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The

Journal

Research

Volume 31

September 1997

Number 3

Contents

A Raptor Survey in the Brazilian Atlantic Rainforest. Santi Manosa and

Vittorio Pedrocchi 203

Breeding Density and Landscape-level Habitat Selection of Common
Buzzards {Buteo buteo) in a Mountain Area (Abruzzo Apennines, Italy).

Vincenzo Penteriani and Bruno Faivre 208

Variations in Breeding Bald Eagle Responses to Jets, Light Planes and

HELICOPTERS. Teryl G. Grubb and William W. Bowerman 213

Productivity of Golden Eagles Wearing Backpack Ratiotransmitters. John

M. Marzluff, Mark S. Vekasy, Michael N. Kochert and Karen Steenhof 223

Crested Caracara Food Habits in the Cape Region of Baja California,

MEXICO. Ricardo Rodriguez-Estrella and Laura B. Rivera Rodriguez 228

Remarkable Saker Falcon (Falco cherrug ) Breeding Records for Mongolia.

David H. Ellis, Merlin H. Ellis and Pu. Tsengeg 234

Spatial Incidence of Barred Owl (Strixvaria) Reproduction in Old-

growth Forest of the Appalachian Plateau, j. Christopher Haney 241

Habitat Associations of the Barred Owl in the Boreal Forest of

SASKATCHEWAN, Canada. Kurt M. Mazur, Paul C. James, Michael J. Fitzsimmons, Gido

Langen and Richard H. M. Espie 253

The Winter Roosting Behavior of Eastern Screech-owls in Central

KENTUCKY. Tara A. Duguay, Gary Ritchison and Jeffrey P. Duguay 260

Nutrient Content of Five Species of Domestic Animals Commonly Fed to

Captive Raptors. Nancy j. cium 267

Short Communications

Juvenal Plumage Characteristics of Male Southeastern American Kestrels {Falco sparverius

paulus). Karl K. Miller and John A. Smallwood 273

Double Brooding by American Kestrels in Idaho. Karen Steenhof and Brit E. Peterson.... 274

First Nest Record of the Bare-shanked Screech-owl (Otus clarkii) . Paula L. Enriquez Rocha,

J. Luis Rangel-Salazar and Joe T. Marshall 276

The Summer Diet of the Little Owl (Athene noctua) on the Island of Astipalaia
(Dodecanese, Greece). Francesco M. Angelici, Leonardo Latella, Luca Luiselli and

Francesco Riga 280

Home Range, Habitat Use and Natal Dispersal of Blakiston’s Fish-owls. Yuko Hayashi 283

Letters 286

BOOK Reviews. Edited by Jeffrey S. Marks 290

Abstracts of Presentations Made at the Annual Meeting of the Raptor
Research Foundation, Inc., Held at Gainesville, Florida, 1986 293

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 September 1997 No. 3

J. Raptor Res. 31 (3):203-207
© 1997 The Raptor Research Foundation, Inc.

A RAPTOR SURVEY IN THE BRAZILIAN ATLANTIC RAINFOREST

Santi Manosa and Vittorio Pedrocchi

Departament de Biologia Animal, Facultat de Biologia, Universitat de Barcelona,

Avinguda Diagonal 645,

08028 Barcelona, Catalonia, Spain

Abstract. — We give the results of a raptor survey conducted in August 1994 in the Porque Estadual
Intervales, a well preserved area of Atlantic rainforest in southeastern Brazil. Point counts were more
effective than transect counts. Ten species of raptors were detected. The presence of a pristine popu-
lation of Mantled Hawks ( Leucoptemis polionota). Black Hawk-eagles ( Spizaetus tyrannus) and Ornate
Hawk-eagles (5. ornatus), gives to the area a remarkable interest for the conservation of birds of prey
in Brazil. We also observed Turkey Vultures ( Cathartes aura), Black Vultures ( Coragyps atratus, Tiny Hawks
(Accipiter superciliosus) , Roadside Hawks ( Buteo magnirostris), Short-tailed Hawks (Buteo brachyurus) , Yellow-
headed Caracaras ( Milvago chimachima) , Collared Forest-falcon ( Micrastur semitorquatus ) and possibly
Grey-headed Kites ( Leptodon cayanensis ).

Key Words: Atlantic rainforest, birds of prey, Brazil.

Sequimiento de Rapaces en la Selva Atlantica de Brasil

Resumen. — Se dan los resultados de un seguimiento de rapaces efectuado en agosto de 1994 en el
Parque Estadual Intervales, una zona bien preservada de selva atlantica del sureste de Brasil. Los censos
puntuales se mostraron mas eficaces que los censos lineales. Se detectaron 10 especies seguras de
rapaces. En particular, destaca la presencia de una poblacion saludable de busardo blanquinegro Leu-
copternis polionota, aguila-azor negra Spizaetus tyrannus y aguila-azor galana S. ornatus, lo cual dota a este
area de un notable interes para la conservacion de aves de presa en Brasil. Se observaron tambien el
aura gallipavo Cathartes aura, el zopilote negro Coragyps atratus, el gavilancito americano Accipiter super-
ciliosus, el busardo caminero Buteo magnirostris, el busardo colicorto Buteo brachyurus, el caracara chi-
machima Milvago chimachima, el halcon-montes collarejo Micrastur semitorquatus, y posiblemente el mil-
ano cabecigris Leptodon cayanensis.

[Traduccion Autores]

The Brazilian Atlantic rainforest is considered
among the areas of highest avian endemism in
South America (Cracraft 1985). However, less
than 8% of the original forest is left, and the
remaining forest patches are small and isolated
from one another (Fonseca 1985, Alburquerque
1995, Fundagao SOS Mata Atlantica 1995). Birds
of prey can be good ecological indicators of the
conservation value of these patches, because
some species require large amounts of well-pre-
served habitat to survive, while others increase
in human-altered habitats. Although some spe-

cies are threatened by habitat fragmentation and
destruction (Thiollay 1985), difficulties faced
when studying rainforest raptors (Thiollay 1989)
limit the information needed to design good
conservation strategies. Several monitoring and
research programs are being conducted in the
Neotropical region to fill this gap (Thiollay 1989,
Vannini 1989, Whitacre and Thorstrom 1992),
but the Atlantic rainforest has received little at-
tention. In this paper, we present the results of
a pilot survey conducted in an Atlantic rainforest
area of southeastern Brazil from 1—12 August

203

204

Manosa and Pedrocchi

Vol. 31, No. 3

Figure I. Map of the Parque Estadual Intervales, show-
ing the location of the areas covered by transect counts
(shaded), the location of the point counts (a, b, c, d)
and the possible location of Manded Hawk breeding ter-
ritories (O). The location of the Parque Estadual Inter-
vales Information Center (Sede) and Barra Grande
station are also shown.

1994 to provide baseline data for future moni-
toring and conservation programs.

Study Area and Methods

The survey was conducted in the Parque Estadual In-
tervales, a natural reserve comprised of 383 km 2 of con-
tinuous mature and secondary Atlantic rainforest (Fig.
1 ) . The reserve is situated at the southeastern portion of
the State of Sao Paulo, 80 km from the coast (24°20'S,
48°15'W), and is part of a mountain range about 900 km
long known as Serra do Mar. The Parque Estadual Inter-
vales, together with neighboring protected and private
land (Parque Estadual Turistico do Alto Ribeira-Petar, Es-
ta^ao Ecologica de Xitue and Parque Estadual de Carlos
Botelho), constitutes a 1168 km 2 area of well-preserved
habitat. The area receives as much as 2500 mm of annual
rainfall, concentrated mainly between November-March.
The reserve is not hunted, and the palmito Euterpe edulis,
one of the main components of the forest, is no longer
being exploited. The forest covers all the reserve, except
small openings around hamlets and guard stations. Ma-
ture or nearly mature forest communities cover 40% of
the reserve, 40% is covered by old secondary forest, and
20% by young secondary forest found mainly along the
roads and around inhabited areas (J.C. Guix pers.
comm.). Areas around the Parque Estadual Intervales are
agricultural land and grassland.

Three areas within the reserve were surveyed (Fig. 1).
The Alecrim area ranging from 150-600 m elevation con-
sisted of old secondary forest and included a small ham-
let surrounded by pastures and crops. The Sao Pedro
area (350-860 m elevation) was covered by mature and

old secondary forest. The Fund area (40-150 m eleva-
tion) was covered by old secondary forest on the hill
slopes and young secondary forest on the lowest areas
near to agricultural areas. For a detailed accouj^jjfplant
communities found in Intervales see Guix et al. (1992).

Raptor counts were conducted using transect and
point count methods. No playback techniques (Whitacre
and Thorstrom 1992) were used. We conducted 68 tran-
sect surveys on foot with the aim of recording monkeys,
toucans, guans and birds of prey. Although transects were
not specifically designed to count raptors, they allowed
us to obtain an index of detections/km for several spe-
cies. Transect lengths ranged from 1—22 km, but most
were 2-3 km long (x = 4.3 km, SD = 4.0). Except the
longest transect that required a full day to complete,
most surveys were conducted just after dawn or before
dusk and lasted for 1.5—2 hr. The spatial arrangement of
the transects was determined by the distribution of foot
paths laid out by the guard staff to survey the reserve,
but we felt it was representative of all the area. Transect
counts were conducted by teams of 2—6 people. The
weather was variable between counts, from clear to slight-
ly rainy.

Point counts followed the method described by Whit-
acre et al. (1992). Counts were conducted in clear and
calm weather by two observers from elevated points of
the landscape, with a view angle of 60°-145°, and an un-
bounded view radius of at least 1 km. We selected points
along the main tracks, offering good visibility of different
rainforest areas. One count (a) was conducted from a
midslope road in the Alecrim area and the other three
(b, c, d) were conducted in the Sao Pedro area. Counts
in the Sao Pedro area were done from the top of emer-
gent trees that were about 2 km apart and gave unob-
structed views of three different valleys. Counts were ini-
tiated 2. 5-4.5 hr after dawn and lasted for 3-4 hr. The
counting period was divided into 5 min intervals. For ev-
ery interval, all raptors seen were recorded. Using this
method, we obtained a list of species, the minimum num-
ber of groups and individuals observed and the propor-
tion of 5-min intervals in which a species was recorded.

Results and Discussion

On 68 transects, we walked a total of 290 km and
made observations for 121 hr. We recorded birds
of prey on 15 occasions (0.12 contacts/hr) for a
total of 26 individuals of five different species. Rap-
tors were observed on only 12 (17%) of the tran-
sects (Table 1). Mantled Hawks (Leucopternis poh-
onota) were observed in the Alecrim area along the
Piloes-Formoso river, between Alecrim and Sede.
One pair was observed 6 km from Alecrim and an-
other three hawks were observed simultaneously 8
km further along the river. In both cases, the birds
were heard calling and were observed perching in
small forested areas. We concluded that at least
three or four pairs of Mantled Hawks inhabited the
14 km of the Piloes-Formoso river valley that we
surveyed (Fig. 1).

September 1997

Atlantic Rainforest Raptors

205

Table 1. Summary of the results of transect counts in Brazilian Atlantic rainforest. Each figure corresponds to the
number of individuals in one group. Numbers in brackets represent numbers of individuals counted on the same
transect.

Alecrim

Sao Pedro

Funil

# of transects

Total length (km)

121

137

Habitat type

Old 3

Mature b

Young 0

Leucoptemis polionota

(2,1) (1) (2) (1,1)

—

(1)

Buteo brachyurus

—

(1)

—

Buteo magnirostris

(1)

—

Cathartes aura

(1) (1) (1)

—

Coragyps atratus

(7) (2,3)

—

a Old secondary forest.
b Mature or nearly mature forest.
c Young secondary forest.

We conducted four point counts totalling 14.5
hr of observation and 174 5-min census intervals.
Five raptor observations, involving 19 individuals
of four raptor species (0.34 contacts/hr), occurred
during three of the point counts. Of the 174 5-min
census intervals, Black Vultures ( Coragyps atratus)
were seen during seven (4%), Manded Hawks dur-
ing 28 (16%), Tiny Hawks ( Accipiter superciliosus)
during one (0.6%) and Ornate Hawk-eagles (Spi-
zaetus ornatus) during one (0.6%). At point count
b, a pair of Manded Hawks was recorded flying and
perching in a small area of the forest for more
than half the observation period and a single Man-
ded Hawk was observed flying over the forest at
point count c (Table 2, Fig. 1).

Between counts, we also recorded a Black Hawk-
eagle ( Spizaetus tyrannus) flying over the Sao Pedro
region, and a Collared Forest-falcon {Micrastur sem-
itorquatus ) and a possible Gray-headed Kite ( Lepto -
don cayanensis ) in the Fund area. Yellow-headed
Caracaras ( Milvago chimachima) were frequendy
seen in the Sede area in open habitats within the
boundary of the reserve. One Manded Hawk was
recorded in the Barra Grande area near Sede and
a second was seen next to the Sao Pedro station

(Fig. 1).

No raptors were observed during a large pro-
portion of the transect counts and, overall, they
yielded fewer observations per unit time than did
point counts. Although foot surveys detected some

Table 2. Summary of the results of the four raptor point counts, totalling 14.5 hr (174 5-min intervals) conducted
in Brazilian Atlantic rainforest. For each species and count, the proportion of time intervals in which the species was
seen, and the number of groups and individuals (groups, individuals) observed, are given. Habitat types are the same
as in Table 1.

Point A

Point B

Point G

Point D

Area

Alecrim

S. Pedro

Elevation

455 m

615 m

500 m

Date

2 Aug

9 Aug

10 Aug

Solar time (H)

0915-1300

0930-1330

1030-1330

0900-1245

Type

Road

Tree

Habitat type

Old

Mature

Duration

225 min

240 min

180 min

225 min

# intervals

Coragyps atratus

13% (2,9)

2% (1,4)

—

Leucoptemis polionota

—

56% (1,2)

3% (1,1)

—

Accipiter superciliosus

—

3% (1,2)

—

Spizaetus ornatus

—

3%(1,1)

—

206

Manosa and Pedrocchi

Vol. 31, No. 3

species not recorded on point counts, these would
have also probably been detected if more point
counts had been conducted. Except in the Alecrim
area, where the transect followed a road with good
views, the foot surveys were inside the forest where
viewing raptors proved difficult due to dense veg-
etation. In fact, most species found during the
transect surveys were not typical forest raptors ( Bu -
teo, Gathartes, Coragyps), and were seen above the
canopy or in openings next to the road or hamlets.
Although an extra amount of time and effort was
needed to find good census trees and to climb
them, the point count method allowed us to stan-
dardize the counts. However, secretive forest-dwell-
ing raptors also escaped detection in our point sur-
veys, probably because no playback techniques
were used.

Of the species recorded in Intervales, records of
Mantled Hawks were most important due to the
fact that there is very little information on this At-
lantic rainforest endemism. Its breeding range ex-
tends along the Atlantic coast of Brazil from Bahia
to eastern Uruguay and Paraguay (del Hoyo et al.

1994) . Mountain habitats upon which this species
relies have quickly disappeared because of defor-
estation. For this reason, the Mantled Hawk, which
was listed as a species of unknown status (Thiollay
1985, IUCN 1990), is now listed as an endangered
(Thiollay 1994) or near-threatened (Collar et al.
1992, del Hoyo et al. 1994) species. All Mantled
Hawks we observed were in adult plumage and
their calling behavior suggested that the second
half of the winter or dry season corresponded to
the early portion of its nesting season in this area.
This species was also reported in four out of the
seven Sao Paulo State Atlantic rainforest areas vis-
ited by Willis & Oniki (1981), in the Serra do Ta-
buleiro on Santa Catarina State (Alburquerque

1995) , and in different areas of disturbed and un-
disturbed habitats in Rio Grande do Sul, where it
is reported as rare (Alburquerque 1986).

The Black Hawk-eagle and Ornate Hawk-eagle
are typical large rainforest raptors. The Ornate
Hawk-eagle has a higher preference for mature for-
ests than the Black Hawk-eagle. Both species were
found in the Sao Pedro area, which is the most
remote of the sites we surveyed and the one with
the least amount of disturbed forest habitats.

The Tiny Hawk and the Collared Forest-falcon
were new records for the Intervales area and for
the Atlantic mountain rainforest of the Sao Paolo
State (Guix et al. 1992, Willis and Oniki 1981). If

we include the Barred Forest-falcon ( Micrastur ruf-
icollis) which was recorded during previous surveys
(Guix et al. 1992), 12 species of raptors have now
been reported in the Parque Estadual Intervales.
A total of only 15 species was found during an ex-
tensive ornithological survey of seven Atlantic rain-
forest areas of Sao Paulo State (Willis and Oniki
1981). Since no more than 20 diurnal raptor spe-
cies are possible in the region (del Hoyo et al.
1994), we concluded that the Parque Estadual In-
tervales still contains a raptor community represen-
tative of the Atlantic rainforest and the area de-
serves protection from further fragmentation and
destruction.

Acknowledgments

We are grateful to J.C. Guix, who made the expedition
possible and helped us at different stages of the work.
We are also in debt to C. Lopez for teaching us tree-
climbing techniques and for help during the counts.
Thanks are also due to the Parque Estadual Intervales,
especially to Katia Pisciotta and to the guard staff for lo-
gistic support. We also thank A. Calle, P. Canti, M.A. Car-
retero, M. Gonzalez-Martin, O. Gonzalez-Moreno, J. Gon-
zalez-Solis, M.J. Hornero, G. Llorente, S, Lope, C. Lopez,
J.M. Masco, E. Mateos, A. Montori, M. Ontanon, M. Pas-
cual, A. Perez, V. Roca, X. Santos, F.L. de Souza, M.J.
Vargas and M. Ventura for their contribution to the
counts.

Literature Cited

Alburquerque, J.L.B. 1986. Conservation and status of
raptors in southern Brazil. Birds of Prey Bull. 3:88-94.

. 1995. Observations of rare raptors in southern

Atlantic rainforest of Brazil, f. Field Ornithol. 66:363-
369.

Collar, N.J., L.P. Gonzaga, N. Krabble, A. Madrono-
Nieto, L.G. Naranjo, T.A. Parker and D.C. W t ege.
1992. Threatened birds of the Americas. The ICBP/
IUCN Red Data Book. ICBP, Cambridge, UK.
Cracraft, J. 1985. Historical, biogeography and patterns
of differentiation within the South American avifau-
na: areas of endemism. Pages 49-84 in P.A. Buckley,
M.S. Foster, E.S. Morton, R.S. Ridgely and F.G. Buck-
ley [Eds.], Neotropical ornithology. Ornith. Monog.
36. AOU, Washington, DC U.S.A,

Del Hoyo, J., A. Elliot and J. Sargatal [Eds.]. 1994.
Handbook of the birds of the world, Vol. 2. New
World vultures to guineafowl. Lynx Edicions, Barce-
lona, Spain.

Fonseca, G.A.B. 1985. The Vanishing Brazilian Atlantic
Forest. Biol. Conserv. 34:17-34.

Fundaijao SOS Mata Atlantica. 1995. Mata atlantica
ameayada. Bol. Especial SOS Mata Atlantica.

Guix, J.C. , A.A.J. Tabanez, A.N. Silva, C. Lopez, C. Mar-
tinez, E. Matheu, F.L. de Souza, KR. Pisciotta, N.
Bradbury and W.G. Portilho. 1992. Viagem de re-

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Atlantic Rainforest Raptors

207

conhecimento cientifico a algumas areas desconheci-
das da Fazenda Intervales, Estado de Sao Paulo, dur-
ante o periodo de 04 a 16 outubro de 1991. Grupo
Estud. Ecol. Ser. Doc. 4:38-94.

IUCN. 1990. IUCN red list of threatened animals. IUCN,
Gland, Cambridge, UK

Thiollay, J.M. 1985. Falconiforms of tropical rain forest:
a review. Pages 155—165 in I. Newton and R.D. Chan-
cellor [Eds.], Conservation studies on raptors, Vol. 5.
ICBP Tech. Publ., Cambridge, UK

. 1989. Censusing of diurnal raptors in a primary

rain forest: comparative methods and species detect-
ability. / Raptor Res. 23(3):72-84.

. 1994. A world review of tropical forest raptors.

Current trends, research objectives and conservation
strategy. Pages 231-239 in B.U. Meyburg and R.D.
Chancellor [Eds.], Raptor conservation today. Pica
Press, Berlin, Germany.

Vannini, J.P. 1989. Neotropical raptors and deforesta-
tion: notes on diurnal raptors at finca El Faro, Quetz-
altenango, Guatemala./, Raptor Res. 23 (2) :2V— 38.

Whitacre, D.F. and R.K. Thorstrom [Eds.]. 1992. Proy-
ecto Maya: Uso de aves rapaces y otra fauna como
indicadores del medio ambiente, para el diseho y ma-
nejo de areas protegidas y para fortalecer la capacidad
local para la conservacion en America Latina. The
Peregrine Fund, Inc., Boise, ID U.S.A.

, L.E. Jones and J. Sutter. 1992. Censos de aves

rapaces y de otras aves en el bosque tropical: mejoras
hechas a la metodologia. Pages 43-56 in D.F. Whitacre
and R.K Thorstrom [Eds.], Proyecto Maya: Uso de
aves rapaces y otra fauna como indicadores del medio
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Received 12 July 1996; accepted 4 May 1997

J. Raptor Res. 31(3):208-212
© 1997 The Raptor Research Foundation, Inc.

BREEDING DENSITY AND LANDSCAPE-LEVEL HABITAT
SELECTION OF COMMON BUZZARDS ( BUTEO BUTEO) IN A
MOUNTAIN AREA (ABRUZZO APENNINES, ITALY)

Vincenzo Penteriani

Laboratoire d’Ecologie, Universite de Bourgogne, Batiment "Mirande,” B.P. 138, 21004 Dijon Cedex, France and
Stazione Romana Osservazione e Protezione Uccelli ( S.RO.P.U .), c/o Oasi Naturale W.W.F. “Bosco di Palo",

00055 Ladispoli, Rome, Italy

Bruno Faivre

Laboratoire d’Ecologie, Universite de Bourgogne, Batiment "Mirande," B.P. 138, 21004 Dijon Cedex, France

Abstract. — The breeding density and landscape-level habitat selection of Common Buzzards ( Buteo
buteo ) was studied from 1989-93 in a mountain area of Abruzzo Apennines (central Italy). Analysis of
landscape features was based on circular plots (2.5 km diameter) centered on occupied nest trees. A
total of 32 Common Buzzard nesting territories were identified within a 387 km 2 area (8.3 pairs/100
km 2 , mean nearest-neighbor distance 2.5 km). The average altitude of the nest sites was 1399 m above
sea level and 73.1% were oriented NE. Stepwise discriminant function analysis showed significant dif-
ferences between nesting (N = 17) and control sites (N = 15) based on four landscape variables: relief
index, distance from forest edge, distance from paved road and distance from valley bottom. Results
suggest that Common Buzzards select nesting areas in the eastern portion of forests which are distant
from roads but close to valley bottoms, in rugged areas of irregular morphology.

Key WORDS: Common Buzzard-, habitat selection; landscape-level, breeding density; Buteo buteo.

Densidad de crfa y seleccion niveles de paisaje habitat en Buteo buteo

Resumen. — La delicadeza de crfa y nivel del paisaje, seleccion del habitat del Buteo buteo fue estudiado
de 1989-93 en un area de montana de Abruzzo Apennines (central Italy). Analisis del elementos del
paisaje estuvo basado en lugares circulos (2.5 km diametro) centrado en nidos de arbol ocupados. Un
total de 32 B. buteo territories de nido fueron identificados dentro de 387 km2 area, (8.3 pares/100
km 2 , media cerca-vecino distancia 2.5 km). El altitud regular de los nidos fue 1399 m arriba del mar y
73.1% fueron orientados NE. Una funcion discriminante de pasos enseno un analisis con diferencias
significas entre nidos ( N = 17) y sitios de control (N = 15) basados en cuatro paisajes variados: relevo
indicie, distancia de la orilla del bosque, distancia del camino pavimentado y distancia del fondo del
valle. Resultados Sugieren que el B. buteo selecciona areas de nidos en lugares este en el bosque donde
estan muy lejos de caminos pero mas cerca al fondo del valle, en areas toscas de morfologia irregular.

[Traduccion de Raul De La Garza, Jr.]

Nest-site preferences have been described for
the Common Buzzard ( Buteo buteo) (Tubbs 1967,
Glutz von Blotzheim et al. 1971, Tubbs 1974, Arce
Velasco 1987, Taylor et al. 1988), but few studies
have attempted to quantitatively determine the fac-
tors involved in nest-site selection at a landscape
level (Newton et al. 1982, Jedrzejewski et al. 1988,
Kostrzewa 1989, Hubert 1993, Hohmann 1994,
Graham et al. 1995, Cerasoli and Penteriani 1996).
All of these studies have analyzed nest-site selection
at a microhabitat level (nest-tree characteristics
and stand structure) without considering the pos-

sible effects of landscape structure. In this paper,
we present a landscape-level analysis of Common
Buzzard nest sites, which was conducted to identify
the landscape determinants of nest-site selection.

Methods

A population of Common Buzzards was studied from
1989-93 in a mountain area of central Italy (Abruzzi Ap-
ennines). The study covered a 387 km 2 area of beech
( Fagus sylvatica ) forest (typical of the Apennine massifs of
the Abruzzi region) that covers the National Park of
Abruzzi and the Sirente mountains. Elevation of the area
ranges from 1000-2340 m. The landscape has a distinct
mosaic structure with large woodland areas and reforest-

208

September 1997

Landscape-level Habitat Selection by Buzzards

209

ed tracts of Pinus nigra, cropland, pastures and fallow
land from 1000-1800 m elevation.

Occupied nests were located by systematic foot search-
es of the area prior to leafout. We also used playbacks of
recorded Common Buzzard calls during the months of
March-April (prelaying period) and Jun e-July (nestling
and fledgling periods) (Cerasoli &: Penteriani 1992). Ar-
eas where a pair of Common Buzzards was observed dur-
ing the breeding period, but no nest was found, were
classified as possible nesting territories (Jedrzejewski et
al. 1994). A number of nesting territories were identified
by observing adults carrying nesting material, by noting
where the displays of males ended with steep dives into
the woods (Picozzi and Weir 1974) and from alarm calls
of adults and shrill calls of the fledged young.

We used the nearest-neighbor distance method (New-
ton et al. 1977) to estimate nesting density. Regularity in
nest-site spacing was computed with a G-test (Brown &
Rothery 1978). Landscape-level analysis of habitat selec-
tion only considered those Common Buzzard nest sites
where nesLs had been located. Moreover, all nest sites
that changed during the study period due to road build-
ing, cutting of forest tracts or changes in farming were
excluded from the analysis. Analysis of landscape features
was based on circular plots centered on the occupied
nest tree. These plots had a diameter equal to the mean
distance between neighboring nest sites. Each nest site
was characterized using a set of 23 variables: slope ex-
posure, elevation, eight variables describing patch com-
position of the landscape (percentage of woodlands, pas-
tures, fallow land, fallow land with trees, rocks, crops,
crops with trees and built-up patches) , three variables for
horizontal heterogeneity (number of ecotones, number
of different habitats calculated on two orthogonal axes
from the plot center and patch interspersion index [hab-
itat changes/plot area] x 100, calculated on two orthog-
onal axes from the plot center; Baxter and Wolfe 1972) ,
two variables for vertical heterogeneity (maximum differ-
ence in elevation and relief index calculated as the sum
of the number of contour lines crossed by two orthogo-
nal axes from the plot center; Janes 1985, Litvaitis et al.
1994) , and eight variables for distance of nest sites from
surrounding landscape components (forest opening, for-
est edge, valley bottom, built-up area, paved road, path-
ways, cliffs, permanent water) . The number of ecotones,
number of habitats and the interspersion relief indexes
were sampled on two straight lines oriented N-S and W-E
along the plot diameters. Areas of each of the different
habitats were determined on the basis of land use maps
to a scale of 1:25 000. For each nest site, one control plot
was established where we measured the same variables as
in nest site plots, except for slope exposure and elevation
to estimate landscape selection. Each control plot was
centered around a random point located between nest-
site plots. To qualify as control plot, the plot had to lie
within a forested area. Plots which did not have woodland
areas or which had only young plantation areas (where
Common Buzzards do not nest) were not included in our
analysis (Hubert 1993, Jedrzejewski et al. 1994).

Landscape characteristics of nest-site and control plots
were compared by using a stepwise discriminant function
analysis (DFA, Sokal and Rohlf 1981). We used the 5%
level of significance for including variables in each step

of the analysis. The classification of the described sites,
obtained with DFA, was tested with Kappa statistic (Titus
and al. 1984). The robustness of the nest-site selection
model was tested with a jack-knife procedure. We used a
chi-square test to analyze the selection of nest-site slope
exposure.

Results

Nest-site Density. A total of 26 known and 6 sus-
pected Common Buzzard nesting territories were
identified within the 387 km 2 study area, for a den-
sity of 8.3 pairs/ 100 km 2 . Mean distance between
nesting territories averaged 2.5 km (range = 1,62-
4.12 km, SD = 0.54) . Within woodland areas, Com-
mon Buzzard nesting sites were spaced regularly,
as shown by the G-test (G = 0.96).

Landscape-level Habitat Selection. The average
altitude of buzzard nest sites was 1399 m above sea
level (range 1150-1550 m, SD = 131.87). Analysis
of nest exposure ( N — 26) showed that 73.1% ( N
= 19) were oriented NE (x 2 — 33.69, df = 3, P =
0.001), 3.8% ( N= 1) S and SE, and 19.3% (N =
5) SW.

The DFA showed significant differences (P <
0.05) between nesting (N = 17) and control sites
(N = 15) based on the four landscape variables
relief index, distance from forest edge, distance
from paved road and distance from valley bottom
(Table 1). We obtained correct classification for 14
of the control sites (93.3%) and 16 of the Common
Buzzard nesting sites (94.1%). Conversely, there
was one misclassified control site (7%) and one
misclassified nesting site (6%). This classification is
87% better than random (Kappa = 0.874, Z —
4.946, P < 0.0001). The jack-knife classification
showed the robustness of the model with 88.2% of
the nesting sites and 93.3% of the control sites cor-
rectly classified.

Discussion

Common Buzzard nesting density decreases
from 8.3 pairs/ 100 km 2 in the mountain areas of
the Apennines, to 19.7 pairs/100 km 2 in the hills
in the piedmont, to 32 pairs/ 100 km 2 in woodlands
of low-altitude areas (Manzi and Pellegrini 1989,
Manzi et al. 1991). Low nesting densities at higher
altitudes is likely due to the scarcity of prey as ev-
idenced by the lower density of birds in high
mountain areas (36 pairs/10 ha; Bernoni 1995)
than in piedmont (59.2 pairs/ 10 ha; Pandolfi and
Taferna 1991) and plain areas (158 pairs/ 10 ha;
Bernoni et al. 1989). The average nearest-neighbor
distance of 2.5 km was also relatively high when

210

Penteriani and Fajvre

Vol. 31, No. 3

Table 1 . Sample means and standard deviations of landscape habitat variables measured at control and nest sites of
the Common Buzzard. Significant differences determined by Stepwise Discriminant Function Analysis.

Woodland patches (%)

Pasture patches (%)

Fallow patches (%)

Fallow patches with trees (%)

Rocky patches (%)

Cropland patches (%)

Cropland patches with trees (%)
Built-up patches (%)

Number of ecotones

Number of habitats

Interspersion index

Maximum difference in elevation (m)

Relief index

Distance from forest opening (m)
Distance from nearest forest edge (m)
Distance from valley bottom (m)
Distance from built-up area (m)
Distance from nearest paved road (m)
Distance from footpath (m)

Distance from cliffs (m)

Distance from permanent water (m)

* P < 0.05.

Nesting Sites (N = 17) Control Plots (N = 15)

54.5

22.9

40.2

25.3

24.1

H-

16.1

19.8

11.3

6.5

9.4

5.8

3.9

4.1

7.9

4.8

6.5

7.1

7.7

2.7

-h

2.9

3.5

5.6

3.5

8.5

12.4

14.7

0.6

1.3

9.8

4.1

4.5

15.5

5.6

-+-

5.2

-+-

2.4

10.5

-t-

2.1

395.1

H-

165.7

468.3

H-

121.5

47.2

-h

12.5

23.2

9.7*

267.6

155.3

179.3

140.9

269.1

239.22

509.3

383.4*

983.8

487.7

1438.3

845.8*

2827.9

1738.8

2236.7

1049.7

1592.6

-h

1224.4

753.3

610.6*

613.2

632.1

120

88.2

1376.5

H-

706.8

1128

486.8

1560.3

959.5

885.3

443.2

compared with the values of 0.87 and 1.13 km
(Newton et al. 1982) 1.04 km (Jedrzejewski et al.
1994) and 1.9 km (Graham et al. 1995) in other
areas of Europe.

Our landscape level analysis showed that Com-
mon Buzzards did not select habitat at random at
a landscape level, as the majority of nest sites
(94.1%) and control sites (93.3%) were correctly
classified. These results suggest that Common Buz-
zards select nest sites in the eastern part of forests
that are situated on northern slopes. The tendency
to use northern slopes may simply be due to the
fact that NE facing slopes support the tallest beech
trees, but it may also be related to the fact that
these slopes provide cooler temperatures and less
sunlight, as well as a denser canopy cover that may
increase nest protection.

The Common Buzzard is an area-sensitive spe-
cies that requires forested habitats which are dis-
tant from roads but close to valley bottoms in
rugged areas (Robbins et al. 1989). The choice
of nest sites which are far from paved roads has
also been corroborated by Kostrzewa (1989).
Nesting close to valley bottoms may be due to the
fact that most pasture and crop lands are found

there, both of which are favorite hunting
grounds for Common Buzzards. Reliance on
open areas for foraging may also explain why
Common Buzzard nest sites are often near forest
edges (Tubbs 1974, Knuwer & Loske 1980, Weir
and Picozzi 1983, Goszczynski 1985, Jedrzejewski
et al. 1988, Kostrzewa 1989, Hubert 1993, Hoh-
mann 1994, Graham et al. 1995). Open areas
may also be needed because they facilitate court-
ship behavior. Development of higher tempera-
tures and upward thermal air currents over open
habitats (Cone 1962, Jedrzejewski et al. 1988,
Cerasoli and Penteriani 1996) may enhance
courtship flights when pair-bonding takes place
in the early part of the nesting season. Nest-site
selection near forest edges may also be attribut-
ed to ease of access to nests and to a need for
an unobstructed view of the surrounding land-
scape (Roche 1977, Hubert 1993).

Acknowledgments

We are extremely grateful to Fabio Liberatori, Marina
Cerasoli and Francesco Pinchera for their help on the
field work; Javier Bustamante, Christine Hubert, Joseph
K Schmutz, Camille Ferry and Bernard Frochot gave
helpful comments on the manuscript. The Administra-

September 1997

Landscape-level Habitat Selection by Buzzards

211

tion of Abruzzi National Park provided the logistic sup-
port for this work: we thank especially the Director, Fran-
co Tassi and Cinzia Sulli.

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Received 20 May 1996; accepted 27 April 1997

J. Raptor Res. 31 (3):213-222
© 1997 The Raptor Research Foundation, Inc.

VARIATIONS IN BREEDING BALD EAGLE RESPONSES TO JETS,

LIGHT PLANES AND HELICOPTERS

Teryl G. Grubb

USDA Forest Service, Rocky Mountain Forest and Range Experiment Station,

2500 S. Pine Knoll Dr., Flagstaff, AZ 86001 U.S.A.

William W. Bowerman

Gale Gleason Environmental Institute, Lake Superior State University, Sault Ste. Marie, MI 49783 U.S.A.

Abstract. — We analyzed 3122 observations of military jets, light planes and helicopters for three levels
of response (none, alert, flight) by breeding Bald Eagles ( Haliaeetus leucocephalus ) at 13 occupied nests
in Arizona and six in Michigan, 1983-85 and 1989-90, respectively. Helicopters elicited the greatest
frequency of response (47%), followed by jets (31%) and light planes (26%). Frequency of response
(23-61%) and frequency of flight (2-13%) both increased through the nesting season from February
to June. Distance from eagle to aircraft, duration of overflight and number of aircraft and/or passes
were the most important characteristics influencing eagle responses to pooled and individual aircraft
types. Classification tree (CART) models for individual aircraft types provide dichotomous keys of dis-
tance and secondary variables affecting associated response rates, and should facilitate evaluating air-
craft-specific impacts. Our analyses indicate a categorical exclusion of aircraft within 600 m of nest sites
would limit Bald Eagle response frequency to 19%.

Key Words: Bald Eagle, Haliaeetus leucocephalus; disturbance, aircraft, behavior, Arizona; Michigan; mod-

eling, classification trees.

Variaciones en crias de aguilas en reaccion ha aviones militar, aviones ligeros y helicopteros

Resumen. — Nosotros analizamos 3122 observaciones de avion militar, avion ligero y helicoptero para
tres niveles de reaccion (nada, alerta, volar) de aguilas ( Haliaeetus leucocephalus) de crfa en 13 nidos
ocupados en Arizona y seis en Michigan, 1983-85 y 1989-1990, respectivamente. Helicopteros le sacaron
la reaccion con mas frecuencia (47%), seguido por avion militar (31%) y avion ligeros (26%). La
frecuencia de reaccion (23-61%) y frecuencia de volar (2-13%) aumentaron durante la temporada de
poner nidos de febrero ha junio. Distancia de aguilas al avion, tiempo en viento, y numeros de aviones
y/o pases eran los mas importantes caracterfsticos influyendo la respuesta de aguilas ha grupos o solos
tipos de aviones. Modelos (CART) con clasificacion tres para aviones solos proporcionan Haves dicoto-
mias de distancia y variables secundarias afectando respuestas asociadas, y debe facilitar la evaluation
de impactos specificos de aviones. Nuestra analisis indica una exclusion categorico de aviones dentro
de 600M de nidos debe limitar la respuesta de frecuencia ha (19%) en aguilas.

[Traduction de Raul De La Garza, Jr.]

Aircraft come into contact with breeding raptors
in essentially two nonexclusive ways: first, as a po-
tentially disturbing form of ambient human activity
(Smith et al. 1988) and second, as a research/man-
agement tool specifically focused on nest over-
flights to survey breeding populations and monitor
reproductive success (Fuller and Mosher 1987).
For effects on breeding Bald Eagles ( Haliaeetus leu-
cocephalus) , aircraft have been addressed either pas-
sively as part of broader disturbance studies (e.g.,
Grubb and King 1991, McGarigal et al. 1991) or
actively as part of an evaluation of the aircraft type

used in the reported study (e.g., Fraser et al. 1985,
Watson 1993). However, comparative response
data on the three common types of aircraft affect-
ing breeding Bald Eagles and other raptors (low-
level military jet fighters, light planes and helicop-
ters) are scarce (Smith et al. 1988, Watson 1993)
and have not been collected within the context of
a single study.

Our research specifically focused on variation in
breeding Bald Eagle responses to the three com-
mon types of aircraft. It represented a collation
and extension of previously described Bald Eagle/

213

214

Grubb and Bowerman

Vol. 31, No. 3

human disturbance research in Arizona (Grubb
and King 1991) and Michigan (Grubb et al. 1992).
Although these studies showed aircraft to elicit the
lowest response of the five disturbance groups eval-
uated (vehicle 52-74%, pedestrian 45-72%, aquat-
ic 46-53%, noise 38-54%, aircraft 29-33%), the
authors noted any potentially disturbing activity, in
excess or under the right conditions, can alter nor-
mal behavior or induce nesting failure. However,
activities that may not cause nest failure can still
detrimentally impact eagles. Low-level overflights
have caused Bald Eagles to attack (Fyfe and Olen-
dorff 1976) or avoid (Fraser et al. 1985) the air-
craft, or depart the area entirely (Grubb and King
1991), all of which are energetically costly and be-
haviorally disruptive. In Arizona, the death of a
nestling was attributed to frequent helicopter
flights <30 m off a cliff nest; this unusual activity
kept the adults away for long periods and signifi-
cantly reduced prey deliveries (L.A. Forbis pers.
comm.).

Thus, our standardized assessment of nonfailure-
producing effects of the three common aircraft
types on Bald Eagle responses should facilitate
evaluation of potential aircraft disturbances and
encourage disturbance-specific breeding area man-
agement.

Study Area

The central Arizona study area was located in Gila,
Maricopa and Yavapai counties, primarily along the Salt
and Verde River drainages. The area is characterized by
clustered mountain ranges and desert basins, with ele-
vations of 500-1500 m (Chronic 1983). All nest sites were
associated with riparian vegetation consisting of cotton-
wood-willow ( Populus fremontii-Salix goodingii) and mixed
broadleaf ( Platanus wrightii, Fraxinus pennsylvanica, Alnus
ohlongifolia ) series amid prevailing Sonoran desertscrub-
Arizona upland or palo verde-mixed cacti (Cercidium spp.-
Opuntia spp.) series (Brown 1982). Most eagle nests were
located on 50-1 00-m cliffs.

The Michigan study area was located in the northern
lower peninsula along the Au Sable River in Alcona, Ios-
co, Oscoda and Otsego counties, and the Manistee River
in Manistee County. Terrain was flat to rolling with oc-
casional hills; elevation range was 200-400 m. Vegetation
was predominantly continuous mixed hardwood forest of
aspen ( Populus grandidentata and P. tremuloid.es) , oak
( Quercus rubra and Q. alba), maple (Acer rulyrum and A.
saccharum) and birch (Betula papyrifera) , with interspersed
conifer stands of white ( Pinus strobus), red (P. resinosa )
and jack (P. banksiana ) pine. All eagle nests were in trees,
mostly white pine.

Methods

Because of federal threatened and endangered species
restrictions, we observed Bald Eagle responses to passing

aircraft opportunistically, with no manipulative experi-
mentation nor direct control of aircraft. We could not
govern the number or distribution of aircraft among nest
sites, through the breeding seasons, or across years. Nor
could we effectively address apparent variation in respon-
siveness by nest site because of differing numbers, types
and timing of aircraft (Table 1). Therefore, after testing
for differences in the Arizona and Michigan data sets, we
combined observations to maximize sample size for anal-
ysis and modeling of response trends. Arizona data ( N =
2848) were collected during the 1983-85 breeding sea-
sons in the vicinity of 13 Bald Eagle nest sites. Michigan
data (N = 274) were collected during the 1989-90 breed-
ing seasons around six nest sites. Data collection tech-
niques were identical in both states. The combined sam-
ple of 19 nest sites represented 2:45 free-flying Bald Ea-
gles from two populations over five breeding seasons (Ta-
ble 1).

