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Journal

Raptor Research

Volume 31 March 1997 Number 1

Contents

Peregrine Falcon Recovery Along the West Central Coast of the Baja
CALIFORNIA Peninsula, Mexico. Aradit Castellanos, Fernando Jaramillo, Federico
Salinas, Alfredo Ortega-Rubio and Cerafina Arguelles 1

Winter Bald Eagle Distribution is Inversely Correlated with Human
Activity Along the Colorado River, Arizona. Bryan t. Brown and Lawrence e.

Stevens 7

Population Status of the Endangered Hawaiian Hawk. Linnea s. Hall, Michael

L. Morrison and Peter H. Bloom 11

Nest-Site Selection by Four Sympatric Forest Raptors in Southern

Norway. Vidar Selas 16

Distribution and Species Richness of a Forest Raptor Community in

Relation to Urbanization. Thomas Bosakowski and Dwight g. Smith 26

Does Vegetation Structure Limit the Distribution of Northern
Goshawks in the Oregon Coast Ranges? Stephen DeStefano and Jon McCioskey. ... 34
Food Habits of the Lanner Falcon (Falco biarmicus feldeggii) in Central

Italy. Federico Morimando, Francesco Pezzo and Alessandro Draghi 40

Nesting Distribution and Population Status of U.S. Ospre\s 1994. Lawrence

M. Houghton and Larry M. Rymon..., 44

The Delaware Bayshore of New Jersey: A Raptor Migration and Wintering

Site of Hemispheric Significance, clay Sutton and Paul Keriinger 54

Food Habits of Common Barn-owls along an Elevational Gradient in
Andean Argentine Patagonia. Alejandro Travaini, Jose A. Donazar, Olga Ceballos,

Alejandro Rodriguez, Fernando Hiraldo and Miguel Delibes 59

Indicators of Male Quality in the Hoots of Tawny Owls (Strix aluco) .

Bridget M. Appleby and Stephen M. Redpath 65

Role of Refuse as Food for Migrant, Floater and Breeding Black Kites

( MlLVUS MIGRANS ) . Guillermo Blanco 71

Short Communications

Interspecific and Intraspecific Aggression Among Grifton and Cinereous Vultures at
Nesting and Foraging Sites. Guillermo Blanco, Jose M. Traverse, Javier Marchamalo and

Felix Martinez 77

Hunting Synchrony in White-tailed Kites. Molly F. Skonieczny and Jeffrey R. Dunk 79

Blood Parasites of Nestling Goshawks. E.P. Toyne and R.W. Ashford 81

Migration of Flocks of Honey Buzzards in Southern Italy and Malta. Nicolantonio

Agostini, Daniela Logozzo and Charles Colero 84

Nesting-Tree Preference and Nesting Success of Japanese Lesser Sparrowhawks in Japan.

Mutsuyuki Ueta 86

Letters 89

BOOK Review. Edited by Jeffrey S. Marks 90

Manuscript Referees 91

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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Printed by Allen Press, Inc., Lawrence, Kansas, U.SA

THE JOURNAL OF RAPTOR RESEARCH

A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC.

Vol. 31 March 1997 No. 1

J. Raptor Res. 31(l):l-6

PEREGRINE FALCON RECOVERY
ALONG THE WEST CENTRAL COAST OF THE
BAJA CALIFORNIA PENINSULA, MEXICO

Arad n Castellanos

Centro de Investigaciones Biologicas del Noroeste, S.C., Div. De Biol. Terrestre., Apdo. Post. 128,

La Paz , B.C.S., Mexico

Fernando Jaramillo

PG7 Consultores, S.C., Apdo. Post. 1887, Cuernavaca, Mor., Mexico

Federico Salinas, Alfredo Ortega-Rubio and Cerafina Arguelles

Centro de Investigaciones Biologicas del Noroeste, S.C., Div. de Biol. Terrestre., Apdo. Post. 128,

La Paz, B.C.S., Mexico

Abstract. — The central west coast of the Baja California peninsula was an important Peregrine Falcon
(Falco peregrinus) breeding area supporting a population of about 13 breeding pairs. This population
declined drastically during the 1960s and early 1970s. We conducted field surveys and compiled data
on nesting Peregrine pairs from 1980-94 to address the current status of the Baja population. We found
10 pairs nesting in the area indicating the Peregrine population has recovered in the area since the late
1970s. Due to increased human activity in the area, proper management is needed to provide suitable
nesting sites and to minimize human disturbances during the nesting season.

Keywords: Peregrine Falcon; recovery; status ; Baja California; Falco peregrinus.

Recuperation del halcon peregrino en la costa centro occidental de la penisula de Baja California, Mexico.

Resumen. — La costa centro occidental de la peninsula de Baja California, ha sido un area importante de
reproduction del halcon peregrino (Falco peregrinus) , manteniendo alrededor de 13 territorios de anidacion.
Esta poblacion declino drasticamente entre los anos 1960s e inicios de los 1970s. Nosotros realizamos estudios
en el campo y recabamos datos sobre las parejas anidantes de peregrinos entre 1980 y 1994 para determinar
su situation actual. Encontramos diez parejas de peregrinos anidando en el area. Nuestros hallazgos sugieren
que los peregrinos se han estado recuperando desde finales de los anos 1970s. Las actividades humanas
estan creciendo en el area, lo que representa tanto amenazas como oportunidades para los peregrinos. Se
requiere de un manejo apropiado que proveea de sitios adecuados de anidamiento para minimizar las
perdidas reproductivas asociadas al hombre durante la estacion reproductiva.

[Traduction Autores]

Peregrine Falcon ( Falco peregrinus) recovery
has been documented in several regions of the
world (Fyfe 1988, Kiff 1988, Ratcliffe 1993, En-
derson et al. 1995). Nesting territories, deserted
during the decline of the species are now being
reoccupied by breeding pairs. Reproduction has

now returned to pre-DDT levels (Kiff 1988, New-
ton 1979, Ratcliffe 1993). The ban on DDT use
in many countries (Ratcliffe 1993) and the suc-
cess of Peregrine recovery programs (Fyfe 1988,
Ratcliffe 1993) has contributed to the Peregrine’s
removal from endangered status and to a redesign

Castellanos et al.

Vol. 31, No. 1

North

Brccdsyipara

Smgt*

Prob«Wy netting
Inland potential

28 * 00 '

26 * 30 -

Figure 1. Recent records of nesting Peregrine Falcons on the central west coast of Baja California, Mexico. (Shaded
areas are Banks, 1969 records.)

of its management strategies (Walton and Thelan-
der 1988).

The historical and current status of the Mexican
Peregrine Falcon population is not well known
(Banks 1969, Fyfe et al. 1976). Populations in the
Baja California peninsula and the Gulf of Califor-
nia are known to have declined between 1967-84
(Porter et al. 1988). By 1976, the Baja west coast
population, once containing 38 breeding pairs, was
considered on the brink of extirpation (Fyfe et al.
1976). However, recent reports of newly occupied
nesting territories in this area (Daneman and Guz-
man Poo 1992, Castellanos et al. 1994) suggest a
recovery is in process. In this report, we provide
information on current numbers, distribution, and
productivity of Peregrines, and discuss their con-

servation needs on the west central coast of the
Baja California peninsula.

Study Area and Methods

The study area, a part of El Vizcaino biosphere reserve,
covers about 350 km along the west coast of Baja Cali-
fornia, Mexico (Fig. 1). The study area includes: desert
islands, a coastal fringe of mostly sandy beaches and two
big coastal lagoons with extensive wetlands (Massey and
Palacios 1994). Most of the area is covered with halo-
phytic vegetation, 30 cm high. Mountains are restricted
to Benitos and Cedros Islands and some parts of the El
Vizcaino reserve.

We used three data sources on the breeding popula-
tion of Peregrine Falcons in the area. Data from 1980-
85 were obtained by F. Jaramillo during wildlife invento-
ries to promote the creation of the El Vizcaino Reserve
and in 1994 on Cedros and Benitos Islands. Data from
Ojo de Liebre and San Ignacio lagoons were obtained by

March 1997

Peregrine Recovery in Baja California

A. Castellanos, F. Salinas, C. Arguelles and A. Ortega in
1993-94. Nesting surveys of breeding Peregrine Falcons
were conducted by searching the plateaus, canyons, hills
and islands on foot and inspecting all potential nesting
sites using binoculars. Boats were used to explore the
lagoons and to reach the islands. We also used recent
records in the oological collection of the Western Foun-
dation of Vertebrate Zoology (WTVZ) and personal re-
ports from other individual observers. We compared our
data with those of Banks (1969). Productivity of breeding
pairs was estimated by checking three nests in 1993 and
five nests in 1994.

Results and Discussion

We located at least 10 nesting pairs of Peregrine
Falcons in six locations in the survey area, includ-
ing small islands in the open sea, islets inside coast-
al lagoons and an inland mountainous zone (Fig.
1). The maximum number of pairs seen in a single
year was six pairs in 1994 when we observed pairs
on San Roque islands, Laguna Ojo de Liebre (Pie-
dras Island and El Chaparrito navigation channel)
and San Ignacio Lagoon (Pelicano and Ballena Is-
lands) (Fig. 1, Table 1). Because not all the histor-
ical and potential nesting sites were surveyed, we
considered this to be a minimum estimate of tire
total number of breeding pairs in the area. The
historical population estimate for the study area
was about 13 pairs (Banks 1969).

Breeding pairs were found on small cliffs, di-
rectly on the ground, in Osprey nests and on arti-
ficial structures such as channel markers and ship-
wrecks. Ojo de Liebre (Scammon’s) lagoon nest
sites were new (Castellanos et al. 1994). Lack of
inland nesting records from the study area led us
to believe that the cliff-nesting pairs on Pacific
North Vizcaino (near Sierra San Jose de Castro)
were also new to the breeding population. A nest-
ing record in the WTVZ reported by B. Reitherman
in 1981 documented a pair nesting at San Ignacio
Lagoon for the first time. Daneman and Guzman
Poo (1992) also found two pairs nesting there in
1989.

In addition to the 10 nesting pairs, there were
several recent single sightings and two probable
nest site reports from Benitos and Cedros islands
(Fig. 1, Table 1). Nearest nesting site distances var-
ied between 4.4 and 60 km. Nests were 4.4 km
apart in San Ignacio lagoon and 8 and 22 km apart
in Ojo de Liebre lagoon (Fig. 1)

We followed the reproductive success of three
pairs during 1993 and five pairs during 1994, lo-
cated at Ojo de Liebre and San Ignacio Lagoons.
An average of 3 eggs, 1.8 nestlings, and 1.6 fledg-

lings were produced per nest (Table 2). This pro-
ductivity appeared to be within the range of pro-
ductivity for healthy populations (Cade et al. 1988,
Ratcliffe 1993).

The Peregrine Falcon is defined by Mexican law
as a vulnerable species (Diario Oficial 16 de Mayo
de 1994). In spite of its status, lack of national re-
covery programs on threatened species has failed
to produce management plans in their Mexican
range. The use of organochlorine pesticides in the
region is not well documented but it has appar-
ently been reduced since the late 1970s. Studies
conducted then and in the late 1980s in Baja Cal-
ifornia, the Gulf of California and in northwestern
Mexican waterfowl wintering areas show pesticide
levels in eggs and bird tissues are among the lowest
in North America (Spitzer et al. 1977, Mora et al.
1987). This evidence leads us to believe that cur-
rent organochlorine pesticide use does not repre-
sent a major threat for Peregrines in the region.

We did, however, identify other threats to breed-
ing Peregrines in the area. Nests on the ground
are accessible to dogs and cats abandoned by fish-
ermen in the islands. Coyotes ( Canis latrans ) tem-
porarily invade Piedras island in Ojo de Liebre La-
goon destroying the nests of Peregrines, Ospreys
and other birds. W 7 e found coyote tracks at one
failed Peregrine nest in 1994 and presumed the
coyote caused the nest failure. Avian depredation
by Western Gulls ( Larus occidentalis) and Common
Ravens ( Corvus corax ) is also a problem. Gulls and
ravens apparently caused the disappearance of
three eggs from a nest on a tower over the water
in Ojo de Liebre lagoon.

A third and more serious threat to nesting Per-
egrines in the area is from humans when fishing,
tourism and other human activities cause inciden-
tal disturbances at nests (Daneman and Guzman
Poo 1992). The study area is still relatively inacces-
sible and partially uninhabited, thus, habitat de-
struction apparently is not yet a factor of concern.

The coastal lagoons, islands and wetlands within
the study area support a magnificent avifauna
which provides adequate numbers and variety of
Peregrine prey (Massey and Palacios 1993). Nev-
ertheless, the density of breeding Peregrine Fal-
cons in the area seems to be rather low for such
an ideal location. Most of the resident pairs re-
ported are coastal or “small island Peregrines”
(Ratcliffe 1993) nesting on small cliffs or even on
the ground. This is apparently because of a short-
age of suitable natural nesting sites on the main-

Castellanos et al.

Vol. 31, No. 1

Table 1. Historical and recent records of nesting Peregrine Falcons in the central west coast of Baja California.
Recent records are from 1980-94.

Locality

Sources and Date

Individual/Nest

Reference

San Benitos Islands

Walker, 1927 and 1950

4 nests

Banks (1969)

Tyler, 8 April 1930

Nest with 2 eggs

WFVZ, this study

Harrison, 1 April 1938

Nest with 2 eggs

WFVZ, this study

22 March 1992

1 seen

This study

Cedros Island

Kaeding, 1905

Common, breeding

Banks (1969)

Carpenter, 2 April 1932

Nest with 4 eggs

WFVZ, this study

8, 31 July and 18 August 1980

1 seen

This study

April 1991

1 seen

This study

2, 3 and 15 March 1992

1 pair, 2 sightings

This study

22 March 1992

1 female, 1 male

This study

Natividad Island

Sechrist, 12 March 1917

Nest with 4 eggs

WFVZ, this study

Lamb, December 1924

6 pairs resident

Banks (1969)

D.S.D., 6 April 1930

Nest with 3 eggs

WFVZ, this study

Bancroft, 3 April 1932

2 nest with eggs

WFVZ, this study

31 July 1985

1 breeding pair

This study

Ojo de Liebre Lagoon

26 January 1982

1 seen

This study

June, October, November 1984

1 seen

This study

29 July 1985

2 females and 2 males

This study

21 September 1985

1 seen

This study

1993 (Castellanos et al. 1994)

3 breeding pair

This study

San Roque Island

Huey, 20 April 1927

Nest with 2 eggs

Banks (1969)

D.S.D., 5 April 1930

Nest with 3 eggs

WFVZ

Harrison, 6 April 1932

Nest with 4 eggs

WFVZ, this study

L. Flores, 1994

1 breeding pair

L. Flores pers. comm.

Asuncion Island

Walker, 1938

1 pair

Banks (1969)

San Ignacio Lagoon

Reitherman, 11 April 1981

Nest, 2 young, 2 eggs

WFVZ, this study

September 1985

1 pair, one seen

This study

Daneman & Guzman Poo, 1992

2 breeding pairs

Daneman & Guzman Poo, 1992

March-June, 1994

2 breeding pair

This study

Pacific North Vizcaino

13 June, 1984

1 breeding pair

This study

June 1984

1 breeding pair

This study

4 November 1984

2 seen

This study

March and September 1985

2 seen

This study

March 1997

Peregrine Recovery in Baja California

Table 2. Peregrine Falcon reproductive success during
1993-1994 on the Scammon’s and San Ignacio lagoons.

1993

1994

Scam-

mon’s

Lagoon

Scam-

mon’s

Lagoon

San

Ignacio

Lagoon

Occupied nests checked

Productive nests

Total eggs laid

Eggs failed to hatch

—

Eggs disappeared

—

6 a

—

Eggs broken

—

l b

Average clutch size

3.3

2.0

Nestlings

Nestling disappeared

Total young fledged

Nesdings/ occupied nest

1.0

1.5

Fledglings/ occupied nest

2.6

0.6

1.5

Fledglings/productive nest

2.6

2.0

3.0

a Probably avian and coyote depredation.
b Unknown causes.

land coast, inland and on small islands, a factor
that limits cliff-nesting raptors in other areas (Rat-
cliffe 1993). Peregrines do take advantage of suit-
able artificial nesting sites when they were available
in the study area (Castellanos et al. 1994)

Given these circumstances, we suggest the imple-
mentation of a Peregrine Conservation Program in
the El Vizcaino Reserve, focusing on the following:
a) regular surveys of historical and new nesting ter-
ritories to assess the current population status and
trends; b) studies of biology and nesting ecology
to enhance knowledge of this poorly known resi-
dent population; c) installation of artificial nesting
substrates to increase the population on the main-
land coast and at coastal lagoons; and d) an edu-
cation program to promote public awareness of
this and other species of wild birds.

Acknowledgments

We thank L. Kiff for permitting us to use the Western
Foundation of Vertebrate Zoology oological collection,
M. Acevedo for his field assistance and L. Flores for un-
published data on Peregrines. The research was support-
ed by the CIBNOR, S.C., CONACYT (Project 3031N), Ex-
portadora de Sal, S.A. and the SEMARNAP. Thanks to W.
Wehtje, M.F. Campana for his academic support and R.
Bowers and Dr. Ellis Glazier for editing of this English
language paper.

Literature Cited

Banks, R.C. 1969. The Peregrine Falcon in Baja Califor-
nia and the Gulf of California. Pages 81-91 in JJ-

Hickey [Ed.], Peregrine Falcon populations. Umv.
Wisconsin Press, Madison, WI U.S.A.

Cade, T., J. Enderson, C. Thelander and C.M. White.
1988. The role of organochlorine pesticides in Pere-
grine population changes. Pages 463-468 in T. Cade,
J. Enderson, C. Thelander and C. White [Eds.], Per-
egrine Falcon populations: their management and re-
covery. The Peregrine Fund, Boise, ID U.S.A.

Castellanos, A., F. Salinas-Zavala and A. Ortega-Ru-
bio. 1994. Status and reproduction of the Peregrine
Falcon at a coastal lagoon in Baja California Sur, Mex-
ico. J. Raptor Res. 28:11 0-1 1 2.

Daneman, G.D. and J. GuzmAn Poo. 1992. Notes on the
birds of San Ignacio Lagoon, Baja California Sur,
Mexico. Western Birds 23:11-19.

Diario Oficial de la Federacion. 1994. Tomo
CDLXXXIX, No. 10, 16 de Mayo de 1994. Secretaria
de Gobernacion.

