

As with the Northern Hawk Owl, evidence pre-
sented by James Duncan, Seppo Sulkava and Kau-
ko Huh tala show the Great Gray Owl responds fa-
vorably to relatively small and complex cuts that
provide suitable foraging perches along edges and
suitable cover for nesting and protection in the ad-
jacent forest habitat. We do not know how the spe-
cies would respond to intermediate-sized cuts but,
based on the species’ ecology, large clear-cuts with
no hunting perches would be of little use. Larger
cuts with well-distributed hunting perches, convo-
luted edges and adjacent areas that provide cover
and nesting may be suitable. Little is known of the
size requirement of a forest area for nesting or cov-
er. In addition, the nesting forest requirements of
large raptors which produce most nesting plat-
forms for the Great Gray Owl also need to be con-
sidered.

Residuals. With the possible exception of the
Northern Goshawk and Long-eared Owl, the re-
maining four species require residuals in logged
areas for the species to use this habitat (Table 1).
For species that often use residuals for nesting such
as the Osprey and Boreal Owl, they are essential.
It is unclear to what extent the Northern Goshawk
or Long-eared Owl require residuals as hunting
perches. Certainly these species use them occasion-
ally as hunting perches or resting sites, but their
importance to their overall fitness is unclear.

Based on the evidence from Fennoscandia, the
Boreal Owl, Northern Hawk Owl and Great Gray
Owl all use residuals left within logged areas for
hunting perches to forage for small mammals (es-
pecially Microtus voles). Seppo Sulkava and Kauko
Huhtala suggest that the Great Gray Owl popula-
tion has increased in many parts of Finland be-
cause of the increased populations of Microtus voles
and the ability of the Great Gray Owl to forage in
these logged areas.

The extent to which either the Northern Hawk
Owl or Great Gray Owl use residuals within clear-

cut areas for nesting is unclear. In Finland, some
nests have been found in open habitats (e.g., clear-
cuts) or near openings. Geir Sonerud indicates
that few breeding opportunities exist in recently
cut areas because of the lack of suitable older trees
for nest sites. He points out that the decline in
Northern Hawk Owls in Finland from the 19th
century to the 1950s was thought to be due to the
disappearance of suitable nest trees. Patricia Dun-
can and Wayne Harris suggest that areas that offer
year-round habitat are cut-overs containing
enough stumps and trees for nest structures.
Hence, it would appear that recently logged areas
may be suitable nesting areas for both the North-
ern Hawk Owl and Great Gray Owl if suitable re-
siduals are left. On a local scale it is also possible
to actively manage for these species by placing nest
boxes (Northern Hawk Owl) or nesting platforms
(Great Gray Owl), but several authors point out
that this type of mitigation is impractical at larger
spatial scales.

In northern Europe, Boreal Owls nest success-
fully in a landscape with a high proportion of clear-
cuts when provided with suitable nest boxes. Nev-
ertheless, Hard Hakkarainen and his colleagues
point out that modern forestry practices must pro-
vide suitable snags and patches of old mature for-
est with large trees dense enough to support the
hole-nesting Black Woodpecker ( Dryocopus mar-
tins) . The Black Woodpecker excavates most natu-
ral nest cavities for the Boreal Owl in Finland. Greg
Hayward points out that, in North America, avail-
ability of nest cavities depends upon available nest
trees (especially aspen, Populus spp.), insects and
pathogens necessary to create suitable, weakened
trees and primary cavity nesters such as Pileated
Woodpecker ( Dryocopus pileatus) to create cavities.
In a nest-box experiment in Idaho, he found that
the Boreal Owl selected nest boxes within forests
of more complex structure (e.g., multiple canopy
layers and many tree size classes) and did not use
boxes in forests with a more simple structure (e.g.,
single canopy layer and more uniform tree diam-
eters). More information is needed to address the
combination of nesting, foraging and cover needs
of the Boreal Owl.

In general, residuals in logged areas are clearly
beneficial to a variety of forest raptors, including
most of those considered here. Quantitative data
obtained through replicated field studies are need-
ed to address specific issues on species, sizes, spa-
tial distribution and number of residuals (dead or

June 1997

Raptors and Forestry

195

alive) required in logged areas. For instance, leav-
ing a few dead trees in the middle of a clear-cut
for Ospreys may be detrimental. If a goal of forest
management is to simulate natural forest condi-
tions to the extent possible, then the natural dis-
turbance forces for most of the northern boreal
forests considered here are fire, insect outbreaks
and wind (Pastor et al. 1996). In these systems, re-
siduals in the form of burned trees, patches of un-
burned forest, charred trees from fire, dead trees
from an insect outbreak or trees with broken tops
from excessive wind were much more common in
the past.