For seasonal analyses, Michigan data were standardized
to Arizona data on the basis of incubation dates; one
month was subtracted from Michigan dates to integrate
the later breeding season into the predominant sample.
For general application beyond these two populations,
February to early-March was considered the incubation
period; mid-March to May, the nestling period; and early
June, the fledging period.

As an alternative to unattainable cause-and-effect test-
ing, we monitored variations in Bald Eagle response se-
verity (none, alert/agitated, flight) and response fre-
quency (% none/any) as aircraft overflights occurred.
Alert behavior included head turns, vocalizations and in-
creased movements on or between perches. Grubb and
King (1991) and Grubb et al. (1992) detail data collec-
tion procedures and analytical methods.

We classified aircraft into three generic types: low-fly-
ing, military jet fighters; civilian, propeller-driven, light
planes; and helicopters, civilian or military, mostly single-
rotor. For all aircraft events within 2000 m of nest sites
and less than approximately 305 m overhead (1000 ft,
estimated), we recorded distance-from-affected-eagle-to-
aircraft (m), du ration -o f-ove f flight (min), number-of-
units-per-event (aircraft and/or passes overhead), visibil-
ity-of-aircraft-to-affected-eagle (none/any), and position-
relative-to-affected-eagle (above/below). Distance-to-air-
craft was approximated by plotting flight paths on topo-
graphic maps and measuring distances to reference
eagles. Visibility was based on eagle and aircraft positions
relative to obscuring vegetation and terrain features.

Medians were used in summary statistics to represent
central tendencies because of skewness in data caused by
a preponderance of nearby, short-duration overflights.
Frequencies, descriptive statistics, and nonparametric k-
sample median and goodness-of-fit tests using the chi-
square statistic were calculated with SPSS/PC+ 4.0 (No-
rusis 1990). We used notched box and whisker plots
(Chambers et al. 1983, STSC 1991) to evaluate the rela-
tionship between distance-to-aircraft and response sever-
ity.

We developed classification and regression tree
(CART) models to assess variations in response frequency
associated with pooled aircraft (all three types combined
with no type distinction), pooled aircraft including air-
craft type as a separate variable and for each aircraft type

September 1997

Eagle Responses to Aircraft

215

Table 1. Sample distribution by nest site, minimum number of Bald Eagles, years of data, aircraft type, nesting
season month and associated variability in frequency of Bald Eagle response for 3122 observations of military jet
fighters, light planes and helicopters near 19 occupied nest sites in Arizona (nests 1-13) and Michigan (nests 14—
19), 1983-85 and 1989-90, respectively.

% Response Frequency 3 % Response Frequency 3

Nest (N for Aircraft Type) ( N for Month)

Site

BEs

Yrs

Pooled

Jets

Planes

Helos

Feb

Mar

Apr

May

JUN

37 ( 108 )

20 ( 5 )

31 ( 90 )

85 ( 13 )

27 ( 77 )

40 ( 15 )

67 ( 9 )

100 ( 7 )

- ( 0 )

34 ( 79 )

33 ( 3 )

27 ( 55 )

52 ( 21 )

39 ( 36 )

33 ( 27 )

27 ( 15 )

0 ( 1 )

- ( 0 )

2:2

44 ( 188 )

57 ( 14 )

38 ( 143 )

64 ( 31 )

33 ( 49 )

40 ( 89 )

62 ( 50 )

- ( 0 )

51 ( 215 )

28 ( 40 )

55 ( 122 )

60 ( 53 )

40 ( 126 )

69 ( 58 )

65 ( 23 )

63 ( 8 )

- ( 0 )

90 ( 39 )

- ( 0 )

93 ( 28 )

82 ( 11 )

- ( 0 )

50 ( 2 )

96 ( 28 )

78 ( 9 )

- ( 0 )

20 ( 1286 )

20 ( 215 )

11 ( 631 )

34 ( 440 )

11 ( 493 )

12 ( 396 )

31 ( 194 )

41 ( 116 )

61 ( 87 )

62 ( 24 )

- ( 0 )

58 ( 12 )

67 ( 12 )

77 ( 13 )

46 ( 11 )

- ( 0 )

62 ( 21 )

- ( 0 )

78 ( 9 )

50 ( 12 )

20 ( 5 )

83 ( 6 )

100 ( 1 )

68 ( 9 )

~ ( 0 )

24 ( 345 )

28 ( 168 )

10 ( 150 )

74 ( 27 )

42 ( 48 )

46 ( 74 )

8 ( 185 )

36 ( 36 )

50 ( 2 )

53 ( 49 )

36 ( 14 )

59 ( 17 )

61 ( 18 )

62 ( 8 )

72 ( 18 )

28 ( 18 )

50 ( 4 )

100 ( 1 )

90 ( 39 )

93 ( 14 )

86 ( 21 )

100 ( 1 )

86 ( 7 )

88 ( 25 )

100 ( 2 )

100 ( 5 )

- ( 0 )

44 ( 390 )

45 ( 97 )

36 ( 234 )

73 ( 59 )

10 ( 40 )

63 ( 91 )

41 ( 134 )

45 ( 125 )

- ( 0 )

40 ( 65 )

17 ( 18 )

40 ( 30 )

65 ( 17 )

39 ( 49 )

64 ( 11 )

- ( 0 )

53 ( 17 )

64 ( 11 )

0 ( 3 )

67 ( 3 )

50 ( 2 )

70 ( 10 )

0 ( 3 )

- ( 0 )

30 ( 10 )

25 ( 4 )

33 ( 6 )

- ( 0 )

50 ( 4 )

25 ( 4 )

0 ( 2 )

- ( 0 )

0 ( 1 )

- ( 0 )

0 ( 1 )

- ( 0 )

100 ( 1 )

- ( 0 )

100 ( 1 )

- ( 0 )

-( 0 )

- ( 0 )

100 ( 1 )

- ( 0 )

50 ( 10 )

100 ( 2 )

38 ( 8 )

- ( 0 )

43 ( 7 )

67 ( 3 )

- ( 0 )

29 ( 235 )

32 ( 173 )

9 ( 34 )

36 ( 28 )

- ( 0 )

33 ( 73 )

28 ( 120 )

26 ( 42 )

- ( 0 )

>45

32 ( 3122 )

31 ( 779 )

26 ( 1594 )

47 ( 749 )

23 ( 953 )

34 ( 910 )

33 ( 801 )

44 ( 368 )

61 ( 90 )

“Response frequency (%) = number of responses divided by number of events times 100%.

(California Statistical Software, Inc. 1985; Grubb and
King 1991). Classification analysis provides predictive,
discriminant models in the form of nonparametric, di-
chotomous keys (Brieman et al. 1984; Verbyla 1987). For
each level (branch) of the model, CART selects the in-
dependent (splitting) variable, and the point within its
range, that best separate (classify) remaining data into
classes of the dependent variable (response in our case).
This process of tree growing continues until all data are
classified.

Only the classification tree aspects of CART were used
in our analyses. The first split in each tree separated the
higher response, left side of the models from the lower
response, right side. Each variable used in CART was
ranked for its splitting ability by assigning the first (pri-
mary) splitting variable a value of 100% and expressing
the relative value of secondary variables as a percentage
of the primary variable.

Cross-validation provided an estimate of classification
accuracy (predictability) for each tree on a scale of 0.00-
1.00 (Brieman et al. 1984, Verbyla 1987). For this pro-
cedure, CART randomly divides the data into 10 subsets,
develops a classification tree with nine subsets, estimates
tree accuracy by applying it to the withheld subset, then
repeats the process until all 10 subsets have been with-
held. Averaging results of the 10 mini-tests yields an over-

all estimate of classification accuracy for the tree devel-
oped from the full data set (Steinberg and Colla 1992).

Results

Frequencies for none, alert and flight responses
did not differ between state populations of Bald
Eagles (Arizona — 68, 28, and 4% and Michigan —
69, 26, and 5%, respectively; x 2 = 1.19, P = 0.55).
Although median distance-to-aircraft for alert re-
sponse varied between Arizona and Michigan (350
and 500 m, respectively; x 2 = 10.57, P < 0.01),
median distances for no response (750 and 800 m;
X 2 = 1.45, P = 0.23) and flight response (both 200
m; x 2 < 0.01, P = 0.96) were similar. When “state”
was added as an independent variable to the CART
analyses, it was not included in the resulting mod-
els; state location had no discriminatory value for
partitioning Bald Eagle responses to aircraft.

Our combined sample consisted of 51% light
planes, 25% military jets and 24% helicopters ( N
= 3122, Table 2). Median number-of-aircraft and

216

Grubb and Bowerman

Vol. 31, No. 3

Table 2. Comparison of disturbance and response characteristics among three types of aircraft for 3122 occurrences
within 2000 m of 13 occupied Bald Eagle nests in Arizona and six in Michigan, 1983-85 and 1989-90, respectively.

Disturbance

No Response

Any Response

Frequency

Median

(no. of

No. PER

Distance

Duration

Frequency

Distance

Frequency

Distance

Type

events)

Event

(m)

(min)

(%) a

(m)

(%) a

(M)

Military jets

779

500

600

400

Light planes

1594

700

850

400

Helicopters

749

420

700

250

Total sample

3122

600

800

333

a Response frequency (%) = number of responses divided by number of events times 100%.

duration (min) were similar for all aircraft types.
Helicopters occurred at the closest median dis-
tance and had the highest response rate, followed
by jets, then light planes. All three types typically
occurred closer than the median no-response dis-
tance, yet overall response rate was only 32%. Re-
sponse frequencies at individual nest sites were
highly variable but at the 12 sites where all three

NOTE ALERT FLIGHT

(N - 2.109) (N - 684) (N- 129)

RESPONSE SEVERTTY

Figure 1. Notched box and whisker plot of median dis-
tance to aircraft (military jets, light planes and helicop-
ters) for three levels of response severity for breeding
Bald Eagles at 19 occupied nests in Arizona and Michi-
gan, 1983-85 and 1989-90, respectively. Boxes cover mid-
dle 50% of data. Tops of boxes indicate the distance with-
in which 75% of recorded responses occurred. Whiskers
indicate range but do not exceed 1.5 times box length.
Stars represent oudying observations. Box width is pro-
portional to sample size. Center lines are medians, with
position indicating skewness. Notches are width of 95%
confidence intervals for pairwise comparisons.

aircraft occurred, helicopters consistently elicited
the highest response (Table 1).

Median distance-to-aircraft varied among differ-
ent levels of response severity, with closer proximity
resulting in greater response ( P = 0.05, Fig. 1).
Response frequencies for each type of aircraft also
varied at each response level (Fig. 2). Helicopters
had the lowest rate of no response (x 2 = 292, P <
0.01) and the highest rates of alert response (x 2 =
124, P < 0.01) and flight response (x 2 = 11.55, P
< 0.01). Median distance for flight response was
200 m for all three aircraft types, although fre-
quency of flight from helicopters was more than
three times that from jets and planes.

As the nesting season progressed, Bald Eagles re-
sponded both more frequently and more severely
with more flight. The frequencies of alert and
flight responses increased from February to June

NONE

ALERT

FLIGHT

RESPONSE SEVERITY

Figure 2. Differing response frequencies among three
types of aircraft for three levels of response severity for
breeding Bald Eagles at 19 occupied nests in Arizona and
Michigan, 1983-85 and 1989-90, respectively.

September 1997

Eagle Responses to Aircraft

217

Figure 3. Monthly variations in response frequency for
three levels of response severity for breeding Bald Eagles
at 19 occupied nests in Arizona and Michigan, 1983-85
and 1989-90, respectively.

(X 2 — 448 and 1904, respectively; P < 0.01), with
a compensatory decrease in no-response (x 2 =
6969, P < 0.01; Fig. 3) . Seasonal changes in aircraft
proximity appeared to have little effect on Bald Ea-
gle responsiveness. Distance-to-pooled-aircraft de-
creased through the nesting season (x 2 = 115, P
< 0.01; Table 3), but median distance-to-aircraft
eliciting response did not fluctuate significantly be-

tween February and May (median = 350 m; x 2 =
3.65, P = 0.30).

Although sample sizes became smaller as the
nesting season progressed, responsiveness to
pooled and individual aircraft types started rela-
tively low during incubation (February), leveled at
a higher plateau during the nestling period
(March-May) and increased to the highest levels
after fledging (June, Table 3). May and June data
also indicated that the consistently higher response
to helicopters was more a function of aircraft type
than distance. In May, when the median distance
to both jets and helicopters was 500 m, eagle re-
sponses were 37% and 52%, respectively. In June,
light planes and helicopters both occurred at 200
m, yet eagle responses were 45% and 84%, respec-
tively.

Frequency of eagle response increased as the fre-
quency of aircraft decreased. Nest site No. 6 had
>1200 recorded aircraft overflights, six sites had
between 100-400 and 12 sites had <100 (Table 1).
Response frequencies for these three groups were
20, 38 and 55%, respectively (x 2 = 545, P< 0.01).
Yet, the median distance-to-aircraft-eliciting-re-
sponse was similar between nest groups: alert re-
sponse, 300-400 m (x 2 = 2.25, P — 0.32) and flight
response, 150-200 m (x 2 = 1.82, P = 0.40).

In the CART pooled aircraft model (Fig. 4) , dis-

Table 3. Monthly variation a in sample sizes, response rates and median distances for 3122 military jet fighters, light
planes and helicopters near 19 occupied Bald Eagle nest sites in Arizona and Michigan, 1983-85 and 1989-90,
respectively.

Feb

Mar

Apr

May

JUN

Military jets

199

209

255

Median distance (m)

600

500

600

500

300

% Response

Light planes

515

503

403

144

Median distance

850

700

600

200

% Response

Helicopters

239

198

143

138

Median distance

500

400

440

500

200

% Response

Pooled aircraft

953

910

801

368

Median distance

800

600

500

250

% Response

a On the basis of incubation dates, Michigan data were standardized to Arizona data by subtracting one month.

218

Grubb and Bowerman

Vol. 31, No. 3

44 %

POOLED AIRCRAFT, INCLUDING TYPE

44 % 38 %

Figure 4. Classification tree (CART) models, with associated eagle response frequencies, for pooled and pooled-
wi thin-type aircraft disturbance near breeding Bald Eagles at 19 occupied nests in Arizona and Michigan, 1983-85
and 1989-90, respectively.

tance was the primary and secondary splitting vari-
able, followed by number, duration, and visibility
on the left (high-response) side of the tree, and
duration alone on the right (low-response) side.
When aircraft type was included as a variable in the

pooled tree, it entered the model at the tertiary
level, after the two distance splits. Type influenced
response rates in the midrange distances (166-590
m) , with helicopters partitioned from and showing
greater response rates than jets and planes. Re-

September 1997

Eagle Responses to Aircraft

219

JETS

HELICOPTERS

LIGHT PLANES

33 %

Figure 5. Classification tree (CART) models, with associated eagle response frequencies (%), for military jet, light
plane and helicopter disturbance near breeding Bald Eagles at 19 occupied nests in Arizona and Michigan, 1983-85
and 1989-90, respectively.

sponse rates for both models were 67% at Si 65 m,
44% at 166-375 m, 38% at 376-590 m, and 19%
at >590 m (x 2 = 4179, P < 0.01). Estimated ac-
curacy for the pooled and pooled-with-type models
was 0.63.

Although GART-generated, initial splitting dis-
tances increased from jet fighters, through light
planes, to helicopters, the low-response side of in-
dividual models showed light planes causing the
least response at greater distances (16%) and jets
the highest (26%, Fig. 5). For jets, short overflight
duration (<5 min) and single aircraft appeared to
mitigate the effect of proximity within 525 m,
whereas longer duration within 175 m caused cer-
tain response. Calculated response rates based
solely on distance were 52% at S175 m, 37% at

176-525 m and 26% at >525 m (x 2 = 398, P <
0.01); the first two rates differ from the CART
model because of the incorporation of duration
and number within 525 m. Jet model accuracy was
estimated at 0.60.

Light planes within 165 m elicited 65% response
regardless of any other factors; between 166-260
m, response rate dropped to 45%. Response rates
at 261-590 m and at >590 m were 33% and 16%,
respectively (x 2 — 3888, P < 0.01). Between 261—
590 m, >1 plane or pass/ event or >4 min duration
caused response greater than or equal to close
proximity events. Response to helicopters simply
decreased as distance increased: 75% at si 40 m,
55% at 141-625 m, and 22% at >625 m (x 2 = 399,
P < 0.01). Accuracy estimates for the light plane

220

Grubb and Bowerman

Vol. 31, No. 3

Table 4. Relative importance 3 of independent (splitting) variables in CART analyses for three types of aircraft
disturbance, treated separately and pooled with/without type included as a variable.

Disturbance

Overall

Ranking

Variable

Pooled

Pooled
with Type

Jets

Planes

Helicopters

Distance

100

Duration

T yP e

Number

Visibility

Position

a Standardized so

primary splitting variable = 100% and

secondary variables

are expressed as

a percentage of the

primary variable.

and helicopter models were 0.61 and 0.70, respec-
tively.

CART modeling verified distance as the most
critical determinant between response and no-re-
sponse associated with aircraft (Table 4) . Duration-
of-overflight was a consistent second and number-
of-units-per-event third. Both duration and num-
ber appeared nearly twice as important for re-
sponses to jets as for the other types of aircraft.
Number had the least effect on response to heli-
copters. Overall, position and visibility affected ea-
gle responses to aircraft very little. When included
in the pooled model, aircraft type was ranked third
behind distance and duration.

Discussion

These results are necessarily qualified by the fact
that sample data were not evenly or randomly dis-
tributed across the various parameters measured or
among nest sites. Thus, the distribution of sample
data should be considered when interpreting or
applying our results. For example, repeated air-
craft observations on many of the same eagles may
have reduced the observed variability, frequency
and/or severity of response. However, inherent
limitations are at least partially mitigated by the
size of the data set, the number of eagles and nest
sites involved, the duration of the study and the
standardization of aircraft and response measure-
ments among types.

Greater stimuli typically result in Bald Eagles re-
acting farther away (Grubb et al. 1992). Thus, hel-
icopters might be expected to cause eagle re-
sponses at greater distances than light planes. The
relatively low median response distance for heli-
copters compared to other aircraft was more likely
a result of proximate flights than an indication of

breeding eagle tolerance. Helicopters, because of
their enhanced maneuverability, and military jets,
because of the nature of low-level fighter training,
tended to follow drainages and contours (where
nests were located) more closely than light planes,
especially in the rugged canyon terrain of Arizona.
At very close range, the consistent 200 m, calculat-
ed median flight distance for all three aircraft and
the pooled-with-type CART model, which did not
include aircraft type before 166 m, indicate prox-
imity outweighs type. Comparable minimum split-
ting distances in each of the type models (jets 175
m, planes 165 m and helicopters 140 m) support
this conclusion.

In their review of responses to aircraft by 14 rap-
tor species, Smith et al. (1988) found the impact
of low-level military jets to be brief and insignifi-
cant. In our study, jets and helicopters occurred at
similar distances from nest sites. Yet, jets and light
planes elicited comparable response rates at iden-
tical response distances. The fact that helicopters
caused much greater response, and that CART
split jets and planes from helicopters in the mod-
eling process, argues for type differences. Also, the
CART model for helicopters included no other
variables than distance, suggesting a stimulus of
sufficient magnitude that secondary characteristics
did not influence response. Distances within the
model were consistent with Platt (1977), who re-
corded helicopter overflights at ^160 m altitude
disturbing all adult Gyrfalcons ( Falco rusticolus) and
overflights >600 m disturbing none of the five
pairs tested. Our data confirm the traditional view
that helicopters are the most disturbing type of air-
craft (Watson 1993).

Bald Eagles appeared least responsive to aircraft

September 1997

Eagle Responses to Aircraft

221

early in the nesting season, as indicated by both
their lower response rate and tendency to remain
at or near nests without flying. Increasing response
rates, especially for flight, later in the season sug-
gest adults were more frequently flushed as their
nest attendance requirements diminished. Watson
(1993) noted presence of young nestlings led to
reduced adult response. He also found eagles with
small young were more reluctant to flush in ad-
verse weather, and eagles were disturbed at higher
rates when no young were in the nest. Decreasing
sample size over time is partially attributable to re-
duced adult presence near nests, which typically
declines as nestiings mature (Bowerman 1991).

Grubb and King (1991) concluded breeding
Bald Eagles in Arizona may have become habitu-
ated to aircraft, and in Michigan habituation was
also evidenced at one nest site near a military air
base (Grubb et al. 1992). Our current analysis of
the combined data set indicates variability among
nest sites, with an inverse relationship between fre-
quency of air traffic and frequency of eagle re-
sponse. If habituation occurs with repeated expo-
sure, then our results may underestimate Bald Ea-
gle response at nest sites with limited air traffic and
overestimate at sites with a high frequency of air-
craft.

The relative importance of CART splitting vari-
ables indicates that managing distance, duration
and number of aircraft overflights could effectively
minimize impacts on breeding Bald Eagles. The
higher values for duration and number with jets
may be a result of the tendency for military jets to
fly in groups of two or more, as well as the prox-
imity of the one Michigan nest (No. 19) to an Air
National Guard, air-to-ground firing range where
repeated overflights were common (Grubb et al.
1992). The relative importance of type in the
p o o 1 e d-wi th-typ e model validates using individual
aircraft models to refine distance and potential
management considerations.

Cross-validation indicates our CART aircraft
models should correctly predict breeding eagle re-
sponse for two of every three aircraft events. Model
accuracy might be improved through controlled
experimentation and by the addition and/or re-
finement of independent variables, including con-
sideration of specific eagle activity (Grubb and
King 1991, McGarigal et al. 1991, Watson 1993)
and weather conditions (Schueck and Marzluff
1995) at the time of overflight. Significance and
intensity of prestimulus eagle behavior, as well as

time of the year (e.g., breeding versus nonbreed-
ing season) may also be important factors (Smith
et al. 1988).

Management plans for nesting Bald Eagles typi-
cally include restrictive buffer zones, limiting hu-
man activity within 400 m of nest sites (Grier et al.
1983). Plans may also include restrictions associat-
ed with key habitat areas such as used for foraging
and perching (Isaacs and Silvosky 1981). Aircraft
are typically precluded from flying within these re-
striction zones. GART primary splits at 525, 590,
and 625 m for jets, planes and helicopters and a
secondary split at 590 m on the pooled model, re-
sulting in 19-26% response, suggest that aircraft
would best be categorically excluded from within
600 m of nest sites and key habitat areas during
the breeding season.

When such a categorical limitation is impracti-
cal, our CART models indicate if duration and
number of aircraft and/or passes are limited to <5
min and one, respectively, jet fighters within 200
m of nest sites would cause relatively low expected
eagle response (<33%). Light planes within 275
m, if limited to <4 min duration and one plane or
pass/overflight, would cause 31% expected re-
sponse. Avoiding helicopter overflights within 600
m of nest sites would result in a 22% expected re-
sponse. However, given the advantages and there-
fore inevitable continued closer use of helicopters
for raptor surveys (Watson 1993, Ewins and Miller
1995), we recommend these surveys be flown at
maximum distance (>150 m) and minimum du-
ration (<1 min), with only one overhead pass.
Whenever possible, surveys are better conducted
with light planes, because they typically cause min-
imal disturbance to breeding Bald Eagles (Fraser
etal. 1985).

Acknowledgments

We are indebted to the 71 Forest Service volunteers in
Arizona during 1983-85 without whose dedication and
field assistance this study would not have been possible.
In Michigan, A.J. Bath and J. A. Johnston, along with S.A.
Hogle, E. Malleck, J.T. Painter, B. Richardson, T. Ridley,
B. Rogers, J. Rogers, P. Stefanek, S. Sutton, S. Thompson,
T.J. Warren, D. Weeks and volunteers from EARTH-
WATCH, provided invaluable field assistance. J.J. Wil-
liams, a USDA Forest Service Rocky Mountain Station
biometrician, helped develop CART models. We also
thank D.E. Anderson, P. Beier, R.N. Lehman, M.H. Reiser
and J.W. Watson for helpful reviews. This study was de-
rived from research funded in Arizona by USDA Forest
Service, U.S. Fish 8c Wildlife Service and U.S. Bureau of
Reclamation; and in Michigan by Consumers Power
Company, Michigan Dept, of Military Affairs, Michigan

222

Grubb and Bowerman

Vol. 31, No. 3

State University, USDA Forest Service and EARTH-

WATCH, Inc.

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Received 13 October 1996; accepted 15 May 1997

/. Raptor Res. 31(3):223-227
© 1997 The Raptor Research Foundation, Inc.

PRODUCTIVITY OF GOLDEN EAGLES WEARING BACKPACK

RADIOTRANSMITTERS

John M. Marzluff 1 and Mark S. Vekasy 1
Greenfalk Consultants, 8210 Gantz Avenue, Boise, ID 83709 U.S.A.

Michael N. Kochert and Karen Steenhof

U.S. Geological Service, Biological Resources Division, Forest and Rangeland Ecology Science Center,
Raptor Research Field Station, 970 Lusk Street, Boise, ID 83706 U.S.A.

Abstract. — We examined the association between the presence of backpack radiotransmitters and Gold-
en Eagle ( Aquila chrysaetos) reproduction (percentage of occupied territories producing young, and
number of nestlings produced) over three years. The association between radio-tagging and nesting
success and the number of nestlings produced varied significantly among years. A negative association
with tagging was observed in one of three years, which coincided with low prey (jackrabbit) populations
and a cold spring. However, small sample size and breeding by subadults may confound this result.

Key WORDS: Aquila chrysaetos; Golden Eagle, productivity, radio-tagging, weather.

La productividad de aguilas ( Aquila chrysaetos) con radio emisora

Resumen. — Nosotros examinamos la asociacion entre la presencia de un radio ajustado en la espalda
y la reproduccion (porcentaje de territorio ocupado tenido jovenes, y numeros de pajaritos producidos)
de la aguila ( Aquila chrysaetos ) por tres anos. La asociacion entre marcando con el radio y el desarrollo
de nidos y los numeros de pajaritos producidos variado mucho entre clases de edad. Una asociacion
negativa con marcando fue observado uno de los tres anos, que coincido con poblaciones bajas de
presa y una primavera fria. Sin embargo, muestras pocas y reproduccion minima de subadultos puede
confundir resultados.

[Traduccion de Raul De La Garza, Jr.]

Effects of radio-tagging on behavior should be
considered before making inferences about an an-
imal’s biology (Wanless 1992, Hiraldo et al. 1994).
Radio-tagging may have little effect (Vekasy et al.
1996), or may adversely affect condition and be-
havior by abrading skin, influencing time budgets,
decreasing foraging efficiency, increasing metabol-
ic costs or causing desertion of eggs or nesdings
(Gessaman and Nagy 1988, Massey et al. 1988,
Hooge 1991, Foster et al. 1992). Effects may vary
year to year with weather and prey abundance
(Peitz et al. 1993, Vekasy et al. 1996).

We examined reproductive responses of Golden
Eagles ( Aquila chrysaetos) wearing backpack radio-
transmitters in the Snake River Birds of Prey Na-
tional Conservation Area (NCA) from 1991-94.
Our objective was to determine the influence of
radio-tagging on reproduction and identify other

1 Present address: College of Forest Resources, Univer-
sity of Washington, Seattle, WA 98195 U.S.A.

factors that may have interacted with radio-tagging
to either increase or decrease the magnitude of the
effect.

Methods

Throughout the course of this study 27 Golden Eagles
were captured and 15 were radio-tagged (Table 1). Our
sample during winter 1991-92 included eight eagles at
seven nesting areas (sections of cliffs or powerlines where
nests are found each year, but where no more than one
pair has ever bred at one time). Both members of the
pair were tagged at one site. In 1992—93 our sample in-
creased by two nesting areas where we tagged the male
of one pair and the female of the other pair. We also
radio-tagged two additional birds in our original seven
areas in 1992-93; a female after her mate’s transmitter
failed, and a male where we had previously trapped and
radio-tagged the female. Our sample size was reduced by
two nesting areas during winter 1993—94, when we found
one female dead of unknown cause, and we failed to lo-
cate one male. Captured eagles were weighed and mea-
sured, and we determined sex using weight and footpad
length and observations of copulation (Edwards and Ko-
chert 1986).

Golden Eagle control nesting areas consisted of all oc-

223

224

Marzluff et al.

Vol. 31, No. 3

Table 1. Golden Eagle territories where birds were radio-tagged and productivity was studied during 1991-94 breed-
ing seasons in the Snake River Birds of Prey National Conservation Area.

Territory

Number <

Cap-

tured

of Eagles
Instru-

Individuals Used in Analyses

Years Used in Analyses

MENTED

Sex

Age

Capture Date

1991 1992

1993 1994

A — Black Butte

12 Nov 91

B — Beercase

18 Jan 92

C — Wildhorse

14 Oct 91

16 Dec 92

D — PP&L 119

19 Feb 91

23 Oct 92

Subad

11 Mar 94

E— Pole 369

0 a

Subad

17 Dec 91

F — Grand View

Subad

17 Dec 91

24 Oct 92

G — Ogden

14 Dec 92

H — Beecham

22 Nov 91

I — Cabin

06 Dec 91

12 Apr 94

Total

a Individual moved from Grand View Sand Cliff territory to Pole 369 territory.

cupied nesting areas in the NGA with known nesting out-
comes and without radio-tagged adults (1992, N = 23;
1993, N = 19; 1994, N = 21). A nesting area was consid-
ered “occupied” if we observed territorial activity, court-
ship, brood rearing activity, eggs, young or conspicuous
field sign (e.g., whitewash at a roost). Control and treat-
ment nesting areas were interspersed along the Snake
River Canyon.

We attached transmitters as backpacks using a Teflon®
ribbon harness (after Buehler et al. 1995). Details of har-
ness construction and fitting are found in Vekasy et al.
(1996). A transmitter with harness weighed 75 g, less
than 3% body weight for males (x = 3691.5 g, SE = 98.9,
N = 10), and less than 2% body weight for females ( x =
4412.5 g, SE = 133.4, N= 4).

We observed Golden Eagle nesting areas from a heli-
copter two or three times throughout the season to de-
termine occupancy and egg laying, and number of nest-
lings ^51 d old (brood size). We surveyed nesting areas
from the ground when we could not determine these
parameters by helicopter. We considered pairs as nonlay-
mg if there was no evidence that eggs were laid and a
bird was not seen in an incubating posture on a nest.
The presence of one member of a pair in incubating
posture, or eggs or young in a nest was considered a nest-
ing attempt. Nesting attempts were considered successful
if at least one nestling reached 80% of fledging age
(Steenhof 1987), or approximately 51 d.

We classified degree of exposure at each nest site when
possible. Nest shading was classified as the percent of a
nest in shade between 1200 H and sunset. Nests were
classified as shaded if >25% of a nest was shaded, inter-

mediate if 6—25% was shaded and exposed if S5% was
shaded.

We observed nesting areas with radio-tagged eagles
once every one to two weeks to assess behavior and hab-
itat use during foraging. One observer remained in the
canyon near the nest while the other was positioned out-
side the canyon to follow an eagle by vehicle during for-
ays. We did not follow and observe eagles in control ar-
eas.

We used a three-factor (treatment, year, nesting suc-
cess) log-linear model to test for the effect of radio-tag-
ging (treatment) on nesting success (number of pairs
successful/occupied territory) among years. We used a
one-factor (treatment) ANOVA with a repeated measure
(year) to test for differences between the number of
young produced by control and radio-tagged pairs at oc-
cupied nesting areas. We used a repeated-measure ANO-
VA because the same eagles were monitored each year.
We used a two-factor (year and treatment) ANOVA to
analyze the brood size of successful control and radio-
tagged eagles. Sample sizes were too small to use the re-
peated measures ANOVA for brood size, and treating the
data as independent may have inflated the significance
of this test.

Small sample sizes of radio-tagged and control eagle
nests made conventional significance tests of shading dif-
ferences suspect, so we analyzed differences in shade
characteristics between radio-tagged and control eagle
nests using permutation tests (Manly 1991; StatXact soft-
ware) on each year separately. Nests classified as shaded
or intermediate were combined and compared to ex-
posed nests.

September 1997

Transmitter Effects on Eagles

225

We used a one factor (treatment) AX OVA to compare
the historical likelihood of nesting successfully between
treatment and control nesting areas. Historical likelihood
of successful nesting (number of years successful/all
years occupied) during 1970-91 was calculated for nine
treatment territories and 19 control areas. For this cal-
culation, we excluded controls with more than five con-
secutive vacancies between 1970-91, or consecutive va-
cancies in 1992 and 1993 because such nesting areas were
also avoided during radio-tagging. This is a conservative
bias that excludes extremely unproductive control terri-
tories because such territories would not have been se-
lected for radio-tagging. We also excluded one control
nesting area with a radio-tagged male present from 1975-
80. At nesting areas with past research disturbances, we
excluded cases where productivity might have been influ-
enced, including treatment of nestlings for parasites,
placement of shade devices and trapping and radio-tag-
ging of adults.

Results

Over all years, tagged and control eagles had
similar nesting success (39% of 23 tagged and 51%
of 63 control nests were successful). However, dif-
ferences in nesting success between radio-tagged
and control eagles varied significantly among years
(3-way interaction of treatment, year and fate: G,
= 5.82, P — 0.054, Fig. 1). Radio-tagged eagles had
similar nesting success compared to control eagles
in 1992, but success of radio-tagged eagles was
much lower than control eagles in 1993. In 1994,
radio-tagged eagles had slightly higher nesting suc-
cess than control eagles.

The timing of failures varied among years. In
1992, all seven radio-tagged pairs laid and hatched
eggs (100%). In 1993, eight of nine (88.9%)
tagged eagles laid eggs and four (50%) hatched
eggs. In 1994, six of seven (85.7%) tagged pairs
laid eggs and four (66.7%) hatched eggs. The per-
centage of nonlaying control and radio-tagged
pairs, respectively, was 17.4% (N = 4) and 0.0% (TV
= 0) in 1992, 10.0% (TV = 2) and 11.1% (TV = 1)
in 1993, and 38.1% (TV = 8) and 14.3% (TV = 1)
in 1994.

Number of fledglings produced in occupied
territories was associated with tagging and year
(F 2 22 = 5.07, P = 0.016). Radio-tagged eagles pro-
duced fewer fledglings than control eagles in 1993,
but their productivity was the same or slightly high-
er during 1992 and 1994 (Fig. 1). Combining ra-
dio-tagged and control eagles, brood size did not
vary among years (F 235 = 2.04, P = 0,15).

The degree of shading at nests did not differ
between radio-tagged and control eagles. Between
1992 and 1994, control and treatment groups had

similar proportions of exposed nests (1992, 36.8%,
TV = 19, 28.6%, TV= 7; 1993, 38.9%, TV= 18, 62.5%,
TV = 8; 1994, 38.5%, TV = 13, 40.0%, TV =5 ; G> =
1.15, P = 0.56).

Historical nesting success of treatment and con-
trol territories did not differ (F l 2 e = 0.003, P =
0.95). The nesting success between 1971-91 was
50.2% (TV = 9) for treatment territories and 49.8%
(TV = 19) of control territories.

Discussion

Decreased Golden Eagle productivity (nesting
success, fledglings per occupied territory and
brood size) was associated with the presence of a
radio transmitter, but this was significant during
only the 1993 breeding season. This is in contrast
to Prairie Falcons (Falco mexicanus ), which carried
similar transmitters without negative effects on
productivity (Vekasy et al. 1996). The stress of cap-
ture did not appear to inhibit nesting success, as
most eagles were captured in the winter of 1991-
92, and no radio-tagging association with success
was apparent during the 1992 breeding season.
Male eagles captured at two nesting areas in 1993
both had mates that laid eggs, but both were un-
successful. One female captured in both 1993 and
1994 did not lay eggs in either year. Effects of cap-
ture and handling may be more evident when cou-
pled with other year-dependent stresses. The tim-
ing of capture within a winter or the sex of the
bird tagged may also influence effects, but our
sample size is too small to quantify this.

Golden Eagle productivity appears to be related
to jackrabbit density. The variable effect of radio-
tagging on productivity in eagles may be related to
the dynamics of prey population fluctuations. The
strongest association between tagging and success
occurred during a precipitous decline in jackrabbit
densities (1992-93). We detected no association
between tagging and success during a slight recov-
ery from low jackrabbit densities (1993-94) or dur-
ing years of high jackrabbit densities (1991-92).
Radio-tagged eagles may be especially sensitive to
changes in prey densities. During periods of low
prey densities, foraging opportunities may be re-
duced, and transmitter loads can decrease maneu-
verability (Gessaman and Nagy 1988) and may de-
crease foraging success.

Weather and nest shading may have interacted
with low prey populations to reduce radio-tagged
eagle nesting success in 1993. Although nest shad-
ing did not differ significantly between treatment

226

Marzluff et al.

Vol. 31, No. 3

</>

c n

— i

■

Radio

□

Control

1992

1993

1994

1992

1993

1994

1992

1993

1994

Figure 1. Radio-tagged and control Golden Eagle nesting success for all occupied nesting areas and mean (±SE)
number of fledglings (nestlings ^51 d old) per occupied territory and per successful pair. Sample sizes (numbers of
pairs) are given above error bars.

September 1997

Transmitter Effects on Eagles

227

and control nests, treatment nests in 1993 had the
highest percentage of exposed nests (62.5%).
Aside from having the lowest prey densities during
our study, the spring of 1993 was also very cool and
wet (NOAA 1993). Wet weather has been associat-
ed with poor foraging success in raptors (Adamcik
et al. 1979, Kostrzewa and Kostrzewa 1990), and
low prey and poor foraging conditions may dispro-
portionately reduce foraging success of radio-
tagged eagles compared to controls. Females we
studied left the nest unattended while males were
absent and may have left more frequently or for
greater durations because of food stress. This may
leave eggs and small chicks exposed and could de-
crease their survival during extreme weather con-
ditions (Mosher and White 1976).

Small sample size may have had the greatest in-
fluence on whether or not we detected an effect
of radio-tagging on Golden Eagles. We attempted
to reduce some of the bias associated with small
sample size by comparing historical nesting success
between treatment and control territories. How-
ever, a slight change in the composition of our
sample can have large effects. For example, two
radio-tagged pairs had subadult mates in 1993, and
both were unsuccessful. Steenhof et al. (1983)
found that pairs of Golden Eagles with at least one
subadult member had lower nesting success com-
pared to adult pairs. If the age composition of pairs
in 1993 had been different or both pairs with sub-
adults had been successful, we may not have de-
tected any difference in nesting success between
radio-tagged and control eagles.

Acknowledgments

This study was funded primarily by the Idaho Army
National Guard (IDARNG) under a contract (U.S. Army
contract DAAD05-90-0135) and numerous agreements
administered by W.S. Seegar. The U.S. Bureau of Land
Management (BLM) and the U.S. National Biological
Service provided additional funding and support. This
study was part of the cooperative BLM/IDARNG project.
L. Schueck, J. McKinley, R. Townsend, B. Kimsey and M.
McFadzen (Greenfalk Consultants), and L. Carpenter
and R. Lehman (RRTAC) gave valuable assistance with
data collection, analysis and debugging. Al Harmonster,
Mike McGrady and Jim Gessaman made valuable sugges-
tions to improve earlier drafts of the manuscript.

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Edwards, T.C. and M.N. Kochert. 1986. Use of body
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effects of radio transmitters on the behavior of nest-
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sure of Golden Eagle nests. Can. Field-Nat. 90:356-359.

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MOEN. 1993. Effects of harness transmitters on be-
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, M.N. Kochert and J.H. Doremus. 1983. Nesting

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Received 20 May 1996; accepted 22 April 1997

J. Raptor Res. 31(3):228-233
© 1997 The Raptor Research Foundation, Inc.

CRESTED CARACARA FOOD HABITS IN THE CAPE REGION OF

BAJA CALIFORNIA, MEXICO

Ricardo Rodriguez-Estrella and Laura B. Rivera Rodriguez

Centro de Investigadones Biologicas del Noroeste, km 1 Carr. San Juan de la Costa,

El Comitdn, La Paz 23000 B.C.S., Mexico

Abstract. — The diet of the Crested Caracara ( Caracara plancus ) in the Cape region of Baja California,
Mexico was studied by analyzing 195 pellets collected beneath 10 occupied nests in 1990 and recording
prey brought to two nests containing young in 1990-91. Our results showed that Crested Caracaras fed
opportunistically on mammals (mainly lagomorphs), reptiles (mainly iguanas and snakes), carrion of
domestic animals such as cattle and dogs, and invertebrates (mainly Coleoptera and Orthoptera) . The
caracaras’ ability to kill live prey was denoted both by the high frequency of reptile and bird remains
in pellets, and by the high frequency of remains of recently killed birds, lizards and hares that were
carried to nests. Our observations at nests indicated that Crested Caracaras killed as much as 63% of
vertebrate prey in pellets, while invertebrates may all have been captured alive. Numerically, live prey
comprised about 88% of the diet of caracaras in the Cape region. In terms of ingested biomass, lago-
morphs (both carrion and killed prey) , reptiles (both carrion and killed iguanas, snakes) and carrion
of cattle, represented the most important food sources. We discuss the importance of slaughterhouses,
henhouses and garbage dumps for young caracaras in the Cape region.

Key Words: Caracara plancus; Crested Caracara ; food; Baja California, Mexico.

Habitos alimenticios de Caracara plancus en la region del Cabo, Baja California, Mexico

Resumen. — La dieta del caracara comun ( Caracara plancus) fue estudiada en la region del Cabo, Baja
California, Mexico, analizando 195 egagropilas colectadas bajo 10 nidos activos en 1990 y registrando
las presas traidas a dos nidos conteniendo 3 y 1 polios en 1990 y 1991, respectivamente. El caracara es
una rapaz oportunista que se alimenta principalmente de lagomorfos, iguanas, culebras, carrona de
animates domesticos, e invertebrados (coleopteros y ortopteros). De acuerdo a nuestras observaciones
en nidos y al analisis de egagropilas, estimamos que los caracaras cazaron el 63% de los vertebrados,
mientras que los invertebrados fueron todos cazados. Por lo tanto, en terminos de frecuencia, las presas
vivas represen taron alrededor de 88% de la dieta, aunque fueron lagomorfos y reptiles (presas y carrona
de ambos grupos), y la carrona de ganado quienes proveyeron la mayor biomasa. Se discute sobre la
importancia de los mataderos, granjas avicolas y basureros en la dieta de los jovenes caracaras en la
region del Cabo.