Enderson, J.H., W. Heinrich, L. Kiff and C.M. White.
1995. Populations changes in North American Pere-
grines. Pages 142-161 in Trans. 60th North American
wildlife and natural resources conference. Wildlife
Management Institute, Washington, DC U.S.A.

Fyfe, R.W. 1988. The Canadian Peregrine Falcon recov-
ery program 1967-1985. Pages 599-610 in T. Cade,J.
Enderson, C. Thelander and C. White [Eds.], Pere-
grine Falcon populations: their management and re-
covery. The Peregrine Fund, Boise ID U.S.A.

Fyfe, R.W., S.A. Temple and T.J. Cade. 1976. The 1975
North American Peregrine Falcon survey. Can. Field
Nat. 90:228-273.

Kiff, L.F. 1988. Changes in the status of the Peregrine
in North America: an overview. Pages 123-139 in T
Cade, J. Enderson, C. Thelander and C. White [Eds.],
Peregrine Falcon populations: their management and
recovery. The Peregrine Fund, Boise, ID U.S.A.

Massey, B.W. and E. Palacios. 1994. Avifauna of the wet-
lands of Baja California Mexico: current status. Studies
in Avian Biology 15:45-57.

Mora, M.A., D.W. Anderson and M.E. Mount. 1987.
Seasonal variation of body condition and organochlo-
rines in wild ducks from California and Mexico J.
Wildl. Manage. 51 (1):132— 141.

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

Porter, R.D., M.A. Jenkins, M.N. Kirven, D.W. Anderson
and J.O. Keith. 1988. Status and reproductive per-
formance of marine Peregrines in Baja California and
the Gulf of California, Mexico. Pages 105-114 in T.
Cade, J. Enderson, C. Thelander and C. White [Eds.],
Peregrine falcon populations: their management and
recovery. The Peregrine Fund, Boise, ID U.S.A.

Ratcliffe, D. 1993. The Peregrine Falcon. Academic
Press, San Diego, CA U.S.A.

Spitzer, P.R., R.W. Risebrough, J.W. Grier and C.R. Sin-
delar,Jr. 1977. Eggshell thickness-pollutant relation-
ships among North American Ospreys. Pages 13-19

Castellanos et al.

Vol. 31, No. 1

in J.C. Ogden [Ed.] Trans. North American Osprey
research conference. U.S. Nat. Park Serv. Proc. Ser. 2,
Washington, DC U.S.A.

Walton, B.J. and C.G. Thelander. 1988. Peregrine Fal-
con management efforts in California, Oregon, Wash-
ington and Nevada. Pages 587-597 in T. Cade, J. En-

derson, C. Thelander and C. White [Eds.], Peregrine
Falcon populations: their management and recovery.
The Peregrine Fund, Boise, ID U.S. A.

Received 28 February 1996; accepted 28 November 1996

J Raptor Res. 31(1):7-10

WINTER BALD EAGLE DISTRIBUTION IS INVERSELY
CORRELATED WITH HUMAN ACTIVITY ALONG THE

COLORADO RIVER, ARIZONA

Bryan T. Brown

SWCA, Inc., Environmental Consultants, 56 West 400 South, Suite 201,

Salt Lake City, UT 84101 U.S.A.

Lawrence E. Stevens

Applied Technology Associates, Inc., P.O. Box 22459,

Flagstaff, AZ 86002 U.S.A.

Abstract. — Helicopter surveys for Bald Eagles ( Haliaeetus leucocephalus) were conducted along the Col-
orado River through Glen Canyon National Recreation Area and Grand Canyon National Park, Arizona,
U.S.A., during winter 1990-91. Eagle abundance and distribution were examined for a possible corre-
lation with human activity levels as documented in National Park Service recreational use reports. Twen-
ty-two times more eagles were detected in river reaches with low human use compared to river reaches
with high to moderate human use. Eagle distribution did not correspond to prey abundance, biomass
patterns, or habitat conditions frequently associated with eagle foraging habitat. Moderate to high levels
of human activity may have been responsible for lower eagle abundance in some reaches of the river,
reinforcing the need for continued management of some areas as refugia where species sensitive to
human disturbance can be protected from higher levels of human activity.

Key Words: Bald Eagle, Colorado River, Arizona; Haliaeetus leucocephalus; human disturbance.

La distribution invierna de el Haliaeetus leucocephalus esta correlacionada inverso con actividad humana
enseguida del Rio Colorado, Arizona.

Resumen. — Inspecciones de helicoptero para Haliaeetus leucocephalus fueron conducidas a lo largo del
Rio Colorado por el area Nacional de Recreation del Glen Canyon y el Parque Nacional del Grand
Canyon, Arizona, U.S.A., durante el invierno de 1990-91. La abundancia y distribution del H. leucoce-
phalus fueron examinados por una posible correlation con niveles de actividad humana como fueron
documentado en reportes de el Servicio Nacional de Parques. Vieinte-dos veces mas fueron los H.
leucocephalus descubiertos en tramos del no con uso bajo de humanidad comparado con tramos del no
con uso alto y moderado. La distribution de H. leucocephalus no corresponds con la presa abundante,
patron de distribution de materia biologica, y condiciones de habitat frecuentemente asociada con
habitat de forraje de H. leucocephalus. Niveles altos a moderado de actividad humana puede ser res-
ponsable por la baja cantidad de H. leucocephalus en tramos de el rio, reforzando la necesidad para la
continuation de administration de unos area de refugio donde especie delicadas a molestos humanos
pueden ser protegidos de niveles alto de actividad humana.

[Traduction de Raul De La Garza, Jr.]

Human activities can influence Bald Eagle {Hal-
iaeetus leucocephalus ) behavior and distribution
(Stalmaster 1987). Numerous studies have shown
that eagle distribution and foraging behavior can
be detrimentally affected by unmanaged human
recreational activities such as hiking (Stalmaster
and Newman 1978), fishing (Knight et al. 1991,
Skagen et al. 1991), and boating (Knight and
Knight 1984, McGarigal et al. 1991), and by phys-
ical developments within eagle habitat (Buehler et

al. 1991). These aspects of eagle-human interac-
tion have been addressed primarily through two
types of studies: those directly quantifying eagle
“flush response distance” to different categories of
human activity (e.g. Stalmaster and Newman 1978,
McGarigal et al. 1991), and those indirectly linking
eagle distribution to secondary measures of human
activity such as housing density and development
set-back distance (e.g., Buehler et al. 1991).

The purpose of our study was to test the null

Brown and Stevens

Vol. 31, No. 1

Figure 1 . Map of the Colorado River study area in Glen
Canyon National Recreation Area and Grand Canyon
National Park, Arizona, showing River Reaches One
through Five.

hypothesis that winter distribution of Bald Eagles
along the Colorado River in Arizona was not cor-
related with indirect measures of human activity
obtained from National Park Service (NPS) recre-
ational use reports. We also considered physical
variables and prey differences that may influence
eagle distribution.

Study Area and Methods

Our study was conducted along the Colorado River
through Glen Canyon National Recreation Area
(GCNRA) and Grand Canyon National Park (GCNP) in
northern Arizona, from 2 km downstream of Glen Can-
yon Dam to the confluence of the Little Colorado River
(120.2 km; Fig. 1). The completion of Glen Canyon Dam
in 1963 altered the river’s physical and biological char-
acteristics, and a subsequent introduction of rainbow
trout ( Oncorhynchus mykiss ) benefited wintering Bald Ea-
gles (Brown et al. 1989, Brown and Stevens 1992, Brown
1993). A concentration of wintering eagles began to oc-
cur during the 1980s at Nankoweap Creek, 105 km down-
stream from the dam, when large numbers of spawning
trout were present (Brown et al. 1989).

The study area was divided into five river reaches based
on bedrock geology (modified after Schmidt and Graf
1990: Table 1). Reaches varied in geomorphic width, but
all were in deep canyons bounded by cliffs typically >300
m high. Reach One, from Glen Canyon Dam to the Paria
River, supported a productive rainbow trout fishery and
was heavily used in winter by bank and boat fishermen
from Lees Ferry. Boating activity upstream of Lees Ferry
consisted of motorboats used by fishermen; downstream
of Lees Ferry, motorized and nonmotorized rafts were on
multi-day trips through GCNP. Reach Two, from the Par-
ia River to Soap Creek Rapid, was moderately used by
bank fishermen and hikers that accessed it from Lees
Ferry or by several trails. The number of raft trips de-
parting downstream to pass through GCNP was limited

Table 1. Habitat features, relative human activity levels, and prey parameters potentially influencing winter Bald
Eagle distribution along the Colorado River in Glen and Grand canyons, Arizona.

Parameter

River Reach

Length (km)

22.6

16.7

18.5

28.2

34.2

Surface 3 (ha/km)

12.6

9.3

5.3

5.7

9.2

% riffle /rapid 3

5.7

4.3

13.9

15.8

11.4

Boats/mo b

671

Fishermen / mo c

570

350

Activity level

High

Moderate

Low

Fish d (kg/ha)

190

Water fowl c

6344

157

101

a Unpubl. data, U.S. Bureau of Reclamation.
b Data provided by NPS; Reach 2 estimated.
c Data provided by NPS; Reaches 2-5 estimated.

d Unpubl. biomass data, 1990-1994, from R.A. Valdez and R.J. Ryel, BIO/WEST, Inc., Salt Lake City, UT, U.S.A.
e Number winter waterfowl detected/km 2 /hr of observation (Unpubl. data).

March 1997

Eagle Distribution and Human Activity

Table 2. Numbers of Bald Eagles detected and expected by river reach in winter 1990-1991 along the Colorado
River in Glen and Grand canyons, Arizona. Numbers/reach are presented two ways: the entire study period and that
time when no spawning trout were in Nankoweap Creek in Reach Five.

River Reach

Parameter

Entire study period

No. detected

No. expected

28.5

19.0

22.6

30.3

42.6

Detections/km

0.13

0.18

1.95

2.06

1.26

No spawning trout

No. detected

No. expected

19.3

12.9

15.3

20.6

28.9

Detections/km

0.13

0.12

1.08

1.45

0.91

by NPS regulations. The community of Marble Canyon
was immediately adjacent to the river where Navajo
Bridge on U.S. Highway 89 crossed Reach Two. Reaches
Three, Four, and Five extended from Soap Creek Rapid
to the Little Colorado River and were remote, far from
roads, and accessed only by a few foot-trails, with the
number of bank fishermen and hikers limited by NPS
regulations. Nankoweap Creek was located in Reach Five.

Fifteen helicopter surveys of the study area were con-
ducted, 1/wk from November 1990-March 1991, to de-
termine eagle abundance and distribution. A special re-
search permit was obtained from NPS for helicopter use
because of the potential for helicopters to disturb park
resources and visitors. Most surveys occurred before 1200
H. Air speed was approximately 90 km/hr at a height of
100 m above river level on a flight path directly over the
river. Surveys were initiated from both ends of the study
area. The pilot and front-seat observer detected eagles; a
rear-seat assistant recorded observations and localities.
Numbers of eagles detected were summarized by reach
and compared to expected numbers of eagles/reach us-
ing a Chi-square goodness-of-fit test, with statistical sig-
nificance accepted at P < 0.05. Expected numbers of ea-
gles/reach were derived by multiplying the total number
of eagles detected by the proportion of total river-km/
reach.

Winter boat launches/mo and bank fishermen/ mo for
Reach One from November 1990-March 1991 were ob-
tained from recreational records maintained by NPS at
GCNRA; monthly totals were averaged to obtain mean
use. Winter raft launches/mo for Reach Two were ob-
tained from records maintained by NPS at GCNP. Reach-
es Two through Five had almost identical levels of boat-
ing activity since all raft trips launching at Lees Ferry
must pass through them; Reach Two received slightly
more overall boating use because of motorboat activity
by NPS maintenance and patrol trips. Mean numbers of
bank fishermen/mo for Reaches Two through Five were
estimated based on interviews with NPS personnel and
our personal observations.

Results and Discussion

Twenty-two times more eagles were detected in
Reaches Three through Five than in Reaches One
and Two when total numbers of eagles detected
were compared (Table 2; x 2 ~ 44.0, df = 4, P <
0.001); we found no difference in the number of
eagles detected among Reaches Three through
Five (x 2 = 5.4, df = 2, P = 0.07). Total numbers
of eagles detected/km in Reaches Three through
Five were more than 10 times greater than num-
bers of eagles detected in Reaches One and Two
(Table 2).

Eagle distribution did not correspond to esti-
mates of prey distribution. Fewer eagles were ob-
served in Reaches One and Two despite biomass
indices that indicated fish and winter waterfowl
abundance to be greater in Reaches One and Two
(Table 1 ) . Reaches One and Two were wider with
many pools with shallow margins all along the riv-
er, They also had a smaller percentage of riffle and
rapid habitats making them more suitable for ea-
gles as foraging areas (Hunt et al. 1992).

It appeared that negative effects due to moder-
ate to high levels of human activity in these two
reaches of the river may have reduced their suit-
ability as eagle foraging areas or perhaps disturbed
eagles from perching and roosting habitat. We did
not find any eagles within 1 km of intensively used
areas near Lees Ferry and Navajo Bridge at Marble
Canyon. This negative correlation between human
activity and Bald Eagle distribution was consistent
with that reported in other studies (Buehler et al.
1991, McGarigal et al. 1991) , although reasons why
eagles avoid areas of higher human activity remain

Brown and Stevens

Vol. 31, No. 1

unknown (Buehler et al. 1991). Repeated flushing
by bank fishermen, hikers, or boats could have
caused wintering eagles to avoid those reaches
heavily used by anglers. McGarigal et al. (1991) re-
ported a similar finding on the Columbia River
where eagles avoided areas with heavy boat traffic.
Although apparent habituation of Bald Eagles to
human activity has been reported (Knight and
Knight 1984), scarcity of eagles in Reaches One
and Two of the Colorado River suggested that ha-
bituation did not occur in this area.

An alternative hypothesis was that more eagles
occurred in Reaches Three through Five because
they were attracted to easily accessible spawning
trout in Nankoweap Creek (Reach Five) . Abundant
trout were spawning at Nankoweap from 15 Feb-
ruary until the end of the study period (Brown and
Stevens 1992). To test this hypothesis, we contrast-
ed numbers of eagles detected/reach when spawn-
ing trout were and were not present at Nankoweap.
Prior to 15 February, total numbers of eagles de-
tected in Reaches Three through Five were 18
times greater than in Reaches One and Two (Table
2; x 2 ~ 27.3, df = 4, P < 0.001). Therefore, we
rejected the alternative hypothesis.

Managed boating, hiking, and fishing activities
are an established use of national parklands. How-
ever, we found an inverse correlation between win-
tering Bald Eagle distribution and human activity
along the Colorado River within GCNRA and
GCNP. Although correlation does not prove cau-
sation, we suggest human activity as the most likely
cause. This would represent an apparent contra-
diction of NPS goals to preserve natural resources
for future generations. A tradeoff of resources for
recreation may be acceptable if the status quo can
be maintained in Reaches Three through Five of
our study area. However, the growing popularity of
national parks such as GCNP increases economic
and political pressures to provide more wilderness
recreation opportunities (Frome 1992), and may
result in long-term increases in human activity lev-
els in Reaches Three through Five. Our findings
reinforce the need to continue to manage some
areas as refugia where species potentially sensitive
to human disturbance, such as Bald Eagles, are not
excluded or influenced by higher levels of human
activity.

Acknowledgments

This study was funded by the Glen Canyon Environ-
mental Studies (GCES), U.S. Bureau of Reclamation

(USBR), as part of a study to determine the influence of
fluctuating flows from Glen Canyon Dam on wintering
Bald Eagles. We thank D. Wegner and the staff of GCES
for support, and GCNRA and GCNP (particularly S.
Cherry of the GCNP river office) for providing recrea-
tional activity records. N. Kline, M. Sogge, T. Yates and
others conducted or assisted with helicopter surveys. We
thank pilots B. Barnes, B. Caldewood, S. Chubbuck and
M. Santee of USBR. K. Buck and M. Yard of GCES, and
E. Pemberton and T. Randle of USBR provided unpub-
lished data on river surface area and percent riffle/rapid
habitat. R. Valdez and R. Ryel provided unpublished data
on fish biomass. Drafts of this manuscript were reviewed
and improved by G. Bortolotti, R. Glinski, T. Grubb, R.
McClelland and C. White.

Literature Cited

Brown, B.T., R. Mesta, L.E. Stevens, and J. Weisheit.
1989, Changes in winter distribution of Bald Eagles
along the Colorado River in Grand Canyon, Arizona,
/. Raptor Res. 23:110-113.

AND L.E. Stevens. 1992. Winter abundance, age

structure, and distribution of Bald Eagles along the
Colorado River, Arizona. Southwest. Nat. 37:404—408.

. 1993. Winter foraging ecology of Bald Eagles in

Arizona. Condor 95:132-138.

Buehler, D.A., TJ. Mersmann, J.D. Fraser and J.K.D.
Seegar. 1991. Effects of human activity on Bald Eagle
distribution on the northern Chesapeake Bay./. Wildl.
Manage. 55:282-290.

Frome, M. 1992. Regreening the national parks. Univ.

of Ar izona Press, Tucson, AZ U.S.A.

Hunt, W.G., J.M. Jenkins, R.E. Jackman, C.G. Thelander
and A.T. Gerstell. 1992. Foraging ecology of Bald
Eagles on a regulated river./. Raptor Res. 26:243-256.
Knight, R.L. and S.K. Knight. 1984. Responses of win-
tering Bald Eagles to boating activity. /. Wildl. Man-
age. 48 :999— 1 004,

, D.P. Anderson and N.V. Marr. 1991. Responses

of an avian scavenging guild to anglers. Biol. Conserv.
56:195-205.

McGarigal, KL, R.G. Anthony .and F.B. Isaacs. 1991. In-
teractions of humans and Bald Eagles on the Colum-
bia River estuary. Wildl. Monogr. 115:1-47.

Schmidt, J.C. andJ.B. Graf. 1990. Aggradation and deg-
radation of alluvial-sand deposits, 1965 to 1986, Col-
orado River, Grand Canyon National Park, Arizona.
U.S. Geol. Survey Prof. Pap. 1493, U.S. Govt. Print.
Off., Washington, DC U.S. A.

Skagen, S.K., R.L. Knight and G.H. Orians. 1991. Hu-
man disturbance of an avian scavenging guild. Ecolog-
ical Applications 1:215-225.

Stalmaster, M.V. 1987. The Bald Eagle. Universe Press,
New York, NYU.S.A.

and J.R. Newman. 1978. Behavioral responses of

wintering Bald Eagles to human activity. / Wildl. Man-
age. 42:506-513.