Conclusions

Although only six species of raptors were consid-
ered here, they illustrate that forest management
aimed toward logging wall benefit some species,
while alternative management measures aimed at
the maintenance of mature and old forest will ben-
efit other species. For instance, clear-cutting in
small units (2-5 ha) has increased populations of
Microtus voles in Fennoscandia and this habitat in-
termixed with suitable forested areas for nesting
and cover are beneficial to the Northern Hawk
Owl and Great Gray Owl. There is a potentially
important role of large fields and large dear-cuts
in supplying source populations of Microtus voles
to the smaller, isolated clear-cuts. In contrast, re-
duction in mature and old forest may lead to re-
duced populations of the Northern Goshawk.

The key is to understand predator and prey re-
sponses to forest changes at a variety of spatial
scales including microhabitat, landscapes and land-
scape mosaics. Individual species responses could
then be incorporated into forest change simula-
tion models that consider both spatial and tem-
poral scales (Pastor et al. 1996). These simulation
models will allow us to assess the effects of a range
of management scenarios on species populations,
other species complexes (e.g., plants, insects, mam-
mals, etc.), ecological processes (e.g., nutrient cy-
cles, plant growth and decomposition) and com-
modity production. The models should be devel-
oped with the best available knowledge and ap-
plied with an understanding of the degree of
uncertainty produced with the output. The models
can be improved as our knowledge of these organ-
isms and processes increase. Similarly, factors that
contribute most to output uncertainty should pro-
tide a framework for prioritizing additional re-
search activity.

Raptors, by virtue of their position in the forest
food chain and their potentially important role in
ecological processes of forests, will always be of
high concern in forest resource management de-
cisions. If we are to maintain healthy forest ecosys-
tems, then it is imperative for society to increase
its investment in understanding these systems.

Acknowledgments

We appreciate the constructive comments on these
concluding remarks from many of the contributors to
this issue including Patricia Duncan, James Duncan, Har-
ri Hakkarainen, Greg Ha) ward, Denver Holt, Patricia
Kennedy, Geir Sonerud, Seppo Sulkava and Per Widen.
We thank the following organizations for their financial
support of the symposium and this issue of the Journal of
Raptor Research : US Bureau of Land Management; the
North Central Forest Experiment Station in St. Paul,
Chequamegon National Forest, Chippewa National For-
est, Nicolet National Forest, Ottawa National Forest and
Superior National Forest; Boise-Cascade, White Paper Di-
vision; Georgia-Pacific Corporation; Lake Superior Paper
Industries; National Council of Stream and Air Improve-
ment Inc.; Potlatch Corporation, Northwest Paper Divi-
sion; Minnesota State Legislature (through the Minne-
sota Environment and Natural Resources Trust Fund as
recommended by the Legislative Commission on Min-
nesota Resources); and the University of Minnesota
through its Natural Resources Research Institute, Raptor
Center, Department of Biology and University College.
This is contribution number 207 of the Center for Water
and the Environment, Natural Resources Research Insti-
tute, University of Minnesota, Duluth.

Literature Cited

Hakkarainen, H. and E. KorpimAki. 1996. Competitive
and predatory interactions among raptors: an obser-
vational and experimental study. Ecology 77:1134-
1142.

Hayward, G.D. and J. Verner. 1994. Flammulated, Bo-
real, and Great Gray Owls in the United States: a tech-
nical conservation assessment. USDA For. Ser. Gen.
Tech. Rep. RM-253, Ft. Collins, CO U.S.A.

Jacobsen, B.V. and G. Sonerud. 1993. Synchronous
switch in diet and hunting habitat as a response to
disappearance of snow cover in Tengmalm’s Owl Ae-
golius funereus. Ornis Fenn. 70:78-88.