[Traduccion Autores]

Among caracaras, the Crested Caracara ( Cara-
cara plancus) is the species with the widest distri-
bution in America, ranging from Florida, Texas
and southern Arizona, through most of Mexico,
particularly in deserts and tropical areas (Peterson
and Chalif 1973), south to Tierra del Fuego
(Brown and Amadon 1968). In spite of its wide
distribution, very little is known on the ecology and
feeding habits of this species. Currently, studies on
its breeding ecology are being carried out in dif-
ferent areas of its distribution (Texas, Dickinson
and Arnold 1996; Mexico, Rodriguez-Estrella et al.
unpubl. data; Florida, J. Morrison pers. comm.; Ar-
gentina, A. Travaini pers. comm.). The Crested

Caracara is described to be opportunistic but large-
ly carrion-feeding raptor (Sherrod 1978, Johnsgard
1990), although it may hunt living prey and steal
food from other birds (Bent 1938, Hamilton 1981,
Whitacre et al. 1982, Rodriguez-Estrella and Rivera
1992). Descriptions of its diet have been largely
anecdotical and few quantitative data have been
published on variation in feeding habits through-
out its range (Bent 1938, Haverschmidt 1947,
Sprunt 1954, Glazener 1964, Brown and Amadon
1968, Richmond 1976, Layne et al. 1977, Kilham
1979, Thiollay 1980, Mader 1981, Whitacre et al.
1982, Lyons 1988, Wallace and Temple 1987, Palm-
er 1988, Yosef and Yosef 1992, Dickinson 1995).

228

September 1997

Crested Caracara Diet in Mexico

229

Here, we present information on the diet of a pop-
ulation of Crested Caracaras during the breeding
season in the Cape region of Baja California, Mex-
ico.

Study Area

We studied caracaras in the xerophylous scrub vege-
tation of the Cape region of Baja California (109°60’-
1 1 1°45'W, 25°45'N). The vegetation is characterized by
cardon (Pachycereus pringlei), dagger cactus ( Stenocereus
gummosus ) , mesquite { Prosopis articulata ) , palo verde ( Cer-
cidium microphyllum) , Adam’s tree ( Fouquieria diguetii),
plum tree ( Cyrtocarpa edulis ), copal (Bursera spp,), lomboy
( Jatropha cinerea) and cholla ( Opuntia cholla). The eleva-
tion of the area ranges from 0-250 m. This zone is char-
acterized by a mean annual precipitation of 150.6 mm, a
winter rainy season and an annual temperature range be-
tween 22.1-23. 4°C.

Methods

The breeding season of the Crested Caracara in the
Cape region extends from February-August. We collect-
ed pellets and prey remains during the breeding season
of 1990, particularly in April, May, June and July. Feeding
habits were determined by analyzing 195 fresh and whole
pellets collected in and around 10 occupied nests. As
Chi-square tests did not detect significant differences in
the type of prey appearing in pellets from all nests ( P >
0.05), we pooled all data. Pellets from nests located near
henhouses were eliminated from the analysis to avoid an
overrepresentation of carrion in the diet. For identifica-
tion, we compared remains of skulls, bones, hairs, scales,
feathers and invertebrates with known reference speci-
mens at the Centro de Investigaciones Biologicas del No-
roeste (CIBNOR, Mexico) . Prey remains in pellets were
identified to the closest possible level of taxonomic res-
olution. Food-niche breadth was estimated using the Lev-
ins (B) index (Krebs 1989). Numbers of prey species
were used for computation of niche breadth.

As it was not possible to determine whether mamma-
lian and reptilian prey represented in the pellets were
captured alive or collected as car rion by caracaras, we
made observations from a blind at a nest one day (Nj =
675 min) and at a second nest for seven days (N z = 3102
min) to determine the proportion of prey types trans-
ported to the nest by adults that were freshly-killed (Rich-
mond 1976, Mader 1981). Nest 1, containing three young
near fledging age, was observed on 16 May 1991 and all
prey adults brought to the nest were recorded. In 1992,
we made similar observations at nest 2 which contained
one young approximately two wk old. We observed prey
deliveries at this nest from 25 September-21 October,
when the young caracara fledged. We analyzed our data
in terms of ingested biomass. If a prey item was heavier
than 500 g, we considered that the prey was probably
consumed as carrion. Mean prey weights were obtained
from specimens trapped in the field and from those
stored at CIBNOR.

Additionally, the number and age (immature, adult;
Clark and Wheeler 1987) of caracaras feeding on carrion
at slaughterhouses, henhouses and garbage sites were re-
corded (Rodriguez-Estrella 1996). Whenever caracaras

were found feeding on roadkills, we recorded the species
on which they were feeding.

Results

Crested Caracaras preyed on a broad variety of
vertebrates and invertebrates (B - 6.3). Prey spe-
cies richness was over 60 species (Table 1). The
most important prey in terms of numbers were in-
sects (mainly Orthoptera and Coleoptera) which
represented 68% of the prey items identified (Ta-
ble 1, N = 2152). In terms of biomass, mammals
(mainly Lepus and Sylvilagus) and reptiles (spiny-
tailed iguana [Ctenosaura hemilopha], and snakes)
were the most important prey (almost 80% of in-
gested biomass, Table 1). We considered that small
mammals, birds, small to medium reptiles, and in-
sects, were preyed upon by caracaras as live prey
because we recorded caracaras both carrying re-
cendy killed items to the nest (Table 2) and killing
those prey. Whether mammals and reptiles >500 g
appearing in pellets were captured alive and car-
ried to the nests remains unknown, but our obser-
vations of prey transported to nests by adults made
this seem doubtful. At nest 1, adults delivered one
mouse, five birds, one spiny-tailed iguana, one
piece of a lagomorph and one unidentified lizard.
None of these were >500 g. At nest 2, one kan-
garoo rat ( Dipodomis merriami), two woodrats (Ne-
otoma lepida), one bird, five lizards, one snake and
several pieces of apparendy freshly-killed hares and
rabbits, none of which were >500 g, were delivered
(Table 2). We also observed adult caracaras hunt-
ing, pursuing and killing live prey on six occasions:
two White-winged Doves ( Zenaida asiatica), two Inca
Doves ( Columbina passerina ) and two spiny-tailed
iguanas. Again none of these prey were >500 g.

Adult, but especially immature caracaras were
frequendy recorded feeding on carrion at hen-
houses, slaughterhouses and garbages dumps
(Rodriguez-Estrella 1996). Most roadkills where
caracaras fed were of hares ( N = 20) , cows ( N =
10), horses ( N = 5), small reptiles (N = 5), small
mammals ( N = 5) and domestic dogs (N = 3).
Caracaras also fed on maggots ( Cochliomyia macel-
laria) at carcasses, adding up to 150 items that one
adult ate in 5 min. Caracaras tended at times to
follow tractors in fields being plowed catching
grasshoppers and small mammals killed by plow.

Discussion

The Crested Caracara in the Cape region is an
opportunistic raptor feeding on mammals (mainly

230

Rodri'guez-Estrella and Rodriguez

Vol. 31, No. 3

Table 1. Diet of the Crested Caracara in the Cape region of Baja California, Mexico in 1990 as determined from
195 pellets collected at nests. Totals show the number of individuals per group and the ingested biomass in grams.
Asterisks indicate that computing weights were a maximum of 500 g.

Species

Weight (g)

% Freq. 1

% Biom.

% Appear. 2

Mammalia

Lepus califomicus

500*

3.6

21.9

39.5

Sylvilagus auduboni

500*

2.4

14.8

26.7

Ammospermophilus leucurus

102

0.8

1.0

9.2

Thomomys umbrinus

103

0.8

1.0

8.7

Chaetodipus arenarius

26.0

0.3

0.1

3.6

Dipodomys merriami

42.0

0.3

0.1

3.1

Peromyscus eva

13.8

0.4

0.1

4.1

Peromyscus sp.

13.0

0.05

tr*

0.5

Neotoma lepida

148

0.3

0.5

3.1

Unidentified rodents

25.0

0.8

0.2

8.7

Canis latrans

500*

0.1

0.9

1.5

Spilogale putorius
Total 3

500*

0.05

213

0.3

71959.1

0.5

Aves

Falco sparverius

0.05

0.5

Callipepla californica

189.5

0.1

0.2

1.0

Zenaida asiatica

152.9

0.8

1.6

9.2

Columbina passerina

0.1

tr*

1.0

Geococcyx californianus

210

0.2

0.5

2.1

Melanerpes uropygialis

0.4

0.2

4.1

Colaptes auratus

0.1

1.0

Myiarchus cinerascens

27.4

0.2

0.1

2.1

Aphelocoma coerulescens

0.4

4.1

Campylorhynchus brunneicapillus

0.05

tr*

0.5

Phainopepla nitens

25.4

0.05

tr*

0.5

Cardinalis cardinalis

0.05

tr*

0.5

Icterus cucullatus

31.8

0.3

0.1

3.1

Polioptila californica

0.05

tr*

0.5

Carpodacus mexicanus

0.3

0.1

3.6

Passer domesticus

22.4

0.05

tr*

0.5

Unidentified birds
Total

3.0

131

0.9

7606.4

32.2

Reptilia

Callisaurus draconoides

23.9

0.1

tr*

1.5

Ctenosaura hemilopha

500*

1.9

11.9

21.5

Dipsosaurus dorsalis

60.7

0.6

0.4

6.7

Phrynosoma coronatum

1.9

0.8

20.5

Sceloporus hunsakeri

52.5

0.1

1.5

Sceloporus licki

16.5

0.9

0.2

10.8

Sceloporus monserratensis

0.5

0.1

5.6

Sceloporus zosteromus

29.5

0.1

0.05

1.5

Cnemidophorus maximus

24.9

1.2

0.4

13.3

Lampropeltis getulus

229

0.1

0.4

1.5

Masticophis flagelum

300

0.6

2.2

6.7

Pituophis melanoleucus

280

0.2

0.8

2.5

Salvadora hexalepis

170

0.2

0.4

2.1

Crotalus enyo

500*

0.2

1.1

1.5

Crotalus ruber

500*

0.4

2.3

4.1

Unidentified reptiles

0.2

0.1

2.6

September 1997

Crested Caracara Diet in Mexico

231

Table 1 . Continued.

Species

Weight (g)

% Freq . 1

% Biom.

% Appear . 2

Unidentified snakes

500*

2.7

16.8

28.7

Total

263

67038.1

Invertebrata

Arachnida

0.5

0.05

tr*

0.5

Theraphosidae

5.0

0.05

tr*

0.5

Scorpionidae

2.0

0.6

tr*

6.2

Solifugae

0.5

0.2

tr*

0.5

Chilopoda

2.0

0.1

tr*

0.5

Coleoptera

0.5

0.7

tr*

7.2

Carabidae

0.37

0.7

tr*

5.1

Scarabaeidae

0.5

tr*

3.1

Tenebrionidae #1

0.5

10.9

0.06

40.0

Tenebrionidae #2

0.13

4.3

tr*

20.5

Cerambycidae

1.0

0.3

tr*

2.1

Orthoptera

0.75

0.2

tr*

1.5

Gryllidae

1.0

29.9

0.4

37.9

Acrididae

2.0

11.8

0.3

26.7

Tettigonidae

1.0

0.2

tr*

1.5

Dermaptera

Formiculidae

0.5

3.1

tr*

9.2

Hymenoptera

0.5

0.4

tr*

2.6

Odonata

1.0

0.05

tr*

0.5

Diptera

0.5

0.05

tr*

0.5

Unidentified

1.0

4.1

0.05

22.6

Total

1466

1470.4

Unidentified carrion

500*

3.7

15.6

40.5

Total

27 300.0

GRAND TOTAL

2152

175 374.0

* tr < 0.05% of total prey biomass.

1 Total number of individuals of each prey type X 100 divided by the grand total number of prey.

2 Number of occurrences of each prey type X 100 divided by the total number of pellets; because of this, the sum of frequencies
may be above 100.

3 Total number and biomass per group

lagomorphs) , reptiles (mainly iguanas and snakes) ,
carrion of domestic animals such as catde and dogs,
and invertebrates (mainly Coleoptera and Orthop-
tera) . Its opportunism is evidenced not only by the
breadth of its food niche, but also by the fact that
as many as nine prey species can be found in a
single pellet. The caracara’s ability to kill live prey
is denoted by the high frequency of mobile prey
including reptiles and birds that appear in pellets
and are brought to nests, apparently having been
captured alive. Its predatory ability is also demon-
strated by the six captures we observed of live prey
(doves and lizards) and observations of active prey
pursuit and capture elsewhere (Richmond, 1976,
Layne et al. 1977, Whitacre et al. 1982).

We were not certain as the proportion of lago-
morphs that were taken as carrion or live prey but,

according to our direct observations, we estimated
that caracaras killed about 63% of vertebrate prey
in pellets (considering conservatively that 35% of
lagomorphs were captured as live prey). We as-
sumed that all invertebrates were all captured as
live prey. Based on our numerical analysis of the
diet, we felt that live prey represented 88% of the
diet of caracaras in the Cape region. However, in
terms of ingested biomass, lagomorphs (both car-
rion and killed prey), reptiles (both carrion and
killed spiny-tailed iguana and snakes) and carrion
of cattle, represented the most important food
sources. In Texas, Dickinson (1990) found that the
majority of the caracara’s diet at nest sites consisted
of live-caught prey (61%), with carrion comprising
39%.

Crested Caracaras in Baja California fed nest-

232

Rodriguez-Estrella and Rodriguez

Vol. 31, No. 3

Table 2. Prey items brought to nests of the Crested Caracara in the Cape region, Baja California, Mexico.

Prey

Number

Notes

Nest 1, 16 May 1991
Mammals

Lepus californicus

1 piece of leg

Unidentified rodent

complete item

Birds

Zenaida asiatica

complete item

Carpodacus mexicanus

complete item

Unidentified bird

20-30 g, complete

Reptiles

Ctenosaura hemilopha

complete item

Unidentified lizard

complete item

Total items

Rate/day

Rate/hour

0.80

Nest 2, 25 September-21 October 1992
Mammals

Sylvilagus audubonii

1 leg, 1 head

Lepus californicus

8 pieces of legs, 2 heads

Dipodomys merriami

complete item

Neotoma lepida

complete item

Birds

Unidentified bird

20-30 g, complete

Reptiles

Cnemidophorus sp.

complete item

Sceloporus sp.

complete item

Unidentified snake

ca. 100 g fresh snake

Total items

Rate/day

Rate/hour

0.46

lings mainly with vertebrate prey captured alive, as
observed in studies elsewhere (Richmond 1976,
Mader 1981), but proportions of prey groups dif-
fered. Levy (1988), analyzing 30 pellets collected
beneath a nest in Arizona, found that 26% of the
pellets contained scales of Phrynosoma lizards, 93%
contained arthropod remains, and seldom were
hairs of lagomorphs identified. Dickinson (1990)
reported that invertebrate prey brought to the
nests accounted for only 3% of items.

Immature caracaras seem to depend mainly on
carrion and invertebrates during the postfledging
period as evidenced in our observations that most
immatures foraged near slaughterhouses, hen-
houses, garbage sites (Rodriguez-Estrella 1996)
and cultivated areas rather than in natural areas.
Carrion in human refuse areas is a predictable
source of food and cultivated areas attract high

numbers of invertebrates (mainly Orthoptera) .
Thus, these feeding areas are probably important
for young caracaras in the Cape region. Indeed,
what appeared to be family groups of two adults
and one to three young were commonly observed
during the postfledging period feeding on human
refuse sources and agricultural areas (35.1%, N —
74 group observations). We had the impression
that adults lead juveniles to predictable food
sources. Additional studies on feeding behavior of
immature and adult caracaras during the post-
fledging period deserve further attention in order
to better understand the process by which juveniles
learn to find predictable food sources.

Acknowledgments

Special thanks are given to A. Tejas for advice in the
identification of arthropods. F. Anguiano and E. Mata

September 1997

Crested Caracara Diet in Mexico

233

helped in observations of nests. D. Whitacre and F. Jaksic
made valuable comments that improved the manuscript.
Centro de Investigaciones Biologicas del Noroeste and
CONACyT (1749P-N) provided financial support. LRR
received a grant from CONACyT. RRE received a grant
from CSIC-Spain that helped support completion of this
work.

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Received 20 June 1996; accepted 30 April 1997

J. Raptor Res. 31 (3):234-240
© 1997 The Raptor Research Foundation, Inc.

REMARKABLE SAKER FALCON (. FALCO CHERRUG) BREEDING

RECORDS FOR MONGOLIA

David H. Ellis

US. Geological Survey, Patuxent Wildlife Research Center, Laurel, AID 20708-4019 U.S.A.

Merlin H. Ellis

Institute for Raptor Studies, HC 1, Box 4420, Oracle, AZ 85623

Pu. Tsengeg

Alongolian State University, Ulaanbaatar — 46, P.O. Box 137, Ulaanbaatar, Alongolia

Abstract. — During 1994-95 surveys, we located over 80 Saker Falcon ( Falco cherrug) breeding sites in
Mongolia. Over half of the sites had features that were in some way remarkable or previously unde-
scribed in the scientific literature. Ten were on utility poles, two on bridges, three on abandoned build-
ings and one was on a truck tire on a pole. Seven sites were very near buzzard nests and two more were
in buzzard nests that were used the same season. Five sites were on cliff tops accessible by walking. Four
were on very short cliffs, two were on broken/sloping cliffs and one was at the base of a cliff. Five were
on the tops of stone pillars. Six were in very short elm trees. Nest composition was also sometimes
remarkable: one was a suspended uric acid (excrement) platform without underlying support, three
were trash nests and two were composed largely of bones.

Key WORDS: Saker Falcon ; Falco cherrug; proximal nesting, Mongolian birds; nesting situations ; Upland Buz-
zard; Buteo hemilasius.

Falco cherrug registros de crias en Mongolia

Resumen. — Durante estudios en 1994-95, nosotros localizamos arriba de 80 Falco cherrug sitios de
cria en Mongolia. Arriba de medio de los sitios tenian elementos que estaban en una manera
extraordinario e nunca antes describido en la literatura cientifica. Diez estaban en postes, dos en
puentes, tres en edificios abandonados y uno en una llanta de camioneta en un poste. Siete sitios
fueron cerca de nidos de Buteo buteo, y dos mas estaban en los nidos de Buteo buteo que estaban
usadas esa misma temporada. Cinco sitios estaban en precipicios accesible por caminando. Cuatro
estaban en precipicios cortos, dos estaban en precipicios quebrados/inclinado y uno estaba en el
base del precipicios. Cinco estaban en el cumbre de columnas de pierda. Seis estaban en arboles
muy cortos. Composicion de nidos tambien a veces estaba extraordinario: uno estaba suspendido
en acido uric plataforma sin soporte por de bajo, tres nidos eran de basura y dos estaban componida
por mayoria de huesos.

[Traduccion de Raul De La Garza, Jr.]

The Saker Falcon ( Falco cherrug) normally
nests on cliffs (short and tall) and in tall trees
(Brown and Amadon 1968, Cade 1982). Other
species of large falcons are known to occasionally
breed on man-made structures (Newton 1979)
and one species, the Peregrine Falcon ( F. peregri-
nus), regularly does so, at least in some parts of
its range. Remarkably, in the eastern Sahara Des-
ert, the Lanner Falcon ( F. biarmicus) has been
reported to breed in such odd situations as in
abandoned motor vehicles and on the ground

next to fuel cans (Goodman and Haynes 1989).
The only published record of a saker possibly
nesting on a man-made structure is Baumgart’s
(1978) reference to a pair that he believed “bred
on a ruin” (Baumgart 1980).

During 1994-95 surveys across Mongolia, we lo-
cated over 80 saker nests. At 78 of these sites, pairs
were breeding the year of our visit. This paper
summarizes unusual aspects of these breeding
sites. We previously reported saker productivity us-
ing data from these samples (Ellis et al. 1995). In

234

September 1997

Saker Falcon Nesting in Mongolia

235

Table 1. Features of Saker Falcon nests on man-made structures in Mongolia, 1994—1995.

Support Type

Number Nests

Nest Height (Structure Height) in m

Powerline monopod

14 (23)

Telephone monopod

7 (8)

Powerline bipod

8 (9), 7 (8)

Powerline tripod

9 (11), 8 (10), 8 (9), 6 (8), 8 (9), 9 (9)

Bridges

1.4 (2.3), 2.0 (2.9)

Abandoned buildings

5 (7), 3.1 (3.1), 5 (6)

Truck tire on metal pole

11 (11)

the only previous study of saker breeding in Mon-
golia, Baumgart (1978) found several pairs, most
of which were believed breeding in the montane
forests near Ulaanbaatar.

Methods

From May-July 1994 and 1995, our survey team, using
4-wheel drive vehicles, traversed 10 781 km in two survey
loops beginning in Ulaanbaatar and extending into ex-
treme northwestern Mongolia (1994) and extreme east-
ern Mongolia (1995). Inasmuch as improved roads are
almost nonexistent except near Mongolia’s largest cities,
our meandering route was largely determined by the
presence of potential raptor nesting habitat on the ho-
rizon.

At each site, we measured over 20 descriptive para-
meters. Most measurements were taken directly from
taped segments of climbing ropes or using tape mea-
sures. Longer distances were estimated from photo-
graphs by proportional comparisons with humans of
known height or segments of the cliff or other supports
of known height or length. Access to powerpole tops was
obtained by first shooting a tethered (nylon, monofila-
ment fish line) arrow over a cross arm, hoisting a climb-
ing rope attached to the monofilament, then using con-
ventional climbing ascenders to scale the rope. Poles with
ground wires were not scaled. Horizontal measurements
were sometimes taken by pacing distances between nest
sites. Long distance estimates are believed to be within
5% of their true value. Distances of 10 m or less are ac-
curate to the nearest 2 cm.

Results

We found 10 nests on power or telephone sup-
port structures (Table 1 ) . Most of these were con-
structed by Upland Buzzards ( Buteo hemilasius),
Ravens ( Corvus corax ) or perhaps Black Kites (Mil-
vus migrans) . Only a small proportion of the poles
in Mongolia are used by raptors because pole con-
figurations usually will not support their nests. We
found a few fallen nests below poles and more than
100 Upland Buzzard nests on the ground imme-
diately adjacent to utility poles. Sakers never pre-
empted these ground nests. Remarkably, the buz-
zards were able to fledge young even in areas

where foxes ( Vulpes vulpes and Cynalopex corsac) and
wolves ( Canis lupus) were believed to be common.
The utility pole nests were in central and eastern
Mongolia and were found only where trees and
cliffs were absent. There are records of sakers nest-
ing on utility pylons in Hungary (Bagyura et al.
1994) and a single record for the Lipetsk Region,
south of Moscow, Russia (V.M. Galushin, pers.
comm.).

Six saker nests were in truly remarkable situa-
tions. Two were on very low railroad bridges. One
of these was only 1.4 m above water. A second was
2 m above the ground (Fig. 1). Three were on
buildings. Two of these were in windows and one
was on a rooftop only 3.1 m above ground. One
nest was on a truck tire on top of a pipe at the
edge of an abandoned Russian military post. All of
these unusual nesting situations (Table 1) were in
eastern Mongolia and far from sizeable cliffs or for-
ests.

Although we frequently saw Saker Falcons chas-
ing Upland Buzzards, we found seven situations
where nests of the two species were in close prox-
imity. Although Dementiev and Gladkov (1951)
mention sakers nesting near other raptors, our ob-
servations in Mongolia suggest that sakers rarely
nest within 200 m of buzzards. At the five excep-
tional sites, sakers were nesting 4.4—50 m from Up-
land Buzzards (x = 36 m). The most unusual of
these nests was only 4.4 m from, and directly above,
a buzzard nest (Fig. 2) containing 2 large fledg-
lings. With the oldest saker nestling about 26 days
of age, the female spent very little time on the nest
except when feeding. From her roost, a Buddhist
shrine 250 m away, she harried the buzzards when
they came within 200 m of the nest. However, once
on its nest, the buzzard could remain unmolested
even when the falcon was on hers.

We found two sites where Saker Falcons had
nested in nests that were later that same year re-

236

Ellis et al.

Vol. 31, No. 3

Figure 1. A saker nest 2 m from the ground on an active railroad trestle.

furbished by Upland Buzzards. At one of these, we
found two saker eggs (one dimpled but sloshy and
a second egg crushed and being consumed by der-
mestid larvae) beneath about 10 cm of recently
added sticks. At the second nest, we found large,
bright (not faded) eggshell fragments beneath
about the same depth of sticks. The lack of fresh
whitewash at egg level in both nests suggested that
neither pair of sakers had hatched or fledged
young.

Sakers also occasionally nested near eagles, but
not nearly as close as to Upland Buzzards. We
found two Steppe Eagle (Aquila nipalensis) nests,
each approximately 1.5 km from saker nests. The
nearest occupied Golden Eagle ( Aquila chrysaetos)
nest containing one nestling was estimated to be
within 200 m of a saker brood, occupying one of
the eagle’s alternate nests.

Most Saker Falcon cliff nests are placed in inac-
cessible niches. However, we found five nests
placed at the very tops of cliffs (Table 2). All of

these were approachable from above by walking,
with no climbing or descending required. These
sites were sometimes beside an emergent boulder,
but each nest could be easily entered by a wolf,
and the eggs in one had been burned in a grass
fire.

At four other sites, the nests were inaccessible,
but the cliffs were very short (<6.5 m high, Table
2). Two other nests were on sloping or broken cliffs
that were accessible from above, below, and/or the
side. The most accessible scrape was on bare soil
at the very base of a tiny, sloping cliff. It had none-
theless fledged at least two young just prior to our
visit.

Five saker nests were on stone columns with little
or no shade for either the brooding adult or the
nestlings (Fig. 3). All of these pillars were steep
enough to require climbing, but only the two
tallest were secure from mammalian predators.

Although falcons do not build nests (Ellis 1993) ,
one of our saker sites seems to violate this rule

September 1997

Saker Falcon Nesting in Mongolia

237

Figure 2. A cliff top Saker Falcon nest with an Upland Buzzard nest 4.4 m below.

238

Ellis et al.

Vol. 31 , No. 3

Table 2. Features of Saker Falcon nests on very short or broken cliffs.

Location of Nest

Number Nests

Nest Height (Cliff Height) in m

On accessible cliff top

8 (9), 3.1 (3.4), 3.7 (3.7), 5.5 (5.5), 5.2 (5.8)

On cliff face

3.0 (5.8), 4.0 (4.9), 4.1 (6.3), 3.2 (4.7)

On sloping or broken cliff

2.1 (2.7), 2.4 (2.7)

At cliff base

0.0 (ca 3)

(Fig. 4). It consisted of an unsupported uric acid
(excrement) platform wedged into a crevice. A
twig clinging to its underside evidenced that it was
once underlain and supported by a stick nest.

Many saker nests contained trash collected by
the previous occupants. In three of these, trash
items were conspicuously important in their
composition and long stringers of wire, twine, or
cloth dangled from the rim. On the open steppe
where few natural building materials are avail-
able other than grass, raptors of several species
routinely use cast off clothing, machine parts,

wire, bones, and tools as nesting material. In one
Upland Buzzard nest, we even found paper mon-
ey.

Dementiev and Gladkov (1951) previously re-
ported a Saker Falcon nest in an elm tree ( Ulmus
sp.) in Mongolia. We found six instances of Saker
Falcons using small elms in southeastern Mongo-
lia. These were 2. 7-4.0 m above the ground in elms
ranging from 4.9— 8.5 m tall. All of these were stick
nests probably built by either Black Kites or Up-
land Buzzards. All but one tree provided a closed
canopy, shading the nest.

Figure 3. A nest on a short, broad, unshaded pillar in southeastern Mongolia.

September 1997

Saker Falcon Nesting in Mongolia

239

Figure 4. This nest is an unsupported uric acid platform, formerly underlain with a stick nest.

Discussion

Saker Falcons are now known to breed in a
wide variety of situations in Mongolia. Many of
these were previously unreported for the saker
and some are new for any large falcon. Probably
the most remarkable structural supports were
the two very low, railroad trestles and the elevat-
ed tire. From a behavioral viewpoint, the nest
only 4.4 m from a buzzard nest was most re-
markable. The saker’s adaptability in using a
wide range of breeding situations is, no doubt, a
response to favorable prey populations in areas
lacking large trees and cliffs.

Acknowledgments

Our expeditions were financed by the National
Aeronautics and Space Administration (NASA), Patux-
ent Wildlife Research Center (Patuxent) and the Na-
tional Avian Research Center (NARC) of the United
Arab Emirates. We offer our deep appreciation to Pat-
rick Coronado and Dr. Vincent Solomonson (both at
NASA/Goddard SFC); Dr. Nick Fox (at NARC); Dr.

George F. Gee (at Patuxent), and our Mongolian
hosts. Gansook, our chauffeur in 1994, provided many
indelible memories, and our agent, Batmonkh, facili-
tated logistics, most importantly a Mongolian driver’s
license for the senior author for the 1995 expedition.
Cathy Ellis assisted in arrangements and manuscript
preparation. Helpful reviews were provided by Tracy
Fleming and Drs. Joseph Schmutz, Tom Cade, Clayton
White and Marc Bechard.

Literature Cited

Bagyura, J., L. Haraszthy and T. Szitta. 1994. Methods
and results of Saker Falcon Falco cherrug management
and conservation in Hungary. Pages 391-395 in B.-U.
Meyburg and R.D. Chancellor [Eds.], Raptor conser-
vation today. WWGBP/The Pica Press, London, UK.
Baumgart, W. 1978. Concerning plumage status, and
migration of breeding eastern Saker Falcons. [In Ger-
man], Mitt. Zool. Mus. Berlin 54 (Suppl., Ann. Orn. 2):
145-166.

. 1980. Der Sakerfalke. A. Ziemsen Verlag, Wit-
tenberg Lutherstadt, Germany.

Brown, L. and D. Amadon. 1968. Eagles, hawks and fal-
cons of the world, Vol. 2. McGraw-Hill, NY U.S.A.

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Ellis et al.

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Cade, T.J. 1982. The falcons of the world. Comstock/
Cornell Univ. Press, Ithaca, NY U.S.A.

Dementiev, G. and N.A. Gladkov [Eds.]. 1951. Birds of
the Soviet Union, Vol. 1. Gosudarstvennoe Izdatel’stvo
“Sovetskaya Nauka”, Moskva. Translated from Rus-
sian by the Israel Program for Scientific Translations,
1966 (Vol. 1), Jerusalem, Israel.

Ellis, D.H. 1993. Do falcons build nests? J. Raptor Res.
27:217.

, M.H. Ellis and Pu. Tsengeg. 1995. Productivity

of Saker Falcons ( Falco cherrug) in Mongolia. Pages
117—130 in Proc. Specialist Workshop, Middle East
Falcon Research Group, Abu Dhabi, U.A.E.

Goodman, S.M. and C.V. Haynes, Jr. 1989. The distri-
bution, breeding season, and food habits of the lan-
ner from the eastern Sahara. Natl. Geogr. Res. 5:126-
131.

Newton, I. 1979. Population ecology of raptors. Buteo
Books, Vermillion, SD U.S.A.

Received 20 June 1996; accepted 17 May 1997

J, Raptor Res. 31 (3):241-252
© 1997 The Raptor Research Foundation, Inc.

SPATIAL INCIDENCE OF BARRED OWL ( STRIX VARIA)
REPRODUCTION IN OLD-GROWTH FOREST OF THE

APPALACHIAN PLATEAU

J. Christopher Haney 1

Wildlife Technology Program, School of Forest Resources, The Pennsylvania State University,

DuBois, PA 15801 U.S.A.

Abstract. — Barred Owl ( Strix varia) occurrence and breeding were evaluated in old-growth forest using
Poisson and binomial models constructed with seven spatially-explicit parameters derived from territorial
density. Reproduction was evidenced by owl chicks heard inside cavity nests or being fed by adults in old-
growth deciduous (beech-maple, oak-hickory) and old-growth mixed forest types (hemlock-white pine-decid-
uous). Barred owls nested on 64% of 11 relatively small (6-33 ha) study plots. Probabilities of obtaining this
many cases of breeding or occurrence by chance alone were extremely low in all model executions, ranging
to as little as P = 1.6 X 10 -7 . Compared to managed forests, old-growth forests used by breeding owls typically
had higher snag densities and basal areas, large (>45 cm dbh) eastern hemlock ( Tsuga canadensis), some
large live trees 50-100 cm dbh, and reduced understories. Among old-growth stands, vertical (P = 0.06) and
horizontal complexity (P < 0.01) of the canopy differed significantly between areas used and not used for
breeding. As in other Strix, I infer that spatial juxtaposition of structural features in late successional forest
favors localizing reproductive effort within a small subset of the owl’s home range. Older forest provides large
cavities for nesting, a dense canopy for thermoregulation and protection from mobbing, and sparse ground
cover that may facilitate prey detection and capture. All of these structural features are enhanced by life
history characteristics of eastern hemlock.

Key WORDS: Barred Owl, Strix varia; reproduction; breeding season habitat, habitat use, eastern old-growth ;

Pennsylvania.

Incidencia espacial de Buhos ( Strix varia) reproduccion en bosques viejos en el Appalachian Plateau

Resumen. — Ocurrencia y crianza del Buho ( Strix varia) fueron evaluados en bosque de crecimiento-viejo
usando modelos Poisson y binomial construidos con siete parametros explicitos derivados de densidad ter-
ritorial. Reproduccion fue indicado por buhos chicos oidos dentro de la cavidad de nidos o por los adultos
dandoles de comer en crecimiento-viejo de bosques de hoja caduca (beech-maple, oak-hickory) y bosque
mixtos de crecimiento-viejo (hemlock-white pine-deciduous) . Buhos estaban en nidos en 64% de 1 1 lugares
de estudio relativamente pequenos (6-33 ha). Probabilidades de obteniendo tantas situaciones de cria y
ocurrencias por chanza sola eran muy bajas en modelo executaciones, desde tan poco come P= 1.6 X 10
Comparado con bosques manejados, bosques de crecimiento-viejo usados por buhos que crfan tipi cam ente
tenian densidades altas y areas (basal) , grandes (>45 cm dbh) Tsuga canadensis, unos arboles grandes 50-
100 cm dbh, y vegetation reducidas por de bajo. Dentro de areas de bosques de crecimiento-viejo, vertical
(P = 0.06) y complexidad horizontal (P < 0.01) del dosel vario mucho entre areas usadas y areas no usadas
para cria. Como en otras Strix, Yo digo que espacial yuxtaposicion de elementos estructurad en bosques
sucesional tardes hace favor de localizar esfuerzos reproductive dentro de un lugar chico en el arreo de los
buhos. Bosques maduros mantienen cavidades grandes para nidos, un dosel denso para reglamentacion
termal y protection de una multitud, y un suelo disperso que puede facilitar detection de presa y captura.
Todos estos elementos estructurad mejoran los caracterfsticos historicos de la vida del Tsuga canadensis.

[Traduction de Raul De La Garza, Jr.]

Mature and structurally-complex forest is a com-
mon feature of breeding habitat in North Ameri-

1 Present address: The Wilderness Society, Ecology and
Economics Research Dept., 900 17th Street N.W., Wash-
ington, D.C. 20006 U.S.A.

can Strix owls. Affinities for old-growth forest are
more widely recognized in the Northern (5. occi-
dentalis caurina, Forsman et al. 1984), California ( S .
o. ocddentalis, Gutierrez et al. 1992) and Mexican
Spotted Owls (S. o. lucida, Ganey and Baida 1994),
but Great Gray (S. nebulosa) and Barred Owls (S.

241

242

Haney

Vol. 31, No. 3

varia) have also been linked with late successional
forests with large-diameter trees (Elody and Sloan
1985, Allen 1987, Bull et al. 1988). Extensive tracts
of old-growth containing eastern hemlock ( Tsuga
canadensis ) were identified as important Barred
Owl habitat a century ago (Bolles 1890, Eifrig
1907).

Barred Owls have been chosen as a management
indicator species in several eastern national forests
(USDA 1985, 1986), and are classified as threat-
ened in some states (Bosakowski 1994). Concern
for this species has increased because of its sensi-
tivity to anthropogenic disturbance, including for-
est fragmentation, and because such land-use prac-
tices may indirectly erode integrity of its habitat via
increasing competition with the more disturbance-
tolerant Great Horned Owl ( Bubo virgini-
anus) (Morrell and Yahner 1994, Laidig and Dob-
kin 1995).

Generally an uncommon nocturnal predator,
the Barred Owl occurs at low densities (one terri-
tory per 2.5-6.5 km 2 ; Nicholls and Fuller 1987, Bo-
sakowski et al. 1989). Populations can be moni-
tored by broadcasts of conspecific recordings
(McGarigal and Fraser 1985, Mosher et al. 1990),
but playback may elicit little response from Barred
Owls during incubation and early chick rearing
(Devereux and Mosher 1984, Laidig and Dobkin
1995). Objectives of this study were to develop and
test statistical models that would evaluate Barred
Owl use of breeding sites in old-growth forest using
passive sampling at spatial scales less than the size
of the home range and to describe structural attri-
butes and habitat configuration in the general vi-
cinity of breeding sites (Hunter et al. 1995).

Methods

Study Areas. Potential study areas on the Appalachian
Plateau physiographic province in western and northcen-
tral Pennsylvania (Fig. 1) were first screened by consult-
ing inventories of locations, forest type, management re-
gime and size of remaining old-growth forests (Mickalitis
1956, Erdman and Wiegman 1975, Smith 1989). Because
a major criterion for plot selection was a size sufficient
to contain the minimum recommended area for a Breed-
ing Bird Census (BBC) in forested habitat (10 ha, Lowe
1995), the smallest sites (26%, N = 51) were excluded
from consideration. Two or more study plots were estab-
lished in each of the three largest consolidated tracts of
mixed old-growth forest in Pennsylvania: Cook Forest
State Park (>200 ha); Tionesta Scenic and Research nat-
ural areas (1675 ha) and Heart’s Content, Allegheny Na-
tional Forest (60 ha).

Aerial photos and detailed maps of stand ages were
used visually to establish eleven 10-18 ha plots away from

50 t

40 -

Mixed coniferous-hardwood old-growth
("Henry Run")

50 t

40-

Deciduous old-growth
("Tryon-Weber")

7.6- 15.2- 22.9- 36 1- 53.3- 66.6- 83.8- i 1 01.6

15.2 22.9 38.1 53.3 68.6 83.8 101.6

Diameter size class (cm)

Figure J. Characteristic diameter size-class distributions
of old-growth mixed, old-growth deciduous, and young,
previously-harvested deciduous forest. Black histograms
represent regional averages for all size classes as calculat-
ed with data from the relevant regional unit as summa-
rized in Alerich (1993); different widths to histograms
reflect varying size-class intervals for which data were
available. Stippled histograms represent the size distri-
butions of trees observed on plots in this study.

roads, rights-of-way, habitat edges and extensive wind-
throws, and in areas where vegetation age and composi-
tion were relatively uniform. Due to limited availability,
small size of potential study areas and other logistical
constraints, random selection of study plots within sites
was not feasible. Nevertheless, all plots were chosen with-
out prior knowledge of the presence of Barred Owls.
Variable extent and shape of old-growth forest remnants
also necessitated study plots of different sizes; plot shapes
were usually square or rectangular. Combined area of all
study plots used in this study was approximately 4% of
the total old-growth known to remain in Pennsylvania
(Haney 1996).

Compared to nearby managed forests, old-growth sites

September 1997

Barred Owl Breeding in Old-growth

243

in this study possessed stand ages older than the average
age at which disturbances interrupt succession (200-300
yr), basal areas 30-73 m 2 /ha, large (70-100 cm dbh) live
and dead trees, canopy cover >90% and a primary mode
of disturbance by windthrow. Eastern hemlock made up
37—70% of the canopy at mixed forest sites; codominant
canopy trees included various hardwoods and occasion-
ally a few eastern white pine ( Pinus stratus) . All three
large old-growth study sites were embedded in mostly un-
fragmented landscapes with extensive forest cover
(>3000 ha, Fig. 1).

Internal structure of mixed old-growth sites has never
been altered substantially (Hunter 1989). Each site is
dominated by very old forest. No extensive cutting has
ever been conducted and stand ages (based on coring)
are generally >300 yr. There is some evidence of histor-
ical fire in both Cook Forest and Heart’s Content, but
not in Tionesta (Hough 1936). American chestnut ( Cas -
tanea dentata ) was never prevalent (<10% canopy) or
widespread at study sites (Hough and Forbes 1943,
Abrams and Ruffner 1995) except for Heart’s Content,
where it was once the third most common canopy species
(Lutz 1930). On the other hand, there has been an eight-
fold reduction in total area of this forest type on the
northern Appalachian Plateau since presettlement times
(Abrams and Ruffner 1995).

Due to the regional rarity of deciduous old-growth
(Erdman and Wiegman 1975, Smith 1989), only small
sites with this forest type were available, and two plots did
not meet the minimum preferred size for BBCs. Al-
though possessing large trees, pit-and-mound topogra-
phy, considerable coarse woody debris (CWD) and other
elements of old-growth, the four smaller deciduous old-
growth sites were probably cut selectively sometime late
in the 19th or early 20th century. Deciduous plots were
located in fragmented landscapes; all were bordered on
two or more sides by fields, roads and other open areas.
Three sites were in glaciated northwestern Pennsylvania
where original forest was beech-maple (Fagus-Acer) . The
fourth deciduous plot was dominated by a mixture of
hardwoods, including hickory ( Carya ), oaks ( Qiiercus )
and maples.

Two 15-ha plots were also established in 40-60-yr-old
managed forest on the Appalachian Plateau. Prior to cut-
ting, compositions and basal areas of canopy trees on
these plots were similar to the mixed old-growth forest.
Further details on the vegetation, exact locations and to-
pographic setting of study plots can be found in J. Field
Ornithol. 65(Suppl.):73-74, 88-93, and 66(Suppl.):53-54,
56-59, 70-71, 82-88.

Data Collection. Barred Owls and nest sites were de-
tected during repeated (7-10) visits to each study plot
while territory mapping for BBCs during the 1993-94
breeding seasons; from one to three additional visits per
plot were undertaken to measure vegetation. Each map-
ping visit, lasting from 1. 5-4.0 hr, involved slowly walking
established census lines <100 m apart and delineating
bird territories within gridded plots at 25-50 m resolu-
tion. Order of visitation (date and time of day, whether
dawn, mid-morning or dusk) was randomized. Two visits
at dusk were generally made on each plot. All BBC visits
were conducted between 22 April-5 July, a period coin-

ciding with incubation, brooding and prefledging of the
Barred Owl (Johnsgard 1988).