Received 1 April 1996; accepted 21 October 1996

J. Raptor Res. 31 (1) :1 1 — 15
© 1997 The Raptor Research Foundation, Inc.

POPULATION STATUS OF THE ENDANGERED

HAWAIIAN HAWK

Linnea S. Hall and Michael L. Morrison

Department of Biological Sciences , California State University, Sacramento, CA 95819 U.S.A.

Peter H. Bloom

Western Foundation of Vertebrate Zoology, 439 Calk San Pablo, Camarillo, CA 93010 U.S.A.

Abstract. — We assessed the current abundance and distribution of Hawaiian Hawks (‘io; Buteo solitarius)
on the island of Hawaii to determine if this federally endangered bird should be downlisted to threat-
ened status. We found a density of 0.004 hawks/ha on the island. Using an estimate of 400 000 ha of
suitable ‘io habitat on Hawaii, we estimated a total of 1600 hawks (1120 adults; 560 pairs) on Hawaii,
Based on the wide distribution of ‘io among vegetation types on the island and litde apparent change
in numbers during the past decade, we agreed with the recommendation for downlisting the hawk but
suggested that researchers collect long-term demographic data to better understand the status of this
species.

Key Words: Buteo solitarius, Hawaiian hawk, ‘io, population status.

El estado de poblacion del Buteo solitaruis en peligro de extincion.

Resumen. — Nosotros fijamos la cantidad corriente y distribucion de Buteo solitarius en la isla de Hawaii
para determinar si el pajaro en peligro de extincion por leyes federales debe ser reducido a estado
amenazado. Nosotros los encontramos una densidad de 0.004 halcon/ha en la isla. Usando la estimacion
de 400 000 ha de habitat conveniente en Hawaii, nosotros estimamos un total de 1600 halcones (1120
adultos; 560 parejas) en Hawaii. En base de la distribucion amplia de B. solitarius entre clases de vege-
tacion en la isla y poco cambio aparente en la cantidad durante la decada pasada, nosotros estamos de
acuerdo con la recomendacion para reducir el halcon pero sugerimos que los investigadores junten
datos demograficos de larga duracion para poder entender el estado de este especie mejor.

The Hawaiian Hawk ( Buteo solitarius), or ‘io, was
federally listed as an Endangered Species in 1967
(37 FR 4001, 11 March 1967) based on its restrict-
ed range on the island of Hawaii (hereafter Ha-
waii), its low numbers at the time of listing (Berger
1981), and the perceived threats to its preferred
habitat from agricultural and commercial devel-
opments (U.S. Fish and Wildlife Service [USFWS]
1984) . At the time of listing, no intensive study of
the ecology of the ‘io had ever been conducted,
and anecdotal accounts gave differing reports on
its abundance across the island (Munro 1944,
USFWS 1984).

Uncertainty over ‘io abundance continued
through the next decade. An intensive survey ini-
tiated by the USFWS in 1976 on Hawaiian forest
birds was unable to estimate the ‘io population size
(Scott et al. 1986). After a detailed study of ‘io
breeding biology in <1% of the island’s area, Grif-

[Traduccion de Raul De La Garza, Jr.]

fin (1985) found that the species might be rela-
tively unaffected by habitat modifications com-
pared to many other native bird species after find-
ing that foraging and nesting occurred in agricul-
tural areas and in stands of exotic vegetation
(Baskett and Griffin 1985). Griffin (1985, 1989) es-
timated the population at 900 breeding pairs and
a total of 2700 individuals in 1983. Because of this,
the USFWS proposed downlisting the ‘io from en-
dangered to threatened status (58 FR 41684, 4 Au-
gust 1993). Because of questions over the validity
of basing such a reclassification on 10-year-old
data, the USFWS requested that an island-wide sur-
vey be conducted of the ‘io population to obtain a
more current estimate of the population size.

Herein, we present our survey results and sam-
pling design to provide a baseline for future sur-
veys designed to monitor the size of the ‘io popu-
lation on Hawaii.

Hall et al.

Vol. 31, No. 1

Study Area and Methods

The most efficient way to sample dominant vegetation
types across Hawaii for the occurrence of ‘io was to con-
duct unlimited distance point counts (Blondel et al.
1981) along paved and dirt roads across the island. Point
counts were selected to make our methods generally
comparable to those of Scott et al. (1986), who conduct-
ed the most complete previous census of ‘io on Hawaii
as part of the USFWS’s Hawaiian Forest Bird Survey
(1976-79). We also needed a method that could sample
the ‘io’s use of vegetation ranging from lowland agricul-
tural areas to subalpine woodlands (Scott et al. 1986,
Griffin 1989), and would be applicable to birds with
home range sizes varying from 48 ha in agricultural areas
to 490 ha in forests and mid-elevation pasturelands (Bas-
kett and Griffin 1985). Use of roadways was the only fea-
sible means of satisfying these objectives. Some studies
have indicated that roadside counts can give biased esti-
mates of bird densities and vegetation associations, but
other studies have indicated that road counts can be use-
ful and appropriate when large areas need to be sampled
and monitored long term (Fuller and Mosher 1981).

When possible, we used roads that crisscrossed an area
to more thoroughly sample for ‘io. We conducted point
counts 0.1-16 km off main roads to ensure that traffic
noise did not interfere with the counts and that we more
adequately sampled vegetation that could contain ‘io.
Count stations were located disproportionately among
vegetation types (Table 1), based on information that ‘io
were unlikely (or very uncommon) in shrublands (vege-
tation type 10), upper-elevation mamane-naio ( Sophora
chrysophylla-Myoporum sandwicense) woodlands (vegetation
type 12), and exotic pioneering lava vegetation (vegeta-
tion type 5) (J. Jeffrey andj. Giffin pers. comm.).

All count stations were 0.8-3. 2 km apart, and counts
were conducted by 1-2 observers between 0900-1700 H.
Each point count lasted for exactly 10 min, which was the
same count length used by Scott et al. (1986), and in-
cluded 8 min of listening and watching for hawks, plus 2
mm of playback of taped adult territorial and fledgling
calls of ‘io. After the first minute of the tape elapsed, we
turned it off and observed the area for any hawks for 7
min. Anytime an ‘io responded to the tape, either by
calling or flying to the point, we immediately stopped the
tape, but continued the count to determine if any addi-
tional hawks were observed. We then played the tape
again for 1 min, and watched for the last minute of the
count.

Although surveys have not previously used broadcast
calls, Banko (1980) and Baskett and Griffin (1985) re-
ported that ‘io call and defend their territories in the
winter. On 12-13 December 1993, we tested if broad-
casted territorial calls elicited responses from ‘io by going
to areas known to have ‘io present (J. Jeffrey pers.
comm.). We watched ‘io that were ^200 m away while
we played the taped calls. Eighty percent of the hawks
responded by taking flight, calling or coming to the tape.

No counts were conducted when precipitation exceed-
ed a light rain, or when wind exceeded 24 km/h. We
recorded the distance from the point of initial detection
of all observed hawks, the detection mode (visual, aural
or both), the morph (light, dark, unknown), and the veg-
etation where the bird was observed (Table 1 ) . Although

Table 1. Vegetation descriptions and codes for survey
transects used in analyses of Hawaiian Hawk numbers
across Hawaii in December 1993. a

Code

Fre-

quency 6

Description

Sugar cane fields with exotic and/ or
native trees or shrubs at edges, as
windrows.

Short or tall exotic trees with exotic
shrubs, and sometimes exotic grass-
es.

Macadamia nut or papaya orchard
with native and/or exotic trees or
shrubs at edges.

Grassland with scattered exotic
and/or native trees (especially
o’hia) ; scattered homes.

Pioneer exotic vegetation growing on
lava.

Native trees and native shrubs occa-
sionally with scattered orchard
trees, or exotic understory and
homes.

Mixed exotic and native trees, some-
times with mixed exotic and native
shrubs or grass.

Residential area with scattered exotic
and native vegetation.

Native tree and mixed exotic and na-
tive shrub vegetation on lava, some-
times with scattered homes; a
pioneer community.

Mixed exotic and native shrubs with
scattered native and exotic trees.

Native trees and grassland; non-pio-
neer community.

Mamane-naio vegetation, with grass
and/or exotic shrub understory, or
sometimes with scattered exotic
trees.

“Scientific names of plants listed: sugar cane ( Saccharum offici-
narum), macadamia nut ( Macadamia ternifolia), papaya (Carica pa-
paya), o’hia ( Metrosideros polymorpka), mamane ( Sophora chrysa-
phylla), and naio (Myoporum sandwicense).

b Frequency = total number of times the described vegetation
was recorded along survey routes.

we tried to determine age and sex of hawks in the field,
it was often difficult to make a positive identification, so
in our analyses we combined all sightings.

We used program DISTANCE (Buckland et al. 1993,
Laake et al. 1993) to estimate the densities of ‘io in the
12 major vegetation types recorded during our surveys

March 1997

Status of Hawaiian Hawk

Table 2. Summary of ’io density estimates (all ages together) by vegetation type, calculated by program DISTANCE
(Laake et al. 1993) from survey data across Hawaii, December 1993. a

Vegetation

CODE b

To-

tal

Ef-

fort

No.

Poi-

nts

No.

Obs-

VD.

Esti-

ma-

tor

Mod-

No/ 1

Esti-

mate

Density Estimations 6
%CV 95% Cl df

De-

tect.

Prob.

Enc.

Rate

Density BooTSTRAP f
Est. %CV Runs

All veg types

399

0.004

0.0007

15.9

0.003-0.006

345

0.02

0.24

0.004

23.7

400

Veg 1

0.002

0.0006

29.1

0.001-0.003

0.08

0.41

0.002

56.2

100

Veg 2

0.0004

0.0003

77.1

0.0001-0.002

0.11

0.12

— *

Veg 3

0.003

0.0016

58.8

0.0009-0.008

0.03

0.23

—

Veg 4

0.004

0.0011

29.1

0.002-0.006

0.04

0.42

—

Veg 5

Veg 6

0.003

0.0014

41.3

0.002-0.007

0.02

0.15

—

Veg 7

0.005

0.0013

26.6

0.003-0.008

118

0.02

0.22

0.005

67.6

100

Veg 8

0.009

0.0046

54.0

0.003-0.024

0.01

0.29

—

Veg 9

—

Veg 10

—

Veg 11

0.005

0.0024

50.5

0.002-0.013

0.03

0.44

—

Veg 12

—

a For explanation of DISTANCE program estimations, see text.
b For vegetation code explanation, see Table 1.

c Total effort = the sum of the number of times each point and its corresponding vegetation was sampled.

d Estimator Model No. = the mathematical estimator model selected by program DISTANCE to analyze the point data, where the
chosen model was the one that had the smallest Akaike’s Information Criterion value (AIC).

e Density estimations: Estimate = density in number of ’io/hectare; SE = standard error of the estimation; %CV = percent coefficient
of variation of the estimate; 95% Cl = 95% confidence interval for the estimate; df = degrees of freedom used in the analysis; Detect
Prob. = the estimate of average probability of detecting an ’io; Enc. rate = the number of animals expected to be observed per
point.

f Density Bootstrap values: Est. = bootstrapped density estimate; %CV = percent coefficient of variation of this estimate; Runs = total
number of bootstrap runs conducted.

s “ — ” = too few degrees of freedom to conduct bootstrap analyses.

h “ — ” = no or too few ’io observed along this survey route, so no density analysis could be performed.

(Table 1). Observations of ‘io were entered as the radial
distance to the hawk from the point. We truncated the
distances at 3000 m (the maximum distance at which
most ‘io were observed) to allow all hawk observations to
be entered into the analyses. We instructed the program
to select the most appropriate density estimation model
for each analysis, based on maximum likelihood ratio
tests of the models vs. each other. We also instructed the
program to conduct 400 bootstrap samples for the island-
wide data, to obtain reliable estimates of the variances
around the density estimates, and 100 bootstrap samples
for each of the analyses of density by vegetation type.

We estimated the current population size of the ‘io on
Hawaii based on the density of hawks per vegetation type,
and the estimated percent cover by each vegetation type
on the island of Hawaii (Jacobi and Scott 1985, Cuddihy
and Stone 1990).

Results and Discussion

We sampled 40 transects across Hawaii, with 399
points covering approximately 500 km of roads.
Among these points, 98 different ‘io were ob-

served. Thirty-three hawks were identified as
adults, 7 as immatures, and 58 as unknown-aged.
Forty-five hawks were light morph birds and 14
were dark morphs.

Densities ranged from a low of 0 in vegetation
types 5, 9, 10, and 12 to a high of 0.009 hawks/ha
in vegetation type 8 (Table 2). Most densities were
between 0.003 and 0.005 hawks/ha, with an overall
mean of 0.004 hawks/ha. Vegetation types 5 and 9
had lava as a major ground component, and thus
had poorly-developed tree cover. Type 10 vegeta-
tion was dominated by shrubs, and type 12 was in
mamane-naio woodland. Type 2 vegetation was typ-
ified by exotic trees of various sizes and had very
few hawks. Type 8 vegetation consisted of residen-
tial areas with both native and exotic tree compo-
nents and showed the highest hawk densities.
Grasslands with scattered exotic and native trees
(vegetation type 4) also had moderately-high den-

Hall et al.

Vol. 31, No. 1

sities of birds. Bootstrapped density estimates
matched the model estimates in all cases where ad-
equate degrees of freedom existed.

Surveys found ‘io most commonly in areas with
native and/or exotic tree cover, usually with un-
derstories of exotic grass, and sometimes with na-
tive and/or exotic shrub understories. Although
‘io were not found frequently in small patches of
mixed native and exotic forest surrounded by open
fields or orchards, they were commonly observed
over the open areas, or in open places, with scat-
tered native and/or exotic trees. For example, in
sugar cane fields with ribbons of native or exotic
trees between fields, or with trees extending down
from higher elevation forests; in open pasture land
with scattered native trees; in orchards (especially
macadamia nut) with taller native and/or exotic
trees at the perimeters. This indicated that ‘io are
now using areas that are not pure native forest.
Based on these data and anecdotal breeding rec-
ords from these more open areas, it appears that
they are also able to successfully breed there (J.
Jeffrey and J. Giffin unpubl. data).

Griffin (1985, 1989) estimated that the popula-
tion of ‘io on Hawaii was about 2700 hawks in
1983. Of this, 1800 were adults. This estimate
served as the basis for the Hawaiian Hawk Recov-
ery Plan developed by the USFWS (1984). It used
an abundance of 2,000 hawks (the midpoint be-
tween the 1500 and 2500 adult birds thought to be
needed for a self-sustaining population) as the tar-
get to downlist the species to threatened status.
The island-wide estimate of ‘io density was based
on a total forested area of 343 000 ha (J.M. Scott
pers. comm., Griffin 1989). This value correspond-
ed roughly to the potential ‘io habitat contained
within the Hawaiian Forest Bird Survey area (Ja-
cobi and Scott 1985) . Using this area, and the over-
all estimate of ‘io density from our surveys (0.004
birds/ha, 95% Confidence Interval [C.I.] = 0.003-
0.006), we obtained a total density of 1372 ‘io
(range = 1029—2058) on Hawaii. Much of the low-
land forested areas of Hawaii, including the sugar
cane, macadamia nut and other disturbed areas oc-
cupied by ‘io, were excluded from Griffin’s area
estimate. We therefore modified the Griffin esti-
mate by adding 60 000 ha of mixed sugar cane-
lowland forested area and various other minor veg-
etation types (Cuddihy and Stone 1990), bringing
the total potential ‘io habitat to 400 000 ha. This
raised our estimate of ‘io on the island to about

1600 birds (range = 1200-2400), with 1120 adults
or 560 pairs.

Our estimated density of adults (1120) is about
25% below the lower end of the target range nec-
essary for a stable ‘io population, according to the
Recovery Plan (target = 1500-2500 adult birds).
Assuming that all birds alive during our surveys
survived to breed, the total number of birds we
estimated (1600) is just above the lower end of the
target range, but is still below the mean target val-
ue of 2000. The target value of adults is not en-
closed in the confidence interval around the
0.004/ha value (95% C.I. = 0.003-0.006), but is
enclosed if we assume that all hawks alive breed
(400 000 ha x 0.006 - 2400 hawks).

We found a relatively high number of birds that
were widely distributed among vegetation types on
Hawaii, including heavily-disturbed areas. In addi-
tion, our results were similar to those found 10
years earlier by Griffin (1985, 1989), indicating the
likelihood of a relatively stable population during
the past decade. Thus, we concluded that down-
listing to threatened status was supported.

As other biologists have suggested for the ‘io,
long-term demographic studies are necessary to ac-
curately assess the overall status of the population
(USFWS 1984, Griffin 1989). Our fieldwork did
not assess population trends, reproductive fecun-
dity and success, dispersal or mortality, all of which
have been shown to be problematic for other for-
est birds on Hawaii (Scott et al. 1986). Thus, we
think that the USFWS should initiate a long-term
demographic study so future density estimations
can be evaluated in light of other population data.
Such a study is necessary before delisting from
threatened status is considered.

Acknowledgments

We thank S. Johnston at the USFWS Pacific Islands of-
fice in Honolulu, Hawaii; J. Jeffrey at the USFWS office
in Hilo; L. Katahira at the NPS office at Hawaii Volcanoes
National Park; M. Reynolds, J. Jacobi, and T. Pratt at the
National Biological Service office at Hawaii Volcanoes Na-
tional Park; J. Giffin at the Forestry and Wildlife office
in Kamuela; and all of the associated field assistants in
Hawaii for their assistance in establishing transects and
conducting surveys. We also thank reviewers of earlier
reports and manuscript drafts, including C.R. Griffin,
D.E. Anderson, J.M. Scott, F. Duvall II, S. Fancy, and B.
Harper for significantly improving our analyses and con-
clusions. This research was funded by Cooperative Agree-
ment No, 1448-0001-93676 between the USFW 7 S and the
Western Foundation of Vertebrate Zoology.

March 1997

Status of Hawaiian Hawk

Literature Cited

Banko, W.E. 1980. Part I. Population histories-species
accounts. Forest birds: Hawaiian Hawk ( l io). Natl.
Park Serv. CPSU/UH Avian Rep. 6A, History of En-
demic Hawaiian Birds. Hawaii Volcanoes National
Park, HI U.S.A.

Baskett, T.S. and C.R. Griffin. 1985. Final project re-
port on the biology of the endangered Hawaiian
Hawk: ecology, life history, and environmental pollu-
tion problems. Missouri Coop. Wildl. Res. Unit, Univ.
Missouri, Columbia, MO U.S.A.