Kenward, R.E. 1996. Goshawk adaptations to deforesta-
tion: does Europe differ from North America? Pages
233-243 in D.M. Bird, D.E. Varland and J. Negro
[Eds.], Raptors in human landscapes. Academic
Press, London, UK

Pastor, J- DJ. Mladenoff, Y Haila, J. Bryant and S.
Payette. 1996. Pages 33-69 in H.A. Mooney, J.H.
Cushman, E. Medina, O.E. Sala and E.D. Schulze
[Eds.], Functional roles of biodiversity: a global per-
spective. J. Wiley and Sons Ltd., London, UK
Reynolds, R., R.T. Graham, M.H. Reiser, R.L. Bassett,
P.L. Kennedy, D.A. Boyce, Jr., G. Goodwin, R. Smith

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Vol. 31, No. 2

and E.L. Fisher. 1992. Management recommenda-
tions for the Northern Goshawk in the southwestern
United States. USDA For. Serv. Gen. Tech. Rep.
RM-217, Ft. Collins, CO U.S.A.

Sonerud, G.A. 1986. Effect of snow cover on seasonal
changes in diet, habitat, and regional distribution of
raptors that prey on small mammals in boreal zones
of Fennoscandia. Holarct. Ecol. 9:33-47.

BOOK REVIEWS

Edited by Jeffrey S. Marks

Demography of the Northern Spotted Owl.

Edited by Eric D. Forsman, Stephen DeStefano,
Martin G. Raphael and R.J. Gutierrez. 1996. Studies
in Avian Biology, No. 17. v + 122 pp., 40 figures, 42
tables, 1 appendix. ISBN 0-935868-83-6. Paper,
$20.00. — An unprecedented amount of effort has
gone into studying the demographics of the North-
ern Spotted Owl ( Strix occidentalis caurina ), largely
because of controversy surrounding its use of com-
mercially valuable old forests in the Pacific North-
west. This volume results from a 12-d workshop
held in Fort Collins, Colorado in December 1993.
The workshop was requested jointly by the Secre-
taries of Agriculture and Interior for the purpose
of examining all existing demographic data result-
ing from mark-recapture studies of Northern Spot-
ted Owls. Some researchers declined to participate,
but the results of 11 demographic studies con-
ducted by federal and university scientists were in-
cluded in analyses completed at the workshop. To
give some idea of the magnitude of the effort rep-
resented by these studies, the study areas covered
an area of 45 846 km 2 , including an estimated 20%
of the range of the Northern Spotted Owl. A sum-
mary report was prepared following the workshop,
submitted to the aforementioned secretaries and
included as an appendix in land-management
planning documents. This volume expands on that
treatment by including background papers not in-
cluded in the summary report.

The volume is organized in three main sections:
Introduction and Methods, Demography of the
Northern Spotted Owl and Synthesis. It also contains
a comprehensive list of references and an appendix
that lists the symbols and acronyms used in the text.

The introductory section contains three chapters.
“Biology and Distribution of the Northern Spotted
Owl,” by R.J. Gutierrez, provides a brief overview of
natural history. The level of information provided
seems appropriate for this volume. The review is not
intended to be exhaustive, or to supplant existing

literature reviews, so it will add little for readers al-
ready familiar with the owl. “History of Demograph-
ic Studies in the Management of the Northern Spot-
ted Owl,” by R.J. Gutierrez, E.D. Forsman, A.B.
Franklin and E.C. Meslow, provides a good overview
of watershed events in the management and con-
servation of the owl and of how demographic data
were used in msyor planning efforts and decisions.
This interesting and informative chapter makes a
strong case for the utility of demographic data for
management planning and for stimulating new ar-
eas of research. The third chapter discusses “Meth-
ods for Collecting and Analyzing Demographic Data
on the Northern Spotted Owl” (A.B. Franklin, D.R.
Anderson, E.D. Forsman, K.P. Burnham and F.W.
Wagner). This chapter provides an overview of the
demography study areas included in the workshop,
field methods used in those studies and analytical
methods used in the workshop. Methods, underly-
ing assumptions and potential biases are clearly dis-
cussed, making this section highly relevant to read-
ers interested in the Northern Spotted Owl and to
those interested in demographic analysis in general.

The demography section contains nine chapters
that discuss the 11 study areas included in the
workshop (some chapters include two areas). The
list of authors includes most (but not all) of the
researchers who have been heavily involved with
Northern Spotted Owls. Each chapter includes a
section on study area(s), field and analytical meth-
ods (especially where they deviate from the meth-
ods discussed by Franklin et al.), results, discussion
and summary. Thus, there is considerable redun-
dancy. This was unavoidable given the nature of
the material, however.