Reproduction was determined by beak clapping, hiss-
ing and food-begging calls of young from within nest
trees, or observations of stationary, prefledging juveniles
outside nests begging from or being fed by adults. Adult
owls often flushed from daytime roosts and gave noneli-
cited calls during visits, but adult presence alone was not
considered evidence of reproduction.

Data Attributes and Model Construction. Study plots
(Table 1) were quite small relative to home ranges re-
corded for Barred Owls (86-370 ha, Nicholls and Warner
1972); techniques appropriate for other birds, such as
the BBC, are usually unsuitable for wide-ranging and se-
cretive raptors (Fuller and Mosher 1981). Over spatial
scales at which field work was conducted, occurrence of
Barred Owl nests would be unexpected even if plots hap-
pened to be fortuitously located within an owl territory.
This was not necessarily the case as plots were located
solely on the basis of their old-growth chararacteristics
On the other hand, two or more plots that were close
together might be situated within a single territory and
thus not represent independent sample units.

These elements of the field sampling required devel-
oping a statistical approach that addressed explicitly each
of the data attributes mentioned above. Thus, I chose a
simple probability approach for testing occupancy of hab-
itats by Barred Owls. Binomial models better account for
frequency of occurrence in a set of samples (e.g., “inci-
dence,” Wright 1991), and similar approaches have been
applied to other studies of Strix owls (Azuma et al. 1990,
Gutierrez 1994). The general null hypotheses tested were
that Barred Owl reproduction and territorial occupancy
did not occur in old-growth forest more than expected
by chance.

Given a documented upper limit of approximately 370
ha for the home range (Nicholls and Warner 1972), only
distances sl.O km (the approximate radius of a circle
having area 370 ha) could certainly be supposed to con-
tain biologically-independent territories. Plots separated
by distances less than 1000 m were therefore combined
into a single unit, ultimately reducing sample size from
15 to 11 (Table 1). This interval to independence was of
the same order used in other studies where the survey
scale matched movement distances by the species (Bo-
sakowski et al. 1987, Laidig and Dobkin 1995).

Modeling was approached as follows: if owl nests are
located randomly within a hypothetical home range of
area B, and plot A represents some fraction of this area,
then let p — A/ B. The variable p is the binomial for the
likelihood that reproductive effort will be localized in
area A ( = positive incidence) , and is expected to be quite
small, except for plots of moderately large size (e.g., p =
0.10 if A = 10 ha and B = 100 ha). Values for A were
derived from plot sizes used in the study, including plots
combined due to spatial proximity (Table 1). Parameter
values for Bwere obtained from the literature: minimum,
mean and maximum home range (Nicholls and Warner
1972), and mean annual and mean summer home range
(Elody and Sloan 1985).

Probabilities of owl reproduction on a particular plot
were estimated by dividing its area, A, by each of the
parameter values available for B, For plots studied both

244

Haney

Vol. 31, No. 3

Table 1. Cumulative (observed) probabilities (j 6, q, or 2pq) of the likelihood of Barred Owl reproduction in sample
plots during a 2-yr period in eastern old-growth forests. The subsample (N =11 plots) includes four pairs of plots
that were combined due to spatial proximity (see Methods) . Final probabilities indicate the likelihood of obtaining
as many instances of owl reproduction as were actually observed across all plots. Seven different estimates of Barred
Owl home range size or density were used to develop probabilities.

Plot

Mixed Coniferous-Deciduous

Cathe-

Parameter

Swamp 3
15 ha

Seneca 3
15 ha

dral/ Hill-
side 3
33.2 ha

Henry Run
15 ha

Tionesta
I, II 3
24 ha

Tionesta

III, IV
24 ha

HC I, II
22 ha

BBC area (0.0095/ha)

0.2451 b

0.2451

0.4326

0.1430

0.3529

0.7712

0.2097

BBC incidence (0.1413)

0.2532

0.4417

0.1487

0.3627

0.7620

0.2182

Mean home range (229 ha) c

0.1207

0.2448

0.0645

0.1851

0.8968

0.0946

Min. home range (86 ha) c

0.2880

0.4740

0.1744

0.4024

0.7209

0.2558

Max. home range (369 ha) c

0.0778

0.1634

0.0405

0.1213

0.9351

0.0595

Mean annual home range
(282 ha) e

0.1007

0.2077

0.0532

0.1557

0.9149

0.0780

Mean summer home range
(118 ha) e

0.2219

0.4044

0.1271

0.3240

0.7966

0.1864

a Plot studied during both breeding seasons (1993, 1994).

b Final probability based on multiplication rule, i.e., the product of all cumulative probabilities of owl reproduction across all plots.
c Nicholls and Warner (1972).

d Exact probabilities are 1.6 and 7.4 X 10 -7 for maximum and mean annual home range parameters, respectively.
e Elody and Sloan (1985).

years, the probability of finding reproduction in one, nei-
ther or both study years is given by the binomial expan-
sion: p 2 , q 2 , or 2j 6q, where q = 1 — p (e.g., the probability
that a plot will not have owl breeding; = negative inci-
dence). Because in no plot was reproduction detected in
both years, nor did any plot studied for two years fail to
have reproduction in one of the years, in practice only
p, q, or 2j 6q gave cumulative plot probabilities. The fact
that no plot had nests or Hedgings in both years, and
plots studied for two years had a nest or fledgings in at
least one year, mitigated against violating the indepen-
dence assumption for binomial trials (Snedecor and
Cochran 1980).

In addition to home range size, two other estimates of
p based on published BBCs were available. The first (p =
0 0095/ha) was calculated by dividing the total number
of owl territories by the total area of all study plots in a
sample of 92 BBCs (J. Field Ornithol. 64[Suppl.] and
65[Suppl.]). These 92 BBCs originated solely from within
the species’ range and consisted of all available plots
from potential habitat (completely vegetated plots in up-
land forest). The second estimate (p = 0.1413) was de-
rived by taking the proportion of the 92 BBCs on which
entire or partial Barred Owl territories were registered.
Note that neither BBC estimate for p necessarily implies
that reproduction occurred; rather, it is a measure of ter-
ritorial occupancy.

Statistical Analyses. Each of five home range- and two
BBC-based parameter values for p was used to calculate
a plot-specific probability of reproduction for either one

or two years; that is, the product of plot area with p , q,
or 2 pq. Each of the seven parameter values was subse-
quently used to compute a final cumulative probability
of reproduction using the binomial multiplication rule
(e.g., the product of probabilities in a specified series of
events such as owl reproduction in independent plots).
Use of different parameter values for p acted as a sensi-
tivity analysis in executions of the binomial model to al-
low examining whether results were solely the conse-
quence of parameter outliers.

In a second approach, I used a two-sample test of pro-
portions (Snedecor and Cochran 1980) to evaluate the
probability of obtaining the observed number of Barred
Owl territories in old-growth. If py is the probability of
territorial occupancy in the sample of old-growth plot-
years (where N { = 15), and p^ is the probability of terri-
torial occupancy in a sample of BBCs (iV 2 = 92 plot-
years), then the test statistic for differences between two
sample proportions is given by the normal deviate, Z,
where:

Z= pi~ p 2 /Vp-q(l/N 1 + 1/N 2 ),

and p and q are the joint probabilities across all BBCs ( N
= 107) of finding and not finding owls, respectively.

Reproduction by Barred Owls at spatial scales em-
ployed in this study should be rare, a condition for which
the Poisson distribution is well-suited. I calculated the ex-
pected number of reproductive events (nests or owl
fledgings) in r = 11 trials (number of combined plots)
using the highest, most conservative parameter value

September 1997

Barred Owl Breeding in Old-growth

245

Table 1. Extended.

Plot

Deciduous

Prince
Gallitzin
10.3 ha

Erie I
6 ha

Erie II
7.5 ha

Tryon-
Weber
9.8 ha

Final P

0.9023

0.9428

0.9285

0.0929

0.000109 b

0.8984

0.9405

0.9256

0.0967

0.000129

0.9559

0.9742

0.9678

0.0419

0.000002

0.8808

0.9302

0.9128

0.1134

0.000247

0.9723

0.9838

0.9797

0.0264

<0.000001 d

0.9637

0.9787

0.9734

0.0346

CO.OOOOOT 1

0.9131

0.9492

0.9364

0.0826

<0.000064

available ( p = 0.1413, Table 1). If owl reproductive events
are distributed randomly with average incidence, p, the
number of events expected in a sample of size C is a
Poisson variable with mean pC (Snedecor and Cochran
1980). If there are more incidences of owl reproduction
than expected, the Poisson model will be a poor fit and
the null hypothesis of randomness will be rejected. Ex-
pected values for the number of reproductive events si
were figured with the Poisson expression:

X P(r) = (ff/r\)e / for all r > 0,

and where e = 2.71828, the base of natural logarithms
(Snedecor and Cochran 1980). Expected values were
then compared to those actually observed using a x 2 test
for goodness-of-fit.

Although I provide exact probability values ( P) for
model runs, these estimates are biased (albeit conserva-
tively so) . For example, if any plot was actually outside an
owl home range, values of p based on area would be in-
flated, increasing the likelihood of falsely accepting the
null hypothesis of no effect of old-growth on owl repro-
duction. Such bias acts to increase the final absolute val-
ue of P Although this increases risk of Type II error, I
was more concerned in these analyses with making false
conclusions regarding Barred Owl use of old-growth.
Thus, P values should be considered as upper limits on
the real chance of committing a Type I error. To guard
against Type II error resulting from small sample sizes,
inferences were considered significant at a = 0.10. When
available, I provide observed significance levels (Forbes
1990).

Vegetation Measurement. On the basis of breeding, I
poststratified plots to compare vegetation characteristics
of forest stands used and not used by owls. Canopy com-
position and shrub stem density on all plots were esti-
mated at randomly-drawn points with 0.04 ha circular
subplots (James and Shugart 1970); sample size for cir-
cular subplots was set uniformly at one per ha of total
plot size (4%). Canopy height was measured at each sub-
plot with a clinometer. Canopy foliage (leaf) cover was

estimated with a concave spherical densiometer (Lem-
mon 1957) based on the average of measurements from
four cardinal directions. Systematic transects were used
to estimate size, total elliptical area and frequency of tree-
fall gaps (Runkle 1985); 10 m X 50 m randomly-chosen
rectangular plots were used to measure snag type and
density, and type, volume and biomass of downed CWD
(Tyrrell and Crow 1994).

Results

Incidence of Reproduction. During both years.
Barred Owls nested on 7 of 15 (47%) original
plots, or 7 of 11 (64%) combined plots (those
<1000 m apart). Nests (N = 1) or prefledging ju-
veniles ( N — 6 instances) were recorded on “Sen-
eca” and “Tionesta I/II” in 1993, and “Swamp,”
“Hillside/Cathedral,” “Henry Run,” “Heart’s
Content I/II” (HC I) and “Tryon-Weber” in 1994
(Table 1). The single nest detected was in a live
eastern hemlock with a broken top. Five of 6 sets
of juveniles (1-3 individuals per brood) were also
being fed in large, old hemlocks. Reproduction oc-
curred on more of the combined plots dominated
by mixed conifer-hardwood old-growth (86%) than
plots dominated by deciduous old-growth (25%; Z
= 2.033, P= 0.05).

Adult owls were recorded as visitors, or had par-
tially-overlapping territories, on other plots and/or
during other years: “Hillside/Cathedral” in 1993,
and “Seneca,” “Tionesta I/II” and “Erie II” in
1994. In none of these instances was reproduction
confirmed, although it could have occurred nearby
in similar forest surrounding most plots.

Model Results. With the first model, some pa-
rameter values for p gave significant incidences of
reproduction on single plots within a single year.
Reproductive incidence on the “Tryon-Weber”
plot alone was significant for all but the minimum
home range parameter (p > 0.10). Greater than
expected reproduction in a single year also oc-
curred when the model was executed with param-
eter maximum home range (5 plots), mean home
range and mean annual home range (3 plots),
BBC area, BBC incidence, and mean summer
home range (1 plot). No plot had a significantly
greater than expected incidence of reproduction
within a single year when the model was executed
with the minimum home range parameter.

Observed number of reproductive events in old-
growth was highly unlikely due to chance alone
(Table 1). No final cumulative probability with the
binomial model exceeded P = 0.000247, and one
cumulative probability (using the model parameter

246

Haney

Vol. 31, No. 3

Table 2. Comparison of observed and expected num-
ber of breeding incidences by Barred Owls in some east-
ern old-growth forest. Expected numbers were generated
with a Poisson model of rare events in 11 trials (plots).

Breeding

Incidences

Expected

Observed

9.551

1.449

Total

11.000

maximum home range) fell to P = 1.6 X 10 7 .
When all plots were analyzed jointly, each param-
eter value for p gave a highly significant final result,
giving no indication that results came from outliers
(extreme values) in model parameters.

Other statistical models gave similar results.
There were more incidences of reproduction than
expected under the Poisson model (x 2 = 24.47, P
< 0.0001; Table 2). Based on a two-sample test of
proportions, there were also more occurrences of
territory occupancy in plots located in old-growth
(80%) compared to younger, managed forests
(14%; Z = 5.63, P < 0.0001).

Vegetation Characteristics. Relative to the entire
regional landscape, diameter size distributions of
canopy trees were different in old-growth plots
used for breeding (Fig. 1 ) . Both mixed and decid-
uous old-growth plots had more diverse diameter
size classes in canopy trees, and were skewed to-
ward trees in larger size classes. Most plots used by
owls had at least some very large trees (70—100 cm
dbh). No evidence of owl reproduction or of ter-
ritorial occupancy was found in younger forest.
Power to detect avoidance of this habitat type was
very low, however. Analyses indicated that with the
binomial model N ^ 12 15-ha plots would be re-
quired to detect whether owls used younger forest
less than expected.

Canopy complexity created by tree-fall gaps dis-
tinguished old-growth sites used and not used for
breeding (Table 3). Owls bred where on average
such canopy gaps opened up 8% of the stand; no
breeding was observed where less than 5% of the
stand was in tree-fall gaps. No significant differ-
ences were detected in the size class distributions
of canopy gaps (Kolmogorov-Smirnov x 2 = 3.34,
maximum difference 0.133, P — 0.361; Fig. 2).

Table 3. Comparison of forest structure at old-growth sites used and not used for breeding by Barred Owls.

Not Breeding

Breeding (N =7) (N = 9 a or 10) Comparison

Structural Characteristic

Range

Z b

P b

Tree stems (per ha)

499

50.4

348-644

473

41.6

317-697

-0.342

31.5

38.5

0.732

Basal area (m 2 /ha)

2.7

30-49

4.1

31-73

-0.441

30.5

39.5

0.659

Hemlock basal area (m 2 /ha)

3.2

<1-25

4.1

0-30

-0.587

0.557

Canopy height (m)

2.0

21-34

1.8

20-37

-0.532

26.5

36.5

0.595

Range canopy height (m)

1.5

10-20

1.9

6-24

-1.865

0.062

Variation canopy height (CV)

1.9

11-25

1.2

8-19

-1.747

0.081

Canopy gaps (%)

1.0

5-13

0.8

0-9

-2.733

0.006

Mean canopy gap size (m 2 )

116

34.7

37-301

159

71.5

0-728

-0.489

0.625

Largest canopy gap (m 2 )

430

107

133-915

658

253

0-2261

-0.195

0.845

Foliage cover (%) c

1.1

92-99

0.7

93-99

-0.401

0.689

Snag stems (per ha)

5.6

12-54

6.0

20-73

-0.977

0.329

Snag basal area (m 2 /ha)

1.2

1-9

0.9

1-8

-0.683

0.495

Snag volume d (m 3 /ha)

13.5

3-100

21.8

4-222

-0.586

0.558

Volume CWD e (m 3 /ha)

152

47.8

20-408

142

57.9

8-612

-0.586

0.558

Biomass CWD (10 3 kg/ha)

27.5

6.6

2.8-58.4

28.2

12.1

1.3-124.6

-0.781

0.435

Shrub stems (10 3 /ha)

4.8

2.7

0.2-20.7

5.9

2.5

1.2-24.6

-0.688

0.491

■* Some missing data for one plot.
b Mann-Whitney C-test corrected for ties.

c Relative cover; high canopy cover in this study mitigated against potential positive biases found in some forest stands measured with
densiometers (see Cook et al. 1995).

d Volume estimates based on decay classes defined in Cline et al. (1980), Tyrrell and Crow (1994).

e CWD = coarse woody debris; biomass of downed tree boles estimated as a function of decay class (Tyrrell and Crow 1994) ,

September 1997

Barred Owl Breeding in Old-growth

247

Table 4. Number of samples 3 (as a function of plot size, in ha) required to detect significantly more incidences ( =
positive incidence) of breeding by Barred Owls than expected by chance.

Alpha Level

a = 0.10

a = 0.05

Parameter Plot Size =

BBC area

BBC incidence 13

x home range

Min. home range

Max. home range

x ann. home range

x summer home range

a Number of samples in a binomial model based on differences in spatial scales between plot size and owl activity (see text).
b Number of samples is derived from a frequency-based parameter rather than a scale difference (see text) .

Size of canopy gap (m )

Figure 2. Canopy gap size-distributions in areas used
and not used for breeding by Barred Owls ( Strix varia).

Rather it was spatial arrangement of the canopy
gaps (e.g., interspersion throughout the stand)
that characterized breeding areas. Breeding sites
on average also had an increase of approximately
25% in variability of canopy height (Table 3).

Plots with breeding owls were more likely to con-
tain large (^45 cm dbh) hemlock snags than plots
not used for breeding (Fig. 3). Some plots on
which owls bred had snags >100 cm dbh. Breeding
owls were also more likely to use stands with higher
densities of large snags (all tree species) and great-
er total snag basal area (all tree species) .

Understory at breeding sites was generally
sparse. Most plots on which Barred Owls bred had
fewer shrubs and sapling trees (stems ^7.6 cm
dbh). Out of 15 original old-growth study plots,
nine were used by owls for either breeding, roost-
ing or foraging, and seven of these (78%) had
shrub densities <3000 stems/ha. Conversely, 67%
of old-growth plots where neither breeding, roost-
ing or foraging was detected had shrub densities
>3000 stems/ha.

Most other vegetation measurements exhibited
little difference between old-growth areas used and
not used by breeding owls (Table 3) . For example,
average tree diameter in all plots used for breeding
(x = 31.7 cm dbh, SD = 5.1, range = 24.4-38.5, N
= 7) was not different than average tree diameter
in plots not used (x = 33.9 cm, SD = 6.9, range =
23.9-44.5, N — 10; Mann-Whitney Utest, Z cor-
rected for ties = —0.684, P = 0.4943).

248

Haney

Vol. 31, No. 3

Plots with Barred Owls
Mtfl Plots with no Barred Owls

Figure 3. Vegetation of forested plots with Barred Owl reproduction compared to plots without reproduction. Bars
indicate +1 SE.

Discussion

Scale and Type of Habitat Use. Barred Owl
breeding was strongly linked to patches of old-
growth hemlock-hardwood forest on the northern
Appalachian Plateau. Given this owl’s low density,
such a large number of breeding events in a rela-
tively small sample was not expected. At plot sizes
ranging from 5-25 ha, however, and regardless of
the home range parameter chosen, no more than
3 plots are required to detect greater-than-expect-
ed incidence of reproduction if all plots are used
for nesting (Table 4) .

Except at Tionesta, breeding territories of the
size typically recorded for the species (Nicholls and
Warner 1972, Elody and Sloan 1985) were unlikely
to have been situated entirely within late succes-
sional forest; remnant patches of old-growth in this
region are usually smaller than Barred Owl terri-
tories (Haney 1996). In silvicultural terms, the spa-

tial scale of habitat use observed in this study cor-
responds to the stand level. Specifically, Barred
Owl use of breeding habitat was detected over
scales on the order of 1-1 Os ha and horizontal dis-
tances of 1 Os-1 00s m.

These scales correspond to an activity center
within the home range. Because habitat use of Strix
owls is quite scale-sensitive (Carey et al. 1992, Hun-
ter et al. 1995), use or selection at the level of nests
or territories may differ. Further study might reveal
whether microhabitat at nest sites used by Barred
Owls is similar to their North American congeners
(Seamans and Gutierrez 1995) via comparison of
nest to random sites (Buchanan et al. 1993),
whether at landscape levels Barred Owl territories
are smaller in or adjacent to old-growth (Carey et
al. 1990) and whether territorial occupancy occurs
in proportion to the availability of different serai
stages. Habitat use is likely to vary also as a func-

September 1997

Barred Owl. Breeding in Old-growth

249

tion of demography (sex, age) , social organization
(population, pair, individual; Carey etal. 1992) and
activity type (foraging, roosting, or nesting; Ganey
and Baida 1994).

Barred Owls and Old-growth. Forest contiguity
and age both influence habitat use by Barred Owls
(Bosakowski 1994, Laidig and Dobkin 1995). Hun-
ter et al. (1995) found that fragmentation adjacent
to nest sites influenced habitat selection of Spotted
Owls. In contrast, several other studies cited by
Hunter et al. (1995) found serai stage heteroge-
neity to be similar between random sites and areas
used by Strix owls. Barred Owls prefer mature to
young forest in patches of similar size (McGarigal
and Fraser 1984). The preference for old-growth is
not a regional artifact. In a follow-up study >800
km away, territorial occupancy and breeding by
Barred Owls occurred in old-growth (>200 yr)
hemlock-hardwood forest more than expected by
chance (P < 0.017 in all model executions; N = 3
plots [12-27 ha each] dispersed across three
Southern Appalachian national forests) .

Seeming inconsistencies in owl use of forested
habitats may arise if all areas studied happen to
meet a threshold of suitability. For example, al-
though I did not find average tree diameter to dif-
fer between sites used and not used for breeding,
my comparisons were restricted largely to old-
growth, and thus all sites may have contained ad-
equate features. Barred Owls avoid forests with av-
erage tree diameters <15 cm (Bosakowski et al.
1987). Average diameter for all forests in my study
region was 20 cm (weighted mean, based on Al-
erich 1993); all sites where I detected breeding
owls had average tree diameters ^30 cm. Despite
trees >50 cm dbh making up <2% of all stems on
the northern Appalachian Plateau (Alerich 1993),
some trees in this size class characterized each site
used by Barred Owls in this study (Devereux and
Mosher 1984).

Barred Owls are thought to prefer mature forest,
including old-growth, due to greater availability of
nest sites, because lower stem densities in the un-
derstory facilitate unimpeded visibility and travel-
ways for foraging, or because dense canopies pro-
vide protection from mobbing (Nicholls and War-
ner 1972, McGarigal and Fraser 1984, Bosakowski
1994) . Dense canopies also foster thermally-neutral
microclimates for some Strix owls (Barrows 1981).
Since all of these structural characteristics were ev-
ident on sites studied here, and I did not measure
availability, it was not possible to identify which fac-

tor (s) were actually selected. Compared to younger
forest, older forest provides other Strix owls with
their preferred prey type, size, or abundance
(Thrailkill and Bias 1989, Waters and Zabel 1995,
Zabel et al. 1995). Barred owls usually have diverse
diets (Bosakowski and Smith 1992), but the prey
base in eastern old-growth would be worthy of de-
tailed study.

Breeding sites were located where the canopy
was more complex. These areas had more vertical
variation in tree heights and greater horizontal
patchiness and internal edge created by tree-fall
gaps. Small openings that are interspersed
throughout the stand yet still near breeding sites
may facilitate foraging by adults who must satisfy
both their own dietary needs as well as provision
chicks. Thus, spatial juxtaposition of diverse eco-
logical characteristics may enhance suitability of
old-growth habitat for Barred Owls.

Any use of older forest by Barred Owls could
have implications for conservation of the Northern
Spotted Owl. Barred Owls have displaced (Sharp
1989) and interbred (Hamer et al. 1994) with
Northern Spotted Owls during the past few de-
cades in the Pacific Northwest. Although the for-
mer species has been implicated as more adapta-
ble, throughout much of eastern North America
the Barred Owl is the more specialized large owl
(Laidig and Dobkin 1995), and its populations are
impacted negatively by forest alterations detrimen-
tal to Northern Spotted Owls, such as fragmenta-
tion and serai truncation (Bosakowski 1994) . I sug-
gest that recent overlap in the ranges of Strix owls
stems at least in part from their broadly-similar
habitat requirements.

Management Considerations. As a codominant
canopy tree (Rogers 1978), eastern hemlock plays
a key role in providing habitat for Barred Owls.
The “eastern hemlock” or “hemlock-white pine-
hardwood” region (Nichols 1935) once stretched
from the Great Lakes, St. Lawrence River Valley
and New England south through the Southern Ap-
palachians. Apparent antibiotic properties of hem-
lock litter (Rogers 1978) and canopy shading both
tend to suppress understory vegetation, maintain-
ing a rather open ground layer that may benefit
foraging owls. After acheiving old-growth condi-
tions at 275-300 yr (Tyrrell and Crow 1994), hem-
locks tend to have snapped tops, broken limbs, cav-
ity inclusions and other signs of decadence that
furnish ample sites for nests as well as perches suit-
able for sit-and-wait foraging. Dense groves of hem-

250

Haney

Vol. 31, No. 3

lock also attract certain hawks, corvids and squir-
rels, all of which construct bulky nests occasionally
appropriated by Barred Owls (Johnsgard 1988).
Hemlock decomposes more slowly than most hard-
woods (Harmon et al. 1986), so snags suitable as
nest sites tend to persist for long periods.

Because hemlock tends to grow well in shade
(Rogers 1978), it ensures a continuous supply of
replacement canopy dominants, thereby exploiting
low-intensity disturbances typical of late-seral com-
munities (Runkle 1982, Ward and Parker 1989).
Hemlock’s longevity (—800 yr; Loehle 1988) and
low frequency of catastrophic stand disturbance
(«sl200 yr; Canham and Loucks 1984, Frelich and
Lorimer 1991) would, historically, have tended to
provide large areas of owl habitat. On the northern
Appalachian Plateau alone, presettlement beech-
hemlock forest covered 2.4 million ha (Bjorkblom
and Larson 1977) . Management practices that pro-
mote stand development or allow expanded cov-
erage of large hemlock (Farr and Tyndall 1992)
are thus likely to benefit Barred Owls.

The Barred Owl’s utility as a management indi-
cator species is predicated on an affinity for older
forest (USDA 1985, 1986). High breeding inci-
dence in the very old stands studied here suggests
that merely extending the rotation ages of timber
harvests to — 110 yr (the criterion for “old-growth”
in many eastern forests) may not in itself provide
optimal habitat for Barred Owls. Further research
is needed on Barred Owl abundance, habitat use
and reproduction across the full spectrum of stand
ages representative of eastern forests.

Acknowledgments

This project was one element of an investigation of
wildlife relationships in eastern old-growth forest. Finan-
cial support was provided by the Center for Rural Penn-
sylvania (CRP) , DuBois Educational Foundation Fund for
Academic Excellence, Pennsylvania State University Re-
search and Development funds and a Challenge Grant
from the Migratory Bird Office, Region 5, U.S. Fish and
Wildlife Service (USFWS). D. DeCalesta,J. Palmer and S.
Stout (U.S. Forest Service), L. Lentz, J. Sowl and D.
Wright (CRP), T. Mountain and D. Pence (USFWS), C.
Schlentner (Cook Forest State Park) and C. Schaadt pro-
vided logistic support, access to study areas or other as-
sistance that greatly facilitated this study. J. Lydic and R.
Williams collected and entered vegetation data and per-
formed many of the summary analyses. For their help
with the Breeding Bird Censuses and other field work, I
thank B. Allison, J. Cheek, L. Hepfner, R. Kaufmann, J.
Lydic, C. Schaadt, J. Seachrist, J. Smreker, S. Weilgosz, S.
Wetzel and R. Williams, M, Bechard, J. Callazo, E, Fors-
man, R. Gutierrez, C. Hunter, D. Lee, D. Smith and an

anonymous reviewer offered many helpful comments on

earlier versions of the manuscript.

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252

Haney

Vol. 31, No. 3

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Received 20 May 1996; accepted 22 April 1997

f. Raptor Res. 31 (3) :253-259
© 1997 The Raptor Research Foundation, Inc.

HABITAT ASSOCIATIONS OF THE BARRED OWL IN THE
BOREAL FOREST OF SASKATCHEWAN, CANADA

Kurt M. Mazur 1

Department of Biology, University of Regina, Regina, Saskatchewan S4S 0A2 Canada

Paul C. James

Saskatchewan Environment and Resource Management, 321 1 Albert Street, Regina, Saskatchewan S4S 5W6 Canada

Michael J. Fitzsimmons

Parks Canada, Prince Albert National Park, P.O. Box 100, Waskesiu Lake, Saskatchewan SOJ 2Y0 Canada

Gido Langen

Prince Albert Model Forest Association, Inc., 77-1 1th Street West, Prince Albert, Saskatchewan S6V 7G3 Canada

Richard H.M. Espie

Department of Veterinary Anatomy, Western College of Veterinary Medicine, 52 Campus Drive,

University of Saskatchewan, Saskatoon, Saskatchewan S7N 5B4 Canada

ABSTRACT. — Little information exists regarding Barred Owl ( Strix varia) habitat requirements in the
boreal forest. During 1993, we located Barred Owls through call-playback surveys in the boreal forest
of central Saskatchewan, Canada. We analyzed habitat found within 1.5 km and 3.0 km radius circles
centered on 25 Barred Owl locations, 100 random locations and 275 survey locations. We compared
habitat found within random circles to that found at survey and owl locations. Habitat at survey locations
differed from random locations for four habitat types, indicating a habitat bias of road-based surveys.
Barred Owls were found associated with old mixed-wood forest, old deciduous forest and water, and
avoided young forest and treed muskeg. As in other portions of its range, the Barred Owl is associated
with old forest in boreal forest.

Key Words: Strix varia; Barred Owl; boreal forest; habitat association; Saskatchewan.

Asociaciones de habitat en buhos ( Strix varia) en bosques boreal en Saskatchewan, Canada

Resumen. — Poca informacion existe con respecto de requisitos de habitat para buhos ( Strix varia) en
bosques boreal. Durante 1993 nosotros localizamos buho con llamadas recordadas en el bosque boreal
de el centro Saskatchewan, Canada. Nosotros analizamos habitat dentro 1.5 km y 3.0 km radio cfrculos
centrados en 25 lugares de 25 buhos, 100 lugares al azar y 275 lugares de encuesta. Nosotros compar-
amos el habitat dentro los cfrculos al azar con los de encuesta y lugares de buho. Habitat en lugares
de encuesta eran diferentes a lugares al azar para cuatro tipos de habitat, indicando una tendencia de
habitat de encuesta con caminos. Buhos fueron encontrados dentro de bosque variables, bosque de
hoja caduca y agua y evitaba bosque jovenes. Como en otras lugares de la pradera, el buho esta asociado
con bosques viejos en bosques boreal.

[Traduccion de Raul De La Garza, Jr.]

Habitat associations of Barred Owls ( Strix varia)
have been quantified for only a portion of their
range, primarily the northeastern U.S. (Nicholls

1 Present address: Grassland and Forest Bird Project,
Box 24, 200 Saulteaux Cresc., Winnipeg, MB R3J 3W3
Canada.

and Warner 1972, Devereux and Mosher 1984, Elo-
dy and Sloan 1985, Bosakowski et al. 1987, Laidig
and Dobkin 1995). In this region, Barred Owls typ-
ically occupy large contiguous tracts of mature to
old-growth hardwood and mixed hardwood/ soft-
wood forests. Some authors have also suggested a
need for swamps and an association with water

253

254

Mazur et al.

Vol. 31, No. 3

(Bent 1961, Bosakowski et al. 1987, Dunbar et al.
1991, Laidig and Dobkin 1995). Its relatively nar-
row habitat requirements have resulted in its adop-
tion as a forest-management indicator in the south-
ern Appalachians (Bosakowski 1994). During this
century, the Barred Owl is believed to have ex-
panded its range into boreal forests to the western
montane forests of Canada and the U.S. (Houston
1959, Taylor and Forsman 1976, Boxall and Step-
ney 1982, Sharp 1989, Dunbar et al. 1991). In the
western portion of their range, Barred Owls were
found in association with old-growth and mature
coniferous and mixedwood forests and riparian
zones (Hamer 1988, Dunbar et al. 1991). Van Ael
(1996) reported Barred Owls in northwestern On-
tario to be found in association with unfragmented
mixed-wood forests. Records from the western bo-
real forest suggest a relationship with old forests,
but this relationship has yet to be quantified (Box-
all and Stepney 1982, Pinel et al. 1991). Our ob-
jective was to identify which habitat in the boreal
forest of Saskatchewan Barred Owls were associat-
ed with, and to compare this to the available hab-
itat.

Study Area

This study was conducted in the southern boreal forest
of Saskatchewan, Canada (53 o 35'-54 0 15'N, 105°05-
106°45'W). The 400 000-ha study area encompassed the
Prince Albert Model Forest including a portion of Prince
Albert National Park. The dominant tree species in the
study area included trembling aspen ( Populus tremuloides ) ,
balsam poplar ( Populus balsamifera) , white birch ( Betula
papyrifera), white spruce (Picea glauca) , black spruce (Pic-
ea mariana), tamarack ( Larix lancina) . jack pine ( Pinus
banksiana ) and balsam fir ( Abies balsamea) . Habitats in-
cluded pure deciduous, mixed coniferous/deciduous
and pure coniferous forest, muskeg and shrub lands. El-
evation ranged from 490—698 m. The topography is gent-
ly rolling, interspersed with numerous lakes and creeks.
The climate is boreal continental, with an average annual
precipitation of 40.1 cm; 28.1 cm as rain and 12.0 cm as
snow. July and January temperatures average 17.6°C and
— 19.7°C, respectively, with annual extreme temperatures
of 36.1°C and — 48.3°C (Environment Canada Parks
1986). A portion of the study area is currently being com-
mercially harvested for wood pulp and lumber. Approx-
imately half of the study area is located within the bound-
aries of Prince Albert National Park.

Methods

Barred Owl locations were estimated through noctur-
nal call-surveys from 28 April-28 May 1993. Call-surveys
were restricted to randomly-selected, vehicle-accessible
roads, and were conducted between one half hour after
sunset and one half hour prior to sunrise. Call-survey
stops were spaced 1 km apart. Thirteen survey routes,
totalling 275 call-survey stops, were each surveyed once.

These call-survey stops represented the survey locations
Territorial calls of a male and a female Barred Owl were
broadcast using a 12-watt battery powered tape recorder
with 4 directional speakers (MTC Electronics), set ap-
proximately 1.5 m above the ground. Surveyors remained
at each survey stop for 8 min consisting of an initial 1
min listening period prior to broadcast, followed by a 2
min broadcast, and concluding with a 5 min post-broad-
cast listening period. McGarigal and Fraser (1985) and
Mosher et al. (1990) found that 70-80% of Barred Owls
detected during the post-broadcast listening period re-
sponded within 5 min of the end of the broadcast period.
Surveys were not conducted during periods of precipi-
tation or when wind speed exceeded 15 km/hr as re-
ported by Environment Canada, or scored 3 or greater
on the Beaufort scale.

At each survey stop where owls responded, we record-
ed the following parameters: the apparent direction to
the owl (to the nearest degree), number and sex of owls
responding, time for owl to respond and if the owl(s) was
observed. Owl locations were determined by triangula-
tion from at least two consecutive survey stops, or by di-
rect observation of the owl, in which case the survey lo-
cation was used as the owl location. One hundred random
locations were generated throughout the study area, in
order to compare available habitats. These random lo-
cations did not include locations on water surfaces.

We characterized habitat within 1.5 and 3.0 km radius
circles (706 and 2827 ha, respectively) centered on 25
owl locations, 100 random locations and 275 survey lo-
cations. Of the 25 owl locations where habitat was char-
acterized, seven represented a pair of owls and 18 rep-
resented a single owl. Area of overlap of adjacent circles
were intersected with Thiessen polygons and the overlap
divided between the two circles to prevent double count-
ing of any habitat area. Therefore, overlapping circles
had a reduced area as the overlapping area was divided
between the two circles.

Although previous studies used smaller circles as an
estimate of the area used by Barred Owls (Laidig and
Dobkin 1995), radiotelemetry data from 14 adult Barred
Owls revealed that annual home ranges (95% MCP) of
Barred Owls in our study area ranged from 692-2489 ha
(x = 1361 ha) (Mazur 1997). We therefore chose circles
of 1.5 and 3.0 km radius which more closely approxi-
mated the area used by Barred Owls in this region. The
circles do not represent an owl’s home range, but rather
provide an area with which an owl is likely to be associ-
ated.

We used the 1993 forest inventories for Prince Albert
National Park (Padbury et al. 1978) and Saskatchewan
Northern Provincial Forest (Lindenas 1985) to classify
the available habitat into 12 types (Table 1). The pro-
portional coverage of each habitat within each circle was
calculated using an ARC/INFO geographic information
system (GIS). As the data did not conform to a normal
distribution we used nonparametric statistics (Zar 1996)
We tested for differences between habitat associated with
owl and random, and survey and random locations for
both 1.5 and 3.0 circles using the Mann-Whitney Gtest
(Zar 1996).

September 1997

Barred Owl Habitat Associations

255

Table 1. Habitat classification of the Prince Albert National Park study area by habitat cover type and age.

Habitat Type

Cover Vegetation Description

Deciduous 1

Mixed-wood 1

Coniferous 1

Treed Muskeg

Open

Water

Trembling aspen +/or balsam poplar +/or white birch
(<20% conifer)

Combination of deciduous and coniferous species: trembling aspen, balsam pop-
lar, white birch, white spruce, black spruce, jack pine, balsam fir
(^20% conifer, ^20% deciduous)

White spruce + /or black spruce + /or jack pine +/or tamarack +/or balsam fir
(<20% deciduous)

Black spruce + /or tamarack, excessive moisture and retarded tree growth
Cut over, burn over, flooded land, sand, clearing, open muskeg, herbs, shrubs
Lakes, rivers, creeks

1 Could occur in three age classes: young (<50 years), mature (50-79 years) and old (80+ years).

Results

Survey Locations versus Random Locations.

Habitat composition surrounding survey locations
(e.g., habitat adjacent to roads) was found to differ
from habitat composition found at random loca-
tions (e.g., habitat throughout the study area)
(Figs, la and lb). Significant differences were
found between the proportions of two habitat types
within the 1.5 circles and four habitat types within
the 3.0 circles. Survey 1.5 circles were found to
have significantly less mature conifer (z = —5.23,
P = 0.000) and treed muskeg (z = —5.06, P =
0.000) than did random 1.5 circles (Fig. la). With-
in survey 3.0 circles, there were significantly more
mature deciduous (z = —2.09, P = 0.025), and sig-
nificantly less mature mixed-wood (z = —3.07, P —
0.001), mature conifer (z = —4.79, P< 0.001), and
treed muskeg (z = —4.10, P < 0.001) compared to
random 3.0 circles (Fig. lb).

Owl Locations versus Random Locations. Barred
Owls were associated with habitat types in different
proportions than expected from the available hab-
itat. Habitat composition of owl 1.5 and 3.0 circles
differed from random 1.5 and 3.0 circles for four
habitat types within the 1.5 km circles and six hab-
itat types within the 3.0 km circles (Figs. 2a and
2b). Within the 1.5 circles, owl locations were
found to have significantly higher proportions of
old mixed-wood (z = —3.53, P< 0.001) than ran-
dom circles, and significantly lower proportions of
young mixed-wood (z = —1.87, P — 0.038), young
conifer (z = —2.27, P = 0.011) and treed muskeg
(z = —3.24, P = 0.001) than random circles (Fig.
2a). Within the 3.0 circles, owl locations were
found to have significantly higher proportions of
old deciduous (z = —2.39, P = 0.014), old mixed-

wood (z = —2.29, P— 0.021) and water (z = —3.82,
P < 0.001) and significantly lower proportions of
young mixed-wood (z = —2.36, P = 0.012), young
conifer (z = —2.44, P — 0.010) and treed muskeg
(z = —3.30, P < 0.001) than random circles (Fig.
2b).

Discussion

Our results indicated that Barred Owls were not
randomly distributed relative to the available hab-
itat. Owls showed a greater than expected associa-
tion with old deciduous forest, old mixed-wood for-
est and water, and an avoidance of young forest
and treed muskeg. This agrees with what has been
recorded previously in the boreal forest (Boxall
and Stepney 1982, Van Ael 1996). Barred Owls are
cavity-nesting owls, requiring relatively large trees
(Johnsgard 1988). In Maryland, Devereux and Mo-
sher (1984) reported an average diameter at breast
height (dbh) of 61 cm for Barred Owl nest trees.
Similarly, in our study area Barred Owl nest trees
average 47 cm dbh. Old mixed-wood forest is likely
the only forest type in the boreal setting that pro-
vides an adequate density of large diameter (>40
cm dbh) trees (Lee et al. 1995). The old mixed-
wood forest is the most structurally and species di-
verse habitat type in the boreal forest (Stelfox
1995) . Therefore, prey diversity and abundance is
likely high in this habitat. The positive association
with water has also been documented in the past
(Sutton and Sutton 1985, Bosakowski et al. 1987,
Pinel et al. 1991). In some areas suitable habitat
for Barred Owls is largely restricted to wet areas
(Devereux and Mosher 1984). In our study area,
the forest was largely continuous, with available
habitat in both upland and lowland areas. We

256

Mazur et al.

Vol. 31, No. 3

35 -

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20 -

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Habitat Type

Figure 1. Comparison of mean percent habitat composition (±SE) within (a) 1.5 km radius circles (706 ha) and
(b) 3.0 km radius circles (2827 ha), centered on 100 random and 275 survey locations. Significant difference * (P
< 0.05).

September 1997

Barred Owl Habitat Associations

257

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Habitat Type

Figure 2. Comparison of mean percent habitat composition (±SE) within (a) 1.5 km radius circles (706 ha) and
(b) 3.0 km radius circles (2827 ha), centered on 100 random and 25 owl locations. Significant difference * (P <
0.05).

258

Mazur et al.

Vol. 31, No. 3

found Barred Owls in both upland and lowland
areas.

Habitat associated with survey locations was
found to be representative of the habitat within the
study area, with the exception of four habitat types.
Typically, roads were built on higher areas, avoid-
ing low-lying muskeg and wetlands. This was evi-
dent as the percentage of treed muskeg associated
with survey locations was significantly lower than
that of random locations. We suggest that when
comparing habitat use to availability, habitat adja-
cent to roads presents an available habitat bias, and
therefore comparisons between habitat use and
random habitat should be made.