Berger, A.J. 1981. Hawaiian birdlife. Univ. Hawaii Press,
Honolulu, HI U.S.A.

Blondel, J., C. Ferry and B. Frochot. 1981. Point
counts with unlimited distance. Pages 414-420 in C.J.
Ralph and J.M. Scott [Eds.], Estimating the numbers
of terrestrial birds. Stud. Avian Biol No. 6.

Buckland, S.T., D.R. Anderson, K.P. Anderson, K.P.
Burnham and J.L. Laake. 1993. Distance sampling:
estimating abundance of biological populations.
Chapman and Hall, London, UK.

Cuddihy, L.W. and C.P. Stone. 1990. Alteration of native
Hawaiian vegetation: effects of humans, their activities
and introductions. Coop. Nad. Park Resour. Stud.
Unit, Univ. Hawaii, Honolulu, HI U.S.A.

Fuller, M.R. and J.A. Mosher. 1981. Methods of de-
tecting and counting raptors: a review. Pages 235-246
in C.J. Ralph and J.M. Scott [Eds.], Estimating the
numbers of terrestrial birds. Stud. Avian Biol No. 6.

Griffin, C.R. 1985. Biology of the Hawaiian Hawk ( Buteo
solitarius). Ph.D. dissertation, Univ. Missouri, Colum-
bia, MO U.S.A.

. 1989. Raptors in the Hawaiian islands. Pages

155-160 in K. Steenhof, M.N. Kochert and M.N.
LeFranc Jr. [Eds.], Proc. Western Raptor Manage.
Symp. Workshop. Natl. Wildl. Fed, Sci. Tech. Ser. No.
12, Washington, DC U.S.A.

Jacobi, J.D. and J.M. Scott. 1985. An assessment of the
current status of native upland habitats and associated
endangered species on the island of Hawaii. Pages 3—
22 in C.P. Stone and J.M. Scott [Eds.], Hawaii’s ter-
restrial ecosystems: preservation and management.
Coop. Nad. Park Resour. Stud. Unit, Univ. Hawaii,
Honolulu, HI U.S.A.

Laake, J.L., S.T. Buckland, D.R. Anderson and KP.
Burnham. 1993. Distance user’s guide, version 2.0.
Colorado Coop. Fish and Wildl. Res. Unit, Colorado
State Univ., Fort Collins, CO U.S.A.

Munro, G.C. 1944, Birds of Hawaii. Tongg Publ. Co.,
Honolulu, HI U.S.A.

Scott, J.M., S. Mountainspring, F.L. Ramsey and C.B.
Kepler. 1986. Forest bird communities of the Ha-
waiian islands: their dynamics, ecology, and conser-
vation. Stud. Avian Biol. No. 9.

U.S. Fish and Wildlife Service. 1984. Hawaiian Hawk
recovery plan. U.S. Fish and Wildl. Serv., Pordand,
OR U.S.A.

Received 15 December 1995; accepted 24 October 1996.

J. Raptor Res. 31 (1) :16— 25
© 1997 The Raptor Research Foundation, Inc.

NEST-SITE SELECTION BY FOUR SYMPATRIC FOREST
RAPTORS IN SOUTHERN NORWAY

VlDAR SELAS

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

P.O. Box 5014, N-1432 As, Norway

Abstract. — Differences between 0.1 ha nest-site plots of Honey Buzzards ( Pernis apivorus), European
Sparrowhawks ( Accipiter nisus), Northern Goshawks (A. gentilis ) and Common Buzzards ( Buteo buteo )
were compared to randomly sampled 0.1 ha control plots within a 400 km 2 area with 80% forest and
<2% agricultural land in southern Norway. At Honey Buzzard nest sites, forests were more productive
than in control plots and there was a higher proportion of spruce, older trees and a higher tree density
at Northern Goshawk nest sites than in control plots. Nests of European Sparrowhawks were also in
sites with higher tree density than expected. Common Buzzard nest sites were situated in steeper terrain
than control plots and more often had a southern aspect. For sparrowhawks, nesting in forests with
high tree density may be an adaptation to avoid goshawks and pine martens ( Martes martes ) which are
their main nest predators. For the larger species, nest-site selection may be a response both to nest
predation risk, microclimate, foraging habitat and food supply.

Key WORDS: Honey Buzzard ; European Sparrowhawk; Common Buzzard ; Northern Goshawk; Accipiter nisus;
Accipiter gentilis; Buteo buteo; Pernis apivorus; forest, nest-site selection; Norway.

Seleccion del nido de cuatro rapaces de bosque sin que no aparean en el sur de Norway.

RESUMEN. — Diferencias entre 0.1 ha parcela de sitio de nido de Pernis apivorus, Accipter nisus, A. gentilis
y Buteo buteo fueron comparados con muestras alazar 0.1 ha parcelas manejadas dentro de una area de
400 km2 con 80% bosque y <2% tierra agricola en el sur de Norway. En nidos de Pernis apivorus, los
bosques fueron mas productivo en las parcelas manejadas y habia una proportion alta de Picea, arboles
maduros y densidad alto de arboles en nidos de A. nisus tambien estaban en sitios con densidad alta
de arboles mas de lo que esperabamos. Nidos de B. buteo estaban situados mas en terreno abrupto que
en parcelas manejadas y con frecuencia tenia aspecto del sur. Para A. nisus , nidos en el bosque con
densidad alta de arboles puede ser un adaptation para evitar A. gentilis y Martes martes que son su
principal depredador de nido. Para la especie mas grande, la seleccion del nido puede ser reaction a
riesgo de depredador al nido, microclima, habitat de forraje y suministro de comida.

[Traduction de Raul De La Garza, Jr.]

Breeding pairs of raptors use relatively large ar-
eas, and thus have a good opportunity to select
nesting places that maximize the probability of suc-
cessful breeding and lifetime reproduction (New-
ton 1979). Interspecific differences in nest-site se-
lection may be due to differences in body size and
flight performance of different species, but it can
also be due to interspecific differences in nest pre-
dation risk, climatic conditions during breeding
and feeding habits (Newton 1979, Janes 1985), or
to interspecific competition for nest sites and ter-
ritories (Newton 1979).

For several bird species, dense foliage close to
the nest both reduces the rate of detection, and
impedes the ability of predators to hunt in the vi-
cinity of the nest (Martin 1993). On the other

hand, dense foliage may decrease the possibility for
breeding birds to detect and escape from preda-
tors (Gotmark et al. 1995). Thus, selection of nest
site may be a trade-off between concealment and
opportunities to escape or attack predators, which
also depend on flight ability, body size or other
characteristics of the species. Selection may also be
affected by a trade-off between current and future
reproduction, since short-lived species with large
brood sizes have more to lose when nesting at-
tempts fail than long-lived species with smaller
brood sizes.

Cover may also be an important factor since it
can shield nests from wind or rain and limit ex-
cessive nocturnal radiation loss or excessive diur-
nal heat-gain from solar radiation (Walsberg 1985) .

March 1997

Nest-site Selection By Four Raptors

Protection from thermal extremes may be the most
important factor in nest-site selection by medium-
and large-sized raptors where nest predation is low
(Newton 1979, Janes 1985). At higher latitudes, the
timing of breeding in these birds should be im-
portant, since early breeders are faced with more
severe climatic conditions than those species which
begin nesting later in spring.

If prey are not evenly dispersed throughout the
landscape, raptors should select nest sites closest to
the best hunting areas in order to reduce time and
energy connected with foraging. Thus, local varia-
tion in the availability of food may influence the
nest-site selection (Janes 1985), and explain inter-
specific differences in nesting habitat.

In Fennoscandia, four raptor species hunt and
nest in forest-dominated landscapes. The Europe-
an Sparrowhawk ( Accipiter nisus ; mean body mass
male 150 g, female 260 g) is the main predator on
small birds (Sulkava 1964a, Selas 1993) , while the
Northern Goshawk (A. gentilis; mean body mass
male 870 g, female 1330 g) primarily feeds upon
larger bird species and mammals (Hoglund 1964,
Sulkava 1964b, Widen 1987, Selas 1989). The Com-
mon Buzzard ( Buteo buteo; mean body mass male
740 g, female 1100 g) is a generalist predator that
responds functionally to changes in populations of
its vole ( Microtus spp.) prey (Suomus 1952, Spids0
and Selas 1988), while the Honey Buzzard ( Pernis
apivorus; mean body mass male 750 g, female 910
g) mainly feeds on the larvae and pupae of social
hymenoptera species (Holstein 1944, Flagen and
Bakke 1958, Itamies and Mikkola 1972).

Several authors have described nest sites used by
sparrowhawks (Tinbergen 1946, Holstein 1950,
Hald-Mortensen 1974), goshawks (Holstein 1942,
Dietzen 1978, Link 1986), Common Buzzards
(Holstein 1956, Knuwer and Loske 1980, Solonen
1982, Jedrzejewski et al. 1988, Hubert 1993) and
Honey Buzzards (Holstein 1944, Amcoff et al.
1994) in Europe. However, no one has compared
nest-site selection of sympatric populations of these
species in a continuous forest habitat. My aim was
to study the importance of different habitat vari-
ables on nest-site selection of these species by com-
paring habitat variables from plots at nest sites with
those from plots placed randomly in the study
area.

Study Area and Methods

The study was conducted from 1985-93 in southern
Norway (58° 43'N, 8°44'E). The study area covers about

400 km 2 and is situated 50—300 m a.s.l. and 10—30 km
inland from the coast, in the boreo-nemoral zone (Abra-
hamsen et al. 1977). The climate is suboceanic, and snow
usually covers the ground from December-April. The
study area is hilly and sharply undulating. It is dominated
by forests (80%), with scattered lakes (10%), bogs (5%)
and less than 2% agricultural land. Forests are character-
ized by a fine-grained mosaic of young-, medium- and
old-aged coniferous, mixed and deciduous stands, with
Scots pine ( Pinus sylvestris), Norway spruce ( Picea abies),
oak ( Quercus spp.), aspen ( Populus tremula) and birch
( Betula spp.) the dominant tree species.

Forestry based on clear cutting, replanting and thin-
ning of the regrowth was introduced to the area in the
1950s. At the time of my study, approximately 30% of the
area had been clear-cut, with most regeneration <20 yr.
The area is divided into numerous small ownerships with
management of forests controlled by each of the land
owners. Most of the properties are managed to provide
a mosaic of forest types. Thus, there is a heterogeneous
environment on a small scale, but a homogeneous, frag-
mented environment on a large scale.

The study area was searched for nest sites each year
(cf. Forsman and Solonen 1984), and habitat variables
were described at one nest site in each nesting territory
located. If possible, the nest site used in 1988 was select-
ed. In territories where the 1988 nest was not found, I
usually described the nest site used in 1989. Alternatively,
the nest site used closest in time to 1988 or 1989 was
described. The breeding density of goshawks increased
during the time of the study. To get a larger number of
nest sites of this species, I first selected one nest site from
each of the nesting territories used since 1985. Then, I
selected one nest site in each of the 11 new nesting ter-
ritories established during 1986-88, even though these
territories substituted five of the existing ones. Since the
goshawks in the five old territories had all been illegally
shot by game keepers, I regarded the data to be inde-
pendent. Thus, I described a total of 48 nest site plots of
the sparrowhawk, 30 of the goshawk, 50 of the Common
Buzzard, and 21 of the Honey Buzzard.

Control plots were described during 1989. Aerial pho-
tographs of the study area taken in spring 1989 (scale 1:
15000) were covered by a grid with 100 numbered inter-
sections of which two were randomly selected as control
plots. Out of 122 selected points, 80 (65.6%) were locat-
ed in forests >20 yr old and 25 (20.5%) were in forests
<20 yr old (clear-cuts and regrowth), while 9 (7.4%)
were on lakes, 4 (3.3%) on bogs, and 4 (3.3%) on agri-
cultural land or developed areas. Measurements were
made only in control plots in habitats apparently suitable
for raptors (i.e., forests >20 yr old, N = 80).

Each of the nest site plots and the control plots cov-
ered 0.1 ha within a circle with a radius of 17.8 m. In
nest site plots, the nest was in the center of the circle.
The following habitat variables were used:

1) Site type, determined from the plant community
(Kielland-Lund 1981, 1994). Plots dominated by Bar-
bilophozio-Pinetum or Vaccinio-Pinetum were classified as
sites with poor productivity, plots dominated by Len-
cobryo-Pinetum, Eu-Piceetum myrtilletosum, or Populo-Quer-
cetum, were classified as sites with intermediate pro-
ductivity, and plots dominated by Melico-Piceetum typi-

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Vol. 31, No. 1

Table 1. Test results (upper, right) and P-values (lower, left) of correlation analyses of habitat variables from ran-
domly-sampled control plots ( N = 80). Categorical variables were tested against each other by use of contingency
table analysis (x 2 value given) and against continuous variables by use of Mann-Whitney U-test (two categories, U-value
given) or Kruskal-Wallis test (more than two categories, H-value given) , while continuous variables were tested against
each other by use of Spearman rank correlation (correlation coefficient given) .

Site

Type

Forest

Type

Forest

Age

Tree

Density

Slope

Aspect

Altitude

Category

Site type (3 categories)

40.60

5.78

28.51

9.70

4.97

5.52

Forest type (5 categories)

<0.01*

1.79

18.12

22.17

11.25

17.30

Forest age (continuous)

0.06

0.77

-0.26

0.01

770.5

2.79

Tree density (continuous)

<0.01*

0.02*

0.13

775.5

7.67

Slope (continuous)

<0.01*

0.91

0.25

778.0

4.00

Aspect (2 categories)

0.08

0.02*

0.81

0.85

0.87

1.30

Altitude category
(3 categories)

0.24

0.03*

0.25

0.02*

0.14

0.52

* Statistically significant.

cum, Melico-Quercetum, Alno incanae-Prunetum padi or
Ulmo glabrae-Tilietum cordatae were classified as sites
with the highest productivity.

2) Forest type, defined according to % pine and spruce
trees with diameters >7 cm at breast height (DBH,
1.3 m above ground). Pine forest was >50% pine and
spruce with pine >67%. Mixed coniferous forest was
>50% pine and spruce with pine and spruce <67%.
Spruce forest was >50% pine and spruce with spruce
>67%. Mixed forest was 25-50% pine and spruce. De-
ciduous forest was <25% pine and spruce.

3) Forest age, defined as the mean age of four trees
judged to represent the age of all trees with DBH >7
cm. Ages were measured using an increment borer at
breast height.

4) Number of trees, regardless of species with DBH >7
cm.

Table 2. Results (P-values) from Likelihood-Ratio tests
in a logistic regression model with nest-site plots and ran-
domly-sampled control plots (N = 80) as responses, and
all habitat variables as explanatory variables. R 2 is the pro-
portion of variation that is explained by the logistic re-
gression model.

Sparrow- Common Honey

Habitat hawk Goshawk Buzzard Buzzard
Variables (N = 48) ( N = 30) (N = 50) (N = 21)

Site type

0.58

0.80

0.54

0.004*

Forest type

0.38

0.023*

0.23

0.35

Forest age

0.94

0.027*

0.39

0.50

Tree density

<0.001*

0.013*

0.15

0.10

Slope

0.82

0.98

<0.001*

0.31

Aspect

0.92

0.06

0.037*

0.05

Altitude category

0.53

0.47

0.20

0.59

R 2

0.90

0.27

0.34

0.47

5) Slope, measured from 0-100°,

6) Aspect, defined as one of two categories: north (1-
100°, 301-400°) or south (101-300°). Nest-site plots
and control plots with slopes <5° were omitted.

7) Altitude, defined as three altitude possible categories
in relation to the altitude variation within a radius of
1 km from the plot. Plots were assigned to the lower
altitude zone if situated in the lower third of the al-
titude difference between the lowest and highest
point within this area. Middle and upper altitude
zones were assigned correspondingly.

When considering the randomly-sampled control plots,
several of the habitat variables were highly correlated
(Table 1). To control for the effect of these correlations
when comparing nest site plots and control plots, I used
likelihood-ratio tests (SAS 1994) in a logistic regression
model, with nest-site plots and control plots as responses
and all habitat variables as explanatory variables (cf. Man-
ly et al. 1993).

Results

Site Type and Forest Type. Of sparrowhawk nest-
site plots, none were on sites with poor productiv-
ity, 66.7% were on intermediate sites and 33.3% in
the highest productivity sites. Corresponding val-
ues for goshawk nest-site plots were 16.7%, 73.3%
and 10.0%; for Common Buzzard 16.0%, 60.0%
and 24.0%, and for Honey Buzzard 0.0%, 57.1%
and 42.9% compared to 42.5%, 47.5% and 10% for
control plots. When controlling for effects of cor-
relations between all habitat variables, there was a
significant difference between Honey Buzzard
nest-site plots and control plots, while the other
species did not differ from the control plots (Like-
lihood-ratio tests, Table 2).

The distribution of goshawk nest-site plots in dif-
ferent forest types differed significantly from that

* Statistically significant.

March 1997

Nest-site Selection By Four Raptors

Figure 1. Distribution of nest-site plots of European Sparrowhawks, Northern Goshawks, Common Buzzards and
Honey Buzzards, and randomly-selected control plots on different forest types. Forest types were defined according
to the frequency of pine and spruce among all trees >7 cm in breast height (1.3 m above ground). The size of each
plot is 0.1 ha.

of control plots, with a higher proportion of nest
sites in spruce forests (Table 2, Fig. 1). Sparrow-
hawk, Common Buzzard and Honey Buzzard nest-
site plots did not differ from control plots with re-
spect to forest type when effects of correlations be-

tween habitat variables were adjusted for (Likeli-
hood-ratio tests, Table 2, Fig. 1).

Forest Age and Tree Density. The mean forest
age was 36.8 ± 18.5 (SD) yr in nest-site plots of
sparrowhawks, 99.3 ± 19.1 yr in those of goshawks,

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Vol. 31, No. 1

Figure 2. Forest age and number of trees (>7 cm in breast height) in nest-site plots of European Sparrowhawks,
Northern Goshawks, Common Buzzards and Honey Buzzards (solid squares), and in randomly-selected control plots
(open squares). The size of each plot is 0.1 ha.

98.5 ± 20.4 yr in those of Common Buzzards and
86.7 ± 28.3 yr in those of Honey Buzzards. When
using likelihood-ratio tests, only nest sites of gos-
hawks differed significantly from control plots,
where the mean forest age was 90.7 ± 29.0 yr (Ta-
ble 2, Fig. 2) .