This demography section is the heart of the vol-
ume in the sense that the chapters present and
discuss the results of the individual studies. The
chapters are interesting and generally well written,
and they contain many details about features
unique to individual studies that are lost in the
summary chapters. Nevertheless, because these
studies parallel each other to such a great extent,
I suspect that the average reader will find the sum-
mary chapters more interesting.

197

198

Book Reviews

Vol. 31, No. 2

The synthesis section consists of two concluding
chapters. In “Meta-analysis of Vital Rates of the
Northern Spotted Owl,” K.P. Burnham, D.R. An-
derson and G.C. White used all of the demograph-
ic data available on the Northern Spotted Owl to
estimate vital rates (survival and fecundity) and
current status of the population. One of the
strengths of the demography studies was that meth-
odology was standardized across studies, so that
data sets could be combined in a meta-analysis (a
technique for combining results of separate but
similar studies) . This analysis has greater statistical
power than analyses based on individual studies
and yielded two lines of evidence suggesting that
the population of Northern Spotted Owls is declin-
ing: an annual rate of population change (X) less
than 1 and a declining trend in survival rates of
adult females. Further, the authors argue that the
spatial extent of the studies combined with repli-
cation across study areas allows inferences derived
from the meta-analysis to be extended to the over-
all population of Northern Spotted Owls, rather
than be restricted to populations of banded birds
on each of the study areas.

These results are extremely important from a
conservation planning standpoint, but the authors
of the chapter on the meta-analysis pointedly re-
frained from speculating on their implications to
land managers. Consequently, a separate set of au-
thors close the volume with a chapter on “Use, In-
terpretation, and Implications of Demographic
Analyses of Northern Spotted Owl Populations”
(M.G. Raphael, R.G. Anthony, S. DeStefano, E.D.
Forsman, A.B. Franklin, R. Holthausen, E.C. Meslow
and B.R. Noon). This chapter provides an excellent
and stimulating discussion of the uses, interpreta-
tion, limitations and potential biases of demograph-
ic data, as well as recommendations for additional
analyses. It should provide abundant food for
thought for land managers and researchers, and the
concepts discussed are relevant to other raptors.

In summary, this is an interesting, useful and im-
portant document. The development of strong re-
search programs and techniques may be the most
positive result of the controversy surrounding the
Northern Spotted Owl and land management. The
demographic studies described in this volume are
unique because of their spatial extent, the inten-
sive long-term sampling (up to nine years) involved
in at least some of the studies and the standardiza-
tion of methods across studies. These studies, many
of which are ongoing, have already made signifi-

cant contributions to land management and to
both applied and theoretical research.

The workshop discussed here also was unique. It
brought together a wealth of expertise on both
Northern Spotted Owls and demographic analysis.
The analytical approach was rigorous and thor-
ough, and methods, assumptions and potential bi-
ases were clearly documented. Consequently, this
volume represents the state-of-the-art in terms of
demographic analysis of Northern Spotted Owls. It
should thus be a valuable reference for anyone
concerned with the biology and conservation of
the Northern Spotted Owl. At least parts of this
document should be of general interest as well, in-
cluding the introductory and synthesis sections.
The methods and concepts discussed are relevant
to anyone interested in analyses of raptor popula-
tions. — Joseph L. Ganey, USDA Forest Service,
Rocky Mountain Forest and Range Experiment Sta-
tion, Flagstaff, AZ 86001 U.SA.

Eagle Studies. Edited by B.-U. Meyburg and R.D.
Chancellor, 1996. World Working Group on Birds
of Prey and Owls, Berlin, Germany, xiii + 549 pp.,
numerous figures and tables, 2 color photographs.
ISBN 3-9801961-1-9. Paper, $30.00.— This volume
contains 64 papers. The first examines genetic dif-
ferentiation in five Aquila species in Europe, and
the final paper discusses satellite tracking of nine
eagle species in Europe, Asia and Africa. The other
studies deal with 10 species (seven papers cover
two species): Osprey ( Pandion haliaetus ; 3 papers),
White-tailed Sea-Eagle ( Haliaeetus albicilla; 18),
Bald Eagle (//. leucocephalus; 2), Steller’s Sea-Eagle
(H. pelagicus', 1), Lesser Spotted Eagle {Aquila po-
marina; 18), Imperial Eagle {A. heliaca; 10), Golden
Eagle (A. chrysaetos ; 8), Greater Spotted Eagle (A.
clanga ; 4), Steppe Eagle (A. nipalensis ; 1) and Bo-
nelli’s Eagle ( Hieraaetus fasciatus; 1). Most of the
papers (46) are in English; 16 are in German and
two in French (the German and French papers
contain brief English summaries) . Papers range in
length from 1 to 44 pages.