Habitat characterization of circles centered on
owl locations contained biases making them not
entirely representative of owl home ranges. Owls
detected may have moved toward the tape play-
back, or the owl may have been detected calling
from the periphery of its home range. However,
Lehmkuhl and Raphael (1993) supported the use
of circles as surrogates for home ranges in the anal-
ysis of habitat pattern associations of Spotted Owls
(Strix occidentalis ) in Washington. Few differences
in habitat composition were apparent between 1.5
km circle comparisons and 3.0 km circle compar-
isons. However, the smaller circles would present a
more conservative estimate of the area that the owl
likely uses. Given that the 3.0 circle approximates
the maximum Barred Owl home range size, this
larger circle size may include large areas of unused
habitat.

Our findings show that in the boreal forest, like
other regions, Barred Owls are associated with old
forest, in this case old mixed-wood forest. This spe-
cies appears to have the potential to serve as a bi-
ological indicator for the management of old
mixed-wood forest in the boreal forest (James
1993). Knowledge of the Barred Owl’s specific hab-
itat and area requirements would allow for man-
agement of an adequate quantity of old mixed-
wood forest, therefore sustaining this highly spe-
cies diverse habitat.

Acknowledgments

This research was funded by the Prince Albert Model
Forest, Prince Albert National Park, the Wildlife Devel-
opment Fund and the Saskatchewan Heritage Founda-
tion. Mauray Toutloff provided assistance in the field,
and Shanna Frith and Terry Breen-Smith helped with
mapping. The Royal Saskatchewan Museum provided the
Barred Owl recording. Comments from Shanna Frith
and Steve Davis improved this manuscript.

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Bosakowski, T., R. Speiser andJ. Benzinger. 1987. Dis-
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Bosakowski, T. 1994. Landsat reveals negative effect of
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Boxall, P.C. and P.H.R. Stepney. 1982. The distribution
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Devereux, J.G. and J.A. Mosher. 1984. Breeding ecol-
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Dunbar, D.L., B.P. Booth, E.D. Forsman, A.E. Hether-
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Elody, B.I. and N.F. Sloan. 1985. Movements and hab-
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Hamer, T.E. 1988, Home range size of the Northern
Barred Owl and Northern Spotted Owl in western
Washington. M.S. thesis, Western Washington Univ.,
Bellingham, WA U.S.A.

Houston, C.S. 1959. First records of the Barred Owl in
Saskatchewan. Blue Jay 17:94.

James, P.C. 1993. Oldgrowth and owls: what have we
learned? Pages 70-79 in D.H. Kuhnke [Eds.], Birds
in the boreal forest. Proceedings of a workshop held
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JOHNSGARD, P.A. 1988. North American owls. Smithson-
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Laidig, KJ. and D.S. Dobkin. 1995. Spatial overlap and
habitat associations of Barred Owls and Great Horned
Owls in southern New Jersey./. Raptor Res. 29:151-157.

Lee, P.C., S. Crites, M. Nietfeld, H. Van Nguyen and
J.B. STELFOX. 1995. Changes in snags and down
woody material characteristics in a chronosequence of
aspen mixedwood forests in Alberta. Pages 49-61 in
J.B. Stelfox [Ed.], Relationship between stand age,
stand structure, and biodiversity in aspen mixedwood
forest in Alberta. Jointly published by Alberta Envi-
ronmental Centre (AECV95-R1 ) , Vegreville, Alberta,

September 1997

Barred Owl Habitat Associations

259

and Canadian Forest Service (Project No. 0001A), Ed-
monton, Alberta, Canada.

Lehmkuhl, J.F. and M.G. Raphael. 1993. Habitat pat-
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Olympic Peninsula, Washington./. Wildl. Manage. 57:
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Mazur, K.M. 1997. Spatial habitat selection by Barred
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Mosher, J.A., M.R. Fuller and M. Kopeny. 1990. Sur-
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Nicholls, T.H. AND D.W. Warner. 1972. Barred Owl
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Received 26 September 1996; accepted 11 May 1997

J. Raptor Res. 31 (3):260-266
© 199V The Raptor Research Foundation, Inc.

THE WINTER ROOSTING BEHAVIOR OF EASTERN
SCREECH-OWLS IN CENTRAL KENTUCKY

Tara A. Duguay, Gary Ritchison and Jeffrey P. Duguay 1

Department of Biological Sciences, Eastern Kentucky University,

Richmond, KY 40475 U.S.A.

Abstract. — The winter roosting behavior of Eastern Screech-owls ( Otus asio) in central Kentucky was
examined from October 1993-March 1994. Eleven owls used 69 roost sites 563 times, with 29 boxes
used 308 times, 25 cavities used 226 times and 15 limbs used 29 times. Most natural cavities were in
black locusts ( Robinia pseudoacacia) , southern red oaks ( Quercus falcata ) and snags; boxes were located
in 15 different species of trees. All conifer limb roosts were in eastern redcedars (Juniperus virginiana).
Frequent use of boxes and cavities during winter is probably the result of owls seeking favorable micro-
climates and concealment from predators. Screech-owls roosted in conifers more frequently when tem-
peratures were above freezing and in boxes and cavities more frequendy on days with rain, drizzle, or
snow, supporting the conclusion that roosting owls seek favorable microclimates. Owls used each roost
site an average of seven times. Female screech-owls were more likely to use boxes and males more likely
to use cavities and conifer limbs. The suitability of boxes as potential nest sites may be one reason for
their frequent use as roost sites by females.

Key Words: Eastern Screech-owl ; Otus asio; roosting behavior, cavities', winter.

La conducta de buhos ( Otus asio ) en centro Kentucky durante el tiempo de percha en el invierno

Resumen. — La conducta de buhos ( Otus asio) durante el invierno en el tiempo de percha en centro
Kentucky fue examinado en Octubre 1993— Marzo 1994. Once buhos usaron 69 sitios de percha 563
veces, con 29 ceyas usadas 308 veces, 25 cavidades usadas 226 veces y 15 ramas usadas 29 veces. Las mas
natural cavidades fueron en Robinia pseudoacacia, Quercus falcata y tocones, y cajas fueron localizadas en
15 diferente especies de arboles. Las ramas de coniferos para percha estaban en Juniperus virginiana. La
frecuencia de uso de cajas y cavidades durante el invierno es probablemente el resulto de buhos bus-
cando microclimas favorable y lugares para esconderse de depredadores. Buhos estaban en percha en
coniferos con mas frecuencia cuando temperaturas estaban arriba de helando y en cajas y cavidades
con mas frecuencia en dias con lluvia, llovizna y nieve, soportando la conclusion que buhos en percha
buscan microclimas favorables. Buhos usaron cada sitio de percha un normal de siete veces. Hembras
eran mas probable usar cajas y machos eran mas probable usar cavidades y ramas de coniferos. La
conveniencia de cajas como sitios de nido puede ser una razon para su uso con regular como sitios de
percha para hembras.

[Traduccion de Raul De La Garza, Jr.]

Many aspects of the behavior and ecology of
Eastern Screech-owls ( Otus asio) have been exam-
ined (e.g., Van Camp and Henny 1975, Belthoff
and Ritchison 1989, Gehlbach 1994), including
their roosting behavior. Belthoff and Ritchison
(1990a) monitored adult and juvenile screech-owls
during the summer (May-July) in central Kentucky
and found that vines (or branches covered to vary-
ing degrees with vines) , cedars and open limbs of
deciduous trees were used as roost sites. These sites

1 Present address: Division of Forestry, P.O. Box 6125,
West Virginia University, Morgantown, WV 26506 U.S.A.

apparendy provided concealment from predators
and favorable microclimates (Belthoff and Ritchi-
son 1990a). Smith et al. (1987) reported that use
of roost sites by screech-owls varied with season,
with open limbs used during the summer and cav-
ities used more often during the fall, winter and
spring. Other investigators have also noted that
screech-owls use cavities for roosting (Merson et al.
1983, Gehlbach 1994).

Although previous work has shown that screech-
owls use different types of roost sites (e.g., open
limbs and cavities), less is known about the envi-
ronmental factors that influence selection of roost

260

September 1997

Screech-owl Roosting Behavior

261

sites or about features of roost sites that might be
important in roost-site selection by screech-owls.
The objective of our study was to examine roost-
site selection by Eastern Screech-owls during late
fall and winter ( Oc tober-March ) in central Ken-
tucky. Specifically, we examined characteristics of
roost sites used by screech-owls, possible relation-
ships between certain environmental conditions
and roost-site selection, and compared frequently
used sites with little used and unused sites in an
attempt to determine which features might be im-
portant in roost-site selection.

Methods

The roosting behavior of screech-owls was monitored
from 11 October 1993-19 March 1994 at the Central
Kentucky Wildlife Management Area, 17 km southeast of
Richmond, Kentucky. This area consists of small decidu-
ous woodlots and thickets interspersed with cultivated
fields and old fields (Sparks 1990, Sparks et al. 1994).
Beginning on 1 1 October, owls were captured from nest
boxes and fitted with radiotransmitters (Wildlife Materi-
als, Carbondale, Illinois). Radio-marked owls were locat-
ed at least four times each week. Each time owls were
located, we noted the temperature (above or below 0° C)
and categorized sky conditions as clear or pardy cloudy,
overcast or overcast with precipitation.

Each roost site was categorized as either a natural cav-
ity, deciduous limb, conifer limb or nest box. For limb
roosts, we noted tree species, roost height, tree height,
diameter at breast height (dbh), roost orientation (po-
sition of owl relative to main bole), distance from main
bole, distance from nearest permanent water and dis-
tance from the edge of the woodlot. For cavities and box-
es, we noted tree species, tree height, dbh and diameter
at cavity height, distance from nearest permanent water
and distance from the edge of the woodlot. Characteris-
tics were also measured for all boxes and accessible cav-
ities, including cavity entrance dimensions (height and
width), cavity depth (total and from bottom of cavity to
entrance), inside diameter (distance from entrance to
back wall) and entrance orientation. Tree, roost and cav-
ity heights were determined with a clinometer.

To determine which features of natural cavities might
influence roost-site selection, we compared the charac-
teristics of 14 frequently used (Sr8 times) cavities with 14
cavities in which owls were not observed roosting. To se-
lect unused cavities, we conducted 14 random line tran-
sects through woodlots used by our radio-tagged owls and
chose the first cavity detected within 10 m on either side
of the transect. Unused cavities selected for comparison
with used cavities had to be large enough to permit entry
by screech-owls (opening >8 cm in height and width).

For both used and unused natural cavities, we mea-
sured the previously listed cavity characteristics plus char-
acteristics of vegetation surrounding the tree (James and
Shugart 1970) . For trees >8 cm dbh located within a 0.04
ha circular plot centered on the cavity’ tree, we recorded
tree species, dbh and height. Shrub density and height
were estimated by making two perpendicular transects

within the plot and counting and measuring the diame-
ter and height of all woody stems <8 cm dbh within 1 m
of each transect. Percent tree canopy and ground cover
were estimated by sampling 10 points along transects in
each of the four cardinal directions from the roost tree
Percent understory cover was measured along the same
transects using the line-intercept method (Brower et al
1977).

All analyses were performed using the Statistical Anal-
ysis System (SAS Institute 1989). Because we made re-
peated observations of the same owls, repeated measures
analysis of variance was used to compare characteristics
(roost height, tree height, dbh and distance to edge and
water) of different types of roosts (conifer limb, natural
cavity and nest box) . Multivariate analysis of variance was
used to compare characteristics of used and unused cav-
ities, characteristics of little used and frequently used cav-
ities and characteristics of cavities used by males and fe-
males. Cavity entrance orientation was analyzed using cir-
cular statistics to test the null hypothesis that orientation
was random. Wilcoxon rank sum tests (which correspond
to Mann-Whitney [/-tests; SAS Institute 1989) were used
to examine possible differences in the roosting behavior
of males and females. Chi-square goodness-of-fit tests
were used to examine differences in frequency of use of
the various types of roosts over time (months) and with
different environmental conditions (temperature, wind
velocity 7 and sky conditions). Results are presented as
mean ±1 SD.

Results

We monitored roosting behavior of 11 radio-
marked owls (3 males and 8 females) . Sex was de-
termined by observations of behavior either during
previous breeding seasons (for previously banded
owls) or the following season. Only two radio-
marked owls were paired. The female of this pair
was only monitored for 14 days and, therefore, no
comparison of the roosting behavior of these owls
was possible. Female and male owls were moni-
tored for an average of 96.8 ± 48.9 days and 131.7
± 22.7 days, respectively. Overall, owls used 69 dif-
ferent roosts 563 times. We located an average of
51.2 ± 19.9 roosts per owl (x = 47.5 ± 22.4 for
females; x = 61 ± 4.6 for males). Six boxes and
five natural cavities were used at different times by
two owls (either by each member of a pair or owls
with adjacent ranges). We located an average of
93,8 ± 53.0 roosts each month, ranging from 33
in October to 189 in December.

Variation among Roost Types. The 69 roost sites
included 29 boxes, 25 natural cavities and 15 limbs.
Fourteen limb roosts were in conifers and one was
in a deciduous tree. The deciduous limb roost was
only used twice and is not considered further. Owls
used boxes 308 times, natural cavities 226 times
and conifer limbs 27 times.

262

Duguay et al.

Vol. 31, No. 3

Conifer roost trees were located closer to the
edge of woodlots than trees with boxes and natural
cavities (F 2 12 = 5.14, P — 0.02). Conifer roosts
were a mean distance of 5.31 ± 4.57 m from edges
while boxes and natural cavities averaged 18.89 ±
11.94 m and 18.81 ± 20.75 m, respectively, from
edges. We found no differences among roost types
in mean distance from water (F 2 12 = 0.51, P =
0.61), with mean distances ranging from 69.5 ±
77.6 m for boxes to 107.2 ± 127.9 m for conifers.
Roost height (e.g., the height of owls in conifers
or the height of the cavity entrance for boxes and
natural cavities) did not vary among the three sites
(F 2 io = 0.51, P — 0.62), with mean heights of 5.7
± 2.4 m for conifers, 5.9 ± 1.5 m for boxes and
6.2 ± 2.3 m for cavities.

The mean diameter (height) of box and cavity
entrances differed (F 1>6 = 51.7, P = 0.0004) as did
the mean depth (distance from the top of the cav-
ity to the bottom) (F 1)6 = 9.98, P = 0.0196), with
natural cavities being deeper (x = 90.6 ± 75.5 cm
for cavities vs. 41.1 ± 13.74 cm for boxes) and hav-
ing taller entrances (x = 20.4 ± 12.5 cm for cavi-
ties vs. 8.2 ±1.6 cm for boxes). In addition, dif-
ferences in the mean cavity depth (distance from
the bottom of the entrance hole to the bottom of
the cavity) and the mean width of cavity entrances
approached significance (cavity depth: F! 6 = 5.32,
P = 0.06; cavity entrance width: F 1>6 = 3.55, P =
0.11). No differences were found either in the di-
ameter of trees at the level of the cavity (Fj 6 =
0.14, P = 0.72) or in the diameter of the cavity (Fj 6
= 0.28, P = 0.62).

The 29 boxes used by roosting screech-owls were
located in 15 species of trees, with most in syca-
mores ( Platanus occidentalis) . The 25 natural cavi-
ties used by owls were in 12 species of trees. Most
natural cavities were in black locusts ( Robinia
pseudoacacia), snags and southern red oaks ( Quer -
cus falcata). All 14 conifer roosts were in eastern
redcedars (Juniperus virginiana ) .

Variation among Individuals and Between Sexes.
The 11 owls used an average of 7.2 ± 3.9 different
roost sites (range = 4—18). We found no correla-
tion between the number of roost sites used and
the number of days that an owl was located (Spear-
man rank correlation; r s = 0.4, P = 0.22). Each
roost site was used an average of 7.0 ± 11.6 times
(range — 1-66).

We found no difference between males and fe-
males in the mean number of different roost sites
used (z = 1.34, P = 0.18; x — 10.7 ± 6.4 for males

■Males

□Females

Conifers Boxes Cavities

Roost type

Figure 1. Use of different roost types by male and fe-
male Eastern Screech-owls.

and 6.0 ±1.9 for females) or the mean number of
times that particular roost sites were used (z =
1.08, P — 0.28; x = 5.7 ± 7.9 times for males and
7.9 ± 13.7 times for females). Males and females
differed in the use of different roost types (x 2 —
13.1, df = 2, P = 0.001). Females were more likely
to use boxes while males were more likely to use
conifers and natural cavities (Fig. 1).

Dimensions of roost trees and natural cavities
used by males and females did not differ (Wilk’s
Lambda = 0.41, F = 1.63, P ~ 0.24). Although
there was no overall difference (i.e., multivariate)
between natural cavities used by males and fe-
males, the mean height of cavities above ground
(one-way ANOVA; F 116 = 6.24, P = 0.024) and the
mean diameter (height) of entrances (one-way
ANOVA; F x 16 = 7.63, P = 0.014) used by males and
females did differ. The mean height of natural cav-
ities was 4.68 ± 1.97 m (N = 11) for males and
7.40 ± 1.89 m for females (N = 13). For cavity
entrances, the mean diameter (height) was 28.13
± 14.15 cm for males ( N = 8) and 14.25 ± 6.60
cm for females (N = 10).

Variation among Months. Use of conifer limbs,
boxes and natural cavities varied among months
(X 2 = 20.2, df = 10, P = 0.028) . Conifers were used
more often in February and March (Fig. 2). Use
of boxes was greatest in November and lowest in
February while use of natural cavities was greatest
in December and lowest in March (Fig. 2) .

Environmental Conditions and Roosting Behav-
ior. Owls used boxes and natural cavities more on
overcast days and days with precipitation (drizzle,

September 1997

Screech-owl Roosting Behavior

<1)

■Conifers
HI Boxes
□Cavities

October November December January February March

Month

Figure 2. Variation in use of different roost types among months.

263

rain or snow; x 2 = 12.3, df = 4, P = 0.015; Fig. 3).
Owls were more likely to use conifers on clear or
partly cloudy days (Fig. 3). Natural cavities were
used more when temperatures were below freez-
ing, and conifers were used more when tempera-
tures were above freezing (x 2 = 8.14, df = 2, P =
0.017).

Characteristics of Used versus Unused Natural
Cavities. We found no differences between used
and unused sites either in the dimensions of roost

trees and cavities (Wilk’s Lambda = 0.60, F = 1.24,
P = 0.34) or in the characteristics of surrounding
vegetation (Wilk’s Lambda = 0.79, F = 0.53, P =
0.83). The mean entrance orientation (direction)
of used and unused roost cavities/boxes was 174
degrees (r = 0.438) and 354 degrees (r = 0.149),
respectively. Neither sample exhibited significant
directionality (Rayleigh’s z-test; used: z = 2.69, P >
0.05; unused: z — 0.27, P > 0.5). Similarly, there
was no significant difference between used and un-

ill

III

Jjl

mm.

111

JJJJ

tltffl

fill

fill!

Ill

Hit

ill

Pill

111

lllll

111

III

111

mm.

■Conifers
□ Boxes
□Cavities

Clear to
Partly cloudy

Overcast (with
no precipitation)

Precipitation

Weather conditions

Figure 3, Variation in use of different roost types with different weather conditions.

264

Duguay et al.

Vol. 31, No. 3

used sites in mean entrance orientation (Watson’s
test; U2 = 0.068, P> 0.5).

Characteristics of Frequently Used versus Infre-
quently Used Natural Cavities and Boxes. For nat-
ural cavities, roost tree and cavity means for fre-
quently used (N ^ 8) and infrequently used (N s
7) sites did not differ (Wilk’s Lambda — 0.75, F —
1.18, P ~ 0.34). Similarly, for natural cavities and
roost boxes combined, roost tree and cavity means
for frequently and infrequently used sites did not
differ (Wilk’s Lambda = 0.76, F = 1.36, P = 0.23).

Discussion

Screech-owls in our study used nest boxes and
natural cavities more frequently than open limbs
during the period from October— March. In con-
trast, Belthoff and Ritchison (1990a) found that
screech-owls in the same study area roosted almost
exclusively in open sites during summer (May-
July) . Previous investigators have also reported sea-
sonal changes in types of roosts used (Smith et al.
1987, Gehlbach 1994). The shift from open sites
in summer to boxes and cavities in winter is prob-
ably the result of owls seeking favorable micro-
climates and better concealment from predators.
Hayward and Garton (1984) found that Western
Screech-owls (Otus kennicottii ) roosted only in co-
nifers during late winter and early spring (prior to
leaf out) and suggested that concealment was the
most important factor in roost-site selection. These
authors suggested that screech-owls roosted in cav-
ities “only when sufficient protective cover for con-
cealment is not available” and further noted that
cavity-roosting owis would be protected from aerial
predators but might be vulnerable to predation by
arboreal mammals (Hayward and Garton 1984).
Roosting in conifers might provide adequate con-
cealment from hawks and other owis plus the op-
portunity to escape approaching mammalian pred-
ators (Hayward and Garton 1984).

Gehlbach (1994) found that use of boxes by
screech-owls during December in central Texas
corresponded significantly to mean air tempera-
ture and suggested that thermoregulation was the
primary factor in roost-site selection. Further, he
(1994) observed three male screech-owls during
the period from November-February and found
that mean ambient temperatures were low r er when
these males were in boxes and higher when in co-
nifer roosts (junipers) . Similarly, we found that am-
bient temperatures were usually above freezing
when screech-owis used conifers for roosting, and

that owls were more likely to use conifers in Feb-
ruary and March when temperatures are begin-
ning to increase.

Eastern Screech-owls in our study roosted in
boxes more than in natural cavities. Availability
may have been one reason for the greater use of
boxes. However, differences in microclimate may
have been another factor, i.e., screech-owis may
have used boxes more frequently during winter to
reduce thermoregulatory costs (see McComb and
Noble 1981).

We found that the height of roost sites in coni-
fers did not differ from the height of the entrance
holes of boxes and cavities used by roosting owls.
Gehlbach (1994) reported similar results and
found that open roosts were an average of 3.8 m
high while entrances of boxes and cavities were an
average of 3.1 m high.

The height of roost sites might be influenced by
the risks of predation. For example, Nilsson (1984)
found a low r er rate of predation on nest cavities
located higher in trees for six species of birds and
Albano (1992) found that Carolina Chickadees
(Pams carolinensis) nesting in lower cavities suf-
fered higher rates of predation. Thus, screech-owls
may not use roost sites below r some minimum
height because of the increased risk of predation.
In addition, Gehlbach (1994) suggested that
screech-owls refrain from using very high roost
sites, possibly because such sites may be more ex-
posed to the elements and flying up to higher
roosts would require more energy (Collias and Col-
lias 1984, Korol and Hutto 1984).

Individual screech-owls used an average of more
than seven different roost sites during our study.
Smith et al. (1987) observed that “an owl may use
a roost site for several days . . . then move to a new
site.” Merson et al. (1983) also reported that
screech-owls used a variety of roost sites. Using dif-
ferent roost sites may reduce the chances of pre-
dation (Belthoff and Ritchison 1990a). Screech-
owls in our study area sometimes lose boxes and
cavities to other species such as eastern gray squir-
rels ( Sciurus carolinensis) and southern flying squir-
rels ( Glaucomys volans), and occasional reuse by
owls might also reduce the chances that cavities
will be usurped by these other species.

Screech-owls in our study used each roost site an
average of seven times. Other investigators have re-
ported the repeated use of certain roost sites by
screech-owls (Merson et al. 1983, Smith et al. 1987,
Gehlbach 1994) and other species of owls (e.g.,

September 1997

Screech-owl Roosting Behavior

265

Barrows 1981, Bosakowski 1984, Hayward and Gar-
ton 1984). In contrast, Belthoff and Ritchison
(1990a) found that screech-owls usually did not use
the same roost site on successive days during the
post-fledging period (May-July), possibly indicat-
ing that many suitable sites are available (Belthoff
and Ritchison 1990a). In contrast, reduced cover
from leaf fall during the autumn months plus the
possible need to use sites providing favorable mi-
croclimates limits the number of suitable roost sites
available during the winter (Belthoff and Ritchison
1990a). Such limits may contribute to the repeated
use of particular roost sites (boxes and cavities)
during the winter.

We found differences in the roosting behavior
of male and female screech-owls. In contrast, Bel-
thoff and Ritchison (1990a) found no differences
in the characteristics of open roost sites used by
male and female screech-owls. At least two factors
may have contributed to differences in the roost-
ing behavior of males and females. First, the avail-
ability of the different types of roosts may have var-
ied among the ranges of males and females. Sec-
ond, the suitability of boxes or cavities used by fe-
male screech-owls may be based in part on their
potential as nest sites. Perhaps as a result, cavities
used by female screech-owls were higher and had
smaller entrances than those used by males. As dis-
cussed previously, higher cavities suffer lower rates
of predation and may be preferred by nesting fe-
males. In addition, nesting screech-owis may avoid
cavities with large entrances (Belthoff and Ritchi-
son 1990b) because cavities with smaller entrances
will exclude some potential nest predators (Sone-
rud 1985).

We found no significant differences between
characteristics of used and unused cavities or be-
tween frequently and infrequently used cavities,
suggesting that screech-owls exhibit little selectivity
in their choice of roost cavities. Smith et al. (1987)
also reached this conclusion and, regarding the
use of roost cavities by screech-owls, stated that
“the sizes of both the cavity entrance and the in-
terior were quite variable. ...” Smith et al. (1987)
also noted that the entrances of some roost sites
were elongated slits while others were large open-
ings created when the tops of trees or limbs had
broken off.

In contrast, Belthoff and Ritchison (1990b)
found that Eastern Screech-owls were selective in
their use of nest cavities, perhaps because variation
in the characteristics of nest cavities may influence

the risks of predation. The apparent tendency of
screech-owls to be less selective in the use of roost
cavities suggests that the risks of predation may be
lower during the nonbreeding season. At least one
group of potential predators, snakes, (Bent 1938)
is either less active or not active during the non-
breeding season. In addition, nestling screech-owls
are more vulnerable to predation than adults.
Therefore, adult owls must select nest cavities that
minimize the risks of predation. During the non-
breeding season, less vulnerable adults may not be
as selective because they are better able to defend
themselves and to escape from potential predators.

Acknowledgments

We thank Sunni Lawless, Carlo Abbruzzese and Joe
Metzmeier for assistance in locating roost sites. Eric Fors-
man, Dwight Smith and an anonymous reviewer provided
many useful comments on the manuscript. Financial sup-
port was provided by Eastern Kentucky University.

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Received 13 October 1996; accepted 15 May 1997

NUTRIENT CONTENT OF FIVE SPECIES OF DOMESTIC ANIMALS

COMMONLY FED TO CAPTIVE RAPTORS

NancyJ. Clum 1

The Peregrine Fund, 566 W. Flying Hawk Lane, Boise, ID 83709 U.S.A.

Marianne P. Fitzpatrick and Ellen S. Dierenfeld

Department of Nutrition, Wildlife Conservation Society, 185th St. and Southern Blvd., Bronx, NY 10460 U.S.A.

ABSTRACT. — The objective of this work was to provide a basis for more informed evaluation of diet
options with respect to the nutritional needs of captive raptors. We compared nutritional content of
five domesticated species that are most commonly fed to captive raptors; quail ( Coturnix coturnix japon-
ica ), chickens ( Callus domesticus), rats ( Rattus norvegicus), mice ( Mus musculus) and guinea pigs ( Cavia
porcellus). We measured proximate composition (moisture, lipid, protein, ash), vitamin A, vitamin E,
copper, iron, zinc, magnesium, manganese, calcium and potassium. Significant species differences were
found in lipid and in vitamins A and E, and differences approached significance in iron and manganese
concentrations. Differences in nutrient content between species did not correspond to differences in
nutrient levels of diets consumed by prey. All species contained adequate amounts of protein, lipid,
vitamin A, calcium, magnesium and zinc. However, whole domesticated prey were potentially inadequate
sources of vitamin E, copper, iron and manganese.

Key WORDS; body composition] minerals-, nutrition-, vitamins ;; raptor diet.

Contentos de nutrimento para cinco especies de animales domesticos frecuentamente dados para comer
ha rapaces captivos

Resumen. — El objetivo de este trabajo fue para proporcionar un base para una evaluacion mas infor-
mada de opciones de dieta con respeto con la necesidad de alimentacion de rapaces cautivas. Nosotros
comparamos el contento de nutrimiento de cinco especie domesticadas que esten frecuentemente dadas
de comer a rapaces cautivos: codorniz ( Coturnix coturnix japonica) , gallinas ( Gallus domesticus) , rata, ( Rat-
tus norvigicus), raton (Mus musculus ) y cobayo (Cavia porcellus) . Nosotros medimos composition proximo
(humedad, grasa, proteina, ceniza), vitamina A, vitamina E, cobre, fierro, zinc, magnesio, manganeso,
calcio y potasio. Diferencia significas de especies fueron encontradas en grasa y en vitamina A y E y
diferencias estaban significante en concentraciones de fierro y manganeso. Diferencias en alimento
entre especie no correspondieron a diferente niveles de nutrimiento de dietas consumidas para la presa.
Todos contienen suficiente cantidad de proteina, grasa, vitamina A, calcio, magnesio y zinc. Sin embar-
go, presa domesticada fueron pontenciamente insuficiente de vitamina E, cobre, fierro y manganeso.

[Traduction de Raul De La Garza, Jr.]

The diets of most wild raptors consist of a wide
variety of prey species (Palmer 1988). Of necessity,
raptors maintained in captivity are usually fed a
very limited array of domesticated species. The
diet of captive birds is therefore artificial in both
the type and variety of species consumed. Few stud-
ies have been done regarding the nutritional status
of free-ranging birds, but the data that do exist sug-

1 Present address: Environmental Science Program,
DePaul University, 1036 West Belden Ave., Chicago, IL
60614.

gest that wild birds may differ significantly from
captive animals of the same species (Dierenfeld et
al. 1989, Dierenfeld 1994). This is of concern to
zoos, private breeders and conservation organiza-
tions that engage in captive propagation because
nutritional status affects health (Gershwin et al.
1985, Sklan et al. 1995), growth (Lavigne et al.
1994a), reproduction (NRC 1984, Naber and
Squires 1993) and longevity (Good and Gajjar
1986). Undernutrition can also have long-term ef-
fects (Bedi 1987, Grantham-McGregor 1987, Lavig-
ne et al. 1994b), and can, therefore, potentially in-
fluence the viability of reintroduced populations.

267

268

Clum et al.

Vol. 31, No. 3

For most individuals and organizations, no fea-
sible alternative exists to feeding artificial diets. For
financial and logistical reasons, options are usually
restricted to prepared commercial diets or to one
or more domesticated species. Relatively little in-
formation is available on the nutrient content of
whole vertebrate prey (see Dierenfeld et al. 1994
for review) to facilitate comparison of dietary op-
tions. Furthermore, existing nutritional informa-
tion focuses primarily on macronutrients such as
lipid, protein, ash and fiber which are less likely to
be limiting in the diet of captive animals than vi-
tamins or minerals.

This study compares nutritional content of five
domesticated species that are among the most
commonly fed to captive raptors: quail ( Coturnix
coturnix japonica) , chickens ( Gallus domesticus), rats
( Rattus norvegicus ) , mice (Aim musculus ) and guin-
ea pigs ( Cavia porcellus). We measured proximate
composition (moisture, lipid, protein, ash), vita-
min A, vitamin E, copper (Cu), iron (Fe), zinc
(Zn), magnesium (Mg), manganese (Mn), calcium
(Ca) and potassium (K). These results provide the
basis for a more informed evaluation of diet op-
tions with respect to the nutritional needs of cap-
tive raptors.

Methods

Experimental Design. We analyzed five species of do-
mesticated animals. Both male and female quail were an-
alyzed, but only males of other species were used because
females are typically retained for breeding stock at our
facility. Birds ( N = 50, each species) were raised from
hatch to 6 wk of age in brooders. Mammals were raised
in litters until weaning. Three individuals from each
mammalian species (from different litters) were then
randomly selected and placed together in new cages.
Mice were raised to 12 wk, rats were raised to 11 wk and
guinea pigs were raised to 10 wk in standard laboratory
mammal cages. The following complete commercial
products were fed, exclusively and ad libitum : quail, Pur-
ina Turkey Starter; chickens, Purina Meatbuilder; rats
and mice, Purina Formulab Chow; guinea pigs, Purina
Guinea Pig Chow (all manufactured by Purina Mills, St.
Louis, MO U.S.A.).

Laboratory Analyses. Three individuals of each species
(and each sex for quail) were ground separately. Feathers
were removed from birds, as most raptors pluck their
prey and the majority of feathers consumed are regur-
gitated in pellets; for this study we assume that nutrient
intake from feather digestion is negligible. Guinea pigs
were also decapitated as even the largest eagles held at
our facility failed to consume the craniums of this spe-
cies. Four samples were immediately taken from each in-
dividual; two for duplicate vitamin analyses and two for
duplicate moisture, lipid, ash and mineral analyses. The
remainder of the ground sample was frozen, and two

samples were taken at a later time for duplicate protein
analyses. One sample was also taken from each type of
feed fed to each species.

Moisture content was determined by drying samples to
a constant weight in a vacuum oven at 60°C. Lipid con-
tent of dried samples was determined indirectly using
Soxhlet extraction (Ellis 1984). Fat-free dry samples were
ashed in a muffle furnace at 550°C for three days (Ellis
1984) to determine ash content. Protein content of
thawed wet tissues was assayed by the Biuret method
(Florwitz 1975); samples were corrected for any moisture
loss during freezing by redrying a second set of samples.
Tissue extraction and analyses of retinol and alpha- and
gamma-tocopherol were modifications of the general
methods of Taylor et al. (1976) as described in Douglas
et al. (1994), using high performance liquid chromatog-
raphy. Extraction of feed was performed according to the
method described by Combs and Combs (1985). Vitamin
A activity was calculated as 0.3 g all-trans retinol = 1 IU
(Olson 1984). Vitamin E was calculated by summing al-
ph a- and gamma-tocopherols, where 1 mg alpha-tocoph-
erol = 1.1 IU and 1 mg gamma-tocopherol = 0.1 IU
(Machlin 1984). Ashed samples were prepared for min-
eral analysis according to the method of Parker (1963).
Ca, Cu, Fe, Zn, Mg and Mn levels were measured on a
Perkin-Elmer atomic absorbance spectrometer.

Statistical Analyses. Species differences in nutrient con-
tent were analyzed using a one-way ANOVA in SYSTAT
(Wilkinson 1990). Sex differences and comparisons be-
tween pairs of species were analyzed using the Mann-
Whitney [^statistic or the Student’s /-test. Comparisons
among more than two species were analyzed with a Krus-
kal-Wallace test. Where the same test was performed on
multiple dependent variables, critical P-values were cor-
rected for multiple comparisons using a sequential Bon-
ferroni method (Rice 1989). Significance was assigned at
the level of (corrected) P < 0.05.

Results

Female quail were 17% heavier than male quail
at 6 wk of age (mass males = 121.6 g, SE = 12.6,
mass females — 146.5 g, SE = 8.9, t = 5.91, P ~
0.00001). No sex differences were found in proxi-
mate composition, vitamin A and vitamin E con-
tent, or mineral levels (Table 1), although females
had consistently higher levels of all vitamins and
minerals (Sign test, g. = 2.5, P = 0.008). Values for
male and female quail were therefore combined in
subsequent analyses.

Significant species differences were found in lip-
id (Table 2), vitamin A and vitamin E (Table 3)
and differences approaching significance (adjusted
P < 0.06) in Fe and Mn concentrations (Table 3) .
Lipid levels were lowest in mice and highest in
guinea pigs and chickens. Mice were 10 times high-
er in vitamin A than rats (Mann-Whitney, U — 18.0,
P = 0.02) , the species containing the next highest
vitamin A values. Rats, quail and chickens did not

September 1997

Nutrient Content of Whole Prey

269

Table 1, Mean nutritional content of whole male and female Japanese Quail. 3

Male

Female

P b

Moisture (%)

65.1 (3.1)

65.6 (1.8)

0.827

Protein (%DM)

64.9 (14.6)

71.6 (6.8)

0.524

Lipid (%DM)

33.2 (6.3)

26.3 (3.2)

0.050

Ash (%DM)

9.6 (1.3)

12.0 (1.7)

0.127

Retinol (IU/kg)

32 989 (10 951)

66 444 (30 525)

0.127

Alpha-tocopherol (IU/kg)

41.6 (13.3)

79.3 (0.4)

0.050

Calcium (mg/kg)

32 685 (4178)

43 615 (6561)

0.127

Copper (mg/kg)

2.66 (0.61)

3.02 (0.77)

0.827

Iron (mg/kg)

85.07 (7.93)

112.40 (33.94)

0.275

Magnesium (mg/kg)

578.6 (255.2)

752.7 (209.3)

0.513

Manganese (mg/kg)

6.61 (2.11)

8.45 (4.31)

0.513

Zinc (mg/kg)

55.01 (9.13)

54.30 (26.66)

0.827

J All data except moisture content presented on a dry matter basis. Values are means and one standard deviation. N = 3, each sex
b Unadjusted P -values, Student’s 2-test. No comparisons significant following correction for multiple comparisons.

differ in vitamin A content ( Kruskal-Wallace , H =
0.641, P = 0.73). Guinea pigs were 50% lower in
vitamin A than chickens (Mann-Whitney, U = 9.0,
P = 0.05), the species with the next lowest values.
Guinea pigs also had vitamin E levels that were at
least 50% lower than quail (Mann-Whitney, U =
18.0, P = 0.02); quail, mice and chickens were not
significantly different in vitamin E content (Krus-
kal-Wallace, H = 1.55, P = 0.46). Rats were three
times higher in vitamin E than mice (Mann-Whit-
ney, U= 9.0, P = 0.05).

Chicken and quail were not significantly differ-
ent in Fe content (Mann-Whitney, U = 10.0, P =
0.80) or Mn content (Mann-Whitney, U = 15.0, P
= 0.12), but the avian species were significantly
higher than the mammalian species in both Fe
(Mann-Whitney, U — 64, P — 0.04) and Mn (Mann-
Whitney, U = 68, P = 0.02). Within the mammals,
mice contained more Fe than guinea pigs (Mann-
Whitney, U — 9.0, P = 0.05) or rats (Mann-Whit-
ney, U = 9.0, P = 0.05), but rats and guinea pigs
did not differ from each other (Mann-Whitney, U
— 4.0, P = 0.827). Guinea pigs and mice had sim-

ilar levels of Mn (Mann-Whitney, U — 5.0, P —
0.275) and were both higher in this nutrient than
rats (Mann-Whitney, U = 16.0, P = 0.05). Differ-
ences in nutrient levels of feeds did not corre-
spond to nutrient differences between species in
any case (Table 4) .

Discussion

The differences between 6-wk male and female
quail were not significant in this study; however, it
is worth noting that females had consistently high-
er levels of most nutrients, as well as lower lipid
levels, than males. We have also found that at 16
wk of age nutrient levels in male quail are un-
changed relative to 6-wk old birds, but levels in fe-
male quail (mobilizing resources for egg produc-
tion) have almost doubled (unpubl. data). These
data suggest that sex differences in nutrient con-
tent may be detectable with larger samples sizes or
at different ages.

With the exception of lipid content, little differ-
ence was observed in proximate composition
among species. Our results are similar to published

Table 2. Proximate composition of whole domestic species. 3

Quail

Chicken

Rat

Mouse

Guinea Pig

Moisture (%)

65.4 (2.3)

67.7 (1.3)

64.3 (2.4)

66.9 (2.6)

69.3 (1.8)

0.075

Protein (%DM)

67.6 (11.4)

64.0 (15.1)

63.4 (14.3)

64.4 (20.8)

58.9 (14.9)

0.955

Lipid (%DM)

29.7 (5.9)

47.2 (5.3)

34.9 (5.2)

23.7 (8.8)

45.4 (11.0)

0.005 b

Ash (%DM)

10.8 (1.9)

10.4 (2.0)

7.5 (2.1)

9.2 (1.6)

8.9 (0.6)

0.155

a All data except moisture content presented on a dry matter basis. Values are means and one standard deviation. N = 3, each species
b P-value significant after correction for multiple comparisons.

270

Clum et al.

Vol. 31, No. 3

Table 3. Vitamin and mineral content of whole domesticated species. 3

Quail

Chicken

Rat

Mouse

Guinea Pig

Retinol

(IU/kg)

49 716 (27504)

35 588 (15 309)

68 244 (23 220)

657 344 (196 887)

19 989 (3000)

<0.00001 b

Alpha-tocopherol

(IU/kg)

60.4 (29.8)

61.4 (5.6)

210.5 (68.7)

74.4 (18.2)

29.8 (0.9)

0.00013 b

Calcium

(mg/kg)

38 150 (7748)

24 546 (2864)

22 856 (4636)

32 076 (6185)

29 458 (4458)

0.01841

Copper

(mg/kg)

2.8 (0.7)

2.7 (0.1)

1.3 (0.4)

3.8 (0.2)

6.0 (4.2)

0.04781

Iron

(mg/ kg)

98.7 (31.6)

97.6 (10.2)

43.0 (3.9)

76.4 (0.4)

51.9 (6.8)

0.00675

Magnesium

(mg/kg)

665.6 (229.5)

535.9 (71.3)

247.3 (134.9)

431.9 (54.2)

637.3 (39.6)

0.02099

Manganese

(mg/kg)

7.5 (3.2)

11.0 (1.2)

2.9 (0.9)

5.3 (1.7)

6.6 (0.5)

0.00688

Zinc

(mg/kg)

54.7 (17.8)

74.1 (21.1)

35.0 (10.0)

44.0 (5.7)

64.4 (23.7)

0.09748

a All data except moisture content presented on a dry matter basis. Values are means and one standard deviation. N = 3, each species
b P-values significant after correction for multiple comparisons.

values for these species, which range between 55-
68% for water content, 43-66% (DM) for protein
content and 7-10% (DM) for ash content (Medway
1958, Lepore and Marks 1971, Brisbin and Tally
1973, Bird and Ho 1976, Thonney et al. 1984, La-
vigne et al. 1994a). Lipid content appears to be the
most variable component of proximate composi-
tion ranging between 19-49% (Lepore and Marks
1971, Brisbin and Tally 1973, Bird and Ho 1976,
Perrigo and Bronson 1983, Thonney et al. 1984,
Lavigne et al. 1994a), but there is no consistent

pattern of lipid content with respect to species, as
might be expected with a labile body component.

Vitamin and mineral content in this study were
much more variable than proximate composition.
Although few comparative data are available, spe-
cies differences in vitamin A and vitamin E content
have also been found by Douglas et al. (1994), and
species differences in mineral content appear to be
present in the results of Bird and Ho (1976) and
Lavigne et al. (1994a), although no statistical anal-
ysis of these data was presented. The pattern of

Table 4. Composition of commercial diets and relation between diet and body composition 3 .