The mean number of trees was 190.4 ± 47.7 in
nest-site plots of sparrowhawks, 84.9 ± 27.7 in
those of goshawks, 73.3 ± 19.9 in those of Com-
mon Buzzards and 86.4 ± 33.4 in those of Honey
Buzzards. The number of trees in nest-site plots of
sparrowhawks and goshawks was significantly high-
er than in control plots, where the mean number
of trees was 62.3 ± 26.3 (Likelihood-ratio tests, Ta-
ble 2, Fig. 2).

Topographical Variables. The mean slope was
8.6 ± 5.6° in nest- site plots of sparrowhawks, 15.1
± 9.1° in those of goshawks, 28.6 ± 13.3° in those
of Common Buzzards, 16.3 ± 7.2° in those of Hon-
ey Buzzard and 15.0 ± 10.5° in control plots. Only
nest sites of Common Buzzards differed signifi-
cantly from control plots (Likelihood-ratio tests,
Table 2, Fig. 3).

Sparrowhawk nest sites were on south-facing
slopes 34.1% of the time while 63,3%, 76.0%,
33.3% and 53.8% of goshawk, Common Buzzard,
Honey Buzzard, and control plots were on south-
facing slopes, respectively. Only nest sites of Com-
mon Buzzards were on south-facing significantly
more than control plots (Likelihood-ratio tests, Ta-
ble 2, Fig. 3).

None of the nest sites of the four raptor species
differed significantly from control plots in terms of
their altitude (Likelihood-ratio tests, Table 2, Fig.
4).

Discussion

Site Type and Forest Type. Only the Honey Buz-
zard showed a significant preference for nesting in
sites with the highest productivity. This finding
agreed with that of Amcoff et al. (1994). Unlike oth-
er raptor species, Honey Buzzard males do not pro-
vision females with food during the egg-laying and
incubation period (Holstein 1944), possibly because
their prey are too small to be profitably transported
to the nest. Because of small size of its prey, short

March 1997

Nest-site Selection By Four Raptors

Figure 3. Slope (0-100°) and aspect (NE, SE, SW, NW) of nest-site plots of European Sparrowhawks, Northern
Goshawks, Common Buzzards and Honey Buzzards. The distance from the origin reflects the slope while the direction
reflects the aspect of the plot. The size of each plot is 0.1 ha.

distances between nesting and foraging areas during
incubation may be especially valuable for this spe-
cies. Highly productive forests may be important for
Honey Buzzards because they support high densities
of juvenile passerines (Tiainen 1981, Helle 1985,
Stokland 1994) which appear to be important prey
in early stages of the breeding season (Amcoff et al.
1994). These forests also support high biomass of
invertebrates (Birkemoe 1993, Stokland 1994) on
which Honey Buzzards may also rely.

Preference for forest type was significant only for
the goshawk, which selected spruce forest for nest-
ing. This preference may be related to the larger
number of important winter and spring prey spe-
cies such as squirrels ( Sciurus vulgaris, Andren and
Delin 1994), Hazel Grouse ( Bonasa bonasia, Swen-
son and Angelstam 1993), and Capercaillie ( Tetrao

urogallus, Swenson and Angelstam 1993) in spruce
forest. However, preference for spruce may simply
be related to the fact that it gives the best cover
and thus the best protection against the main pred-
ator of the goshawk, the Eagle Owl ( Bubo bubo, Ut-
tendorfer 1952, Mikkola 1983).

Forest Age and Tree Density. Goshawk nest sites
were situated in older forests than control plots.
Old forest is an important hunting habitat for the
goshawk (Widen 1989) and it provides large trees
for nest building (Dietzen 1978, Anonymous 1989,
Siders and Kennedy 1996, Squires and Ruggiero
1996). Goshawk nests were also found in forests
with a higher tree density than control plots. Gos-
hawks may reduce the risk of predation by nesting
in dense forests, since Eagle Owls prefer to hunt
in open or semi-open landscapes (Mikkola 1983).

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Vol. 31, No. 1

100 -t

Lower zone

4 —*

"(0

1—

75 H

Sparrowhawk Goshawk Common Honey

n = 48 n = 30 Buzzard Buzzard

n = 50 n = 21

E!!!] Middle zone
1ZI Upper zone

Control Plots
n = 80

Figure 4. The distribution of nest-site plots of European Sparrowhawks, Northern Goshawks, Common Buzzards
and Honey Buzzards, and of randomly-selected control plots by altitude categories. A plot was assigned to the lower
altitude zone if situated in the lower third of the altitude difference between the lowest and highest point within 1
km from the plot, to the middle zone if situated in the middle third of this altitude difference, and to the upper
zone if situated in the higher third.

For the smallest species investigated, the spar-
rowhawk, the only variable that discriminated be-
tween nest-site plots and control plots was tree den-
sity. I obtained similar results after a thinning ex-
periment, where the reuse of nest stands in
thinned young forests was lower than of nest stands
in young forests not thinned (Selas 1996). Place-
ment of nests in dense forest could hardly be prof-
itable with respect to food of sparrowhawks, be-
cause the density of passerines is usually low here
(Haapanen 1965, 0degaard 1982, Glowacinski and
Weiner 1983, Helle 1985). Probably, predation is
the most important aspect in the nest-site selection
of sparrowhawks (Selas 1996), since its main pred-
ators, goshawks and pine martens ( Martes martes),
both prefer mature forest rather than young, dense
forest when hunting (Pulliainen 1984, Widen 1989,
Storch et al. 1990) . Actually, dense forest seems to
be less important as nesting habitat for the spar-
rowhawk when the goshawk is absent (Bomholt
1983, Newton 1986, T0mmeraas 1994). Pine mar-
tens will probably find raptor nests easy, because
of the smell from pellets and prey remains. Since
the pine marten is also known to remember dif-
ferent sites of food resources (Sonerud 1985), it is
likely to be familiar with most of the old raptor
nests within its home range. This may be one rea-
son for why sparrowhawks rarely use nests for two
successive years, unlike goshawks and Common
Buzzards which are probably less vulnerable to
pine marten predation due to their large size.

Goshawks and pine martens are also important
predators of Honey Buzzard nestlings (Kostrzewa
1991). The Honey Buzzard seems to prefer spruce,
which gives best cover, as nest trees (Amcoff et al.
1994). In contrast to the other species studied,
Honey Buzzards are usually silent when disturbed
by humans at the nest site (Holstein 1944, Hagen
1952, Kostrzewa 1985). Rather than selecting nest-
ing habitats to avoid nest predation, Honey Buz-
zards appear to behave as cryptically as possible at
the nest site, possibly because they are less efficient
than other raptors in defending their nests against
predators. In addition, low annual mortality and
low clutch size of the Honey Buzzard (Holstein
1944, Kostrzewa 1985, Tjernberg and Ryttman
1994) may make nest defense less profitable than
for Common Buzzards and goshawks.

Topographical Variables. The only species which
showed any preference for slope was the Common
Buzzard, which usually nested in steep terrain. Sim-
ilar results have been found for the Red-tailed
Hawk ( Buteo jamaicensis, Titus and Mosher 1981,
Speiser and Bosakowski 1988). Flight energetics
may be more favorable on steeper slopes for larger
soaring raptors like eagles and large Buteos (Speis-
er and Bosakowski 1988). It may however also be
important that these broad-winged species can best
escape, or attack, predators in this habitat. Even
though the Common Buzzard is able to rob prey
from the goshawk (Fischer 1980, Jorgensen 1983),
its breeding success has been found to be nega-

March 1997

Nest-site Selection By Four Raptors

tively correlated with the distance to goshawk nests
(Kostrzewa 1991). One reason for the difference
in nest-site selection between the Common Buz-
zard and goshawk may be that the goshawk, which
is better adapted for flight and foraging in dense
forest, is more dangerous to Common Buzzard in
dense forest.

Common Buzzards also preferred nest sites with
southern aspects. There was also a tendency for a
higher percentage of nest-site plots of goshawks to
have southern aspects than expected, while those
of Honey Buzzards tended to have northern as-
pects. Common Buzzards and goshawks start their
breeding nearly one month earlier than sparro-
whawks and more than one month earlier than
Honey Buzzards (Forsman and Solonen 1984), at
a time of the year when the temperatures may still
be far below freezing in southern Norway. Nests of
both species w T ere most often found at sites with a
southeastern aspect, which are the first heated by
the morning sun when nest building occurs (Hol-
stein 1942, 1956). Also in Alaska, goshawks have
been found to favor southern slopes (McGowan
1975), while in more temperate areas, southern ex-
posures are avoided (Dietzen 1978, Reynolds et al.
1982, Moore and Henny 1983, Link 1986, Speiser
and Bosakowski 1987). A similar pattern has been
observed for nest sites of Golden Eagles ( Aquila
chrysaetos, Mosher and White 1976, Pfaff 1993).

In Central Europe, Common Buzzards place
their nest near forest edges (Knuwer and Loske
1980, Spitzer 1980, Hubert 1993), probably be-
cause they hunt from perches in open areas or
from forest edges (Widen 1994). In my study area,
open areas were usually covered by snow when
Common Buzzards arrived to breed, making the
field vole ( Microtus agrestis ), which is the most im-
portant prey species in this habitat (Hansson 1978,
Spids0 and Selas 1988), nearly unavailable (Hans-
son 1982, Sonerud 1986). Early snow-free areas
available for vole hunting in the spring are found
on southfacing slopes and in steep terrain, where
Common Buzzard nests are usually found.

Goshawks, Common Buzzards and Honey Buz-
zards rarely used nest sites with southwestern as-
pect, possibly because too much sun may be harm-
ful to newly-hatched nesdings (c.f. Holstein 1942,
Hald-Mortensen 1974, Reynolds et al. 1982, Link
1986). Unless there is good shelter, as in the dense
young forests used by sparrowhawks, nest sites with
a southwestern aspect are probably unprofitable re-
gardless of when egg-laying begins.

The observed interspecific differences in nest-
site selection between the raptor species investigat-
ed may be explained by interspecific differences in
body size and flight performance, nest-predation
risk, time of breeding and feeding habits. The risk
of predation probably affects nest-site selection or
breeding habits of all these species, but mostly
sparrowhawks and Honey Buzzards which were
most vulnerable to nest predation. Common Buz-
zards, goshawks, and Honey Buzzards also showed
nest-site preferences which could be explained as
an adaptation to microclimate. For these three spe-
cies, nest-site selection could also be connected to
the availability of food in the early stage of the
breeding season. These species may have a broader
habitat choice and it is possible that factors other
than the habitat variables I selected for study may
have been of importance. This may have been es-
pecially true for the goshawk, which builds larger
nests than the other species and may be influenced
by characters directly connected to the nest tree.

Acknowledgments

I wish to thank I. Selas and K.O. Selas for assistance
with the field work, G.A. Sonerud for valuable advice dur-
ing the analyses of the data, and T. Bosakowski, S.M. Brai-
nerd, S. Dale, G.A. Sonerud and two anonymous referees
for constructive comments on the manuscript. The study
was supported by the Nansen Endowment.

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Received 18 June 1996; accepted 1 December 1996

f. Raptor Res. 31(l):26-33
© 1997 The Raptor Research Foundation, Inc.

DISTRIBUTION AND SPECIES RICHNESS OF A FOREST
RAPTOR COMMUNITY IN RELATION TO URBANIZATION

Thomas Bosakowski 1

Department of Biological Sciences, Rutgers University, Newark, NJ 07102 U.S.A.

Dwight G. Smith

Biology Department, Southern Connecticut State University, New Haven, CT 06515 U.S.A.

Abstract. — We studied the species richness and distribution of the forest raptor community in a New
Jersey watershed in relation to urbanization. Raptors were systematically surveyed using high volume
broadcasts of conspecific and heterospecific calls during the breeding season at a total of 81 survey
stations. Ten habitat variables relevant to urbanization were measured at each survey station using to-
pographic maps and aerial photographs. Results showed a community composed of 10 species of breed-
ing raptors. Buteo lineatus, Accipiter gentilis and Strix varia showed a significant avoidance of suburban
habitat, whereas B. jamaicensis and Bubo virginianus had a greater tendency to occupy such areas. Lowland
habitat was significantly selected by S. varia, B. lineatus and A. cooperii, a habitat usually most susceptible
to development in the study region. Raptor species richness showed a strong positive correlation (r =
0.79, P < 0.01) with wilderness area size. No wilderness area less than 1000 ha had more than four
raptor species while four to eight species were found in areas from 1000-8000 ha. Utilization of three
increasing size classes of wilderness areas showed increasing trends for B. lineatus, A. gentilis and S. varia,
and decreasing trends for B. jamaicensis and Bubo virginianus.

Key WORDS: forest raptors ; community; urbanization; forest fragmentation; wilderness', survey.

Distribucion y fertilidad especie de una comunidad bosque de rapaces en relacion a urbanizacion.

Resumen. — Nosotros estudiamos la fertilidad de la especie y distribucion de una comunidad de rapaces
de bosque en una linea divisoria de dos cuencas en New Jersey en relacion a urbanizacion, Los rapaces
fueron inspeccionados sistematicamente usando llamadas de conespecificos y heteroespecificos trans-
mitidas en alto volumen durante la temporada de cria en un total de 81 estaciones de inspeccion. Diez
variables de habitat pertinente al urbanizacion fueron medidas en cada estacion de inspeccion usando
mapas topograficos y fotografia aerea. Los resultados ensenaron una comunidad compuesta de 10 es-
pecie de rapaces en cria. Buteo lineatus, Accipiter gentilis, y Stix varia mostraron un aversion significante
al habitat suburbio, mientras B. jamaicensis y Bubo virginianus tuvieron una tendencia mayor para ocupar
tales areas. Habitat de tierra baja fue sensiblemente escogido por S. varia, B. lineatus y A. cooperii el
habitat por lo general mas susceptible para el desarrollo en el estudio de la region. La de la especie
riquesa de rapaces mostro una correlation fuerte y positiva ( r = 0.79, p < 0.01) con la area del tamano
del bosque. Ningun area de bosque menos de 1000 ha tuvo mas de cuatro especie de rapaces mientras
cuatro a ocho especie fueron encontrados en areas de 1000-8000 ha. La utilization de tres clases
aumentando y de diferente tamano de areas de bosque mostraron una tendencia de aumento para B.
lineatus, A. gentilis y S. varia y una tendencia decreciente para B. jamaicensis y Bubo virginianus.

[Traduction de Raul De La Garza, Jr.]

Raptors are secondary and tertiary consumers so
trophic theory suggests that they will be fewer in
abundance and lesser in diversity than other breed-
ing bird communities. Factors that tend to pro-
mote high species diversity in raptor communities

Present address: Beak Consultants, Inc., 12931 NE 126th
Place, Kirkland, WA 98052.

are high prey diversity and high habitat heteroge-
neity (White 1974). Conversely, forest fragmenta-
tion and reduced forest interior tend to reduce
raptor community diversity (Thiollay and Meyburg
1988). In Maryland, Robbins et al. (1989) exam-
ined the effect of forest area on breeding bird
communities and found at least one raptor (the
Red-shouldered Hawk, Buteo lineatus ) was impacted

March 1997

Forest Raptors and Urbanization

by habitat fragmentation and urbanization and
qualified to be categorized as an “area-sensitive
species.” Titus and Mosher (1981) examined nat-
ural habitat variables of four sympatxic hawk spe-
cies in Maryland, but urbanization and variables
associated with habitat development were not in-
cluded in their analysis. Nevertheless, Red-shoul-
dered Hawks have been shown to avoid nesting
near human dwellings (Bosakowski et al. 1992a)
and Northern Goshawks ( Accipiter gentilis) nest sig-
nificantly farther from paved roads and human
dwellings than randomly-selected sites (Bosakowski
and Speiser 1994).

In this study, we examined a wider variety of ur-
banization variables than previously published for
forest raptors and examined their association with-
in a diverse community of breeding raptors. Such
data could provide further insight into the pres-
ervation and management of viable forest ecosys-
tems for diverse raptor communities rather than
using a single species approach.

Study Area

The study was conducted in the Highlands Physio-
graphic Region (Braun 1950) which extends southwest
to northeast across the New York-New Jersey border. The
study area was part of the Pequannock Watershed, which
is owned and operated by the city of Newark, New Jersey.
The study area includes approximately 16 100 ha and
includes parts of Passaic, Morris and Sussex Counties.
This hilly, mostly forested terrain is part of the Eastern
Deciduous Forest Biome (Shelford 1963).

Nearly all of the Highlands forests have been previ-
ously cut or burned within the last 200 years (Ohmann
and Buell 1968, Russell 1981), resulting in largely second
growth forest dominated by oaks ( Quercus spp.) and oth-
er subclimax hardwood trees (Buell et al. 1966, Russell
1981). Older forest (>100 years of age) is rare and is
typically limited to small remnant stands surrounded by
younger forest.

The study area includes some of the last remaining
wilderness in the northern half of New Jersey. Residential
and commercial development is limited to about 10% of
the study area. The remaining land area is composed of
a mosaic of submature second growth and mature forest,
typically ranging from about 40-80 years of age. About
75% of the forest is deciduous habitat, about 20% con-
sists of hemlock-white pine ( Tsuga canadensis-Pinus stro-
bus) stands, and 5% is mature conifer plantations. Aquat-
ic habitats include five major reservoirs, several smaller
impoundments, beaver ( Castor canadensis) ponds, marsh-
es, shrub swamps and wooded swamps, the latter occur-
ring in many areas. The study area is periodically
thinned, but clear-cutting/burning is not permitted.
Since access to these forests is regulated by recreational
permits and motorized boats are not permitted on the
reservoirs, human disturbance is greatest directly adja-
cent to some suburban areas.

Methods

Sampling design. The spatial design of the survey sys-
tematically covered the Pequannock Watershed study
area with a grid pattern of 81 broadcasting stations,
spaced at approximately 1.2 km intervals. The spacing of
stations was not used to estimate population size, since
this investigation was aimed solely at determining occu-
pancy in different habitat types. The survey stations were
plotted on 15 min USGS quadrangle topographic maps
of the study area. Four surveys were conducted at each
of the 81 broadcasting stations. Surveys were conducted
on fair weather days with low wind velocity (<15 km/hr)
and no precipitation. The order of sampling of calling
stations was different for each of the four surveys due to
weather and wind conditions, but stations in the four car-
dinal quadrants of the study area were worked as equally
as possible to avoid regional bias.