June 1997

Book Reviews

199

Most of the papers were presented at three meet-
ings: the International Symposium on the White-
tailed Sea Eagle and the Lesser Spotted Eagle, in
1991 at Zielonka, Poland; the IV World Conference
on Birds of Prey and Owls, in 1992 at Berlin, Ger-
many; and the Third International Meeting of the
Imperial Eagle Working Group, in 1993 at Kiralyret,
Hungary The majority of studies originated in Eu-
rope or Asia, including the former Soviet Union,
the former Eastern Block, Scandinavia, Japan, Pa-
kistan, India and Israel. The editors recognize the
variable quality of the papers and state in the Pref-
ace: “Since a large number of the manuscripts were
not written in their authors’ mother tongue a con-
siderable amount of editorial work was also re-
quired. ...” Considering this enormous task, I
found no difficulty overlooking the occasional punc-
tuation errors, awkwardness and verbosity.

Causes of eagle population declines or extirpa-
tion and partial recoveries in some areas are re-
ported in many of the papers. Conservation pro-
grams described range from the need to protect
nest trees to more extensive problems with forag-
ing or migration habitats. For example, for the
White-tailed Sea-Eagle in Southern Moravia, au-
thors Mrlik and Horak stress: . . the preservation

of sufficiently extensive tall and old forests near
large water surfaces . . . and the elimination of any
wood cutting or other disturbance at breeding sites
during nesting . . . .” In most areas the problems
are complex and broad. Author Rodziewiez re-
ports: “. . .in Poland the nests and their surround-
ings have been protected by law since 1984, and
foresters have a positive attitude towards this pro-
tection, at least in some regions ... in 1987 the Ad-
ministration of State Forests in Olsztyn (northeast-
ern Poland) employed two ornithologists to deal
solely with the protection of rare raptors. Thus
more dangerous are the threats to foraging habi-
tat. The great political and consequent economic
changes will result in development of agricultural
methods with greater intensity. So the problem is
not of protecting individual territories, but one of
general agricultural policy in the regions of highest
Lesser Spotted Eagle density.” The essential im-
portance of sustaining prey populations is interest-
ingly described in several papers. Authors Vlachos
and Papageorgiou characterize the key to the fu-
ture of the Lesser Spotted Eagle in Dadia, Greece,
as habitat management that will continue to sus-
tain a high density of reptiles. Authors Bahat and
Mendelssohn describe fascinating habitat in the Ei-

lat Mountains (southern Negev), Israel, in which
two pairs of Golden Eagles foraged primarily on
the spiny-tailed lizard ( Urornastix aegyptius ) .

The book could be criticized for lacking any ap-
parent theme in a hodgepodge of studies and status
reviews. However, there is interesting information
on eagle behaviors and distributions that previously
has been unknown, or nearly so. The document also
may provide valuable baseline records for many ar-
eas, especially in countries where research has been
difficult to accomplish because of political turmoil.
Authors Abuladze and Eligulashvili describe a con-
servation program they believe is needed for the
White-tailed Sea-Eagle in the Transcaucasus, but la-
ment that the program: “. . . is at present impossible
because of various political, economic, and social
problems.” Author Abuladze describes a long-term
program for raptor conservation in the Republic of
Georgia. He states: “Unfortunately the present po-
litical and economic situation prevents this from be-
ing implemented. In such conditions, problems
concerning wildlife are of little concern. Due to the
most recent political circumstances, the research
group of professional ornithologists has broken up.
One can only hope that the present crisis will not
last for ever, so that one day the work begun can be
continued with greater efficiency.”

Considering the logistical and political difficul-
ties involved in many of the areas, it is impressive
that so much good work has been done. In that
context, I view this book as an apt tribute to per-
severing researchers, naturalists, managers and lay
people who continue to struggle for the apprecia-
tion and protection of raptor populations and
their habitats in Europe and Asia. Although this
book may not be a high priority for personal pur-
chase in most (cases, it should be in university li-
braries. — B. Riley McClelland, P. O. Box 366, West
Glacier, MT 59936 U.S.A.