Turkey

Starter

Meat-

Builder

Formulab

Chow

Guinea Pig
Chow

P h

Moisture (%)

9.3

7.7

8.9

9.4

0.900

Lipid (%)

1.1

4.2

2.0

2.4

0.192

Protein (%)

20.4

18.3

15.2

16.1

0.274

Ash (%)

7.0

5.8

7.6

8.4

0.270

Vitamin A (IU/kg)

3500

4500

6133

29 733

0.282

Vitamin E (IU/kg)

11.8

4.2

14.8

15.9

0.730

Calcium (mg/kg)

17079

12 584

13 762

15 124

0.085

Copper (mg/kg)

18.7

14.5

13.4

14.1

0.872

Magnesium (mg/kg)

1285.1

1218.5

1068.0

1757.4

0.202

Iron (mg/kg)

161.9

154.6

239.6

290.4

0.855

Manganese (mg/ kg)

76.4

78.2

16.3

54.7

0.520

Zinc (mg/kg)

127.3

124.4

99.8

90.4

0.058

a All data except moisture content presented on a dry matter basis. N = 1, all diets.
b Unadjusted P-values for regression of diet composition on body composition.

September 1997

Nutrient Content of Whole Prey

271

species differences in these studies, however, is not
consistent with the pattern that we observed. For
example, we observed rats to be generally low in
mineral content, while Bird and Ho (1976) did
not. Also, our values for vitamin E were up to 50%
greater, and our values for vitamin A were up to
two times greater than those of Douglas et al.
(1994). Variation in nutritional content can result
from differences in diet (Thonney and Ross 1987,
Dierenfeld et al. 1989, Clum et al. 1996), genetics
(Lepore and Marks 1971), age (Brisbin and Tally
1973, Bird and Ho 1976, Thonney and Ross 1987,
Douglas et al. 1994) or sex, all of which have been
demonstrated to cause significant changes in prox-
imate composition and/or vitamin and mineral
content. Diet formulation in particular has almost
certainly changed over the two decades that these
studies encompass, and may, therefore, be a signif-
icant source of variation. Manner and length of
storage can also affect nutrient levels, particularly
of vitamins, which are more labile than minerals
or proximate composition. Storage may have
caused the observed differences in vitamin levels
between our study and that of Douglas et al.
(1994), as their animals were purchased frozen
from breeders whereas ours were freshly killed.

Our study suggests that species differences in nu-
tritional content are not readily predictable. Com-
parative work on digestive efficiency of birds of
prey has shown that the Common Buzzard ( Buteo
buteo ), a generalist species, has high efficiency on
a wider variety of prey than the Peregrine Falcon
( Falco peregrinus), a specialist species (Barton and
Houston 1993). Such variation in the ability to ex-
tract nutrients may partially explain the food pref-
erences of birds in captivity. However, the prey that
has the closest physical resemblance to wild prey
does not necessarily bear the closest nutritional re-
semblance for the reasons mentioned above. If dif-
ferent species require prey with different nutrition-
al content as Barton and Houston (1993) have sug-
gested, then it is necessary to provide prey that are
not only taxonomically acceptable, but nutrition-
ally compatible for optimal breeding.

Generally, when authors allude to food quality
or nutritional content they are referring to proxi-
mate composition. Although lipid content of prey
may be of critical energetic importance in wild
birds (Blem 1990) and does have the ability to limit
egg number (Drobney 1980), lipid reserves are un-
likely to be a limiting factor in the energetics or
reproduction of captive birds that experience both

lower energy demands and more regular access to
food. A greater potential problem in captivity is
egg and chick viability, which is not limited by lipid
and protein reserves, but can be severely affected
by vitamin and mineral content of food (NRG

1984, Naber and Squires 1993).

All prey analyzed in this study met known re-
quirements of domestic mammalian carnivores for
vitamin A, Ca, Mg and Zn (vitamin A, 2440-10 000
IU/kg; Ca, 0.4-1. 2%; Mg, 0.04-0.1%; Zn, 30-50
mg/kg; NRC 1985, 1986, Robbins 1983). Copper
levels were inadequate in all species except guinea
pigs, Fe was below recommended levels in rats and
guinea pigs and Mn was lower than suggested in
rats (Cu, 5. 0-7. 3 mg/kg; Fe, 60-100 mg/kg; Mn,
5-10 mg/kg; NRC 1982, 1985, 1986). Manganese
deficiency has recently been documented in cap-
tive raptor chicks fed exclusively rats (C. Sandfort,
pers. comm.). Although all species except guinea
pigs met recommended levels of vitamin E for
mammalian carnivores (20-80 IU/kg, NRC 1982,

1985, 1986), it has been suggested that raptors may
require up to 10 times more vitamin E to protect
against deficiencies (Calle et al. 1989, Dierenfeld
et al. 1989). Other differences between nutrient
requirements for domestic mammalian carnivores
and nondomestic avian carnivores may exist.

Acknowledgments

We would like to thank A. Sirles and G. Thomas for
care and maintenance of research animals. We also thank
J. Rigg, B. Bammel and J. Munger for advice and access
to additional laboratory space and equipment and L.
Pearson and D. Barker for their laboratory assistance
This research was made possible by donations of equip-
ment from Perkin-Elmer, Corning, Milton Roy, Precision
Scientific and Nalge. This manuscript benefited from
helpful comments by D. Bird, T. Cade, G. Duke, J. Ges-
saman, J. Iinthicum and C. Marti.

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Sklan, D., D. Melamed and A. Friedman. 1995. The ef-
fect of varying dietary concentrations of vitamin A on
immune response in the turkey. Br. Poult. Sci. 36:385-
392.

Taylor, S.L., M.P. Lamben and A.L. Tappel. 1976. Sen-
sitive flourometric method for tissue tocopherol anal-
ysis. Lipids 11:530-538.

Thonney, M.L. and D.A. Ross. 1987. Composition of
gain of rats fed low or high protein diets and grown
at controlled rates from 80 to 205 grams . J. Nutr. 117:
2135-2141.

, AM. Oberbauer, D.J. Duhaime, T.C. Jenkins and

N.L. Firth. 1984. Empty body component gain of
rats grown at different rates to a range of final
weights. / Nutr. 114:1777-1786.

Wilkinson, L. 1990. SYSTAT: The system for statistics.
Systat, Inc., Evanston, IL U.S.A.

Received 3 August 1996; accepted 22 April 1997

Short Communications

Juvenal Plumage Characteristics of Male Southeastern American Kestrels

{Falco sparverius paulus)

Karl E. Miller

Department of Wildlife Ecology and Conservation, University of Florida, P.O. Box 110430,

Gainesville, Florida 32611 U.S.A.

John A. Smallwood

Department of Biology, Montclair State University, Upper Montclair, NJ 07043 U.S.A.

Key Words: American Kestrel, Falco sparverius; Florida;
aging, plumage pattern.

Determining ages of American Kestrels {Falco sparver-
ius) can be problematic. The first prebasic molt in Amer-
ican Kestrels is incomplete; juvenal body plumage is re-
placed in the late summer or early fall of the hatching
year, while juvenal remiges and rectrices are retained.
However, some males retain a few too many juvenal body
feathers through the first prebasic molt (Smallwood
1989). Retention of juvenal body feathers was common
in male American Kestrels {F. s. sparverius) wintering in
southern Florida; of 18 males known to be immature be-
cause of distinctive fault bar patterns (Hamerstrom 1967,
Smallwood 1989), four retained their juvenal body plum-
age after the first prebasic molt was completed and four
others were undergoing delayed body molt as late as No-
vember (J. Smallwood, unpubl. data) . Thus, a substantial
portion of immature male American Kestrels can be aged
after the first prebasic molt.

Many sources have reported that heavy streaking on
the breast and dark barring on the anterior dorsum are
diagnostic characters of the juvenal plumage of male
American Kestrels (Parkes 1955, Bird and Palmer 1988,
Smallwood 1989, Wheeler and Clark 1995). In his key for
age and sex determination of American Kestrels, Small-
wood (1989) used the absence of bars on the “upper
one-third to one-half of back” as a diagnostic character
to distinguish males in basic plumage from hatching-year
males.

Existing keys for aging American Kestrels are based on
F. s. sparverius. Little has been published about the biol-
ogy of the Southeastern American Kestrel ( F. s. paulus),
which breeds in Florida and the southern portions of
South Carolina, Georgia, Alabama, Mississippi and Loui-
siana (Smallwood 1990). This nonmigratory race under-
went a marked decline in recent decades (Hoffman and
Collopy 1988) and is currently listed as threatened in
Florida (Collopy 1996). The objective of this study was to

examine the plumage characteristics of male Southeast-
ern American Kestrel nestlings in northcentral Florida
and to compare them to those observed throughout the
better studied portion of the species’ range.

Study Area and Methods

We examined the plumage characteristics of nestling
male Southeastern American Kestrels in Levy County,
Florida, during May-July 1994 and May 1995. Nestlings
ranged in age from 14—27 d at the time of banding, but
some nestlings younger than 17 d of age were not suffi-
ciently feathered to include in our analysis. Therefore,
we characterized the juvenal plumage of nestlings ^17-d
old. We defined the “back” of the kestrel as the area
extending from the rump to the nape, including the in-
terscapular region (U.S. Fish and Wildlife Service 1980).
Each nestling was classified as belonging to one of four
categories based on a visual assessment of the extent of
barring on its back: (1) barring restricted to the posterior
third of the back, (2) barring extending beyond the low-
er one-third but not beyond the lower one-half of the
back, (3) barring extending throughout the lower two-
thirds of the back or (4) barring extending throughout
the entire back or nearly so.

Results and Discussion

We examined 33 male nestlings from 20 nest boxes.
Mean age of the nestlings examined was 22.4 d. Fifteen
(45%) of 33 male nesdings lacked the diagnostic barring
on the anterior half of the dorsum. Several had no bar-
ring at all. Only nine males (27%) had barring through-
out the entire dorsum as indicated in couplet 2A of the
key (Smallwood 1989). Moreover, brood mates did not
share the same barring pattern; of 1 1 nests containing at
least two males, only four nests had brood mates belong-
ing to the same dorsal plumage category.

Bloom (1973) stated that immature birds of either sex
in southern California could not be distinguished from
adults by feathering. However, most authors reported
that juvenal males had heavy streaking on the breast and

273

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Vol. 31, No. 3

dark barring on the anterior dorsum, whereas males in
basic plumage had immaculate to lightly spotted upper
breasts (Parkes 1955, Bird and Palmer 1988, Smallwood
1989, Wheeler and Clark 1995). We found that the
amount of streaking on the breast was variable in male
nestlings as has been observed in juvenal plumage of F.
s. sparverius. In contrast, we found that nearly half of the
male nestlings we examined in our study area lacked the
dark barring on the anterior dorsum diagnostic in F. s.
sparverius. We recommend both characters be assessed in
determining age in F. s. paulus.

Resumen. — Nosotros comparamos los caracteristicos del
plumaje de pajaritos machos Falco sparverius paulus en el
norte centro de Florida con esos observados durante
todo el estudio de la especies pradera. 15 (45%) de 33
machos pequenos les faltaba la barra en el parte anterior
de la espalda que ha estado reportando como diagnos-
tico para F. s. sparverius. Muchos no tenian barras. No-
sotros recomendemos que una variada de plumaje este
valorada en determinando edad en F. s. paulus.

[Traduccion de Raul De La Garza, Jr.]

Acknowledgments

We thank N. Dwyer, J. Harris, K. Long, K. McPherson,
R. Melvin and S. Smitherman for building, erecting and
monitoring nest boxes. This study was supported by a
grant to J. Smallwood and M. Collopy from the Florida
Game and Fresh Water Fish Commission’s Nongame
Wildlife Program. We are grateful for the cooperation of
the Florida Power Corporation, which authorized the use
of selected utility poles for mounting nest boxes. D. Bird,
G Bortolotti and K. Steenhof provided helpful com-
ments that improved the manuscript. This paper is con-
tribution No. R-05838 of the Journal Series, Florida Ag-
ricultural Experiment Station, Gainesville.

Literature Cited

Bird, D.M. and R.S. Palmer. 1988. American Kestrel
{Falco sparverius). Pages 253-290 mR.S. Palmer [Ed.],
Handbook of North American birds, Vol. 5, Diurnal
raptors (Part 2). Yale Univ. Press, New Haven, CT
U.S.A.

Bloom, P.H. 1973. Seasonal variation in body weight of
sparrow hawks in California. West. Bird Bander 48:17-
19.

Collopy, M.W. 1996. Southeastern American Kestrel.
Pages 211-218 ra J.A. Rodgers, H.W. Kale and H.T.
Smith [Eds.], Rare and endangered biota of Florida,
Vol. V, Birds. Univ. Presses of Florida, Gainesville, FL
U.S.A.

Hamerstrom, F. 1967. On the use of fault bars in aging
birds of prey. Inland Bird-Banding News 39:35-41.

Hoffman, M.L. and M.W. Collopy. 1988. Historical sta-
tus of the American Kestrel {Falco sparverius paulus )
in Florida. Wilson Bull. 100:91-107.

PARKES, K.C. 1955. Notes on the molts and plumages of
the sparrow hawk. Wilson Bull. 67:194—199.

Smallwood, J. A. 1989. Age determination of American
Kestrels: a revised key . J. Field Ornithol. 60:510-519.

. 1990. Kestrel and Merlin. Pages 29-37 in B.A.

Giron Pendleton [Ed.], Proc. of the southeast raptor
management symposium. Natl. Wildl. Federation,
Washington, DC U.S.A.

U.S. Fish and Wildlife Service. 1980. North American
bird banding manual, Vol. II (revised edition). U.S
Government Printing Office, Washington, DC U.S.A

Wheeler, B.K and W.S. Clark. 1995. A photographic
guide to North American raptors. Academic Press,
London, UK.

Received 24 October 1996; accepted 10 May 1997

Double Brooding by American Kestrels in Idaho
Karen Steenhof and Brit E. Peterson

Snake River Field Station , Forest and Rangeland Ecosystem Science Center, Biological Resources Division,
U.S. Geological Survey, 970 Lusk Street, Boise, ID 83706 U.S.A.

Key Words: Falco sparverius; American Kestrel, renesting,
Idaho ; double brooding.

American Kestrels {Falco sparverius ) sometimes raise
two broods in a single nesting season in captivity (Porter
and Wiemeyer 1970, 1972), and double brooding by wild
kestrels has been recorded in Florida and Central Mis-

souri (Howell 1932, Toland 1985). Evidence for double
brooding elsewhere, however, has been mainly circum-
stantial (Stahlecker and Griese 1977, Black 1979, Sutton
1979), and there have been no reports of double brood-
ing by kestrels north of 40° latitude. During a long-term
study of kestrel nest box occupancy, productivity and site
fidelity, we confirmed that a pair of kestrels successfully

September 1997

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275

raised two broods in southwestern Idaho (43° N, 116° W)
during a single breeding season.

In 1996, we captured the same marked pair of adults
at two different nest boxes, both of which had young that
reached fledging age. We captured the female on an in-
complete set of three eggs at the first box on 25 March
and captured the male in a mist net (Steenhof et al.
1994) placed by the same box on 17 May. We banded five
young from this box on 17 May. Ages of the young at
banding ranged from 15—25 d, based on a comparison
with a photographic aging key (Griggs and Steenhof
1993). We recaptured the female on 18 June in a box
with six eggs, 800 m from the first box. We caught the
same male in this box on 28 June with three eggs and
three young. We banded five 22- to 26-d-old young from
this box on 23 July. We assume that all 10 young fledged
from the boxes because we found no dead young in or
below the boxes during subsequent checks.

Both members of the pair were at least 2-yr-old in 1996,
and both had nested successfully in the area in 1995. The
female was first captured on 5 February 1995 on a bal-
chatri midway between her two 1996 nesting efforts. In
1995, she raised young in the same box where she raised
her second brood in 1996. The male was first captured
as a breeding adult in 1995, paired with a different fe-
male at a box approximately 1.7 km from his nearest
1996 nesting attempt.

The distance between nesting efforts in Idaho (800 m)
was much greater than the distances in Missouri (0-300
m, Toland 1985), possibly due to fewer available nesting
sites in Idaho. Both boxes used in Idaho were mounted
on boards attached to fenceposts in open agricultural
and rangeland habitats. There were no nest boxes or nat-
ural cavities nearer either box. The second clutch size (6
eggs) in Idaho was bigger than any recorded in Missouri,
and in contrast to Toland’s (1985) findings, the second
clutch in Idaho was larger than the first clutch (5 eggs).

The estimated hatching dates of young produced by
the pair that raised two broods in 1996 were 24 April and
28 June. During our 11-yr study, estimated hatch dates
have been as early as 17 April and as late as 24 July (x =
25 May, SD = 18.5 d, N = 247). We have identified five
broods with earlier hatch dates than the first brood of
the pair that raised two broods and 10 broods with later
hatch dates than their second brood, for all years com-
bined. In 1996, the first brood of the renesting pair was
the second earliest nesting effort in our study area, and
the second brood was the second latest. The individuals
that raised two broods in 1996 probably only raised one
brood each in 1995 because their 1995 nesting chronol-
ogy was closer to the long-term mean. The male’s 1995
brood hatched on 14 June, and the female’s hatched on
11 May.

Whether a pair will attempt to raise two broods in a
single season likely depends on food availability, weather
conditions and nesting experience. Both food availability
and prior nesting experience may have increased the

likelihood of successful double brooding in 1996. Prey
remains in the two nest boxes consisted mainly of voles
( Microtus spp.) , and our subjective observations indicated
that voles were unusually abundant in 1996. Both mem-
bers of the renesting pair in Idaho had successfully bred
in the area the prior year. As in Poland’s (1985) study,
dottble brooding may be possible only for early breeders
in Idaho. The climate in southwestern Idaho provides
just enough time for kestrels to raise two broods. Henny
and Brady (1994) found that permanent residents nest
earlier than migrant kestrels in the Pacific Northwest.
The female that raised two broods in our area was known
to have spent at least part of one winter near her nesting
territory.

Although this was the first and only documented case
of double brooding during our 11-yr study, it may have
occurred before. We would have missed other cases of
double brooding if kestrels used natural nest sites in trees
that we did not monitor for one of their nesting attempts.
We also might have missed cases if we did not capture
and/or mark both adults during one of their nesting ef-
forts. In 1996, we knew the identities of 63% of the males
and 93% of the females nesting in boxes; the propor tions
of unidentified individuals were higher during the first 7
yr of our study. The fact that kestrels used different boxes
for nesting makes it difficult to confirm double brooding
if the parents are not individually marked. It also raises
doubts about some suspected cases of double brooding
reported in the literature. The presence of a second
clutch in the same box does not constitute evidence for
renesting by a particular individual or pair (Sutton 1979) .
During our study, we knew the identity of females in five
“renestings” following failures during incubation. In
three cases, females whose clutches failed during incu-
bation moved to other boxes. In two other situations, a
new female nested in the same box where a different
female had failed during incubation.

American Kestrels probably require a minimum of 120
d to raise two broods successfully: at least 5 d for each
laying period, 27 d for each incubation period and 30 d
for each brood-rearing period (Porter and Wietneyer
1972). In southwestern Idaho, kestrels begin laying eggs
as early as mid-March, and young have fledged as late as
early to mid-August, a window of approximately 150 d.
Theoretically, pairs with young that hatch earlier than 15
May could produce a second brood, and broods with
hatch dates later than 15 June could be second broods.
In our 11-yr study, 31% of broods hatched on or before
14 May, suggesting that almost one-third of the popula-
tion nests early enough to produce two broods. However,
only 15% of broods hatched after 15 June, indicating that
at least half of the early nesters do not produce a second
brood. In addition, some of the late broods represent
pairs that nest late for other reasons, including renesting
after failures during incubation. During our study, we
knew of six renestings following failures, only two of
which were successful. The young from these nesting at-

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Vol. 31, No. 3

tempts hatched on 15 and 19 June, 9-13 d earlier than
the second brood from the double brooding pair. If we
assume conservatively that only those broods with hatch
dates after 28 June (the hatch date of the second brood
we confirmed) were second broods, then approximately
4% of the kestrel pairs in southwestern Idaho raise sec-
ond broods. Continued monitoring of marked adults
should provide more insight about the frequency of dou-
ble brooding in northern latitudes.

RESUMEN. — Una pareja marcada de Falco sparverius crio
dos nidadas de cinco en una temporada en dos cajas de
nidos diferentes en el sur oeste de Idaho. Los dos padres
tenian el minimo de dos anos y tenian exito con nidos
en el lugar antes. Las dos crias eran primera y la mas
tarde en la area de estudio, pero sospechamos que hasta
4% de parejas en el sur oeste de Idaho crian dos crias
cada temporada.

[Traduccion de Raul De La Garza, Jr.]
Acknowledgments

This paper is a contribution from the Snake River Field
Station, Forest and Rangeland Ecosystem Science Center,
Biological Resources Division, U.S. Geological Survey
(formerly the National Biological Service’s Raptor Re-
search and Technical Assistance Center). We thank
George Carpenter and Julie Heath for building boxes
and collecting background data that allowed us to make
these observations.

Literature Cited

Black, E.A. 1979. American Kestrel possibly two-brood-
ed in central Oklahoma. Bull. Okla. Omithol. Soc. 12'
29-30.

Griggs, G.R. and K Steenhof. 1993. Photographic
guide for aging nestling American Kestrels. Unpubl.
rep., Raptor Res. Tech. Asst. Cen., U.S. Dept. Interior,
Bur. Land Manage., Boise, ID U.S.A.

Henny, C.J. and G.L. Brady. 1994. Partial migration and
wintering localities of American Kestrels nesting in
the Pacific Northwest. Northwestern Naturalist 75 : 3 7-43

Howell, A.H. 1932. Forida bird life. Florida Depart-
ment of Game and Freshwater Fish, Tallahassee, FL
U.S.A.

Porter, R.D. and S.N. Wiemeyer. 1970. Propagation of cap-
tive American Kestrels. J. Wildl. Manage. 34:594-604.

AND . 1972. Reproductive patterns in captive

American Kestrels (sparrow hawks). Condor 74:46-53.

Stahlecker, D.W. and H.J. Griese. 1977. Evidence of
double brooding by American Kestrels in the Colo-
rado high plains. Wilson Bull. 89:618-619.

Steenhof, K, G.P. Carpenter and J.C. Bednarz. 1994.
Use of mist nets and a live Great Horned Owl to cap-
ture breeding American Kestrels. J. Raptor Res. 28:
194-196.

Sutton, G.M. 1979. Is the American Kestrel two-brood-
ed in Oklahoma? Bull. Okla. Ornithol. Soc. 12: 30-31.

Toland, B.R. 1985. Double brooding by American Kes-
trels in central Missouri. Condor 87:434—436.

Received 6 September 1996; accepted 10 May 1997

First Nest Record of the Bare-si tanked Screech-owl ( Otus clarkii)

Paula L. Enriquez Rocha 1 and J. Luis Rangel-Salazar 1
Programa Regional en Manejo de Vida Silvestre, Universidad Nacional, Apartado 1350-3000, Heredia, Costa Rica

Joe T. Marshall

National Museum of Natural History, 10th & Constitution Ave. N.W., Washington, DC 20560 U.S.A.

Key Words: Bare-shanked Screech-owl, Otus clarkii; nest, Cos-
ta Rica.

Most New World tropical forest raptors are poorly
known, especially those restricted in distribution and

1 Present address: Depto. Ecologia y Sistematica Te-
rrestre, ECOSUR, Apartado Postal 63, 29290 San Cristo-
bal de Las Casas, Chiapas, Mexico.

habitat. An estimated one-half of Neotropical raptors,
their nests, eggs and voices have never been described
(Thiollay 1985). Most of the world’s owl species occur in
the tropics and their ecology and biology are little known
(Clark et al. 1978) .

Costa Rica contains 9.9% (17 species) of the 172 owl
species (Monroe and Sibley 1993). The Bare-shanked
Screech-owl ( Otus clarkii) is a resident from the central
mountains of Costa Rica (Central Cordillera, Talamanca

September 1997

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277

Cordillera) , Panama (W Chiriqui, Veraguas and Darien)
through NW Colombia (Cerro Tacarcuna in NW Choco)
(AOU 1983). It has been recorded in the highlands of
Monteverde, Poas Volcano, Tapanti, Chirripo, Cerro de
la Muerte, Cerro Ghompipe and Villa Mills (Stiles and
Lewis 1980). It is the only species of the genus Otus that
inhabits cloud forest and humid forest at high altitudes
(900-2350 m) in Costa Rica (Slud 1964). Its numbers are
unknown, but it has been reported to be uncommon or
rare in Costa Rica (Stiles and Skutch 1989). Almost no
information is available on its breeding biology and nests
remain undescribed (Wetmore 1968, Stiles and Skutch
1989). The only previous evidence of breeding by Bare-
shanked Screech-owls in Costa Rica comes from the pres-
ence of brood patches on museum specimens (Stiles and
Skutch 1989) . In this paper, we describe what we consider
to be the first Bare-shanked Screech-owl nest located in
the mountains of central Costa Rica.

From 15-17 April 1994, we heard and observed what
appeared to be a breeding pair of Bare-shanked Screech-
owls in a pasture near the Tapanti Hotel, approximately
71 km south of Cartago City, Provincia de San Jose, Costa
Rica (9° 35'N, 83° 45'W). It is close to Tapanti National
Park and above Cerro de la Muerte at the top of the
northwest Talamanca Cordillera with an elevation of 2490
m. Mean annual temperature in the area is 6°C and mean
annual precipitation is 6500 mm. Typical vegetation is
cloud forest and subalpine paramo, and includes oaks,
bromeliads, orchids, mosses and ferns.

On 15 April 1994 at 1900 H, w^e heard a deep whistled
huu-huu-huu much like the typical call of a Bare-shanked
Screech-owl. When it stopped, we imitated the call and a
small owl flew to a mossy branch approximately 5 m away
from us. We identified the owl to be an adult Bare-
shanked Screech-owl and recorded its calls that night. On
the morning of the next day, a search of two isolated
trees nearby failed to locate any roosting owls or any sign
that owls had recently used the trees. However, at about
one-half hour after sunset, we observed an adult female
Bare-shanked Screech-owl flying and a male perched and
calling in the two trees. We distinguished the female by
her higher pitched call (Fig. 1). At approximately 1830
H, the female flew to a nest in a live oak ( Quercus copey-
ensis) . A fork in the trunk created a natural cavity where
the bird nested. The tree had a dbh (diameter at breast
height) of 65 cm and was 23 m tall. The nest was 3.3 m
high and the cavity was 35 cm long and 64 cm wide.

On 17 April 1994, we observed a single nestling cov-
ered with down in the cavity that we estimated to be ap-
proximately 3-wk-old (Fig. 2). No nesting material was
found but the nestling was on a large clump of moss.
Habitat surrounding the nest, tree consisted of scattered,
tall oaks laden with epiphytes. Ground cover consisted of
meadow grasses.

Both parents brought food to the nestling. We were
unable to identify the prey that were delivered, but the
items appeared to be large insects such as orthopterans

and coleopterans. Many Otus species are mainly insectiv-
orous (Ross 1969). Several times, the female flew from
the nest to capture insects on the ground and returned
quickly to give the food to the nestling. Occasionally the
male perched on a lamp from which it caught insects.
No pellets were found below the nest and we did not
search for pellets inside the nest in order to avoid dis-
turbance to the nest.

Otus is the largest genus in the order Strigiformes, and
Monroe and Sibley (1993) list 46 species for this genus
Marshall (1967) lists seven species of Otus occupying
North and Middle America where they overlap without
interbreeding. Most of them live in the world’s tropical
regions, except in Australasia, and many restricted pop-
ulations of screech owls are now endangered species
(Hekstra 1973). Our observations indicate that Bare-
shanked Screech-owls probably breed from middle Feb-
ruary (egg laying) through early May (fledging) in the
Tapanti region. We estimated the breeding chronology
based on Flammulated Owl (Otus flammeolus) in Colora-
do (Reynolds and Linkhart 1987). These findings are
comparable with those reported by Stiles and Skutch
(1989).

In view of limited geographic distribution of the Bare-
shanked Screech-owl and its unknown breeding status,
more information is needed on its nesting biology, nest-
ing density and habitat affinities to address questions
concerning its possible management and conservation
Currently habitat loss is a major problem that threatens
all raptor populations, and cloud forest habitats in cen-
tral Costa Rica have been affected seriously by develop-
ments related to the dairy industry on highlands. Based
on our limited knowledge of the breeding biology of the
Bare-shanked Screech-owl, it may actually be the devel-
opment of dairy farms which increase the numbers of
isolated trees and lampposts for feeding. Any useful con-
servation strategy for the protection of Bare-shanked
Screech-owls should at least protect woodlots as potential
breeding sites.

Resumen. — Reportamos el primer registro de anidacion
de la Lechucita Serranera ( Otus clarkii), encontrado en
las tierras altas de Costa Rica. El nido se localizo en un
arbol de encino ( Quercus copeyensis) a una altura de 3.3
m en una cavidad natural con las siguientes dimensiones:
35 cm de largo y 64 cm de ancho. El nido contenia un
polio con una edad estimada de 3 semanas. Ambos pa-
dres alimentaban al polio con insectos del orden coleop-
tera y ortoptera. No encontramos egagropilas dentro ni
fuera del nido. Se necesita mas informacion sobre den-
sidad poblacional y aspectos ecologicos para la Lechucita
Serranera. Sin embargo, para establecer estrategias de
conservacion para esta especie, es importante incluir la
proteccion de lotes arbolados para su reproduccion.

[Traduccion de Autores]

O t u s

278

Vol. 31, No. 3

Figure 1. Three kinds of Bare-shanked Screech-owl calls digitally recorded over 14 min the night of 15 April 1994 near the Tapanti Hotel, approximately
71 km south of Cartago City, Provincia de San Jose, Costa Rica: a) duet of the pair, female higher pitch, b) pair duet in flight, and c) male solo.

September 1997

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279

Figure 2. The nest and nestling of the Bare-shanked Screech-owl pair near the Tapanti Hotel, Costa Rica.

Acknowledgments

We wish to dedicate this article to the memory of Elsie

Marshall.

We thank D.W. Holt, J.R. Belthoff and M. Ritchinson

for their helpful comments to this manuscript.

Literature Cited

American Ornithologists’ Union. 1983. Checklist of
North American birds, 6th edition. AOU, Smithsoni-
an Inst., Washington, DC U.S.A.

Clark, R.J., D.G. Smith and L.H. Kf. l so. 1978. Working
bibliography of owls of the world. Washington DC,
Natl. Wild. Fed. Sci. Tech. Publ. No. 1.

Hekstra, G.P. 1973. Scops and Screech owls. Pages 94-
155 mJ.A. Burton [Ed.], Owls of the world. Eurobook
Limited, The Hague, Netherlands.

Marshall, J.T. 1967. Parallel variation in North and
Middle American Screech-owls. Western Foundation
of Vertebrate Zoology. Monogr. 1. Los Angeles, CA
U.S.A.

Monroe, B.L. and C.G. Sibley. 1993. World checklist of
birds. Yale Univ. Press, New Haven, CT U.S.A.

Reynolds, R.T. and B.D. Linkhart. 1987. The nesting
biology of Flammulated Owls in Colorado. Pages 239-
248 in R.W. Nero, R.J. Clark, R.J. Knapton and R.H.

Hamre [Eds.], Biology and conservation of northern
forest owls. Winnipeg, Manitoba, Canada.

Ross, A. 1969. Ecological aspect of the food habits of
insectivorous Screech-owls. Proceedings of the West-
ern Foundation of Vertebrate Zoology Vol. 1, No. 6.
Los Angeles, CA U.S.A.

Slud, P. 1964. The birds of Costa Rica, distribution and
ecology. Bull. American Mus. Nat. Hist., Vol. 128. New
York, NY U.S.A.

Stiles, G. and T.J. Lewis. 1980. Locational check-list of
the birds of Costa Rica. Costa Rica Expeditions, San
Jose, Costa Rica.

and A. Skutch. 1989. A guide to the birds of

Costa Rica. Cornell University Press, Ithaca, NYU.S.A.

Thiollay, J.M. 1985. Falconiforms of tropical rain forest:
a review. Pages 155-165 in I. Newton and R.D. Chan-
cellor [Eds.], Conservation studies on raptors. ICBP
Tech. Publ. No. 5. Paston Press England Ltd., Lon-
don, UK.

Wetmore, A. 1968. The birds of the Republic of Pana-
ma. Smithsonian Misc. Coll., Vol. 150, Part 2. Wash-
ington, DC U.S.A.

Received 12 July 1996; accepted 25 April 1997

280

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Vol. 31, No. 3

The Summer Diet of the Little Owl (Athene noctua) on the
Island of Astipalaia (Dodecanese, Greece)

Francesco M. Angelici and Leonardo Latella
Dipartimento di Biologia Animale e delTUomo, Universita di Roma “La Sapienza,
viale delVUniversitd 32, 1-00185 Roma, Italy

Luca Luiselli

Dipartimento di Biologia Animale e delTUomo,

Universita di Roma “La Sapienza, ” via A. Borelli 50, 1-00161 Roma, Italy

Francesco Riga

Istituto Nazionale della Fauna Selvatica, via Ca ’ Fornacetta 9,
1-40064 Ozzano dell’Emilia (Bologna), Italy

Key Words: Athene noctua; Little Owl, diet, Dodecanese,

Greece.

Widespread and easy to study taxa are ideal models for
analyses of life-history divergence, because they permit
comparisons that are not confounded by genetically-cod-
ed divergence in other morphological, behavioral and
ecological traits (Luiselli et al. 1996a, 1996b). The prob-
lem, however, is to find species whose life history traits
have been adequately studied in different portions of
their range. In general, Palearctic owls have a great deal
of potential in this area because several aspects of their
biology such as food habits have been studied in detail
(Herrera and Hiraldo 1976, Cramp 1985). The exception
to this is the Mediterranean Islands, especially islands in
the Aegean and East Mediterranean Seas, where virtually
nothing is known concerning the food habits of owls (Ut-
tendorfer 1952, Niethammer 1989). In particular, the
diet of the Little Owl ( Athene noctua) is litde known.
There are some dietary data available for islands in the
western Mediterranean (Contoli et al. 1988, Lo Verde
and Massa 1988) but nothing is known about what Little
Owls eat in the Aegean and East Mediterranean Seas.

In the present paper, we report detailed information
on the summer diet of the Little Owl from a Mediterra-
nean island of Dodecanese, Greece.

Study Area and Methods

Data were collected in late June 1990 on Astipalaia,
an island of Dodecanese, Greece (36 o 30 , -36°36'N,
26T4'-26°30'E, Fig. 1). The island is mainly mountain-
ous (highest elevation, 506 m) with calcareous soils on
the eastern and exterior western sides, and arenaceous
and schistous soils in the remaining parts. The vegetation
is poor, and characterized by chaparral with spiny shrubs,
olive-groves, orchards, vineyards and cereal growings. De-
tailed faunistic studies for Astipalaia have already been
done (Angelici et al. 1990, 1992). The island is inhabited
by two species of owls, the Little Owl and Barn Owl ( Tyto

alba ) . We recently reported the first records of Barn Owls
on the island (Angelici et al. 1992).

Owl pellets were collected in abandoned buildings and at
a few rocky sites. The collected material was identified in
the laboratory. Small mammals and reptiles were identified
by skull and mandibular remains, and arthropods by chitin-
ous exoskeleton remains. We counted, in the most parsi-
monious way possible, the frequency of occurrence of each
prey species in the diet. Although it was not possible to
identify Crocidura remains to species level, we assumed they
all belonged to C. suaveolens, a species widespread in the
Dodecanese islands (Niethammer 1989).

Statistical analyses were performed by a STATISTICA
(version 4.5, 1993) for Windows PC package, with a set
at 5%. All data were checked for homoscedasticity before
statistical analyses and normalized if necessary. If this pro-
cedure also failed in obtaining a normal distribution,
nonparametric tests were used. Dietary diversity was as-
sessed by applying Simpson’s (1949) and Levins’ (1968)
formulas to the numerical frequency of occurrence of
the various prey types in the pellets.

Results

We collected a total of 33 complete and an undetermined
number of incomplete Litde Owl pellets, containing 1068
prey remains. Excluding the incomplete pellets from the
analysis, the mean number of prey per pellet was 23.3. Litde
owls preyed on both vertebrates (0.56% of the total number
of prey eaten) and invertebrates (99.44%) (Table 1). Con-
tingency-table analysis showed that Litde Owls fed on inver-
tebrates significandy more frequentiy than on vertebrates
(X 2 = 1044.135, df = 1, P< 0.00000001). All invertebrates
eaten were insects, and most of them were earwigs ( Forficula
lurida ) which accounted for over 70% of the total number
of prey items ingested. Litde Owls preyed significandy more
often on earwigs than on all the other prey categories com-
bined (x 2 = 251.24, df = 1 ,P< 0.000000001). Moreover,
the mean number of earwigs per pellet was statistically high-
er than that of any other prey type in the diet (paired t, in
all cases P < 0.00001). Beeties (belonging mainly to the

September 1997

Short Communications

281

Figure 1. Location of Astipalaia Island (Dodecanese, Greece). Symbols: black triangles = high points in elevation;
black circles = villages or towns.

family Tenebrionidae) were also frequently eaten (18.16%
of the total number of prey items eaten). Some ants were
eaten, all of them winged forms. Little Owls preyed occa-
sionally also on small vertebrates (lizards, birds and shrews) .

Dietary diversity was relatively low either using Levins’
index ( L = 0.094) or Simpson’s index ( B = 1.751).

Discussion

Our data show the summer diet of Astipalaia Little
Owls consists almost entirely of insects. This finding is
consistent with Mikkola’s (1983) suggestion that the pro-
portion of insects in the diet of the Litde Owl increases
from the central European regions to the Mediterranean
regions due to the lower availability of microtine rodents
in the Mediterranean. An apparent exception has been
shown in Sicily, where Microtus savii is widespread and is
frequently preyed upon (16.4% of the total number of
prey items) by Little Owls (Lo Verde and Massa 1988).

Our data collection was restricted to the summer season
so it is not surprising that Little Owls would be eating large
numbers of insects like earwigs which were readily available.
Earwigs have been cited as important prey for Little Owls
in other areas, including Denmark (Cramp 1985). The Lit-
tle Owl diet on Astipalaia is probably greatly affected by
seasonal fluctuations in the availability of various types of
prey (Cramp 1985, Arias 1994), Therefore we feel that anal-

ysis of prey remains collected over an entire year would
show a larger proportion of small mammals in the diet. We
were surprised that we did not find remains of murids of
the genus Mas in the Little Owl diet. These small-sized ro-
dents (on average 17 g in mass) are the most common small
mammal in Astipalaia (Angelici et al. 1992) and are fre-
quent prey species for Little Owls elsewhere (Arias 1994). It
is likely that Little Owls on Astipalaia become more depen-
dent on murids later in the season when insects are not as
abundant (Zerunian et al. 1982).

Resumen. — Los costumbres de comida del Buho ( Athene noc-
tua) fue estudiado durante el verano en la Isla de Astipalaia,
una isla arida en Dodecanese, Grecia donde la ecologfa de
este especies todavia esta completamente sin conocer. Un
total de 1068 pedazos de presa fueron colectados. La dieta
de buho consiste casi totalmente de insectos, especialmente
tijeretas (. Forficula lurida ) . Escarabajos y hormigas con halas
tambien fueron frecuentemente comidas. Vertebrados casi
nunca fueron cazados y muy pocos ratones chicos del genio
Mus, que estaban muy abundante en los labores, casi nunca
fueron cazados.

[Traduccion de Raul De La Garza, Jr ]
Acknowledgments

We thank J. Angelopoulos (Athens) for helpful field
assistance, and C. Marti and R.J. Clark for the helpful

282

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Vol. 31, No. 3

Table 1. Summer diet of the Little Owl on Astipalaia
Island (Dodecanese, Greece).

Prey Type

% N

Vertebrata

Reptilia

Podarcis erhardii

0.28

Aves

Passer domesticus

0.09

Mammalia
Crocidura sp.

0.19

Arthropoda

Insecta
Dermaptera
Forficula lurida

793

74.25

Orthoptera

Tettigonidae

3.93

Coleoptera

Tenebrionidae

8.80

Curculionidae

0.84

Cerambicidae

0.66

Scarabeoidae

0.09

Carabidae

4.12

undetermined

3.65

Hymenoptera

Formicidae

3.09

comments on the manuscript. This paper is the contri-
bution No. 226 of the “Ricerche zoologiche delle Univ-
ersita di Roma nel vicino Oriente.”

Literature Cited

Angelici, F.M., M. Capula and F. Riga. 1990. Notes on
the herpetofauna of Astipalaia island (Dodecanese,
Greece). Brit. Herp. Soc. Bull. 34:31-33.

, F. PlNCHERA AND F. Riga. 1992. First record of

Crocidura sp. and Mus domesticus and notes on the
mammals of Astipalaia Island (Dodecanese, Greece).
Mammalia 56:159-161.

Arias, J.M. 1994. Nota sobre alimentacion de mochuelo
(Athene noctua L., Aves: Strigiformes) . Doriana, Acta
Vertebrata 21:183-185.

Contoli, L., G. Aloise and M.G. Filippucci. 1988. Sulla
diversificazione trofica di Barbagianni Tyto alba e Civ-
etta Athene noctua in rapporto al livello diagnostico
delle prede. Avocetta 12:21-30.

Cramp, S. [Ed.]. 1985. The birds of the western Palearc-
tic, Vol. IV. Oxford Univ. Press, Oxford, UK

Herrera, C.M. and F. Hiraldo. 1976. Food-niche tro-
phic relationships among European owls. Ornis Scand.
7:29-41.

Levins, R. 1968. Evolution in changing environments.
Princeton Univ. Press, Princeton, NJ U.S.A.

Lo Verde, G. and B. Massa. 1988. Abitudini alimentari
della civetta ( Athene noctua) in Sicilia. Naturalista sicil.
12 (suppl.): 145-149.

Luiselli, L., M. Capula and R. Shine. 1996a. Food hab-
its, growth rates and reproductive biology of grass
snakes Natrix natrix (Colubridae) in the Italian Alps.
J. Zool. (Lond.) 240: in press.

, AND . 1996b. Reproductive out-
put, costs of reproduction and ecology of the smooth
snake ( Coronella austriaca ) in the eastern Italian Alps.
Oecologia 106:100-110.

MlKKOLA, H. 1983. Owls of Europe. T. & A.D. Poyser,
Cal ton, U.K.