Target species. Previous pilot studies (Benzinger et al.
1988, Bosakowski et al. 1989a) revealed 11 potentially
breeding raptor species in the study area (excluding Ca-
thartid vultures) . To reduce the number of surveys re-
quired to survey these 11 species, vocalizations of 2-3
hawk or owl species were broadcast on each survey, re-
sulting in a total of two night surveys for owls and two
daytime surveys for hawks. The selection of species for
each survey was based on regional nesting phenology
(Bull 1964, Bosakowski et al. 1989b, Speiser and Bosa-
kowski 1991, and Bosakowski 1990). Broadcasts were or-
dered from the smallest to the largest raptor, to avoid
potential inhibitory effects of large raptors on the re-
sponse behavior of smaller species (Call 1978). About
half of the raptors responded to heterospecific broad-
casts, so every species was actually sampled twice during
the day (or night) at all calling stations. Furthermore,
some raptors were detected by visual observations or call-
ing prior to broadcasts on either survey, regardless of the
species that were broadcast.

Broadcast vocalizations and equipment. Eastern
Screech-owl ( Otus asio), Great Horned Owl ( Bubo virgi-
nianus). Northern Saw-whet Owl ( Aegolius acadicus) ,
Barred Owl (Strix varia). Red-shouldered Hawk, Red-
tailed Hawk {Buteo jamaicensis ) and Broad-winged Hawk
(Buteo platypterus) vocalizations were obtained from the
National Geographic Society guide to Bird Songs (Eva-
tone Soundsheets, Inc., Clearwater, FL, 1983). Northern
Goshawk, Sharp-shinned Hawk ( Accipiter striatus ) and
Long-eared Owl ( Asio otus) vocalizations were from the
Peterson Field Guide records, Cornell University, Labo-
ratory of Ornithology. A Cooper’s Hawk {Accipiter coopeni)
tape of the female alarm call was obtained from R.N.
Rosenfield (Rosenfield et al. 1985).

Tapes were broadcast from a Contec portable stereo
cassette tapedeck (Model V83, Japan) rated at 10 watt
output with two removable speakers. The units were pow-
ered by 8 “D” cell batteries. Cassette tapes (normal bias)
were broadcast at a standard volume setting, approxi-
mately 95% of full capacity, but without detectable dis-
tortion. The speakers were high fidelity, each containing
a woofer and tweeter component for more accurate
sound replication of calls. The two speakers were mount-
ed back-to-back to provide bidirectional broadcasting of
vocalizations at all times.

Field protocol. Two surveys were conducted at night

Rosakowski and Smith

Vol. 31, No. 1

for the five target owl species. These surveys began at
least V 2 hr after sunset and were terminated by 0100 H
the following morning. The first owl survey was conduct-
ed from 21 March 1989-16 April 1989 for Eastern
Screech-owls, Long-eared Owls, and Great Horned Owls.
Species on the second owl survey, conducted from 16
April-19 May 1989, included the Northern Saw-whet Owl
and Barred Owl. Hawk surveys were conducted from at
least 2 hr after sunrise to no later than 2 hr before sunset.
Species included on the first hawk survey conducted
from 7 April-4 May 1989 included the Red-shouldered
Hawk, Northern Goshawk, and Red-tailed Hawk. The sec-
ond hawk survey from 18 May-20 June 1989 included the
Sharp-shinned Hawk, Cooper’s Hawk, and Broad-winged
Hawk.

On each survey, tape-recorded vocalizations of three
raptor species were played during an 18-min period. Vo-
calizations of each species were recorded on cassette tape
for a 3-min period followed by a 3-min period of silence.
The silent period served a dual function as a listening/
watching period for vocal or visual responses of raptors
and also as a refractory period prior to broadcasts of the
next raptor species. The double speakers were hung on
low tree branches about 1.5 m above ground during
broadcasts.

Macrohabitat measurements. To measure habitat at the
81 calling stations, their location was plotted on topo-
graphic maps and their position noted on aerial photo-
graphs (1:8000) taken during the winter of 1982. Habitat
at each calling station was quantified in a 300 m radius
circle centered on the calling station. This distance was
chosen as representative habitat of the calling station as
all raptors were detected within this distance and most
were detected within 100 m. On aerial photographs, a
dot-grid overlay was used to quantify the suburban area
within the 300 m radius circle. To measure the length of
paved roads and forest edge within the habitat circles, w T e
overlayed them with a fine thread, then the thread was
straightened and measured. Forest edge can be consid-
ered as any abrupt change (Small and Hunter 1989) to
open habitat (e.g., field, marsh, river or lake) that are
easily discernable on the aerial photographs. We mea-
sured distances to forest openings (>0.5 ha), paved
roads, human habitation and wetlands (>0.5 ha) from
aerial photographs using a metric ruler with a mm scale.
Topographic maps were used to calculate slope (rise in
elevation over 300 m baseline) . For the purposes of this
paper, “wilderness” was defined according to a standard
operational definition supplied by Webster’s Dictionary
as “any uninhabited, uncultivated region.” Wilderness
area was calculated as the total area of contiguous, un-
cultivated, uninhabited habitats bounded by paved roads
and/or housing developments.

Statistical analysis. A total of 10 habitat variables rele-
vant to urbanization were used to describe the 81 calling
stations. Each raptor species had a unique set of stations
where they were recorded and these data sets were used
to calculate habitat means and standard deviations. In
order to increase the sample size for Eastern Screech-owl
( N — 3) and Northern Goshawk ( N = 2), two screech-
owl sightings and three goshawk nests from previous
years were added to the data set. These additional raptor
locations occurred in the mid- to late eighties and fell

Table 1. Species richness values for breeding raptor
communities studied in North America.

Study

Area (ha)

No. OF
Species

New Jersey (this study)

16,100

Utah (Smith and Murphy 1973)
Wyoming (Craighead and

20,700

Craighead 1956)
Michigan (Craighead and

3,100

Craighead 1956)

9,600

Idaho (BLM 1979)

53,200

well within five of the 81 habitat circles described. No
additional sites were known for the Long-eared Owl
which was found at only two sites, or the Sharp-shinned
Hawk which was found at one site, so they were dropped
from any habitat analyses.

Each species was compared to the set of stations which
did not have any raptor detections during the study (un-
occupied habitat, N =■ 20) to provide a measure of hab-
itat selection. For statistical comparison, we used a non-
parametric test (Mann-Whitney U-test) since some of the
data were percentages or nonnormal in distribution (Zar
1974). Species richness was determined for all wilderness
areas with four or more survey stations and a logistic re-
gression curve was calculated (Excel Software, Microsoft
Corp., Redmond, WA, Version 5.0) to test the strength
of the relationship. To detect area relationships for in-
dividual species, wilderness areas were grouped into
three size class categories (0-1000, 1000-2000 and 2000-
8000 ha) for all broadcast stations. The percentage of
occupied stations by a species in each size class (% usage)
was subtracted from the percentage of all stations sam-
pled (% availability) to determine habitat utilization
(Johnson 1980) for each wilderness area size class. Usage
is said to be selective if resources are used disproportion-
ately to their availability (Johnson 1980). Proportions in
each category were tested for increasing or decreasing
trends in relation to wilderness area size using an Armi-
tage (1955) proportion trend test.

Results and Discussion

Species Richness of the Raptor Community. We

compared raptor species richness of the New Jersey
raptor community with studies of other raptor
communities (Table 1 ) . Of these studies, the lowest
species richness was found in Michigan (Craighead
and Craighead 1956) which was mostly farmland
(11% wooded). New Jersey forestland and the
spruce-fir-pine slopes and sagebrush benches in
the Snake River of Wyoming (Craighead and
Craighead 1956) had slightly higher richness but
both were surpassed by Utah scrub juniper desert
(Smith and Murphy 1973). Highest species rich-
ness was reported for Snake River Canyon in Idaho

March 1997

Forest Raptors and Urbanization

Table 2. Urbanization habitat variables for sites occupied by forest raptors in a northern New Jersey watershed. Top
number represents the mean and bottom number represents the SD (* = P < 0.05, + = P< 0.10). Sample size for
each species given in parenthesis. GHOW = Great Horned Owl, BAOW = Barred Owl, ESOW = Eastern Screech-
owl, RTHA = Red-tailed Hawk, RSHA = Red-shouldered Hawk, BWHA = Broad-winged Hawk, COHA = Cooper’s
Hawk, NOGO = Northern Goshawk.

Variable

GHOW

(16)

BAOW

(27)

ESOW

(5)

RTHA

(22)

RSHA

(9)

BWHA

(16)

COHA

(10)

NOGO

(5)

Unoccu-

pied

Habitat

(20)

Distance to Human

426.6

671.9*

778.0

477.3

888.9*

769.4

651.5

676.0+

505.0

Habitation (m)

325.9

488.0

659.4

411.6

569.7

704.4

672.0

533.5

496.2

Number of

4.19

1.26*

0.60

3.32

3.06

1.20

2.35

Houses/Bldgs. (#)

7.30

3.14

0.89

5.55

4.35

2.25

4.35

Suburban Area (%)

2.7

1.0+

0.2

2.4

2.5

0.9

1.5

4.1

2.5

0.3

3.7

3.6

1.9

2.4

Distance to Paved

292.8

468.0

458.0

406.6

546.1

343.8

259.0

482.0

343.8

Road (m)

258.0

465.6

393.9

345.9

549.9

293.5

141.0

243.0

294.4

Road Mileage (m)

404.7

299.4

64.0

367.5

206.7

269.4

352.5

78.0

268.4

620.2

321.4

143.1

528.7

226.3

330.5

380.5

174.4

294.1

Edge Length (m)

623.7

370.0

652.0

595.9

492.2

374.1

656.5

369.0

467.0

632.7

524.2

536.6

617.6

618.8

374.9

420.5

550.1

526.6

Distance to Wetland

563.1

197.0+

662.0

442.0

62.2*

360.3

252.0

164.0

369.8

(m)

518.2

247.9

581.8

585.0

49.7

377.8

203.1

207.5

333.4

Distance to Forest

195.0

199.7

197.0

163.9

80.6*

141.6

112.0+

147.0

240.2

Opening (m)

237.3

206.2

261.5

192.2

91.4

148.1

109.6

141.3

195.4

Slope (%)

6.5

6.1

10.0

6.8

5.1 +

6.9

4.4*

6.8

8.0

4.4

4.8

4.9

4.7

5.3

6.3

4.1

6.7

5.5

Wilderness Area (ha)

2291.2

3766.9

5094.0

2498.2

4388.9

3144.0

2271.6

5278.0

2729.5

2731.4

3164.4

3543.4

2971.8

3179.1

3204.8

2181.3

3320.5

2636.0

(BLM 1979) with its vertical cliffs and sagebrush
desert, probably reflecting the high structural hab-
itat diversity as a result of vertical and horizontal
habitat partitioning. Overall, cultivated land of
Michigan (circa 1950s farming techniques) ap-
peared to slighdy reduce raptor diversity and abun-
dance compared to contiguous forestland in New
Jersey.

Urbanization and Forest Fragmentation. Urban-
ization and its ultimate effect in fragmenting for-
ests into smaller wilderness areas is an unnatural
factor which reduces raptor abundance and diver-
sity. Robbins (1979) noted the disappearance of
several nesting bird species including the Broad-

winged Hawk after 30 yr of severe fragmentation.
In our study area, the proximity of human habita-
tion showed marked differences in the habitat suit-
ability for several forest raptors. The Red-shoul-
dered Hawk was most sensitive to human distur-
bance and it occupied sites significantly further
from human habitation than unoccupied habitat
and it showed a complete lack of suburban habitat
within the 300 m radius habitat circles examined
(Table 2). The Northern Goshawk had the second
largest distance to human habitation ( P < 0.10)
and similarly showed a significant lack of suburban
habitat within the 300 m radius habitat circles ex-
amined. Additional data from 16 Northern Gos-

Bosakowski and Smith

Vol. 31, No. 1

Figure 1. Species richness of forest raptors with respect to seven wilderness areas of varying size. Curve represents
best-fit logistic regression function.

hawk nest sites in the study region also showed that
the species nests further from human habitation
and paved roads than expected (Bosakowski and
Speiser 1994). In the study region. Northern Gos-
hawk nests were significantly farther from human
habitation than were Cooper’s Hawk nests (Bosa-
kowski et al. 1992c) or Red-tailed Hawk nests
(Speiser and Bosakowski 1988). The Barred Owl
was similar to the Red-shouldered Hawk and
Northern Goshawk, occupying sites that were sig-
nificantly farther from human habitation, with sig-
nificantly fewer houses, and a tendency for less sub-
urban habitat area (P < 0.10).

With regard to wetlands, the Red-shouldered
Hawk occupied areas significantly closer to wet-
lands which resulted in a significantly lower slope
percentage as well as a significantly closer distance
to forest openings. The Barred Owl also had a ten-
dency to be closer to wetlands (P < 0.10). The
Cooper’s Hawk occupied sites with significantly
lower slope percentages and was often closer to
forest openings (P< 0.10), but not necessarily due
to a preference for wetlands since it often used
suburban forest edge as well. Overall, these three
lowland species had a greater vulnerability to de-
velopment pressures as valley bottoms and flat ter-
rain are generally the first areas targeted for roads,
houses and commercial buildings (Tiner 1985).

Distance to paved roads, road mileage, edge
length and wilderness area did not show any sig-
nificant differences among any species compared
to unoccupied habitat. Four species, Great horned
Owl, Red-tailed Hawk, Eastern Screech-owl and

Broad-winged Hawk did not have any variables that
were significantly different from unoccupied hab-
itat, suggesting that they were less sensitive to ur-
banization.

Forest Area Relationships. Area-sensitive species
respond negatively to decreasing forest size and
show predictable declines or absence as the area
of the forest shrinks (Robbins 1979, Ambuel and
Temple 1983, Robbins et al. 1989). Within our
study area we found a strong correlation (r = 0.79,
P < 0.01) for species richness of forest raptors and
increasing size of wilderness areas (Fig. 1). This is
likely the result of the inclusion of area-sensitive
species in large wilderness areas and their exclu-
sion in smaller forest fragments. No area less than
1000 ha had more than four raptor species while
4-8 species were found in areas from 1000-8000
ha in size. Thiollay and Meyburg (1988) also noted
a positive correlation between the size of reserves
and the abundance index of diurnal raptors (Fal-
coniformes) on the Island of Java.

When wilderness areas were grouped into size
classes, several distinct trends emerged among the
species. We calculated percent utilization of three
wilderness area size classes and determined the
probability of increasing or decreasing trends for
raptors. Several species revealed increasing trends
with increasing wilderness area size (Red-shoul-
dered Hawk, P = 0.97; Northern Goshawk, P =
0,84; and Barred Owl, P = 0.82), whereas several
species revealed decreasing trends (Red-tailed
Hawk, P — 0.95 and Great Horned Owl, P = 0.85,
Fig. 2). Craighead and Craighead (1956) noted

March 1997

Forest Raptors and Urbanization

-J

0 .

-10

-20

ES0U

<0.56.

0.44)

BA0W

(0.82.

0.18)

■

GHOU

(0 15.

0.85)

C0HA

<0.70.

0.30)

(MIA

<0 33.

0 67)

■

RSHA

<0.97.

0 03)

N0G0

<0.84.

0.16)

■

RTHA

(0.05,

0.95)

-30

0-1000

1-2000

2-8000

WILDERNESS AREA (ha)

Figure 2. Relative utilization (use-availability) of different wilderness area size classes for eight sympatric forest raptor
species (see Table 2 for species acronyms) . Numbers in parentheses represent probability of species increasing and
decreasing (I, D) in relation to wilderness area size (Proportion Trend Test — Armitage 1955).

that Red-tailed Hawks and Great Horned Owls
were generalist predators and the results of this
study indicate that they clearly benefit from forest
fragmentation and urbanization (at least to the ex-
tent found in the study area) . On the other hand,
Red-shouldered Hawks, Northern Goshawks and
Barred Owls usually prefer extensive remote areas
of deep woods (Bosakowski 1989, Bosakowski et al.
1992a, Bosakowski and Speiser 1994).

The northern half of our study area was virtually
all wilderness and contained all 10 species. How-
ever, the southern half had nearly all of the sub-
urban areas and a four-lane highway, and was miss-
ing three species (Northern Goshawk, Sharp-
shinned Hawk and Red-shouldered Hawk) . Results
of this study predict the expansion of dominant,
disturbance-tolerant Great Horned Owls and Red-
tailed Hawks after forest fragmentation, and a re-
duction in raptor diversity.

A decrease in forest area can result in decreased
bird species diversity (Lovejoy et al. 1986), distur-
bance to species in adjoining wetlands, reduced
buffering against human disturbance and in-
creased predation (Chasko and Gates 1982, Yahner
1988). It is also becoming increasingly clear that
“edge effect” is beneficial to only a limited num-
ber of wildlife species and, by and large, it has a

strong negative impact on other members of forest
communities (Robbins 1979, Wilcove et al. 1986,
Yahner 1988, Robbins et al. 1989). For tropical
rainforests, Thiollay (1984) found that raptors are
among the first species to disappear in the process
of human population growth and exploitation and
are thus suitable indicators of habitat disturbance.
The situation appears to be similar for temperate
forests disrupted by urbanization and agriculture
(Craighead and Craighead 1956).

Large raptor communities are needed as popu-
lation reserves to maintain genetic diversity and to
provide constant recruitment to marginal habitats
(White 1974, Wilcove 1987). Results of this study
suggest that only large wilderness areas (2000-8000
ha) can provide the full diversity of forest raptors
necessary to stock marginal habitats. Until other
data on reserve size become available, the present
data could have useful management implications
for conservation of Northeast forest raptors, either
by regulating development and recreation, or de-
ciding how large an area should be set aside for
future reserves. In addition, further research will
be needed to determine the number and distance
of other reserves (Shaffer 1985, Hunter 1990) nec-
essary to support a viable population network for
each species (Wilcove 1987).

Bosakgwski and Smith

Vol. 31, No. 1

Acknowledgments

This study was part of a doctoral thesis of the first au-
thor that was approved by the faculty of Rutgers Univer-
sity under the advisorship of Dr. D.W. Morrison. The re-
search was primarily funded by a grant from the New
Jersey Division of Fish, Game, and Wildlife, Nongame
Wildlife Program. As such, we would like to extend our
appreciation to L. Niles and M. Valent for their interest,
cooperation and comments on the initial design proto-
col. The New Jersey Audubon Society also provided funds
for field work for which we are grateful. We also thank
forester T. Koeppel for use of aerial photographs, forest-
ry equipment and information on the history of Pequan-
nock Watershed forestry. We also thank M.R. Fuller, G.D.
Hayward, K. Titus, W.M. Block and J.A. Mosher for re-
viewing various drafts of the manuscript.