The Striated Caracara Phalcoboenus australis in
the Falkland Islands. By Ian J. Strange. 1996. Avail-
able from I J. Strange, The Dolphins, Stanley, Falk-

200

Book Reviews

Vol. 31, No. 2

land Islands. 56 pp., 17 color photos, 10 black-and-
white photos, 9 figures, 2 tables. Paper, $17.00. —
Printed on glossy paper with two-column format,
this publication resembles a high-quality brochure.
Seventeen full-page color photographs portray the
“Johnny Rook,” as it is known locally — now on a
promontory surveying a vast colony of albatrosses,
now investigating the author’s rucksack for scraps
of food or bright objects to steal, now a flock of 30
or so devouring the beached carcass of a penguin.
Mr. Strange, the author of a book on the Falklands
(where he has lived most of his life) , has studied
caracaras with assistance from the National Geo-
graphic Society and others. Previous observations
on this bird are scanty; some of the best were by
none other than Charles Darwin.

After summarizing the history and distribution
of P. australis, the latter including small islets off
Cape Horn, Mr. Strange touches upon all aspects
of the general natural history of this bird (e.g.,
nesting, fledging, behavior of adults and imma-
tures, food and foraging habits). Smaller, fully la-
beled photographs, as well as the larger ones in
color, convey much information. Once with a
bounty on its head, or rather beak, the Johnny
Rook now exists in tolerable numbers only on a
few islets where one lands at the risk of shipwreck.
Even on these remote locales, the insatiable de-
mands of the world’s teeming masses for protein
and fuel are despoiling the once limitless seas
down to their very depths. The seabirds and seals
are already showing the effects, and if they go, so
goes the caracara. Ian Strange is among the out-
numbered few who are striving to save at least a
vestige of this wildlife. — Dean Amadon, Ornithol-
ogy Department, American Museum of Natural
History, Central Park West at 79th Street, New
York, NY 10024 U.S.A.

Messages from an Owl. By Max R. Terman. 1996.
Princeton University Press, Princeton, NJ. xi + 217
pp., 66 black-and-white photos, 1 table, 1 appen-

dix. ISBN 0-691-01105-2. Cloth, $24.95.— In this
book, Max Terman provides a chronicle of the first
seven years of interaction between himself and an
“imprinted” Great Horned Owl ( Bubo virginianus ) .
The story remains unfinished, with Terman and
“Stripey” (the name provided to the owl by one of
Terman’s students at Tabor College) still in contact
with each other in a barn on Stripey’s territory
near the Flint Hills of Kansas. The book is remark-
able for several reasons, not least of which is that
Terman, through diligence and luck (and the use
of radio telemetry) , was able to follow Stripey’s life
from inept youth in captivity to membership in the
local breeding population of wild owls.

The book is a refreshing blend of natural history
observation, muse and candor; in the process of
sharing in Stripey’s development we get to know
something about Max Terman. Terman begins his
narrative by taking us with him to the Hillsboro
city park to retrieve a four-wk-old owlet, apparently
abandoned by its parents and now starving. From
there, we follow Stripey and Terman through the
next several years of growth and discovery. Along
the way, Terman shares his thoughts regarding the
behavior and development of Stripey and other
“imprints,” his concerns regarding Stripey’s ability
to fit into owl society once released from captivity,
his frustrations with his undertaking (especially the
ups and downs of using radiotelemetry to follow
Stripey’s movements), and his justified satisfaction
when he finally witnesses Stripey’s wild offspring
after years of effort by Stripey to secure a territory
and mate.

Whether or not an imprinted owl can survive
and reproduce in the wild is a question near the
forefront of Terman’s thoughts, and he belabors
this question throughout the book. By book’s end,
we know that a young captive owl can “make it”
in the wild as an adult, but is Stripey really an owl
version of Konrad Lorenz’s greylag geese? I found
myself questioning if an owl already four wk of age
when exposed to humans is an imprint. I suspect
not. I also question that the avoidance of humans
was wired into Stripey’s system (p. 16). Neverthe-
less, because the book is written almost as a dia-
logue of discovery between Terman and himself,
with the imprint question unfolding in parallel
with the development of Stripey, his thoughts on
this topic (especially regarding the period of fledg-
ling dependency) are informative and fun to fol-
low. And, much to my relief, Terman concludes (p.
146) that many birds considered imprints are ac-

June 1997

Book Reviews

201

tually “deficit birds,” birds not imprinted on hu-
mans but lacking a full repertoire of normal ac-
quired behaviors. Terman feels that many deficit
birds can make it on their own, and as successfully
as wild individuals, if they are carefully monitored
upon release and gradually weaned from feeding
stations while they learn to hunt. Whether or not
this is true, these thoughts should be of special
practical interest to rehabilitators.