Niethammer, J. 1989. Gewollinhalte der Schleiereule
(Tyto alba) von Kos und aus Siidwestanatolien. Bonn.
Zool. Beitr. 40:1—9.

Simpson, E.H. 1949. Measurement of diversity. Nature
163:688.

Uttendorfer, O. 1952. Neue Ergebnisse fiber die Er-
nahrung der Greifvogel und Eulen. Eugen Umer,
Stuttgart, Germany.

Zerunian, S., G. Franzini and L. Sciscione. 1982. Little
Owls and their prey in a Mediterranean habitat. Boll.
Zool 49:195-206.

Received 23 April 1996, accepted 25 April 1997

September 1997

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283

Home Range, Habitat Use and Natal Dispersal of Blakiston’s Fish-owls

Yuko Hayashi

Laboratory of Applied Zoology, Faculty of Agriculture , Hokkaido University , 060 Sapporo, Japan

Keywords: Blakiston’s Fish-owl', Ketupa blakistoni; radio-

telemetry, home range, habitat use, natal dispersal ; inbreeding.

Blakiston’s Fish-owls ( Ketupa blakistoni) occur in south-
eastern Russia (Amurland and Ussuriland, Sakhalin and
southern Kuril Islands), northeastern China and northern
Japan (northeastern Hokkaido) (Voous 1988, Br az il and Ya-
mamoto 1989). Although this species was once widely dis-
tributed throughout Hokkaido, it now occurs very locally
(Brazil and Yamamoto 1989) and the present population is
estimated at 80-100 individuals (Brazil and Yamamoto
1989) and with no more than 20 breeding pairs (Clark and
Mikkola 1989). This species is highly dependent on riparian
forest (Burton 1973) and loss of suitable habitat could be
contributing to its decline. However, there is no information
available on the home range and habitat use of this species.
Here, I report the results of a study aimed at describing the
home range size, habitat use and dispersal behavior of
young Blackiston’s Fish-owls.

Study Area and Methods

The study area (43°23'N, 143°20'E) was in the National
Forest Agency and located on the upper Tokachi River
in eastern Hokkaido, Japan. Approximately half of the
study area consisted of a conifer forest plantation con-
sisting of Sakhalin spruce ( Picea glehnii ) , Japanese larch
{Larix leptolepis) and eastern white pine ( Pinus strobus).
The other half included two types of natural forest. One
consisted of mixed coniferous forest, mainly Yezo spruce
( Picea jezoensis), Sakhalin fir ( Abies sachalinensis) and
broad-leaved tree species, such as Mongolian oak ( Quer -
cus mongolica), painted maple ( Acer mono) and basswood
(Tilia japonica) at higher elevations, and the other type
consisted of broad-leaved forest dominated by Japanese
poplar ( Populus maximowczii ) and alder (Alnus spp.)
which occurred along streams at lower elevations. Most
of the ground cover was dwarf bamboo (Sasa nipponica).

Blakiston’s Fish-owls were first observed breeding in
the study area in 1986 and, thereafter, the same pair suc-
cessfully fledged young four times from 1987-91 (N=9
fledglings). Since 1985, the Environmental Agency of Ja-
pan supplemented the food supply of this pair by stork-
ing a pond with fish to prevent possible starvation of the
owls, especially in winter. This pond has become a major
feeding area for the owls. In 1987, both adult owls and

Present address: Chromosome Research Unit, Faculty
of Science, Hokkaido University, 060 Sapporo, Japan.

one of the two young born in 1986 were mist netted and
individually color banded. Since 1987, all fledglings have
also been color marked for individual identification.

To study movements, two young owls, one male and
one female, raised in the study area were captured in
mist nets near the stocked pond on 27 and 30 January
1992, respectively. Radio-transmitters were attached to
the tail according to Kenward (1978), with some modi-
fications. Radios were trimmed to fit the rectrix shaft and
attached to the ventral surface with stainless-steel wire
and epoxy glue. Antennas were 260 mm in length. They
were fastened to the feather shaft using fishing trace wire,
and the ties were sealed with epoxy resin.

Owls were tracked using Yaesu FT-290mkII receivers.
When tracking, a car-mounted whip antenna was used to
determine the general location of an owl. A more precise
location was then determined using a three-element
hand-held Yagi antenna. Bearings were taken from at
least three different sites. If the resulting error polygons
were larger than 1 ha, the location was not used. For each
owl, locations were determined once in the daytime
around noon and three or four times (with an interval
of more than 2 hr) during the night. The minimum con-
vex polygon method (MCP, Mohr 1947) was used to cal-
culate home range sizes. Mean error distance of the di-
rectional bearings from the test transmitters was 46.8 m
(SD = 29.0, range = 0-125, N = 12).

Habitat types in the study area were identified using
topographic maps (Geographical Survey Institute) and
timber-type maps (National Forest Agency) and catego-
rized as: (1) mixed forest (evergreen coniferous and de-
ciduous broad-leaved trees), (2) coniferous plantation
(deciduous and evergreen) , (3) young broad-leaved for-
est, (4) artificial (forest roads, houses and electric pow-
erline right-of-ways) and (5) water area (streams and
lake). The 13.3 km 2 study area was surrounded by moun-
tain ridges so an aerial survey was also conducted to ob-
tain dispersal data.

Results and Discussion

One radio-tagged owl (90M) was a male that fledged
from the nest in 1990. He stayed within his natal area for
one yr, disappeared in late April 1991, and returned on

P 1 flftl XJo i»iop /'arvHit’o/l nnd v «-» rl i o CYCT P rl AD

\J lit VVUJ anu i UH

27 January 1992. After radio-tagging, he stayed within
400 m of the capture site for two d. He then traveled
upstream 6.9 km and then returned to the capture site
where he stayed for the next month. I calculated his
home range to be 6.1 km 2 ( N =11 locations) during the
month of February. After that, he disappeared. On 25

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Vol. 31, No. 3

Table 1. Compositions of the habitat components in the study area and home range, and of the actual habitat used
by one female Blakiston’s Fish-owl.

Vegetation Category

Study Area

Percent Expected
Coverage Numbers

Home Range

Percent Expected .
Coverage Numbers

Radio Locations
Day Night Total

Mixed forest

60.4

66.7

70.0

79.7

77.1

Coniferous plantation

39.8

21.1

24.0

13.8

Young broad-leaved forest

0.8

0.9

2.5

2.7

9.1

Artificial

1.5

1.6

2.3

2.6

Water area

1.3

—

4.1

—

Total

100.0

109

100.0

109

100.0

June (3.5 mo later), the transmitter signal was found 18.7
km west of the capture site during an aerial search of the
study area and the radio, which had fallen off, was re-
covered on 15 July in the same location. On 28 July, I
found 90M roosting at the side of a stream 2.5 km from
the point where the radio had been recovered. He was
not located again after that.

The other owl (86F), a female that fledged from the
nest in 1986, was radio-tagged on 30 January 1992. For
two yr, she had remained within the natal nesting area
before disappearing in March 1988. She returned to her
natal area on 16 November 1988 and spent the next win-
ter there, after which she again disappeared in March
1989 and was absent for two yr. Two mo after the disap-
pearance of this owl’s mother (this probably occurred
sometime in October 1991), she returned to the natal
area and mated with her father in December 1991. She
laid two eggs in early March 1992 and incubated until
early April, during which time she stayed in the nest al-
most continuously except for one or two short trips away
from the nest (<200 m) for 2-20 min. In early April, she
deserted the nest before the eggs hatched.

I did not radio-track 86F egg-laying and incubation pe-
riods but, from 30 January-19 May (when the radio fell
off), I obtained 109 locations for this owl on 49 different
days. Her total home range size was 4.1 km 2 . During the
prelaying period (from February-March) , the home
range was 0.3 km 2 (N = 20 locations) . This area included
the nest and the small area immediately around the nest.
After the nest failed, the home range increased in size to
3.6 km 2 in April ( N = 51 locations) but decreased again
in May to 2.8 km 2 ( N = 38 locations).

Use of the home range by 86F appeared to be affected
by the location of water. Her most distant location was
462.5 m from water and it was only about one third of
the way to the edge in the study area. Daytime roost sites
averaged significantly farther from water (x = 139.29 m,
SE = 18.36, N = 42 locations) than did nighttime roosts
(x = 88.99 m, SE = 13.35, N = 67 locations; Mann-Whit-

ney Utest, U = 1010.5, P < 0.05) indicating that she
tended to hunt around streams and lakes at night.

The distribution of habitats also affected use of the
home range by 86F. All telemetry locations were in the
three forest types (mixed forest, conifer forest plantation
and young broad leaved forest) , and she was never found
using either the open water or artificial habitat catego-
ries. Because there was no significant difference in hab-
itat use between day and night (x 2 = 2.51, df = 2, P >
0.05), all locations were pooled when habitat use was
compared to availability within the overall study area and
home range (Table 1). Because the area of young broad-
leaved forest was small (0.8% of whole study area), lo-
cations in this category were combined with locations in
the mixed forest category. Owl 86F used mixed forest
more often than expected based on its availability within
the study area (two-tailed binomial test, P < 0.01) and
within the home range (P < 0.05).

Because use of mixed forest was possibly related to the
fact that a stream was located adjacent to the area of mixed
forest in the home range, I compared the number of lo-
cations in mixed forest that were within 100 m of water ( N
= 58 locations) to the expected number of locations in
mixed forest based on the availability of this habitat category
(N = 41 locations) and found the difference in use to again
be significant (two-tailed binomial test, P < 0.01). There-
fore, I concluded that the owl selected both the stream and
its surrounding mixed forest habitat.

In Hokkaido, heavy timber cutting from the late 1950s to
early 70s has converted most native forests into conifer for-
est plantations. Obviously this caused a loss of habitat and
thus reduced the number of Blakiston’s Fish-owls in the
area. The father-daughter mating observed in my study was
probably inevitable due to the small size of the fish-owl pop-
ulation. A daughter returning to the parental home range
after a long absence (22 mo) suggests that she could not
find a potential mate nor adequate habitats elsewhere.

Resumen. — El tarnano de la pradera, uso de habitat y el
comportamiento de dispersion de Ketupa balkistoni fueron

September 1997

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285

estudiados en el norte este de Hokkaido, Japon en 1992.
Este especie crfa de febrero-mayo, durante el tiempo, dos
individuales (macho y hembra) fueron observados por uso
de radio-telemetro. Una hembra formo una pareja con su
padre que habia perdido su pareja tres meses antes. Su
pradera calculado por el metodo minimo convexa poligono
(MCP) fue 0.3 km 2 antes de poner, poniendo y tiempos de
incubacion. Ella dejo el nido antes que los huevos salieron
de cascaron. 3.6 km 2 en abril y bajo ha 2.8 km 2 en mayo.
La pradera total medida durante el tiempo de observation
fue 4.1 km 2 . Ella preferia usar bosques mixtos con rfos. El
macho joven se movia en un area amplia despues que es-
taba marcado, y se fue del area de nacimiento.

[Traduction de Raul De La Garza, Jr.]

Acknowledgments

1 would like to thank H. Abe and Y. Saito for their
valuable suggestions and improvements of this manu-
script. I wish to thank also T. Ito, F. Sato, K Tanaka, M.
Tazawa, M. Tomizawa, K. Tsuji, A. Unno and Y. Yama-
moto for helping with field research. Manufacturing the
transmitters were the works of M. Maeda. Special thanks
to the late N. Fuchu for his excellent piloting. A.R. Chit-

tenden cooperated with the English. This study was partly

funded by WWF Japan.

Literature Cited

Brazil, M.A. and S. Yamamoto. 1989. The status and
distribution of owls in Japan. Pages 389-401 in B.-U.
Meyburg and R.D. Chancellor [Eds.], Raptors in the
modern world. WWGB: Berlin, London and Paris.

Burton, J.A. 1973. Owls of the World. A & W Visual
Library.

Clark, R.J. and H. Mikkola. 1989. A preliminary revi-
sion of threatened and near threatened nocturnal
birds of prey of the world. Pages 371-388 in B.-U.
Meyburg and R.D. Chancellor [Eds.], Raptors in the
modern world. WWGB: Berlin, London & Paris.

Kenward, R.E. 1978. Radio transmitters tail-mounted on
hawks. Ornis Scand. 9:220-223.

Mohr, C.O. 1947. Table of equivalent populations of
North American small mammals. Am. Midi. Nat. 37:
223-249.

Voous, K.H. 1988. Owls of the northern hemisphere.
William Collins Sons 8c Co. Ltd., London, UK

Received 20 June 1996; accepted 3 May 1997

Letters

Nest Defense and Mobbing Behavior of Elf Owls

Avian mobbing has been defined as when birds of one or more species assemble near a predator, change perch
locations frequently and emit loud vocalizations (E. Curio 1978, Z. Tierpsychol. 48:175-183). Predator mobbing is the
most widely distributed avian response to predators (A.F. Skutch 1976, Univ. Texas Press, Austin, TX U.S.A.; Curio
1978; I.G. McLean and G. Rhodes 1991, Current Ornithol. 8:173-211), and has been the subject of numerous studies
(Curio 1978; McLean and Rhodes 1991), but little information exists on nocturnal mobbing by either diurnal or
nocturnal species. Such behavior may be rare among diurnal species. For example, Common Terns ( Sterna hirundo)
will group mob Black-crowned Night Herons ( Nycticorax nycticorax) during diurnal periods but flee from them at
night (D.A. Shealer and S.W. Kress 1991, Colonial Waterbirds 14:51-56).

Nocturnal species, such as owls, may be more likely to engage in nocturnal mobbing behavior, but accounts of
owls mobbing natural predators are rare. Screech-owls ( Otus spp.) will make vocal and physical attacks on squirrels,
snakes, domestic cats and humans (A.C. Bent 1938, Pt. 2. U.S. Natl. Mus. Bull. 170; F.R. Gehlbach 1994, Texas A&M
Univ. Press, College Station, TX U.S.A.), and we have captured Western Screech-owls ( O . kennicottii) in a dho-gaza
trap (P.H. Bloom et al. 1992./. Raptor Res. 26:167-178) baited with a Great Horned Owl ( Bubo virginianus) after dark.
Also, Gehlbach (1994) observed a male Eastern Screech-owl (O. asio ) among a flock of songbirds mobbing a black
ratsnake (E. obsoleta) in daylight. Martin (1973, Condor 75:446-456) reported adult Burrowing Owls ( Speotyto cunicu-
lana ) from territories as far away as 300 m approaching and aiding a resident pair in mobbing a Great Horned Owl.

The Elf Owl ( Micrathene whitneyi) is the smallest Strigiform (P.M. Walters 1981, North Am. Bird Bander 6:104—105).
They are territorial but will sometimes nest in close proximity (10 m) to one another (J.D. Ligon 1968, Misc. Pub.
Mus. Zool., Univ. Mich. No. 136, Ann Arbor, MI U.S.A.; M.S. Goad and R. W. Mannan 1987, Condor 89:659-662). If
their nest is approached by a human, Elf Owls may make scolding vocalizations, fly closely by and possibly even strike
the intruder (Ligon 1968). However, Elf Owls are little studied and virtually no information is available on their
defensive behavior toward natural predators. Herein, we report the defensive behaviors of Elf Owls toward two
different predators. The dates of the observations correspond with late incubation and early nestling stages for Elf
Owls (Ligon 1968). Thus, we suggest these observations are examples of mobbing as a nest-defense behavior.

On 20 June 1995, at approximately 1950 H, we observed a 91-106 cm long gopher snake ( Pituophis melanoleucus )
climbing a honey mesquite tree ( Prosopis velutina) at our field station 1.6 km south of Fairbank, Arizona, in the San
Pedro Riparian Conservation Area. We knew from adult vocalizations that Elf Owls had been nesting in the tree but
we had not located their nest cavity. The snake was approximately 5.5 m above the ground when we visually located
an Elf Owl making scolding cheeur vocalizations (Ligon 1968) from its perch in the canopy of the tree. Moments later
a second Elf Owl flew from a cavity as the snake approached the entrance. When the snake entered the cavity with
its head and 8-10 cm of its body, we heard the trilling vocalizations of nestling Elf Owls. By this time it was dark and
all further observations were made with the aid of flashlights. Both adult Elf Owls repeatedly changed perches within
the canopy of the tree and continued vocalizing, but did not approach the snake when it was in the cavity. After 12-
15 min, the snake withdrew from the cavity and began to descend the tree. The Elf Owls increased their vocalization
rate and made repeated passes at it, striking its head at least four times. The strikes were powerful enough to propel
the snake’s head 5-10 cm sideways. The snake stopped at a main crotch of the tree where it was relatively protected
from the Elf Owls. The vocalizations of the owls gradually subsided, and we ended our observations at 2022 H. The
snake was no longer in the tree when we checked at 0430 H the next morning.

Both owls made flights at the snake, but we could not determine if only one or both had actually struck the snake.
During the attacks, a third Elf Owl, presumably from a known adjacent territory, flew to the mesquite and also
vocalized. The third owl repeatedly changed perch locations within the canopy but we were unable to determine if
it also attacked the snake. Despite being a very territorial species (Ligon 1968), there was no indication of intraspecific
aggression between the Elf Owls; all aggression appeared directed toward the snake. We did not observe the defensive
wing drooping postures Elf Owls use during intraspecific territorial interactions (Ligon 1968), but this may have been
due to the owls’ small size and the poor light conditions.

We observed a similar incident in which several Elf Owls attacked a Great Horned Owl. At dusk (approximately
1930 H) on 21 June 1993, we tethered a Great Horned Owl to a perch in our campsite in a riparian woodland near

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Aravaipa Creek, Arizona. Almost immediately at least six Elf Owls began vocalizing from dispersed locations around
our campsite and at least four of them began making low passes at the Great Horned Owl. Before we could relocate
the horned owl to a protected enclosure, it was struck once in the head by an Elf Owl.

Some nocturnal behaviors may not be well known or understood, not because they are rare, but because they are
difficult to observe. This may change with the increased availability of night vision equipment (P. Henson and J A
Cooper 1994, Auk 111:1013-1018). Currently, observations of nocturnal behaviors are likely to be sporadic and
anecdotal, and therefore unreported. Such information, however, may help in understanding a species biology. For
example, other researchers have observed group mobbing by Elf Owls (F.R. Gehlbach, pers. comm.; B.A. Millsap,
pers, comm.), but there are no published reports of the behavior. Our observations, and those of other researchers,
suggest that Elf Owls will join together in mobbing and that they can be physically aggressive when defending their
nests against predators.

We thank A. Duerr, T.S. Estabrook and R.L. Spaulding for assisting with the observations. We also thank T. Brush,
F.R. Gehlbach, R. Glinski, P. Hardy, B.A. Millsap, G. Proudfoot and H.A. Snyder for sharing their observational
information concerning mobbing by small owls. This manuscript benefitted from the constructive reviews of F.R.
Gehlbach, C. Marti, B.A. Millsap and an anonymous reviewer. — Clint W. Boal, Brent D. Bibles and R. William Mannan,
School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721 U.SA.

Griffon Vultures ( Gypsfulvus ) Ingesting Bones at the Ossuaries of

Bearded Vultures ( Gypaetus barbatus )

Some African vultures overcome the calcium deficiency in their diets by ingesting bone fragments, and are depen-
dent on the presence of large predators to supply them (Mundy and Ledger 1976, S. Afr. J. Sci. 72:106-110; Mundy
1982, The comparative biology of southern African vultures, Vulture Study Group, Johannesburg, South Africa; Rich-
ardson et al. 1986, J. Zool. Lond. 210:23-43). Because of the lack of large mammalian carnivores in the Iberian
Peninsula, vultures apparently satisfy their calcium needs by ingesting small bone fragments from carcasses (Konig
1975, Ardeola 21:219-224) or small pieces of limestone (Fernandez 1975, Ardeola 22:29-54; Elosegi 1989, Acta Biol.
Mont. 3, Serie documents de Travail) . This note reports several observations of Griffon Vultures ( Gyps fulvus) making
use of bone splinters obtained from Bearded Vulture ( Gypaetus barbatus) ossuaries, where large bones are deliberately
dropped onto rock slabs (Boudoint 1976, Alauda 44:1-21).

Field work was carried out in the meridional Prepyrenees (northeast of Spain), an area of isolated calcareous
massifs described by Riba et al. (1976, Geografia fisica dels Pai'sos Catalans, Ketres, Barcelona, Spain). The data were
collected while we were monitoring several Bearded Vulture pairs between 1991-95 at eight ossuaries located in five
different nesting areas (Heredia 1991, Pages 78-89 in R. Heredia and B. Heredia [Eds.], El quebrantahuesos Gypaetus
barbatus en los Pirineos, ICONA, Madrid, Spain) selected at random. All ossuaries had Griffon Vulture colonies nearby
(<1 km). We made 126 visits to the nesting areas during the nestling period from February-August.

Griffon and Bearded Vultures interacted at ossuaries in all five nesting areas. Occasionally, Griffon Vultures ex-
plored ossuaries when there had been no previous occurrence of bone drops, but more often they were observed at
ossuaries after Bearded Vultures had dropped bones. Over a 6-d-period, we observed groups of one to seven Griffon
Vultures (x = 2.62, SD = 1.99, N = 21) visiting the sites. During a total of 75 bone droppings, Griffon Vultures
immediately descended to the ossuaries on 13 occasions (17.3 %) in numbers ranging from one to five individuals
(x = 2.30, SD = 1.63, N= 30).

On five occasions, Griffon Vultures attempted to pirate bone fragments from Bearded Vultures. Once, when an
immature Bearded Vulture was dropping a bone, a Griffon Vulture flew in quickly and ingested small bone fragments
next to the place where the impact had occurred before the Bearded Vulture could land. Twice, we observed griffons
trying to overtake Bearded Vultures in flight to recover dropped bones, without success. Once, after a Bearded Vulture
had perched next to the bone it had dropped, three Griffon Vultures attacked it and seized a large bone fragment
which they then proceeded to fight over and ingested. We also saw a Bearded Vulture drop a bone and, once on the
ground, five Griffon Vultures attacked the Bearded Vulture forcing it to flee with the prey.

We also observed three Griffon Vultures inside a Bearded Vulture nest that had been used in the previous breeding

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season. One of them spent 30 min pecking an old sheep or goat bone. Occupation of Bearded Vulture nests by
Griffon Vultures is frequent in the Pyrenees (Fernandez and Donazar 1991, Bird Study 38:42-44; Donazar, pers, obs.).

Our observations suggest that Griffon Vultures living near Bearded Vultures benefit from this association because
Bearded Vultures provide a source of calcium. The Griffon Vulture, like other species that eat mainly soft parts of
carcasses (Brown 1976), is subject to a lack of calcium because his diet contains only 0.01 % of this element (Houston
1978,/. Zool. Lond. 186:175-184). The Bearded Vulture, a species that in the last century was spread over a large part
of the Iberian Peninsula mountains (Hiraldo et al. 1979, El quebrantahuesos Gypaetus barbatus (L.), Monograflas 22,
ICONA, Madrid, Spain) , may have also facilitated the spread of Griffon Vultures as the distribution of both species
was extensively coincident in much of the southern Palaearctic (Elosegi 1989, Acta Biol. Mont. 3, Serie documents de
Travail) .

We thank S. Manosa for many helpful suggestions and criticism for the manuscript’s improvement. We also thank
J.A. Donazar, D.C. Houston and S.R. Wilbur for their review and comments on the manuscript. This study was
supported by the Departament d’Agricultura, Ramaderia i Pesca de la Generalitat de Catalunya. — Joan Bertran 1 and
Antoni Margalida, GEPT (Grup d’Estudi i Proteccio del Trencalos), Ap. 43, E-25520 El Pont de Suert (Lleida), Spain.

1 Present address: Grases, 14-18. entlo la, 08004 Barcelona, Spain.

A Golden Eagle Eats Wild Canada Goose Eggs

Golden Eagles ( Aquila chrysaetos ) prey primarily upon medium-sized rodents, hares, birds and ungulates (S.K.
Carnie 1954, Condor 56:3-12; Boeker and Ray 1971, Condor 73:463-467; M.N. Kochert 1972, M.S. thesis, Univ. of
Idaho, Moscow, ID U.S.A.; P.A. Johnsgard 1990, Hawks, eagles, and falcons of North America, Smithsonian Inst. Press,
Washington, DC U.S.A.). Although they are known to exploit a great variety of prey items throughout their holarctic
range, eggs have not been reported as a food item (A.C. Bent 1961, Life histories of North American birds of prey,
Dover Publications, Inc., New York, NY U.S.A.; Dement’ev and Gladkov 1966, Birds of the Soviet Union, Israel
Program for Scientific Translations, Israel; Brown and Amadon 1968, Eagles, hawks, and falcons of the world, County
Life Books, London, UK; Beecham and Kochert 1975, Wilson Bull. 87:506-513; Matchett and O’Gara 1987,/. Raptor
Res. 21:85-94; Palmer 1988, Handbook of North American birds, Yale Univ. Press, New Haven, CT U.S.A.).

We observed a Golden Eagle raid a Canada Goose ( Branta canadensis ) nest and eat two eggs on 4 April 1995 in
Hell’s Canyon National Recreation Area in western Idaho. At 1150 H, an adult Golden Eagle (gender unknown)
flushed a Canada Goose off a ground nest located on an island in the Snake River. The eagle landed near the nest,
walked to the nest and broke open the eggs by grasping an egg in its foot and placing all of its weight on the egg
until, after two to four attempts, it broke. The eagle ate the contents of the egg (stage of embryonic development
was unknown) and then broke and ate the second egg. The pair of geese that had been displaced from the nest and
four other pairs of nearby geese gave alarm calls during our observations, but never approached the eagle. Two
Black-billed Magpies ( Pica pica) followed the eagle to the nest and scavenged eggshell fragments while the eagle
consumed the contents. The eagle finished eating both eggs at 1206 H and then spent the next 5 min walking and
hopping around the island, possibly searching for more eggs. The magpies followed the eagle on the ground until
1211 H when the eagle flew 50 m downstream and perched on a talus slope. The Canada Goose pair returned to
their depredated nest at 1430 H.

Although Golden Eagles have not been previously observed eating eggs, we speculate that depredation on goose
eggs in Hell’s Canyon may not be uncommon. Perhaps Golden Eagles in Hell’s Canyon eat eggs when more typical
prey for this region (black-tailed jackrabbits, Lepus califomicus) are rare. In contrast, Golden Eagles nesting 128 km
upstream of Hell’s Canyon in the Snake River Birds of Prey National Conservation Area, where black-tailed jackrabbits
were abundant and an important prey species (Steenhof and Kochert 1988,/ Anim. Ecol. 57:37-48), have not been
observed to prey upon goose eggs, even though Canada Geese occasionally nest nearby (W. Bodie, pers. comm.).

This study was funded by the Idaho Power Company. Toni Holthuijzen, Mary McFadzen, Brian Herting and Mike
Kochert made constructive comments on the manuscript. Toni Holthuijzen translated the German papers. James

September 1997

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289

McKinley worked long hours in the field. Mark Fuller and Stephanie Gossett provided administrative support. — Laura
L. Valutis, Department of Biology, Boise State University, Boise, ID 83725 U.S A. and John M. Marzluff, Sustainable
Ecosystems Institute, 30 E. Franklin Road, Suite 50, Meridian, ID 83642 U.SA.

J. Raptor Res. 31(3):289

Two Plumbeous Kites (Ictinia plumbea) Capture Swallow

The Plumbeous Kite ( Ictinia plumbea) is a common but poorly studied raptor of the neotropics, ranging from
Mexico to northern Argentina and Paraguay (L. Brown and D. Amadon 1968, Eagles, hawks and falcons of the world,
McGraw-Hill Book Co., New York, NY U.SA). This species feeds mainly on insects (A.F. Skutch 1947, Condor 49:25-
31; F. Haverschmidt 1962, Condor 64:154-158), but vertebrates, including birds and bats, make up a small percentage
of its diet (N.E. Seavy et al. 1994,/. Raptor Res. 29:65-66). Likewise, birds, including swallows and swifts, and bats have
been recorded as prey items for the similarly insectivorous Mississippi Kite ( Ictinia mississippiensis ) (J.W. Parker 1988,
pgs. 166-186 in R.S. Palmer [Ed.], Handbook of North American birds, Vol. 4, Yale Univ. Press, New Haven, CT
U.S.A.). We know of no published accounts, however, of either species capturing small birds by tandem hunting.

On 6 June 1994, we were observing a Plumbeous Kite nest in Tikal National Park, Peten, Guatemala. The nest
contained one 21-d-old nestling. Both adults were perched approximately 100 m from the nest in a large cedro
( Cedrela mexicana) tree. The area between the adults and the nest was a large open plaza covered with short grass.
At 0659 H, one of the kites flew from its perch passing within 1 m of a flying Northern Rough-winged Swallow
( Stelgidopteryx serripennis) . The swallow flew down and away and the kite dived unsuccessfully again on the fleeing
swallow, which at this point was no more than 1-2 m above the ground. On a third dive, the kite again missed, and
the swallow took cover, perching in the short grass. As this kite was making a fourth dive, the second adult kite also
dived from its perch toward the grounded swallow. As the first kite dived, the swallow flushed and was caught by the
second kite in its feet no more than 2 m above the ground. The first kite followed the second kite for a short distance
and then returned to perch in the cedro. The second kite flew to the nest and fed the swallow to the nestling.

Cooperative hunting can allow raptors to take larger or more elusive prey with increased success compared to solo
hunting (D.P. Hector 1986, Ethology 73:247-257; J.C. Bednarz 1988, Science 239:1525—1527). Based on the social
foraging classes defined by Ellis et. al. (1993, Bioscience 43:14—20), our observation qualifies as either “pseudocoop-
erative hunting” (group attacks by a variable number of individuals on large or elusive quarry, without division of
labor or sharing of prey, though success is enhanced) or “cooperative pair hunting” (involving only two birds, clear
division of labor and at least limited prey sharing) .

Tandem hunting occurred only once during 127 foraging attempts we observed from perches. Most attempted
prey captures were directed at insects. In comparison, 29% (102 of 349) of all Aplomado Falcon (Falco femorahs)
foraging attempts observed by Hector (1986, Ethology 73:247-257) involved pursuit by two falcons. Of these tandem
hunts 66% were directed at birds and only 2% at insects. Though probably not important in the pursuit and capture
of insects and other small prey, tandem hunting may allow the Plumbeous Kite to increase success in occasional
attacks on elusive prey such as birds.

This is a contribution of the “Maya Project,” a conservation research effort of the Peregrine Fund, Inc. Financial
support was provided by Robert Berry, Crystal Channel Foundation, Fanwood Foundation, Gold Family Foundation,
KENNETEGH/U.S. Windpower, the John D. And Catherine T. MacArthur Foundation, Mill Pond Press, National
Fish and Wildlife Foundation, Norcross Foundation, Hank and Wendy Paulson, Pew Charitable Trusts, Andres Sada,
Joe and Flinda Terteling and the U.S. Agency for International Development. P.H. Bloom, D.H. Ellis, L. Kiff and K.
Meyer provided helpful comments on an earlier draft of this manuscript. — Nathaniel E. Seavy, 17142 Lemolo Shr.
Dr. N.E., Poulsbo, WA 98370, U.SA., Mark D. Schulze, Botany Dept., 208 Muellor Bldg., Penn. State Univ., University
Park, PA U.SA. and David F. Whitacre, The Peregrine Fund, Inc., Boise, ID 83709 U.SA.

BOOK REVIEW

Edited by Jeffrey S. Marks

The Golden Eagle. By Jeff Watson. 1997. T. 8c
A.D. Poyser, London, U.K. xx + 374 pp., 76 figures,
73 tables, 6 appendices, color frontispiece. ISBN
0-85661-099-2. Cloth, $49.95. — This long-awaited
volume from T. & A.D. Poyser originates in Scot-
land, where Golden Eagles ( Aquila chrysaetos ) have
been studied for many years. In The Golden Eagle,
Jeff Watson combines details of his own research
on Golden Eagles in Scotland with information
from studies of Golden Eagles and other Aquila ea-
gles conducted throughout the world. Using this
approach, Watson provides a comprehensive re-
view of the ecology of the Golden Eagle and a gen-
eral overview of the ecology of Aquila eagles. A tre-
mendous amount of general and technical infor-
mation is presented in the text and accompanying
figures, tables and appendices; however, the book
is relatively easy to read. Most chapters begin with
an introductory statement and conclude with a
brief summary. Each chapter is illustrated with
beautiful black-and-white drawings by Keith Brock-
ie and wash landscapes by Donald Watson. Two col-
or plates, one by each artist, appear at the begin-
ning of the book.

In his acknowledgments and opening chapter,
Watson reflects on his experiences writing this
book. As I read the book, I was struck with a great
appreciation for how much work went into gath-
ering, compiling and organizing the information
for it; I think that readers will quickly gain a similar
appreciation. Seton Gordon once said “I cannot
imagine anyone studying the ways of the eagle
without admiring the nobility of the bird.” Jeff
Watson’s admiration, knowledge and enthusiasm
for Golden Eagles come across loud and clear
throughout the book.

Chapter 2 presents a review of field characteris-
tics, reversed sexual size dimorphism, taxonomy
and general ecology of Golden Eagles. The distri-
bution of the Golden Eagle is reviewed in Chapter
3, and Watson introduces readers to the Scottish

Highlands in Chapter 4. Most of Watson’s work was
conducted in the Scottish Highlands, an area rich
in contemporary and historical Golden Eagle re-
search and conservation. This chapter provides a
good background for discussions of Watson’s re-
search in Scotland through the rest of the book.

In Chapter 5, Watson describes the hunting be-
havior of Golden Eagles. This chapter is informa-
tive, despite the scarcity of studies on this subject.
Descriptions of the food habits of Golden Eagles
and other species of Aquila are presented in Chap-
ter 6. The long-standing issue of Golden Eagles
and livestock is discussed at the end this chapter.
Chapter 7 focuses on nesting sites of Golden Ea-
gles, including descriptions of nest structures, nest
types, nest elevations, nest orientation, use of al-
ternative nest sites and interactions with other spe-
cies at nesting sites. I was surprised that interac-
tions between nesting Gyrfalcons ( Falco rusticolus )
and Golden Eagles in western Canada (Platt 1989)
were not mentioned in this chapter.

In Chapter 8, entitled “Ranging Behavior,” Wat-
son describes home ranges and territories of Gold-
en Eagles based primarily on observational studies
from Europe and North America. A brief discus-
sion on the ranging behavior of nonbreeding birds
as determined using radiotelemetry is also pre-
sented. Watson also briefly discusses variation in
home range size, competition with other species,
communal roosting and ranging behavior of mi-
gratory Aquila. Chapter 9 begins with an overview
of the mechanisms driving nest spacing and density
of breeding birds. Using data from Scotland, Wat-
son discusses the relationship between breeding
densities and food supply. He also discusses winter
densities of migratory Golden Eagles in North
America.

Current population estimates and trends of
Golden Eagle populations are addressed in Chap-
ter 10. The strong point of this chapter is the re-
view of the historical and current status of Golden
Eagles in Europe. The weakest point is Watson’s
North America population size estimate. Watson
suggests that a total population estimate of 50,000

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September 1997

Book Review

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to 70,000 individual Golden Eagles in North Amer-
ica would “appear reasonable”; however, I would
argue that insufficient data are available to make
a continent-wide population estimate.

Chapter 11, entitled “The Pre-breeding Season”
begins with a description of Golden Eagle behavior
in winter, and continues with descriptions of ter-
ritorial flights, nest building, courtship, mating and
unusual mating systems. Chapter 1 1 concludes with
a discussion on faithfulness to mates in Golden Ea-
gles. Watson reminds readers that without empiri-
cal data, testing the assumption that Golden Eagles
form lifelong pair bonds is difficult.

The breeding season is covered in Chapters 12
and 13. In Chapter 12, Watson describes Golden
Eagle eggs and reviews nesting phenology, clutch
size, replacement clutches, incubation period, be-
havior of adults during incubation and reasons why
pairs fail to lay eggs. He also examines the rela-
tionship between latitude and median egg-laying
dates. Watson states that “in the most northerly
populations of Alaska and Siberia (65-70°N) laying
does not commence until the first 10 days of May.”
This contradicts several published studies that doc-
umented mean laying dates for Golden Eagles in
arctic Alaska and Canada from mid- to late April
(Ritchie and Curatolo 1982, Poole and Bromley
1988, Young et al. 1995). In Chapter 13, Watson
draws heavily on studies conducted in the western
United States to describe activities associated with
the nestling period. A review of the postfledgling
period and time to independence is presented in
Chapter 14. Few studies have focused on the be-
havior and activities of Golden Eagles from the
time they leave the nest to the time they are re-
cruited into the breeding population. Watson re-
views the available information and is quick to
point out that more study is needed to describe
this portion of the Golden Eagle’s life cycle.

In Chapter 15, Watson examines the factors that
influence breeding performance of Golden Eagles,
including food and weather. At the end of the
chapter, Watson asks why Golden Eagles in North
America typically breed more successfully than
those in Scotland and includes an interesting anal-
ysis of the relationship between dietary breadth
and reproductive performance. Based on an anal-
ysis using data from 24 studies, Watson suggests
that breeding success is likely to be high when ea-
gles can specialize on one or two types of prey in
the “optimal size range.” This is an interesting sug-
gestion. Given the limitations of determining food

habits of Golden Eagles using prey remains col-
lected at nests, however, more studies are needed
before general conclusions regarding the dietary
breadth of Golden Eagles can be made.

Patterns of molt and age-specific plumage char-
acteristics are reviewed in Chapter 16. In Chapter
17, Watson discusses the movement and migration
of Golden Eagles and other Aquila eagles using
data collected through banding and telemetry
studies in Europe and North America. Chapters 18
and 19 cover mortality and threats to Golden Ea-
gles, respectively. In the first part of Chapter 18,
Watson discusses the difficulty' of estimating mor-
tality rates of Golden Eagles using banding and
marking studies. The major causes of Golden Eagle
mortality' are covered in detail in Chapter 18. The
take-home message of this chapter is that most
known Golden Eagle deaths in Europe and North
America are attributable to humans. In Chapter
19, Watson lists the known threats to Golden Ea-
gles including direct persecution by humans, un-
intentional human disturbance, pesticides and pol-
lutants, powrer poles and land-use changes. Watson
also describes the effects of large-scale afforesta-
tion of the Golden Eagle population in Scodand
and the relationship between Golden Eagles and
grazing animals. The last part of the chapter will
be interesting to anyone managing lands where
grazing animals and Golden Eagles interact.

Chapter 20, entitled “Conservation,” is a good
follow-up to the previous chapter. Watson address-
es the various management and educational tools
used for countering the threats outlined in Chap-
ter 19. In Chapter 21, Watson draws on informa-
tion from Europe, North America and central Asia
to describe relationships between Golden Eagles
and humans and to describe how these relation-
ships have changed over time. Historically, Golden
Eagles held high status in all three geographic ar-
eas. With the introduction of firearms, however,
many eagles in Europe and North America were
persecuted. Watson ends this chapter on a positive
note and hopes that “some of the empathy for the
natural world which was implicit in so many tradi-
tional societies, will be rekindled and embraced
again by people in the so-called ‘developed
world.”’

In the closing chapter, Watson provides “food
for thought” and makes recommendations for fu-
ture Golden Eagle research. These recommenda-
tions include learning more about the distribution
of Golden Eagles in the eastern Palearctic and re-

292

Book Review

Vol. 31, No. 3

mote areas of northern North America and de-
scribing ranging behavior of nonbreeding and mi-
gratory Golden Eagles. In closing, Watson stresses
the need to understand how land-use changes af-
fect Golden Eagle populations. This is a worldwide
concern that should be addressed sooner rather
than later.

The 22 Chapters are followed by six appendices
that provide additional information on the distri-
bution and ecology of the genus Aquila, the Scot-
tish Highland landscape, calculating dietary
breadth, assessing food supply, calculating nearest-
neighbor distances and densities and the scientific
names of plants and animals mentioned in the
book.

Overall, I enjoyed the book very much. The il-
lustrations by Donald Watson (Jeff Watson’s father)
and Keith Brockie are superb. The Golden Eagle con-
tains an enormous amount of technical and gen-
eral information. At times, however, I felt that Wat-
son tried to cover too much territory, that some of
his statements were too general and that some of
his statistical analyses were based on insufficient
data to draw conclusions. Despite these minor

shortcomings, The Golden Eagle contains a wealth of
information on Golden Eagle ecology, and I highly
recommend it to anyone interested in raptors.
Raptor bibliophiles will want this book for their
libraries, and the book also is a must for public and
university libraries. — Carol L. McIntyre, National
Park Service, P.O. Box 74680, Fairbanks, AK 99707
U.S.A. and Department of Fisheries and Wildlife,
104 Nash Hall, Oregon State University, Corvallis,
OR 97331 U.SA.

Literature Cited

Platt, J.B. 1989. Gyrfalcon courtship behavior and early
breeding behavior on the Yukon North Slope. Socio-
biology 15:43-69.

Poole, K.G. and R.G. Bromley. 1988. Interrelationships
within a raptor guild in the central Canadian Arctic.
Can. J. Zool. 66:2275-2282.

Ritchie, R.J. and J.A. Curatolo. 1982. Notes on Golden
Eagle productivity and nest site characteristics, Por-
cupine River, Alaska, 1979-1982. Raptor Res. 16:123-
127.

Young, D.D., Jr., C.L. McIntyre, P.J. Bente, T.R. McCabe
and R.E. Ambrose. 1995. Nesting by Golden Eagles
on the North Slope of the Brooks Range in North-
eastern Alaska. /. Field Ornithol. 66:373-379.

Abstracts of Presentations Made at the Annual Meeting of the
Raptor Research Foundation, Inc., Held at Gainesville, Florida, 1986

The Impact of Falconry on Wild Raptor Populations
Preface

Robert Kenward. Institute of Terrestrial Ecology, Furze-

brook Road, Wareham, Dorset BH20 3 AS U.K.

At the 1986 annual conference of the Raptor Research
Foundation held in Gainesville, Florida, Jim Mosher or-
ganized a mini-symposium to consider the impact of fal-
conry on wild raptor populations, as the basis for a po-
sition statement. An ad-hoc committee including Jim Mo-
sher (Chair), Jim Brett, Robert Kenward and Ian Newton
prepared a draft position statement that was modified at
the annual conference in St. Paul, Minnesota in 1988,
and was then approved by a postal vote of the member-
ship early in 1989. The six expanded abstracts that follow
provide pointers to further literature on each of the main
issues of the position statement.