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Received 28 February 1996; accepted 18 November 1996

/ Raptor Res. 31 (1) :34— 39
© 1997 The Raptor Research Foundation, Inc.

DOES VEGETATION STRUCTURE LIMIT THE
DISTRIBUTION OF NORTHERN GOSHAWKS IN THE

OREGON COAST RANGES?

Stephen DeStefano 1

National Biological Service, Oregon Cooperative Wildlife Research Unit,

104 Nash Hall, Oregon State University, Corvallis, OR 97331 U.S.A.

Jon McCloskey 2

Bureau of Land Management, Salem District Office, 1717 Fabry Road, Salem, OR 97306

Abstract. — Northern Goshawks ( Accipiter gentilis ) breed in a variety of forested areas throughout the
Pacific Northwest. Nevertheless, they were only first found breeding in the Coast Ranges of Oregon in
1995, despite apparently suitable habitat and abundant prey. We document the rarity of goshawks in
the Coast Ranges by reviewing previous and current survey results for nests of goshawks and other forest
birds since the 1960s, examining sightings of goshawks since 1980 and reporting on a survey we con-
ducted in 1994. We suggest that nesting goshawks are rare in the Coast Ranges because of the vegetative
structure of the area and its influence on prey availability.

Key Words: Northern Goshawk ; Accipiter gentilis; distribution', habitat, foraging, reproduction ; Oregon.

Puede ser que la estructura de vegetacion limite la distribucion de Accipiter gentilis en la sierra costa de
Oregon.

Resumen. — Accipiter gentilis se crian en una variedad de areas de bosque en todas partes del noroeste
pacifico. Sin embargo, la primera vez que los encontraron en la sierra costa de Oregon fue en 1955, a
pesar de suficiente habitat conveniente y presa abundante. Nosotros documentamos la rareza de A.
gentilis en la sierra costa examinando anterior y corriente resultados de estudios para nidos de A. gentilis
y otros pajaros de bosque desde los 1960s, examinando observaciones de A. gentilis desde 1980 y repor-
tando un estudio que nosotros conducimos en 1994. Nosotros pensamos que los A. gentilis que hacen
nido en la sierra costa es raro por la estructura de vegetacion en el area y su influencia en la disponi-
bilidad de presa.

[Traduccion de Raul De La Garza, Jr.]

Northern Goshawks {Accipiter gentilis) are distrib-
uted across northern North America and through-
out forested areas of the western U.S. (Palmer
1988). They nest in a variety of forest types, includ-
ing boreal, deciduous and western coniferous for-
ests. In Oregon, goshawk nests are found through-
out forested areas east of the Cascade Mountain
Range, on east and west slopes of the Cascade
Range, in the Siskiyou Mountains of southwestern
Oregon and even in isolated stands of aspen ( Pop-
ulus spp.) in mountain draws and valleys in the

1 Present address: USGS, Arizona Cooperative Fish and
Wildlife Research Unit, 104 Biological Sciences East, Uni-
versity of Arizona, Tuscon AZ 85721.

2 Present Address: Winema National Forest, 38500 High-
way 97 N, Chiloquin, OR 97624.

Great Basin region of southeastern Oregon (Mar-
shall 1992). Goshawks were not known to breed in
the Coast Ranges of western Oregon, even though
their nests are found in all other areas of the state
(Reynolds et al. 1982, Marshall 1992). Incidental
sightings of goshawks have been reported in the
Coast Ranges, but it was not until 1995 that two
pairs of goshawks were found breeding there
(Thrailkill and Andrews 1996).

Reynolds (1975, 1978) found breeding pairs of
Cooper’s Hawks {Accipiter cooperii) and Sharp-
shinned Hawks (A. striatus) , but not goshawks after
an extensive search of the Coast Ranges from
1968-78. He speculated that forest conditions, spe-
cifically dense understories which may interfere
with a goshawk’s ability to hunt, precluded gos-
hawks from breeding in this area (Reynolds and

March 1997

Vegetation Structure and Goshawks

Figure 1. The Coast Ranges (shaded area) of western
Oregon and areas (circles, which may represent ^ 1 sur-
vey blocks) surveyed for Northern Goshawks during
June-August 1994.

Wight 1978, Reynolds et al. 1982) . No one has con-
ducted systematic searches for goshawk nests in the
Coast Ranges since, and no one has used broad-
casts of goshawk vocalizations to survey for the
presence of breeding pairs over large areas of the
Coast Ranges (Woodbridge, USDA Forest Serv. un-
publ. rep. 1990; Kennedy and Stahlecker 1993).
Herein, we report results of a study conducted to
document the presence of breeding Northern Gos-
hawks and to assess vegetative conditions that
might influence their distribution in the Oregon
Coast Ranges.

Study Area

The Coast Ranges of western Oregon lie north of the
Coquille River and west of the Willamette Valley, and are
separate from the Siskiyou Mountains in the southwest-
ern corner of Oregon (Fig. 1). Topography is steep and
dissected by many streams. Elevations range from sea lev-
el to 450-750 m at main ridge summits, with scattered
peaks as high as 1250 m. Climate is characterized by mild,

wet winters and relatively dry summers (Franklin and
Dyrness 1973).

Historically, the Coast Ranges were densely forested
with sitka spruce ( Picea sitchensis), western hemlock ( Tsu-
ga heterophylla) and western redcedar ( Thuja plicata)
(Franklin and Dyrness 1973). Most of the mature forest
(>80 yr old) has been logged or burned during the past
150 yr (Franklin and Dyrness 1973). Much of what re-
mains are stands of second-growth trees with older stands
occurring as islands, fragmented by clearcut logging. As
a result of these disturbances, and because of tree plant-
ing, Douglas-fir ( Pseudotsuga menziesii) is now the major
component of the forests in this area. Western hemlock
and western redcedar are common coniferous species
and red alder ( Alnus rubra ) , vine maple ( Acer circinatum )
and bigleaf maple (A. macrophyllum ) are common hard-
wood species (Franklin and Dyrness 1973, Forsman et al.
1996b).

Methods

Goshawk Surveys. We surveyed for Northern Goshawks
in 24 survey blocks, totaling 3285 ha (range 60-335 ha)
during June-August 1994. Surveys covered the east and
west slopes of the Coast Ranges and included federally
administered public lands, the MacDonald-Dunn State
Forest managed by Oregon State University and areas
originally searched by Reynolds (1975, 1978). To maxi-
mize the potential for locating nesting goshawks, survey
blocks were chosen based on our knowledge of goshawk
nesting habitats (DeStefano et al. 1994), past sightings of
goshawks, recommendations from agency biologists fa-
miliar with local conditions and habitat and examination
of aerial photographs and topographic maps to deter-
mine accessibility of potential goshawk habitat. Whenever
possible, older (>80 yr), larger contiguous forested
blocks were surveyed. Because we were more interested
in documenting the presence of breeding goshawks rath-
er than calculating an unbiased estimate of nesting den-
sity, we focused on areas with the greatest potential for
nesting habitat, based on the published literature and
our experience in Oregon. We did, however, survey a va-
riety of forest types and serai stages.

We used taped vocalizations of Northern Goshawks to
elicit responses from adults and juveniles (Woodbridge,
USDA Forest Serv., unpubl. rep. 1990; Kennedy and Stah-
lecker 1993); the adult alarm call was used during the
nesting period in June-July, and both adult alarm and
juvenile begging calls were used during the post-fledging
period in July-August. Road and foot transects were first
delineated on maps and aerial photographs. Foot tran-
sects were 200 m apart with broadcast stations every 300
m; stations along adjacent transects were staggered (Joy
et al. 1994). Broadcast stations on roads were 250 m
apart. At each station, vocalizations were broadcast in
three directions (60°, 180°, and 300°) for 10 sec, with 30
sec between each call. This procedure was conducted at
each station by one or two observers. Presence, location
and behavior of raptors were recorded.

Historical Sight in gs. To document past sightings of gos-
hawks in the Coast Ranges, we searched records and data
bases compiled from 1980—1995 by state and federal land
management and conservation agencies and local bird-
watchers. We assessed the reliability of these sightings

S. DeStefano and J. McCloskey

Vol. 31, No. 1

and categorized them as 1 (questionable - observer had
no or litde experience identifying birds, or experience
could not be assessed) , 2 (reliable = experienced birder)
and 3 (confirmed = experience observing raptors, pro-
fessional biologist). Records were searched for date ob-
served, behavior of adults, presence of immatures and
other clues that might indicate reproductive activity. We
also questioned biologists who have been conducting ex-
tensive surveys for Northern Spotted Owls ( Strix occiden-
talis) and Marbled Murrelets ( Brachyramphus mamoratus)
in the Coast Ranges for the past 10-15 yr (see Nelson
and Sealy 1995 and Forsman et al. 1996a for background
and methods).

Vegetation Surveys. We examined vegetation in the 24
survey blocks for forest stand structure and composition,
understory conditions and landscape patterns. Stand can-
opy structure, % canopy cover and tree species were re-
corded from ground surveys and aerial photos. After con-
ducting ground assessments at each survey block, we
then used a Geographic Information System (GIS) to cal-
culate % cover of forest type and amounts of mature
(>80 yr) and second-growth forest in survey blocks. We
recorded the presence and % cover of dominant under-
story plant species at each survey site. Understory vege-
tation was classified into six associations, according to
Franklin and Dyrness (1973), and represented a gradient
from dry to wet conditions. These associations were (1)
ocean-spray-salal ( Holodiscus discolor-Gaultheria shallon ) as-
sociation found in dry, relatively open sites, (2) Pacific
rhododendron-Oregon grape ( Rhododendron macrophyl-
lumrBerberis nervosa) found on dry, exposed ridge tops, (3)
big huckleberry-beargrass ( Vaccinium membranaceunbXero-
phyllum tenax ) on shallow, stony soils at high elevations,
(4) vine maple-salal in cool, moist sites with moderately
dense tree cover, (5) swordfern ( Polystichum munitum ) in
moist sites associated with mature overstory conditions,
and (6) swordfern-Oregon oxalis ( Oxalis oregano) along
streamside slopes.

Results

Goshawk Surveys. We found no Northern Gos-
hawks in the entire 3285 ha survey area. However,
we did find two Sharp-shinned Hawks, seven Red-
tailed Hawks ( Buteo jamaicensis) and two unidenti-
fied raptors (too small to be goshawks). We also
found two small, unoccupied accipiter nests (prob-
ably Sharp-shinned Hawks built in previous years) .

Vegetation Surveys. Of the 24 survey blocks cov-
ered, 23 were dominated by a Douglas-fir overstory.
The one remaining block was a fire-regenerated
stand dominated by noble fir ( Abies procera ); this
site was undisturbed by logging, except for access
roads. For all 24 survey blocks combined, 63% of
the area was in older (>80 yr) Douglas-fir with
85% canopy cover, 24% was conifer-hardwood mix
(Douglas-fir, western hemlock, western redcedar,
red alder, vine and bigleaf maple) with <65% can-
opy cover, 7% was open mixed conifer (Douglas-
fir, western hemlock, western red cedar) with

2^65% canopy cover, 3% was young (<80 yr) Doug-
las-fir with ^85% canopy cover, 2% was older
Douglas-fir with <30%) canopy cover, and 1% was
clearcut areas, meadows or water.

Ground cover within the survey blocks was dom-
inated by understory types 4 and 5; these two types
were present in 20 and 14 of the 24 stands, re-
spectively. Vine maple, salal and swordfern were
the most common species overall, and each survey
block, with the exception of the stand of noble fir,
had a dense shrub layer with 45-100% ground cov-
er (* - 81%, SE = 3 %, N= 23). Twenty-three of
the 24 blocks were previously disturbed by logging
and fire.

Historical Sightings. Records of previous sight-
ings indicated that goshawks have been seen
throughout the Coast Ranges every year, and dur-
ing each month in any given year, for the past 15
years (Table 1), with apparent peaks in sightings
in the spring and fall (Fig. 2). This would coincide
with dispersing or migrating hawks. Numbers of
observations were low, however, and prior to 1995,
no confirmed evidence of reproductive activity had
been documented. Few reports have described ag-
gressive, territorial behavior or repeated sightings
of adults in a particular area during the breeding
season. Goshawk sightings in the Coast Ranges
have been much lower than for other parts of the
state (Fig. 3).

Discussion

Determining the “absence” or rarity of a species
in a geographic region, and whether or not this is
meaningful ecologically, can be a difficult task, es-
pecially when the species is normally secretive and
present in low densities. At least two elements are
involved in making this determination. The first is
that large-scale searches (both spatially and tem-
porally) must be conducted because occupation of
habitat by a species can vary over space and time
(Morris 1987, Block and Brennan 1993, DeStefano
et al. 1994, Keane and Morrison 1994). Our survey
for goshawks over a single breeding season is ob-
viously inadequate to address this concern, but
coupled with surveys in the 1960s and 1970s, the
consistent and widespread searches for Northern
Spotted Owl and Marbled Murrelet nests during
the 1970s-90s, and the reports of birdwatchers and
agency biologists for the past 15 years, the tempo-
ral and spatial scope of the search for goshawk
nests in the Coast Ranges has been broad.

The second element involves a determination,

March 1997

Vegetation Structure and Goshawks

Table 1. Sightings of Northern Goshawks, grouped by year, in the Coast Ranges of western Oregon, 1980—95. Only
records by experienced agency biologists or birders (levels 2 and 3; see text) were used. Unknown age or behavior
indicates observer was unsure or information was not reported.

Year

Adult

Age

Imm.

Unk.

Observed Behaviors

SOURCE a

1995

2 nests located on BLM lands

BLM, OCWRU

1994

Hunting, calling, respond to owl tape,
fly-by

Private, OCWRU, BLM

1993

Unknown

OB 19:57, BLM, ONHP, Private

1992

Fly-by, perched, soaring, unknown

OCWRU, BLM, Private

1991

Unknown

BLM, Private

1990

Near spotted owl nest, chasing spotted
owl, unknown

OB 16:94,185, BLM, Private

1989

Fly-by, unknown

BLM, Private

1988

Unknown, soaring

Private

1987

Unknown, aggressive interaction with
red-tailed hawk

OB 13:232,314, Private, Crannel and
DeStefano 1992

1986

Unknown

OB 12:212

1985

Unknown

USFS, Private

1984

Unknown, soaring

Private

1983

Unknown

OB 10:130, Private

1982

Unknown

Private

1981

Unknown

Private

1980

Unknown

Private

“Source codes: BLM = Bureau of Land Management; OB = various issues (vol.:page) of the journal Oregon Birds (individual
issues not listed in Lit. Cited); OCWRU = Oregon Cooperative Wildlife Research Unit; ONHP = Oregon Natural Heritage Pro-
gram; Private = records of private individuals; USFS = U.S. Forest Service.

probably on both objective and subjective bases,
that a species is absent from a region because im-
portant components of the habitat are lacking.
Otherwise, the absence of a species from a region
is meaningless. Our contention is that the absence,
or at least rarity, of nesting goshawks in the Coast
Ranges of western Oregon is an interesting phe-

Figure 2. Distribution of Northern Goshawk sightings
by month, consolidated for the years 1980-95, in the
Coast Ranges of western Oregon. Adults and young from
two nest sites located in June 1995 are not included.

nomenon, and one that suggests something about
vegetation structure on the distribution of this spe-
cies.

The Northern Goshawk has been called a forest
habitat generalist (Reynolds et al. 1992). The lo-
cation of the Coast Ranges of western Oregon in
relation to the geographic range of the goshawk in
North America, and the general forest conditions
(mixture of mature and second-growth coniferous
forest and openings) that exist there, appear to in-
dicate that the Coast Ranges should be part of the
breeding range of this species. Rarity is common
along the geographic boundaries of a species’
range, yet goshawks nest in areas north, south, and
east of the Coast Ranges.

Why is there a gap in the breeding range of this
species and what is it about the Coast Ranges of
Oregon that prevents goshawks from nesting
there? It could be related to climate. The Coast
Ranges receive 150-300 cm of annual rainfall
(Franklin and Dryness 1973), much of it falling
during the spring breeding season. However,
Northern Goshawks breed commonly on the

S. DeStefano and J. McCloskey

Vol. 31, No. 1

Figure 3. Relative abundance of Northern Goshawks by
county in Oregon, based on records of the Oregon Au-
dubon Society and Oregon Department of Fish and Wild-
life. Abundance ratings are common (black), occasional
(dark gray), rare (light gray), and unknown (white).
Note that coastal regions of Lane and Douglas Counties
would likely be rated as having rare (light gray) sightings
of goshawks, but both counties are rated as occasional
(dark gray) because large portions of each reach into the
Cascade Mountain foothills and range, where goshawks
are seen more often.

Olympic Peninsula of Washington (E. Forsman
pers. comm.) and in southeastern Alaska (K. Titus
pers. comm.), where annual precipitation equals
or exceeds levels received in Oregon’s Coast Rang-
es.

A second possibility might involve predation or
competition from other raptors. Great Horned
Owls ( Bubo virginianus) and Red-tailed Hawks fre-
quently interact with Northern Goshawks and of-
ten use their nests (Moore and Henny 1983, Cran-
nell and DeStefano 1992, Rohner and Doyle 1992) .
It is unlikely, though, that this form of predation
and/or competition is more intense in the Coast
Ranges than other parts of the goshawks’ geo-
graphic range.

The third explanation involves vegetation struc-
ture as it relates to prey availability. Goshawks tend
to hunt in the ground-shrub and shrub-canopy
zones of the forest (Reynolds and Meslow 1984). A
dense shrub layer is characteristic of most forest
areas of the Coast Ranges and disturbances such
as logging and fire have decreased mature oversto-
ry canopy closure, allowing more sunlight to reach
the ground. These conditions, coupled with high
levels of rainfall, have resulted in increased under-

story stem densities and dense, lush undergrowth
on many sites (Franklin and Dyrness 1973). Prey,
which are varied and abundant in the Coast Rang-
es, include such species as snowshoe hares ( Lepus
americanus), brush rabbits ( Sylvilagus bachmani),
Douglas squirrels ( Tamiasciurus douglasii ), Ruffed
Grouse ( Bonasa umbellus). Mountain Quail ( Oreor -
tyx pictus ) , Northern Flickers ( Colaptes auratus) and
other woodpeckers, and Stellar’s and Gray Jays (Oy-
anocitta stellari and Perisoreus canadensis, respective-
ly) . Many of these prey species may be difficult for
goshawks to capture because of the dense under-
story conditions that exist throughout most of the
Coast Ranges (Reynolds and Meslow 1984). In ad-
dition, the larger biomass prey species (lago-
morphs, grouse) may be more important to breed-
ing goshawks than smaller prey (jays, woodpeck-
ers) (Reynolds et al. 1992), and low availability of
larger prey may depress reproductive potential
(Alaska Dept. Fish and Game 1993). Dense under-
story conditions would make the larger, ground-
dwelling species more difficult to capture (Reyn-
olds et al. 1992).