A few comments peripheral to the book’s main
topic are incorrect. The Sutton Avian Research
Center near Bartlesville, Oklahoma is not a raptor
rehabilitation center, but was a center for raptor
research, especially the reestablishment of the
Southern Bald Eagle ( Haliaeetus l. leucocephalus ) ,
and has since expanded its work to include popu-
lation-level studies of prairie birds. Margaret Morse
Nice was not “reborn as a naturalist” in Columbus,
Ohio, but experienced her ornithological epiph-
any several years earlier on the banks of the Ca-
nadian River near Norman, Oklahoma. These in-
accuracies detract not a whit from Terman’s story
of Strip ey.

The book is mostly free of typos; I noted only
one (“opprotunity”, p. 190). The black-and-white
photographs enhance the narrative (who else has
pictures of Stripey on his territory in Kansas?) and
help make the bird and setting more personal.

Some, such as Stripey catching food (hot dogs and
such) in the air (p. 76), are downright enlighten-
ing, especially for people unfamiliar with the ability
of Great Horned Owls to capture bats on the wing.
A few photographs are of such poor quality, how-
ever, that I question whether they should have
been published. For instance, the leg-mounted
transmitter shown on p. 17 is difficult to discern in
the fuzzy photograph, leaving someone unfamiliar
with such devices still unfamiliar; the photograph
of the author radio-tracking Stripey (p. 84) also
contributes litde. The computer-generated map of
Terman’s farm and surroundings (where Stripey
was released) helps the reader put place-names
mentioned in the text in geographic perspective
(although I don’t understand why north was ori-
ented to the right), but the visual attractiveness of
the map suffers.

Messages from an Owl is entertaining, educational
and exposes the reader to the thought processes
of a scientist trying to get answers to some per-
plexing questions. It should appeal to the same
professional and lay readership that enjoyed Bernd
Heinrich’s One Man’s Owl, although the story of
Stripey is carried much farther than that of
“Bubo.” Also, there is a “surprise ending” that I
won’t divulge. — Paul Hendricks, Montana Natural
Heritage Program, 909 Locust Street, Missoula,
MT 59802 U.S.A.

THE RAPTOR RESEARCH FOUNDATION, INC.

(Founded 1966)

OFFICERS

PRESIDENT: David M. Bird SECRETARY: Betsy Hancock

VICE-PRESIDENT: Michael N. Kochert TREASURER: Jim Fitzpatrick

BOARD OF DIRECTORS

EASTERN DIRECTOR: Brian A. Millsap
CENTRAL DIRECTOR: Robert N. Rosenfield
MOUNTAIN & PACIFIC DIRECTOR:

Karen Steenhof

CANADIAN DIRECTOR: Gordon S. Court
INTERNATIONAL DIRECTOR #1:

Jemima ParryJones

INTERNATIONAL DIRECTOR #2:

Michael McGrady

DIRECTOR AT LARGE #1: Patricia L. Kennedy
DIRECTOR AT LARGE #2: John A. Smallwood
DIRECTOR AT LARGE #3: Keith L. Bildstein
DIRECTOR AT LARGE #4: CLsar MArquez
DIRECTOR AT LARGE #5: Petra Bohall Wood
DIRECTOR AT LARGE #6: Katherine McKeever

EDITORIAL STAFF

EDITOR: Marc J. Bechard, Department of Biology, Boise State University, Boise, ID 83725 U.S.A.

ASSOCIATE EDITORS

Keith L. Bildstein Fabian Jaksic

Gary R. Bortolotti Daniel E. Varland

Charles J. Henny Javier Bustamente

BOOK REVIEW EDITOR: Jeffrey S. Marks, Montana Cooperative Research Unit, University of Montana,

Missoula, MT 59812 U.S.A.

The Journal of Raptor Research is distributed quarterly to all current members. Original manuscripts
dealing with the biology and conservation of diurnal and nocturnal birds of prey are welcomed from
throughout the world, but must be written in English. Submissions can be in the form of research articles,
letters to the editor, thesis abstracts and book reviews. Contributors should submit a typewritten original
and three copies to the Editor. All submissions must be typewritten and double-spaced on one side of 216
X 278 mm (816 X 11 in.) or standard international, white, bond paper, with 25 mm (1 in.) margins. The
cover page should contain a title, the author’s full name(s) and address (es). Name and address should be
centered on the cover page. If the current address is different, indicate this via a footnote. A short version
of the title, not exceeding 35 characters, should be provided for a running head. An abstract of about
250 words should accompany all research articles on a separate page.