A publication that is long in gestation risks being over-
taken by events. Nevertheless, the conclusions of the po-
sition statement have so far been strengthened rather
than contradicted. After three further years of data from
a Prairie Falcon ( Falco mexicanus ) harvest study, D.E. Run-
de (pers. comm., see too Conway et al. 1995) was “com-
fortable that removal of 10-20% of nestlings is a safe sus-
tainable yield.” Radio-tagging has shown that banding
can substantially overestimate first-yr mortality, and the
resulting new models indicate that sustainable yields for
some species could be more than 30% of the young
(Kenward pp. 295-296). Three cases of hybrid falcons
displacing normal peregrines breeding in Germany (H.
Reilman pers. comm.) reinforce the position statement
recommendation that such birds should at the least be
imprinted on humans before being used in falconry.

Literature Cited

Conway, C.J., S.H. Anderson, D.E. Runde and D. Ab-
bate. 1995. Effects of experimental nestling harvest
on Prairie Falcons./. Wildl. Manage. 59:311-316.

Controlled Harvest of Nestling Prairie Falcons: A
Field Experiment

Douglas E. Runde . 1 Wyoming Cooperative Fishery and
Wildlife Research Unit, Department of Zoology and Physiol-

1 Present address: Florida Game and Fresh Water Fish
Commission, 620 South Meridian Street, Tallahassee, FL

32399 U.S.A.

ogy, University of Wyoming, Box 3166, University Station,

Laramie, WY 82071 U.S.A.

Raptors have been removed from the wild for use in fal-
conry for centuries, but sustainable levels of harvest have
not been clearly demonstrated. As the recreational de-
mand for raptors focuses primarily on the younger age
classes (nestlings and juveniles) , standard models for es-
timating maximum sustainable yield (MSY) are inappro-
priate. The MSY concept is based on density-dependent
population growth models, which typically require a re-
duction in population size well below carrying capacity
in order to stimulate maximal population growth and al-
low maximal levels of harvest. For raptors, a more appro-
priate goal is to maintain stable populations near carry-
ing capacity while allowing conservative harvests.

One approach to estimating a sustainable yield (SY)
for a raptor population is based upon a comparison of
reproductive success and mortality. For the Prairie Falcon
{Falco mexicanus ), mean productivity (from 15 studies
spanning more than 20 years) is 2.5 young pair 1 yr 1
(Runde 1987). A series of 15 survival schedules, derived
from banding data, indicated that an average of 2.0
young pair 1 yr 1 are needed to maintain stable popula-
tions through time (Runde 1987). Theoretically then, an
average surplus of 0.5 nestlings is produced by each
breeding pair each year.

From this, a SY for a local Prairie Falcon population is
easily calculated by dividing the number of breeding
pairs by 2. To do so requires an estimate of breeding
population size. However, it may be impractical to survey
the population each year and then set harvest levels. If
an estimate of the number of breeding territories, or
maximum number of breeding pairs is available, then
average breeding population size can be calculated. A
conservative estimate of occupancy rate (based on 9 field
studies) is 65% (Runde 1987). If previous surveys indi-
cate that 100 breeding territories are present, then 65
pairs are expected to occupy territories and 32.5 surplus
nestlings will be produced in an average year. Due to
normal fluctuations in populations, this approach will
lead to recommended harvest levels that are above SY in
some years and below SY in others.

My approach is based upon life-table estimates of sur-
vival rates from band return data. Such estimates are un-
avoidably suspect due to many potential sources of bias
(Burnham and Anderson 1979). Thus, a field test of this
approach was proposed and an experimentally-con-
trolled harvest of nesding Prairie Falcons in southwestern
Wyoming was begun in 1982. A 2420 km 2 study area was
divided into a harvest area (with 20-26 breeding pairs)
and an adjacent control area (with 45-55 pairs) .

293

294

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Vol. 31, No. 3

Table 1. Summary of nestling Prairie Falcon removals
in SW Wyoming, 1982-86.

Year

Number

Removed

Harvest
Rate (%)

Productivity
After Removals 1

1982

1.95

1983

—

1.55

1984

1.88

1985

1.90

1986

1.70

Totals

1.80

1 Number of young per occupied breeding territory.

Experimental harvest involved removing enough nest-
ling falcons to reduce breeding success to below 2 young
pair 1 each year (Table 1). Nestlings were fostered into
nests far removed from the study area (>225 km to the
east) , and hacked at an artificial nest site in southeastern
Montana. None were removed in 1983 as natural nest
success was very low.

If harvest exceeds SY, a decline in the population may
eventually result. Excessive harvesting may lead to a de-
cline in falcons available to fill vacant nest sites, in which
case the number of occupied territories should decline.
Therefore, breeding territories in the harvest and control
areas were monitored each year to compare trends in
territory occupancy. To avoid biases due to the discovery
of additional nesting territories, occupancy rates were cal-
culated from a subset of sites visited every year.

Although there was no evidence of a change in pop-
ulation size from 1982-86, it is too early to draw firm
conclusions. Effects of the harvest will be detectable only
after falcons fledged during the experiment dominate
the breeding population. Trapping of breeding adults in-
dicated that the recruitment of these cohorts began in
1985. As annual mortality of adults has been low (13-
19%) (Runde 1987), recruitment will be slow. Complete
turnover of the breeding population will require about
eight yr.

Immigration may compensate for reduced breeding
success and maintain the population even if SY has been
exceeded. In an attempt to measure immigration into
the harvest area, an extensive banding program has been
conducted. More than 500 nestling and 100 adult falcons
have been banded in or near the study area. If immigra-
tion is high and there is no decline in numbers of breed-
ing pairs, a precise level of sustainable harvest will not
have been demonstrated. However, the presence of a har-
vestable surplus will be shown and the approach taken
may be applicable on a local scale.

Literature Cited

Burnham, K.R and D.R. Anderson. 1979. The compos-
ite dynamic method as evidence for age-specific wa-
terfowl mortality./ Wildl. Manage. 43:356-366,

Runde, D.E. 1987. Population dynamics and movement
patterns of the Prairie Falcon ( Falco mexicanus) . Ph.D.
dissertation, Univ. Wyoming, Laramie, WY U.S.A.

Falconry Harvest in the United States

James A. Mosher. Savage River Consulting,

17811 Lappans Road, F airplay MD 21733 U.S.A.

Falconry, most simply defined, is the taking of game with
the aid of a trained raptor. Many raptors used in falconry
are birds taken from wild populations. There are numer-
ous opinions about the sport or potential impacts on wild
populations from this harvest. The purpose here is to
present some data concerning raptor harvest, to put the
harvest in perspective with regard to population numbers
and to make some reasoned management recommen-
dations. I believe that biologists and falconers alike will
be drawn to similar conclusions by these data. The data
came from two sources. First, an unpublished report by
Brohn in 1986 for the International Association of Fish
and Wildlife Agencies (IAFWA) Nongame Wildlife Com-
mittee included summaries of numbers of falconers and
of raptors harvested, based on survey responses from 42
states. Second, I summarized falconers’ annual reports
for 23 states covering the 1- or 2-yr reporting periods
ending in 1986. Copies of these reports were kindly pro-
vided by Walter Steiglitz, Assistant Director for Refuges
and Wildlife of the United States Fish and Wildlife Ser-
vice (USFWS). In order to protect the privacy of the in-
dividuals, much information was obscured in these re-
ports. Where this resulted in a range of possible values,
I used the high estimate for numbers harvested, and the
low estimate for numbers returned to the wild. Because
so many Peregrine Falcons {Falco peregrmm ) and Harris’
Hawks {Parabuteo unicinctus ) were captive bred, and that
information was obscured on most reports, I excluded
those species from the USfWS data. They are, however,
included in the IAFWA data.

Brohn reported that 2 776 falconers harvested 737 rap-
tors of 15 species from the wild during 1986. Of these
raptors, 367 were returned to the wild, either intention-
ally or accidentally, for an estimated net annual harvest
of 370 birds. My review of USFWS data from 23 states
yielded 350 birds harvested, 66 released and 118 acci-
dentally lost, for a net harvest of 166 birds from wild
populations. The IAFWA survey gave a net harvest rate
of 8.8 birds state 1 yr \ while the USFWS reports gave a
net harvest rate of 7.3 birds state 1 yr -1 . Further, the
USFWS reports record that 330 young birds (6.9 state -1
yr -1 ) were produced by captive propagators during the
1985 reporting year. Even allowing for no benefit from
raptors returning to the wild from any source, the max-
imum annual harvest is estimated between 15.2 and 17.5
birds in each state.

Almost 56% of all raptors harvested were Red-tailed
Hawks {Buteo jamaicensis) or Prairie Falcons {Falco mexi-

September 1997

Abstracts

295

canus), species certainly not threatened or endangered.
Regionally, California reported the highest harvest, with
128 birds taken and 118 returned to the wild, giving a
net loss of 100 birds from the wild.

G.S. Butcher, M,R. Fuller and J.L, Ruos (unpubl. data)
found significant increases from the early 1970s to the
early 1980s in Christmas Bird Count (CBC) numbers of
Northern Goshawks ( Accipiter gentilis) , Red-tailed Hawks,
Merlins ( Falco columbarius) , Prairie Falcons and Gyrfal-
cons (Falco rusticolus), using the most conservative data.
Their estimates of continental population numbers, ex-
trapolated from CBSs for Red-tailed Hawks and Prairie
Falcons are 80 000 and 13 000, respectively, for winter
1982-83.

My estimates of density of breeding raptors in the east-
ern forests, based on complete censuses of 32 km 2 study
areas distributed from Maryland to Minnesota, approxi-
mate to 1 pair of Broad-winged Hawks ( Buteo platypterus )
in 5 km 2 and 1 pair each in 25 km 2 of Red-shouldered
Hawks ( Buteo lineatus), Red-tailed Hawks and Cooper’s
Hawks (Accipiter coopmi). In the northeastern U.S., where
these study areas are located, there are approximately
575 000 km 2 of forested land. Some of it is certainly not
suitable breeding habitat for one or more of these spe-
cies. Likewise, portions of the areas I censused did not
provide suitable breeding habitat. If only half of the avail-
able forest land is occupied, these data can be extrapo-
lated to over 10 000 breeding pairs of the least dense
species and almost 60 000 pairs of Broad-winged Hawks.

International trade in raptors is also dwarfed by these
numbers. The annual report of the convention on Inter-
national Trade in Endangered Species (CITES) of wild
fauna and flora for 1986 reports 213 468 birds imported
to the U.S.A. Only 36 individuals were raptors of falconry
interest, and 9 of them were for falconry. For the same
period, 5684 birds were exported, which included 16 rap-
tors (15 hybrid falcons and 1 Peregrine Falcon reexport-
ed to Canada). The total number of imports, including
species not covered by CITES (all raptors are covered)
was estimated to be more than 700 000.

In the light of these data, I agree with the IAFYVA that
the harvest of wild raptors by falconers has no significant
biological impact on the resource. It does not seem that
substantial expenditures of time and money by state and
federal regulatory agencies are needed to protect raptor
populations from falconry harvest. In fact, when captive
propagation by falconers is considered, the net effect
may be a gain rather than a loss for some species in some
areas. As noted by the IAFWA, there is scope for simpli-
fication of regulations and a reallocation of federal and
state funding priorities. The limited funds available for
management of raptor populations would be far better
spent on regional and national monitoring programs and
for research on the impacts of land use changes.

In particular, I note that in the U.S. it would be con-
sistent with other managed migratory bird populations
to remove state barriers to harvesting raptors. In 1986,

Wisconsin required only a nonresident small game li-
cense to permit harvest by nonresident falconers. Re-
porting and banding requirements could be eliminated
for all species except those of special concern. Interna-
tionally, experience in the U.S. supports the licensing of
falconers based on demonstrated competency and ex-
perience, with possession limits based on the class of li-
cense. If standards of competency for falconers similar to
the U.S. system were adopted internationally, noncom-
mercial exchange of raptors might be permitted among
licensed individuals of any countries adhering to such
standards.

Inferring Sustainable Yields for Raptor Populations

Robert E. Kenward. Institute of Terrestrial Ecology, Ware-

ham, Dorset BH20 5 AS U.K

Sustainable yield levels for raptors can be estimated in
three main ways: (1) from data on populations harvested
for falconry, (2) from data on stable populations in which
a known proportion was killed by man and (3) by study-
ing the dynamics of artificially depressed populations.

Ideally, harvest data should be obtained for at least 10
yr from populations where compensatory immigration
can be discounted. The only such data are for Gyrfalcons
(Falco rusticolus ): records of nestlings which were taken
from Iceland for four centuries would represent 25-50%
of young from the present, saturated population (Cade
1968). More recently, an average 22% of Peregrine Fal-
con ( Falco peregrinus ) nestlings were taken from the
Queen Charlotte Islands during five yr in the early 1960s
(Blood 1968). There was no immediate marked popula-
tion decline, but a slight downward trend would have
been undetected in this short period. Similarly, the ex-
perimental 9-27% harvest of young Prairie Falcons ( Falco
mexicanus ) in Wyoming seems to have caused no popu-
lation decline (Runde 1987).

Although the proportion killed by man has ranged
from 40-92% of recoveries in at least 27 banding studies
(Newton 1979), this must partly reflect recovery bias:
47% of recovered Northern Goshawk ( Accipiter gentilis)
rings were from killed hawks on a Swedish island during
1975-85, but man caused only 36% of the deaths among
352 radio-tagged hawks in the same period (Kenward et
al. 1993). To obtain a minimum estimate of man’s im-
pact, the number of birds killed can be expressed as a
proportion of the number banded, and not just the re-
covered bands. In this case 14% of peregrines and 19-
21% of goshawks were killed in Fennoscandia prior to
1962 (Nordstrom 1963, Hoglund 1964), and 16% of
North American Cooper’s Hawks ( Accipiter cooperii ) dur-
ing the 1930s (Henny and Wight 1972). The Fennoscan-
dian goshawk population has remained large, with “best
estimates” that about 30% were being killed in Finland
(Haukioja and Haukioja 1970).

Data on increase rates for depressed raptor popula-

296

Abstracts

Vol. 31, No. 3

tions provide minimum estimates of sustainable yield, be-
cause the increase may stem from alleviation rather than
removal of the depressive factors. Increase rates of 12%
per annum in Britain and 16% in West Germany have
been recorded for peregrines as a result of reduced per-
secution or pollution (Ratcliffe 1980, Newton 1988). In
Holland, goshawk numbers increased by 19% annually
during 1963—80 as organochlorine use was restricted
(Marquiss 1981), and the reintroduced British goshawk
population grew at an annual rate of 21% during 1964-
80 (Thissen et al. 1981) . The increases probably stemmed
in part from breeding by birds which would not repro-
duce in saturated populations. Thus, 12% of goshawks
bred in their first year in a German population where
many adults were killed (Ziesemer 1983, Looft 1984),
whereas none have in the Swedish island study (Kenward
et al. 1991). If the German reproduction data are used
in the Swedish population model, there is a 27% annual
increase. Moreover, the Swedish females have a lower
mortality than males, and thus a 1.67:1 excess in the adult
population: removing 36% of young females would
equalize the adult sex ratio.

These studies show that healthy peregrine and gos-
hawk populations can sustain the removal of at least 10%
of their young, and in some cases more than 20%. The
same probably applies to many other raptor species. The
impact of allotting native raptors for falconry is likely to
be less than the gross take, because 50-93% may even-
tually be released or lost into the wild (Kenward 1974).
This process can even benefit raptor conservation: it was
a cheap and successful way to reestablish goshawks in
Britain (Kenward et al. 1981, Marquiss 1981).

Healthy raptor populations can probably sustain at
least a 10% harvest of juveniles, and in some cases per-
haps more than 20%. The actual number of birds avail-
able from a given population would depend on the pop-
ulation’s size, which should be monitored continuously
to ensure that no decline results from the harvest. Since
population monitoring is useful for raptor conservation,
but costly, it may make more sense to encourage falcon-
ers to contribute to data collection, as the price for their
harvest, than to channel their resources into the captive
breeding of species which are unthreatened in the wild.

Literature Cited

Blood, D.A. 1968. Population status of Peregrine Fal-
cons in the Queen Charlotte Islands, British Colum-
bia. Can. Field Nat. 82:169-176.

Cade, T.J. 1968. The Gyrfalcon and falconry. Living Bird
7:237-240.

Haukioja, E. and M. Haukioja. 1970. Mortality rates of
Finnish and Swedish Goshawks ( Accipiter gentilis).
Finn. Game Res. 31:13—20.

Henny, C .J. and H.M. Wight. 1972. Red-tailed and Coo-
per’s Hawks: their population ecology and environ-
mental pollution. Patuxent Wildlife Research Center,
MD U.S.A.

Hoglund, N. 1964. Der Habicht Accipiter gentilis in Fen-
noscandia. Viltrevy 2:195-270.

Kenward, R.E. 1974. Mortality and fate of trained birds
of prey./. Wildl. Manage. 38:751-756.

, M. Marquiss and I. Newton. 1981. What hap-
pens to goshawks trained for falconry. J. Wildl. Man-
age. 45:802-806.

, V. Marcstrom and M. Karlbom. 1991. The gos-
hawk ( Accipiter gentilis) as predator and renewable re-
source. GibierFaune Sauvage 8:367-378.

, AND . 1993. Causes of death in ra-
dio-tagged Northern Goshawks. Pages 57-61 in P.T.
Redig, J.E. Cooper, D.J. Remple and D.B. Hunter
[Eds.], Raptor biomedicine. Univ. Press, Minneapolis,
MN U.S.A.

Looft, V. 1984. Die Entwicklungen des Habichtbe-
standes (Accipiter gentilis) in Schleswig-Holstein 1968—
1984. Corax 10:395-400.

Marquiss, M. 1981. The goshawk in Britain — its prove-
nance and current status. Pages 43-57 in R.E. Ken-
ward and I.M. Lindsay [Eds.], Understanding the gos-
hawk. Int. Assoc. Falconry Cons. Birds of Prey, Ox-
ford, U.K.

Newton, I. 1979. Population ecology of raptors. T. &
A.D. Poyser, Berkhamsted, U.K

. 1988. Regulation of peregrine populations.

Pages 55-101 in T.J. Cade, J.H. Enderson, C.G. The-
lander and C.M. White [Eds.], Peregrine Falcon pop-
ulations: their management and recovery. The Pere-
grine Fund, Boise, ID U.S.A.

Nordstrom, G. 1963. Einige Ergebnisse der Vogelber-
ingung in Finnland in der Jahren 1913-1962. Ornis
Fenn. 40:81-124.

Ratcliffe, D. 1980. The Peregrine Falcon. T. Sc A.D
Poyser, Berkhamsted, U.K.

RUNDE, D.E. 1987. Population dynamics and movement
patterns of the Prairie Falcon (Falco mexicanus). Ph.D
dissertation, Univ. Wyoming, Laramie, WY U.S.A.

Thissen, J., G. Muskens and P. Opdam. 1981. Trends in
the Dutch Goshawk Accipiter gentilis population and
their causes. Pages 28—43 in R.E. Kenward and I.M.
Lindsay [Eds.], Understanding the goshawk. Int. As-
soc. Falconry, Birds of Prey, Oxford U.K

Ziesemer, F. 1983. Untersuchungen zum Einfluss des
Habichts ( Accipiter gentilis) auf Populationen seiner
Beutetiere. Beitrage zur Wildbiologie 2. G. Hartmann,
Kronshagen, Germany.

Comments on Hybridization in Raptors

Jimmie R. Parrish 1 and Clayton M. White. Department
of Zoology, Brigham Young University, Provo, UT 84602
U.S.A.

1 Present address: Avocet Consulting, Inc., 1065 East
Canyon Road, Avon, UT 84328 U.S.A.

September 1997

Abstracts

297

The concept of hybrid raptors has interest to both the
evolutionary biologists (systematist), because of the im-
plications of hybridization to the understanding of phy-
logenetic relationships, and also to the falconer, because
of the blending of characteristics that hybrids may man-
ifest, some of which may be particularly desirable in the
sport. At the writing of this paper, hybrids in many com-
binations of species are a major source of raptors for the
falconer. As a group, falconers thus have specific interest
in the phenomenon, in part because the concept of pro-
ducing hybrids has come under question by some envi-
ronmentalists, conservationists, biologists and others.

A basic understanding of taxonomic concepts, as well
as criteria defining hybridization, is critical to adequately
address hybridization involving raptors. We defined these
concepts as pertaining to avian populations in general.
We then defined the species using the classical and time
honored characteristic notion of reproductive disconti-
nuity (Mayr 1970, Bush 1975), as outlining the limits of
a species, recognizing that such a definition may become
obsolete as more and more data and analyses, especially
molecular data, are available. Within this context, how-
ever, hybridization is the mixing of “alien” genes from
one Mendelian population to another (Sibley 1957, Ris-
ing 1983) in both natural and artificial schemes. The hy-
brid is then the offspring of a cross between genetically
dissimilar (at some level) individuals or populations. The
word hybrid may conjure bad connotations (Cade 1983)
while the word “purebred” gives good feelings. Pure-
breds, however, are nothing but channeled mixtures of
genotypes. We used examples of hybrids that may occur
in stable hybrid zones in the wild, among such nonrap-
torial birds as flickers (Colaptes spp.), jays (Cyanocitta spp.)
and meadowlarks (Sturnella spp.) (Rising 1983). We fur-
ther explored the influence of the natural spread of
“alien” genes throughout the range of a species; for ex-
ample, the Mallard ( Anas platyrhynchos ) is reproducing
with and swamping out genes in related species such as
the American Black Duck ( Anas rubripes ) (Ankney et al.
1986) and Pacific Black Duck ( Anas superciliosa) . Impor-
tant questions, as they applied to the above nonraptorial
species, but also the raptorial species discussed, include:
what constitutes hybrid vigor (heterosis)? What is the ef-
fect of a hybrid swarm? How is fecundity of a given taxon
affected by hybridization? What other effects should be
considered when introduction of a hybrid occurs in a
population? Is the question of hybridization among wild
raptors an important one?

Most of these questions are not easily answered. At
present, some cannot be. A relative paucity of data exists
for evaluating effects of hybridization among wild raptor
populations. Therefore, we discussed the kinds of data
needed to formulate effective management questions in-
volving hybrid raptors. An early record suggested the nat-
ural cross between a male Northern Goshawk ( Accipiter
gentilis) and a female Common Buzzard ( Buteo buteo)
(Gray 1958). Recently, there are at least five cases of in-

trageneric natural hybrids in raptors: Otus asio x Otus ken-
nicotti, Buteo jamaicensis X Buteo buteo, Falco tinnunculus X
Falco naumanni, Accipiter fasdatus X Accipiter novaehollan-
diae, Milvus milvus X Milvus migrans and Falco peregrinus
X Falco mexicanus (Marshall 1967, Wobus and Creutz
1970, Sylven 1977, Hollands 1984, Olsen and Olsen 1985,
Bjilsma 1988, Oliphant 1991). Two other natural hybrids
have been suggested. Ellis (1995) speculated, based pri-
marily on plumage, that the so-called Altay falcon ( Falco
altaicus or Falco cherrug ?) of the mountains of central Asia
resulted from hybridization of Falco cherrug X Falco rusti-
colus. Seibold et al. (1993), based on DNA sequence data
showing two distinct mitochondrial hyplotypes within the
currently recognized Falco cherrug, suggested that one of
the hyplotypes may have resulted from hybridization of
Falco cherrug X Falco peregrinus. Any special circumstances
surrounding each of these examples is briefly discussed.

Some of the most interesting hybrids are those pro-
duced in captive breeding situations. The list of species
that have been bred in captivity often with artificial in-
semination, is, of course, considerable. Of 83 species of
diurnal raptors successfully bred in captivity as of 1985,
23 were falcons, eight buteos and seven accipiters (Cade
1986). Currently, hybrids are commonplace within the
falconry community (Haak 1980). Certain combinations
of falcons seem to be better for the sport than either of
the parental types and indeed, some types of hybridiza-
tion may confer a certain evolutionary fitness over either
parental species (Grant and Grant 1992). We do not have
good data on all the hybrid falcons that have been pro-
duced nor the combinations (either species involved or
whether a tri- or more hybrid cross) , and thus not much
of an assessment can be made. Some of the karyotype
and chromosomal differences in parental species within
large native North American Falco were discussed
(Schmutz and Oliphant 1987).

The inevitable question concerns the fate of such hy-
brid raptors if lost to the wild. Since we now live in hab-
itats that are highly modified, a sort of hybrid environ-
ment, the question of what fits best into the environment
is moot. Hundreds of “exotic” raptors have been lost
into the environment without any discernable long-last-
ing affects. For example, Saker Falcons have bred with
Peregines (Stevens 1972) and yet sakers lost to the wild
in North America seem never to show up again; their
genes certainly do not seem to be represented in wild
breeding native populations of other North American
Falco unless the haplotype situation mentioned above
could be detected. Certainly, genes modifying morphol-
ogy are not evident. Some intrageneric hybrids, where
one of the parents is an exotic species, may be of con-
cern, however. Buteo jamaicensis, an exotic in the U.K., has
mated in the wild with Buteo buteo and this could pose a
problem in the future as with the Mallard X black duck
example.

As with most other management-oriented questions,
the answers to questions surrounding hybridization are

298

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Vol. 31, No. 3

to be found within the natural realm only after some
periods of observations. We can provide logical expecta-
tions on effects of artificial hybridization to wild raptor
populations, and the affects seem to be of little conse-
quence. In our discussion, particular emphasis was
placed on taxa within the genus Falco.

Literature Cited

Ankney, C.D., D.G. Dennis, L.N. Wischard and J.E. Seeb.
1986. Low genic variation between black ducks and
Mallards. Auk 103:701-709.

Bijlsma, R.G. 1988. Unidentified Kenyan kites — Hybrid
Black X Red? Gabar 3:19-20.

Bush, G.L. 1975. Modes of animal speciation. Ann. Rev.
Ecol. Syst. 6:339—364.

Cade, TJ. 1983. Hybridization and gene exchange
among birds in relation to conservation. Pages 288-
309 in C.M. Schonewald-Cox, S.M. Chambers, B.
MacBryde and W.L. Thomas [Eds.], Genetics and
conservation. The Benjamin/Cummings Publ. Co.,
Inc., Menlo Park, CA U.S.A.

. 1986. Propagating diurnal raptors in captivity: a

review. Int. Zoo Yearb. 24/25:1-20.

Ellis, D.H. 1995. What is Falco altaicus Menzbier? J. Rap-
tor Res. 29:15-25.

Grant, P.R. and B.R. Grant. 1992. Hybridization of bird
species. Science 256:193-197.

Gray, A.P. 1958. Bird hybrids. A checklist with bibliog-
raphy. Commonw. Bur. Anim. Breed. Genet. Tech. Com-
mun. Edinburgh 13:1-390.

Haak, B. 1980. Hybrid falcons./. N. Am. Falconers Assoc.
18/19:74-83.

Hollands, D. 1984. Eagles, hawks and falcons of Aus-
tralia. Thomas Nelson Australia, Melbourne, Victoria,
Australia.

Marshall, J.T., Jr. 1967. Parallel variation in North and
Middle American Screech-owls. Monog. No. 1, West.
Found. Vert. Zool., Los Angeles, CA U.S.A.

Mayr, E. 1970. Populations, species and evolution. Har-
vard Univ. Press, Cambridge, MS U.S.A.

Oliphant, L.W. 1991. Hybridization between a Pere-
grine Falcon and a Prairie Falcon in the wild .J. Raptor
Res. 25:36-39.

Olsen, P.D. and J. Olsen. 1985. A natural hybridization
of the Brown Goshawk ( Accipiter fasciatus) and Grey
Goshawk (A. novaehollandiae) in Australia, and a com-
parison of the two species. Emu 85:250-257.

Rising, J.D. 1983. The great plains hybrid zones. Pages
131-157 in R.F. Johnston [Ed.], Current ornithology,
Vol. 1. Plenum Press, New York, NY U.S.A.

Schmutz, S.M. AND L.W. Oliphant. 1987. A chromo-
some study of Peregrine, Prairie and Gyrfalcons with
implications for hybrids. J. Tiered. 78:388-390.
Seibold, I., A.J. Helbig and M. Wink. 1993. Molecular
systematics of falcons (family Falconidae). Naturwis-
senschaften 80:87-90.

Sibley, C.G. 1957. The evolutionary and taxonomic sig-

nificance of sexual selection and hybridization in
birds. Condor 59:166-191.

Stevens, R. 1972. B.P.I.E. No. 27. Peregrine Falcon-Saker
cross. Raptor Res. 6:18-21.

Sylven, M. 1977. Hybridisering mellan glada Milvus mil -
vus och brunglada M. migrans i Sverige 1976. VarFa-
gelvdrld 36:38-44.

Wobus, U, and G. Creutz. 1970, Eine erfolgreiche
Mischbrut von Rot und Schwarzmilan (Milvus milvus
X Milvus migrans ). Zool. Abh. 31:305-313.

Contributions of Rehabilitation/Education
Programs in Raptor Management

Patrick T. Redig, Gary E. Duke and Marc Martell.
Raptor Research and Rehalnlitation Program, 295 Animal
Science/Veterinary Medicine Building, 1988 Fitch Avenue,
St. Paul, MN 55108 U.S.A.

The rise of rehabilitation of raptors has occurred con-
currently with the increase in general efforts to manage
and conserve raptors. Prior to the mid-1960s there was
little evidence of rehabilitation being undertaken on any
scale that might impact aspects of raptor management.
Similarly, prior to 1970, there was a dearth of specific
veterinary information available to be utilized in provid-
ing state-of-the-art medical care for raptors. Since then,
a significant development in the number and scope of
organizations for rehabilitating raptors and other wildlife
has occurred among both lay and professional sectors.
Many of these projects include public education and re-
search, both basic and applied, among their objectives,
so that the total impact of these efforts can potentially
have a sizeable positive influence on the survival of rap-
tors. Using data derived largely from the research and
rehabilitation effort maintained at the University of Min-
nesota since 1974, we reached a number of conclusions.
(1) Combined research and rehabilitation programs can
provide effective means for detecting naturally occurring
diseases and for assessing the importance of various caus-
es of mortality among raptors. Fourteen years of data col-
lected systematically show in general that the occurrence
of natural disease is low in raptors, whereas the incidence
of traumatic injuries from man-made factors constitutes
the majority of the admissions. Among the latter, the
greatest number of injuries arose from collisions with
moving vehicles and powerlines. (2) Rehabilitation can
result in complete recoveries with successful releases to
the wild and subsequent survival. Data from banding rec-
ords and telemetry studies show survival in excess of sev-
en yr for some rehabilitated raptors and distances of
more than 1000 miles traveled over the course of five mo
following release. Data are also available which document
successful nesting of released Bald Eagles ( Haliaeetus leu-
cocephalus ) , through the finding of color-marked feathers
in and below occupied nests. The influence of these re-
covered birds on wild populations varies with the num-

September 1997

Abstracts

299

bers involved, the number of wild birds present in a pop-
ulation and the effectiveness with which rehabilitated
raptors are assimilated back into the wild. (3) Reintro-
duction and translocation projects for Bald Eagles and
Peregrine Falcons ( Falco peregrinus ) have benefited by the
rearing of young, and also through the assessment of
health status and medical treatment of those that have
become ill or injured during the release process. (4) Re-
search into the utilization of crippled raptors for breed-
ing purposes has produced positive results. Young of Bald
Eagles and several owl species have been produced by
crippled parents for release projects.

Other impacts of rehabilitation projects are farther
reaching, but less measurable, than those mentioned
above. Since 1980, 18 senior veterinary students have
completed internships ranging from three wk to three
mo at this program, and several have gone on to establish
research and rehabilitation projects at other veterinary
colleges. Additionally, raptor biologists from Spain, Mex-
ico, France, England, Denmark, New Zealand and Israel
have served internships during which they gained valu-
able experience in capture, restraint, blood sampling and
other procedures that enhance their ability to gather
field data about raptors. Further, the program now main-
tains an active list of more than 100 volunteers working
in clinical, educational and public relations areas which
not only further the immediate work of the program, but
also provide the volunteers with lifetime experiences that
will stimulate their understanding and make them effec-
tive communicators for raptor conservation in the future.

The most immeasurable thrust is in the area of public
relations and education. Uncountable hundreds of
thousands of people are being informed about the ongoing
need for conservation of raptors and wildlife resources. Re-
habilitation statistics indicate the effectiveness of such ef-
forts. In the period 1972-75, 35% of the admissions to the
program occurred due to projectile injuries; since 1981, 4%
or fewer of admissions have come from projectile injuries.
Additionally, public awareness of the need for eagle winter-
ing habitat caused the reevaluation of an airport improve-
ment project in St. Paul, MN that would have resulted in
the felling or topping of trees on an island in the Mississippi
that was used by Bald Eagles. This population of eagles was
found by radio-tracking a rehabilitated bird that had recov-
ered from a trap injury.

Influencing public policy and legislation are other arenas
in which rehabilitation projects have had an impact. The
current trend toward elimination of lead shot for waterfowl
hunting has gained impetus from the realization that Bald
Eagles are affected by lead poisoning, a fact that came to
light from the admission of lead-poisoned eagles to rehabil-
itation facilities as well as the USFWS Health Laboratory in
Madison. Additionally, several states in the Midwest have en-
acted legislation to eliminate the use of open-baited steel-
jawed traps for small mammal trapping after recognizing
the numbers of eagles admitted to rehabilitation projects
that had been caught in traps.

The cost-effectiveness of rehabilitation is only measur-
able in terms of the number of benefits one is willing to
apply against the actual medical costs of rehabilitation
The Minnesota project computes a cost of about $75 per
bird admitted to the clinic, amortized over a total admis-
sion of 4000 raptors in 14 yr. At an average release rate
of 42%, the cost per released bird is about $150. Cost
factors associated with other means of raptor manage-
ment are not available, so direct comparisons cannot be
made. However, given the wide array of benefits afforded
raptors by the global efforts in conservation mediated
through rehabilitation and education projects, we con-
clude that this area of endeavor is a viable and worth-
while tool for their management.

Development of Captive Breeding and Release
Techniques

TomJ. Cade and Martin J. Gilroy. The Peregrine Fund,

World Center for Birds of Prey, 5666 West Flying Hawk Lane,

Boise ID 83709 U.S.A.

Aldo Leopold (1933) began what can be called the “eco-
logical tradition” in wildlife management, with its emphasis
on habitat. Its principle is that the preservation and manip-
ulation of all environmental factors that are necessary to
support wildlife populations is more important than direct
manipulation of the animals themselves. This approach has
continued to the present date and is certainly the best policy
whenever it can be pursued. The preservation of suitable
habitats for birds of prey should be our paramount concern,
as it is for all wildlife, since the more natural areas and
ecosystems we can set aside and preserve in the unaltered
state, the greater will be the abundance and diversity of rap-
tors in the future. However, we all recognize that despite
our best intentions and efforts, natural habitats of all sorts
foil continue to shrink in size and to deteriorate in their
capacities to support a diversity of species, under the con-
tinuing influence of human population pressures and
needs. Such passive preservation measures that aim to pre-
serve the status quo are delaying actions at best, and alone
will not suffice, simply because they will not occur on a large
enough scale to take care of everything. Increasingly in the
future, the strategy of biological conservation will need to
combine strict habitat preservation with preservation of in-
dividual species, by using manipulative techniques (such as
captive propagation and reintroduction) to help species to
adjust and to survive in the increasingly human-dominated
world.

Propagation

It is curious that the captive propagation of raptors is a
quite recent activity, given the long tradition of human in-
volvement with these species in the sport of falconry and as
tribal and national totems. The first Peregrine Falcon ( Falco
peregrinus) known to be raised from captive parents was pro-
duced as recently as 1942, and even as late as 1965 only

300

Abstracts

Vol. 31, No. 3

about 23 species of diurnal raptors had successfully been
bred in captivity, mostly on a casual basis.

The situation has changed markedly in the last two
decades. When it became evident in the late 1960s that
many raptor populations in north temperate regions had
suffered major declines, owing to DDT and related pes-
ticides or to other forms of environmental degradation,
an interest emerged (particularly among falconers) to
perfect techniques of captive breeding for some of these
species, especially the peregrine. More than a quarter of
all falconiform species have now been bred in captivity.
At least 12 species have produced more than 100 progeny
in captivity since 1975, some having produced thousands;
the number of peregrines produced worldwide certainly
exceeds 5000. It is probably safe to conclude that most,
if not all, diurnal birds of prey can be bred in captivity
given sufficient knowledge of their needs and sufficient
resources to carry out the work.

Among the explanations for these breakthroughs is the
zealous nature of raptor breeders. Most of them are falcon-
ers, building on 3000 years of knowledge about handling
and training hawks and falcons. A second factor contribut-
ing to the success of these projects has been the rapid and
free exchange of information among breeders through or-
ganizations such as the Raptor Research Foundation, North
American Falconers Association, the Hawk and Owl Trust
and the British Falconers’ Club, to name a few. Finally,
much is owed to the application of basic scientific infor-
mation on avian reproductive physiology 7 and breeding be-
havior and ecology. A quick example is the now well-known
development of human-imprinted “semen donors” for ar-
tificial insemination, solving infertility problems owing to in-
compatibilities between mates. A thorough review of captive
propagation is available in Cade (1986).

Reintroduction

Raptor reintroduction programs, which are often tech-
nically “restocking” in that the original population is not
truly extinct, have employed three general methods: (1)
fostering captive-bred or harvested wild young into the
nests of conspecific surrogates, (2) cross-fostering into
the nests of other species and (3) hacking by modifica-
tions of the traditional falconers’ methods. Details are
available in Sherrod et al. (1981), Cade et al. (1988) and
Barclay and Cade (1983). As these techniques have been
refined, there has been a rapid increase in the number
of reintroduction programs for raptors.

If a program is to be successful, its goals need to be
specifically stated, based on reproductive and survival
data from similar projects or from natural populations in
other parts of the species’ range so that accurate projec-
tions of the required commitment can be made, in terms
of birds, work, time and money. Such projects should not
be started merely because it is now comparatively easy to
do so, or is good publicity, or makes an agency available
for federal funding. Experience to date indicates that the

establishment of self-sustaining populations in vacant
range takes a lot of birds and a lot of time.

A concerted, cooperative, regional approach can maxi-
mize the return on species restoration efforts. Clustering
release sites so as to saturate a region increases the likeli-
hood of pair formation, and may be accomplished through
cooperation of several states. Toward that end, an active,
enthusiastic recovery team approach has worked well in the
eastern peregrine reintroduction. Besides their role in co-
ordinating the multitude of state and federal agencies that
carry out this work, they have helped to expedite the reg-
ulatory burden and moderate the political aspects that ac-
company a large-scale program.

The cost of conducting raptor restoration programs in
the coming decades will be high, since they are so labor
intensive, especially when captive-produced birds are in-
volved. Taking the Eastern Peregrine Recovery Program
as a case in point, the Peregrine Fund has spent about
$2.8 million to propagate and release peregrines in the
eastern states. Figuring in the expense of cooperating
agencies probably brings this cost to about $3.5 million,
perhaps more, and this is but one of four regional re-
covery programs in the U.S. Though this may seem a
staggering amount at hrst, it is not really that expensive
relative to many of the other things people are willing to
spend our public and private wealth to obtain. Compared
to the $10 million one individual recently paid for a sin-
gle untrained racehorse, or the $15 million purses of
championship prize fights, or the billions of dollars spent
on Star Wars technology, saving endangered species
seems a bargain.

These costings underscore the need for sound eco-
nomic projections in the planning stages of a reintro-
duction program, and the need for continued support
for the duration of the program. Complete restoration
may not be achieved until years after the initial enthusi-
asm of the program has waned. Moreover, the required
support extends beyond money alone, to agency support.
The success in establishing initial small populations can
lead to an attitude of complacency, for example, so that
states just entering a program become ineligible for the
federal funds that got the program started. Government
labs can become reluctant to analyze eggs to monitor the
factors responsible for the species’ original decline.

The involvement of the skilled private sector is one way
of reducing some of the costs of reintroduction programs.
Members of local bird clubs and individual falconers have
helped survey and monitor falcons in the east. Many falcon-
ers have provided young for the peregrine recovery effort.
Because of production problems at our facility in Boise in
1986, more than 15% of the birds released in the east were
donated by private breeders. Others provided falcons for
release in the Upper Mississippi region.

As natural environments become fragmented and de-
graded, it is up to those of us who care about these birds
to convince the rest of humanity that they are worth the
cost of saving. So long as people are willing to commit

September 1997

Abstracts

301

the necessary time, effort and money, the creative use of
management techniques like captive breeding and rein-
troduction can be made to work for particular species of
concern. The future is not bleak, as some pessimists
would have us think; rather, it is a challenge.

Literature Cited

Barclay, J.H. and T.J. Cade. 1983. Restoration of the
Peregrine Falcon in the eastern United States. Biol.
Cons. 1:3—40.

Cade, T.J. 1986. Using science and technology to rees-
tablish species lost in nature. Pages 279-288 in E.O.

Wilson [Ed.], Biodiversity. National Academy Press,
Washington DC U.S.A.

, J.H. Enderson, C.G. Thelander and C.M.

White. 1988. Peregrine Falcon populations, their
management and recovery. The Peregrine Fund Inc.,
Boise, ID U.S.A.

Leopold, A. 1933. Game management. Charles Scrib-
ner’s Sons, New York, NY U.S.A.

Sherrod, S.K, W.R. Heinrich, W.A. Burnham, J.H. Bar-
clay and TJ- Cade. 1981. Hacking: a method for re-
leasing Peregrine Falcons and other birds of prey. The
Peregrine Fund, Boise, ID U.S.A.

THE RAPTOR RESEARCH FOUNDATION, INC.

(Founded 1966 )

OFFICERS

PRESIDENT: David M. Bird
VICE-PRESIDENT: Michael N. Kochert

SECRETARY: Betsy Hancock
TREASURER: Jim Fitzpatrick

BOARD OF DIRECTORS

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

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: Cesar MArquez
DIRECTOR AT LARGE #5: Petra Bohall Wood
DIRECTOR AT LARGE #6: Katherine McKefver

INTERNATIONAL DIRECTOR #2:

Karen Steenhof

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

Jemima ParryJones

EDITORIAL STAFF

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

ASSOCIATE EDITORS

Charles J. Henny

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

Missoula, MT 59812 U.S.A.

SPECIAL PUBLICATIONS EDITOR: Daniel E. Varland, Rayonier, 3033 Ingram Street, Hoquiam, WA

98550

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 (8 V 2 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 outlined in “Information for contributors,”/. Raptor Res., Vol. 27(4), and are available from the editor.

Allen M. Fish
Gary R. Bortolotti

Fabian Jaksic
Daniel E. Varland

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, title 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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