Tables, one to a page, should be double-spaced throughout and be assigned consecutive Arabic numer-
als. Collect all figure legends on a separate page. Each illustration should be centered on a single page
and be no smaller than final size and no larger than twice final size. The name of the author (s) and figure
number, assigned consecutively using Arabic numerals, should be pencilled on the back of each figure.

Names for birds should follow the A.O.U. Checklist of North American Birds (6th ed., 1983) or another
authoritative source for other regions. Subspecific identification should be cited only when pertinent to
the material presented. Metric units should be used for all measurements. Use the 24-hour clock (e.g.,
0830 H and 2030 H) and “continental” dating (e.g., 1 January 1990).

Refer to a recent issue of the journal for details in format. Explicit instructions and publication policy
are oudined in “Information for contributors,”/. Raptor Res., Vol. 27(4), and are available from the editor.

1997 ANNUAL MEETING

The Raptor Research Foundation, Inc. 1997 annual meeting will be hosted by Georgia Southern
University and will be held October 30 through November 2 at the Marriott Riverfront in Savan-
nah, Georgia. Details about the meeting and a call for papers will be mailed to Foundation
members in the spring of 1997. For more information, contact Michelle Pittman (912/681-5555,
e-mail: meeden@gsvms2.cc.gasou.edu) or Steve Hein (912/681-0831) at Georgia Southern Uni-
versity.

Raptor Research Foundation, Inc., Awards
Recognition for Significant Contributions 1

The Dean Amadon Award recognizes an individual who has made significant contributions in the field of
systematics or distribution of raptors. Contact: Dr. Clayton White, 161 WIDE, Department of Zoology,
Brigham Young University, Provo, UT 84602 U.S.A. Deadline August 15.

The Tom Cade Award recognizes an individual who has made significant advances in the area of captive
propagation and reintroduction of raptors. Contact: Dr. Brian Walton, Predatory Bird Research Group,
Lower Quarry, University of California, Santa Cruz, CA 95064 U.S.A. Deadline: August 15.

The Fran and Frederick Hamerstrom Award recognizes an individual who has contributed significantly to
the understanding of raptor ecology and natural history. Contact: Dr. David E. Andersen, Department
of Fisheries and Wildlife, 200 Hodson Hall, 1980 Folwell Avenue, University of Minnesota, St. Paul,
MN 55108 U.SA. Deadline: August 15.

Recognition and Travel Assistance

The James R. Koplin Travel Award is given to a student who is the senior author of the paper to be
presented at the meeting for which travel funds are requested. Contact: Dr. Petra Wood, West Virginia
Cooperative Fish and Wildlife Research Unit, P.O. Box 6125, Percival Hall, Room 333, Morgantown,
WV 26506-6125 U.SA. Deadline: established for conference paper abstracts.

The William C. Andersen Memorial Award is given to the student who presents the best paper at the annual
Raptor Research Foundation Meeting. Contact: Ms. Laurie Goodrich, Hawk Mountain Sanctuary, Rural
Route 2, Box 191, Rempton, PA 19529-9449 U.SA Deadline: Deadline established for meeting paper
abstracts.

Grants 2

The Stephen R. Tully Memorial Grant for $500 is given to support research, management and conservation
of raptors, especially to students and amateurs with limited access to alternative funding. Contact: Dr.

Kimberly Titus, Alaska Division of Wildlife Conservation, P.O. Box 20, Douglas, AK 99824 U.SA Dead-
line: September 10.

The Leslie Brown Memorial Grant for $500-$l,000 is given to support research and/or the dissemination
of information on raptors, especially to individuals carrying out work in Africa. Contact: Dr. Jeffrey L.
Lincer, P.O. Box 1675, Valley Center, CA 92082 U.SA Deadline: September 15.

1 Nominations should include: (1) the name, tide and address of both nominee and nominator, (2) the
names of three persons qualified to evaluate the nominee’s scientific contribution, (3) a brief (one page)
summary of the scientific contribution of the nominee.

2 Send 5 copies of a proposal (<5 pages) describing the applicant’s background, study goals and methods,
anticipated budget, and other funding.

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