Others have found the distribution of raptor for-
aging to be inversely related to the density of plant
cover (Southern and Lowe 1968, Wakely 1978, Ba-
ker and Brooks 1981, Bechard 1982, Collopy and
Bildstein 1987). Preston (1990) summarized these
findings by describing raptor hunting distribution
as a function of both prey abundance and avail-
ability, which in turn is a function of a suite of
environmental factors, including habitat character-
istics (e.g., vegetation structure). In fact, suitable
foraging habitat may be more important than nest-
ing habitat in determining the distribution of gos-
hawks in boreal forest (Widen 1989). The impor-
tance of prey in the distribution and management
of northern goshawks has been emphasized in the
management guidelines of Reynolds et al. (1992).

If a relationship between vegetation structure
and availability of prey does indeed exist, then the
forest conditions present in the Coast Ranges of
Oregon may limit prey availability to goshawks and
thus prevent or depress reproductive activity, de-
spite potentially suitable nesting substrate and ad-
equate populations of prey.

Acknowledgments

We thank W. Dean, D.S. Hopkins and J.A. Reid (Bu-
reau of Land Management) , E.D. Forsman and S .J. Mad-
sen (U.S. Forest Service), C. Bruce and T, O’Neil (Ore-
gon Department of Fish and Wildlife) , M. Hansen, S.K
Nelson, J.A. Thrailkill, and M. Wilson (Oregon Cooper-

March 1997

Vegetation Structure and Goshawks

ative Wildlife Research Unit), and E. Schauber and S.
Spean (Oregon State University) for providing field sup-
port and/or information on goshawk sightings. Special
thanks to R. Bayer, E. Eltzroth, D. Faxon, G. Gilson, T.
Mickel and H. Nehls for sharing their private records.
The manuscript benefited from reviews by R Beier, S.K.
Daw, S.M. Desimone, E.D. Forsman, M.T. McGrath, M.L.
Morrison and R.T. Reynolds. Additional logistical support
was provided by the Oregon Cooperative Wildlife Re-
search Unit and the Department of Fisheries and Wildlife
at Oregon State University.

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Received 1 April 1996; accepted 23 November 1996

J. Raptor Res. 31(1) :40-43
© 1997 The Raptor Research Foundation, Inc.

FOOD HABITS OF THE

LANNER FALCON (. FALCO BIARMICUS FELDEGGI1) IN

CENTRAL ITALY

Federico Morimando, Francesco Pezzo

Dipartimento di Biologia Evolutiva, Gruppo di Etologia e Ecologia Comportamentale,
Universita di Siena, via P.A. Mattioli 4, 53100 Siena, Italy

Alessandro Draghi

Proeco Gestione Fauna E Ambiente Studio Associato, via Uopini 19, 53100 Siena, Italy

Abstract. — The diet of the Lanner Falcon (Falco biarmicus feldeggii) was studied in an area of central
Italy for one yr. Diet composition differed according to the methodology used for data collection with
mammals, small passerines and insects found more frequently or exclusively in pellets. Conversely, anal-
ysis based on pluckings and observations of prey taken to the nest indicated the diet consisted of only
nonpasserine and large passerine birds. Overall, the birds including European Starling (Sturnus vulgaris) ,
Green Woodpecker ( Picus viridis) , and Jay ( Garrulus glandarius) were the most common prey taken by
Lanner Falcon. Mammals taken included wood mouse (Apodemus sp.), common dormouse ( Muscardinus
avellanarius ) and Savi’s pine vole ( Microtus savii ).

Key WORDS: Lanner Falcon; Falco biarmicus ; diet; feeding ecology; Italy.

Habitos alimenticios del Falco biarmicus feldeggii en la region central de Italia.

Resumen. — La dieta del Falco biarmicus feldeggii fue estudiada durante un ano en un area de la region
central de Italia. Se observo que la composicion de la dieta variaba de acuerdo con la metodologia
utilzada para la recoleccion de datos, de tal manera que se encontraron pequenas aves passerinas,
marmferos e insectos mas frecuentemente o exclusivamente en las egagropilas. Por el contrario, al
analizar la presa en el nido y al realizar desplumamiento, se hallo que la dieta consistfa unicamente de
aves no passerinas y grandes passerinas. Sobre todo aves como Sturnus vulgaris, Picus viridis, Garrulus
glandarius, fueron la presa mas comun de F biarmicus feldeggii. Los marmferos incluidos en la dieta eran
Apodemus sp., Muscardinus avellanarius y Microtus savii.

[Traduccion de Agustina Lanusse]

The European subspecies of the Lanner Falcon
(Falco biarmicus feldeggii ) is limited in its distribu-
tion to southern Italy and the Balkans with a pop-
ulation estimated to be just a few hundred pairs
(Gensb0l 1992). There are <60 pairs in the Italian
peninsula but, in Sicily, the population consists of
>80 pairs (Massa et al. 1991). In Tuscany, central
Italy, five breeding pairs are known in a 8300 km 2
area, with a density of 1 pair/ 1660 km 2 .

The biology of the Lanner Falcon in Europe is
poorly studied (Cramp and Simmons 1980). Most
work has been done on the African subspecies
(Cade 1965, Maclean 1984, Goodman and Haynes
1989, 1992) and very few studies have been con-
ducted in the Mediterranean area (Mebs 1959,
Bonora and Chiavetta 1975, Massa et al. 1991).
Lanner falcons nest on cliffs, making it difficult to

find perches and roosts where food remains may
be collected for dietary studies (Massa et al. 1991,
Chiavetta 1992). Apart from studies conducted in
Sicily (Mebs 1959, Massa et al. 1991), no detailed
research has been done so far on the food habits
of Lanner Falcons. Herein, we present results of
an analysis of the diet of a pair of Lanner Falcons
in central Italy based on remains collected at
plucking areas and the nest site and observations
of prey delivered to the nest.

Study Area

The study was conducted in a rural area named “Crete
Senesi” in Tuscany, central Italy, at 200-300 m elevation.
The climate of the area is temperate, locally subarid, with
an average annual temperature of 18°C. It is very hilly,
open and eroded at the bases of hills which has created
an abundance of small clay/sand cliffs ranging from 10-

March 1997

Lanner Falcon Food Habits

Passeriformes
□ Non Passeriformes
■ Mammalia

Figure 1. Composition of the diet of Lanner Falcons during the breeding season (December-June) using three
different methods: direct observations of prey taken to the nest, pellets, and plucked remains. Numbers above the
bars refer to sample size for each method.

40 m in height. These small cliffs are used for nesting by
Lanner Falcons (Morimando et al. 1994). The vegetation
of the area is dominated by pastureland with small oak-
woods ( Quercus pubescens ) interspersed with cereal crops
and small patches of olive trees ( Olea europea ).

Methods

We observed a pair of Lanner Falcons through two re-
productive seasons from spring 1994-summer 1995. We
made direct observations on prey brought to the nest
and we collected prey remains at the bases of nest cliffs.
By watching activity patterns, we also found two main
perches where the falcons plucked and ate their prey.
From June 1994, we collected prey remains at these
perches twice a month by searching the ground carefully
under each perch. Prey were often identified in the field
based on feathers and fresh remains. Remains that were
not readily recognized as well as pellets were examined
using a dissecting microscope and remains were identi-
fied using museum specimens and identification guides
(Brown et al. 1987, Svensson 1992). We also used taxo-
nomic keys (Chaline et al. 1974) and a sample of speci-
mens collected locally to compare bones and skeletal re-
mains.

We calculated the % occurrence and % biomass of
each prey category in the diet based on each of the three
methods. Prey biomass was determined based on the av-
erage weight of prey taxa reported in the literature
(Cramp 1977-94, for birds; Macdonald and Barret 1993,
for mammals). We calculated the mean prey weight
(MPW) using the formula: Total Biomass/Total Numbers
of each prey species.

Results

We observed the nest 16 times for a total of 96
hr. We also collected plucked remains 26 times and

collected 59 pellets under perches. Altogether, we
identified 15 prey items delivered to the nest, 78
prey items from pellets, and 71 prey items from
plucked remains (Table 1). The mean number of
prey per collecting event was 0.94 prey for obser-
vations, 1.32 for pellets and 2.73 for pluckings.

Direct observations at the nest showed that pas-
serine birds were delivered to the nest more fre-
quently than nonpasserines (Table 1). Although
most of these birds were not identified, the main
prey species taken to the nest appeared to be the
European Starling ( Sturnus vulgaris ) and Blackbird
( Turdus merula ) . The MPW of prey brought to the
nest was 121 g (SE = 38.27). During nest obser-
vations, we twice saw the male storing small prey
in a cliff hole that was used as a cache.

Nearly identical percentages of passerines and
nonpasserines were identified from plucked re-
mains (Table 1). The MPW of prey based on this
type of analysis was 182.01 g (SE = 216.94), con-
siderably larger than that of prey taken to the nest
{t = 2.47, df = 20; P < 0.02). Based on this type
of analysis, the main prey were Green Woodpecker
( Picus viridis ) , Jay ( Garrulus glandarius) and Euro-
pean Starling. Most of the biomass in the Lanner
diet was from nonpasserines of which the Green
Woodpecker was the largest and most frequently
taken species.

Bone fragments in pellets were small and diffi-

Morimando et al.

Vol. 31, No. 1

Table 1. Numbers (N), percentage of biomass (%B) and percentage of occurrence (%0) of prey items in Lanner
Falcon diet in Tuscany, Italy, according to collecting methods.

Pellets

Pluckings

Observations

Prey Item

MAMMALS

Muscardinus avellanarius

0.38

1.28

Apodemus spp.

0.96

3.85

Microtus savii

0.38

1.28

Rodentia ind.

0.9

5.13

Total mammals

2.62

11.54

BIRDS

Falco tinnunculus

6.19

5.63

Phasianus colchicus

10.83

2.82

38.57

6.67

Columba livia

11.49

3.85

16.25

9.86

16.53

6.67

Columba palumbus

5.74

1.28

3.48

1.31

Streptopelia turtur

19.1

1.28

3.48

4.23

Athene noctua

1.08

1.41

Apus apus

0.31

1.41

Picus viridus

35.10

14.10

30.95

22.54

Total nonpasserines

54.24

20.51

72.58

49.30

55.10

13.33

Alauda arvensis

0.57

2.82

Turdus merula

6.13

7.69

0.62

1.41

13.22

Turdus philomelos

1.58

4.23

Garrulus glandarius

14.47

15.49

Pica pica

6.89

3.85

4.18

4.23

Corvus monedula

10.74

6.67

Sturnus vulgaris

28.33

47.44

5.11

15.49

16.53

33.33

Emberiza calandra

0.29

1.41

Anthsu pratensis

0.14

1.41

Passeriformes ind.

1.79

8.97

0.46

4.23

4.41

26.67

Total passerines

43.14

67.95

27.42

50.70

44.90

86.67

Total birds

97.38

88.46

100

cult to recognize. Nevertheless, this form of anal-
ysis showed that mammals also occur in the diet of
the Lanner Falcon (Table 1). Mammals identified
were wood mouse ( Apodemus spp.), common dor-
mouse ( Muscardinus avellanarius ) and Savi’s pine
vole ( Microtus savii). Also, some pellets were made
solely of insects remains, mainly Formicidae and
small Coleopterans. A careful inspection of pellets
showed that most insects were inside the gizzard
remains of Green Woodpeckers and Starlings. Pas-
serines were the most frequent prey found in pel-
lets, but nonpasserine species accounted for slight-
ly more biomass using this method of analysis.
Main prey species represented in the pellets were
the European Starling, Green Woodpecker and
Blackbird. The MPW in pellets was 100.44 g (SE =

163.01) which was smaller than the MPW found in
pluckings ( t — 2.80, df = 20; P < 0.01).

Medium-sized passerines (x = 98.41 g) and large
nonpasserines (x = 268.70 g) were the primary spe-
cies found in plucked remains. In pellets, mammals
(x = 22-71 g) and mainly passerines of small size (x
= 63.70 g) were present. Small passerines (x = 62.69
g) and few nonpasserines of large size (x = 265.62
g) were taken to the nest. The differences among
plucked remains, pellets and direct observations as
to the three main categories of prey found (passer-
ines, nonpasserines and mammals) , were highly sig-
nificant (x 2 - 13.46, df = 4, P < 0.009).

Discussion

We concluded that use of a single method to
determine the diet of Lanner Falcon biases the re-

March 1997

Lanner Falcon Food Habits

suits. Data from plucking sites provided more in-
formation about diet composition per searching ef-
fort than other collecting methods. Pellets are also
important because they provided more complete
information on the composition of prey, especially
that of mammals and insects. Direct observations
of prey taken to the nest appear to be unnecessary
since they add little to quantitative information on
the Lanner Falcon’s diet. The use of plucking data
alone may result in overestimates of large- and me-
dium-size birds, as generally longer and heavier
feathers fall directly beneath the perches, while the
light feathers of small birds tend to disperse (e.g.,
in the wind) . Analysis of pellets alone tends to over-
estimate the amount of small passerines and mam-
mals. Lanner Falcons generally swallow small birds
almost entirely, whereas they tear flesh and feath-
ers from larger birds. This probably accounts for
the high frequency of small passerine remains
found in pellets. We feel that it would be best to
evaluate information obtained from all the collect-
ing methods to most accurately assess the diet of
Lanner Falcons.

We found the diet of Lanner Falcon in central
Italy to be qualitatively different from that of Lan-
ner Falcon in other areas. Birds comprised 88-
100% of the diet in our study area depending on
the method of diet analysis used. These results
were consistent with those of Cramp and Simmons
(1980), except we found for the first time that
Green Woodpecker is taken by Lanners. We also
confirmed that they eat small mammals but that
they do not appear to eat reptiles and insects as
reported by Massa et al. (1991). In Sicily, birds
(90.4%), reptiles (4.1%) and mammals (2.7%) are
in the diet of Lanners (Mebs 1959), with Jackdaw
( Corvus monedula ) , Lesser Kestrel ( Falco naumanni)
and feral Pigeon ( Columba livid) preyed upon the
most. Magpie ( Pica pica) and Spanish Sparrow
{Passer hispaniolensis) are also preyed upon in Sicily
(Massa et al. 1991) and small mammals and rep-
tiles account for only a small percentage of the diet
(4% and 2.3%, respectively).

Acknowledgments

We are grateful to G. Fratalocchi, R. Nardi and G. Cap-
pelli for their help with fieldwork. We are indebted also
to G. Manganelli and N. Baccetti for their invaluable as-
sistance in identifying feathers, bones and skeleton re-
mains of birds. Thanks are also due to F. Cancelli for
allowing us to get free access to the material of the Zoo-
logical Museum of the “Accademia dei Fisiocritici” in

Siena and to P. Galeotti, G. Bogliani and F. Spina for
critical comments on the manuscript. S. Lovari made use-
ful suggestion on the first draft of the paper and im-
proved the English text. J. B. Buchanan and two anony-
mous referees provided useful suggestions and improved
data presentation.

Literature Cited

Bonora, M. and M. Chiavetta. 1975. Contribution a
l’etude du Faucon lanier Falco biarmicus feldeggi en It-
alie. Nos Oiseaux 33:153—168.

Brown, R., J. Ferguson, M. Lawrence and D. Lees.
1987. Tracks and signs of the birds of Britain and
Europe. Croom Helm, Bromley, London, UK.

Cade, T.J. 1965. Relations between raptors and colum-
biforms at a desert water hole. Wilson Bull. 77:340-
345.

Chaline, J., II. Baudvin, D. Jammon and M.C. Saint Gi-
rons. 1974. Les proies des rapaces. Doin, Paris,
France.

Chiavetta, M. 1992. II Lanario. Pages 674-678 in P. Bri-
chetti, P. De Franceschi and N. Baccetti [Eds.], Fauna
d’ltalia-AVES I. Vol. I. Edizioni Calderini, Bologna, It-
aly.

Cramp, S. [Ed.]. 1977—94. The birds of the Western Pa-
learctic, Vol. I-IX. Oxford Univ. Press, Oxford, UK.
Cramp, S. and K.E.L. Simmons [Eds.]. 1980. The birds
of the Western Palearctic, Vol. II. Oxford Univ. Press,
Oxford, UK.

Gensbol, B. 1992. Collins guide to the birds of prey of
Britain and Europe with North Africa and the Middle
East. Harper Collins, London, UK
Goodman, S.M. and C.V. Haynes. 1989. The distribu-
tion, breeding season and food habits of the Lanner
from the Eastern Sahara. Nat. Geogr. Res. 5:126-131.

. 1992. The diet of the Lanner Falco biarmicus in

a hyper arid region of the Eastern Sahara. J. Arid En-
viron. 22:93-98.

Macdonald, D.W. and P. Barret. 1993. Mammals of
Britain and Europe. Harper Collins, London, UK.
MacLean, G. L. 1984. Robert’s birds of Southern Africa.

New Holland Publishers, London, UK
Massa, B., F. Lo Valvo, M. Siracusa and A. Ciaccio.
1991. II lanario ( Falco biarmicus feldeggi Schlegel) in
Italia: status, biologia e tassonomia. Naturalista siciliano
15:27-63.

Mebs, T. 1959. Beitrag zur Biologie des Feldeggsfalken
Falco biarmicus feldeggi.. Vogelwelt 80 : 1 42—1 49 .
Morimando, F., F. Pezzo, A. Draghi and G. Frataloc-
chi. 1994. Prima nidificazione di Lanario Falco biar-
micus in provincia di Siena e note sulla locale distri-
buzione storica. Avocetta 18:157-159.

Svensson, L. 1992. Identification guide to European pas-
serines. Stockholm, Sweden.

Received 2 April 1996; accepted 1 December 1996

J Raptor Res, 31(1):44— 53
© 1997 The Raptor Research Foundation, Inc.

NESTING DISTRIBUTION AND POPULATION
STATUS OF U.S. OSPREYS 1994

Lawrence M. Houghton 1 and Larry M. Rymon

Biology Department, East Stroudsburg University, East Stroudsburg, PA 18301

Abstract. — Ospreys ( Pandion haliaetus) once nested throughout most of the U.S. The decline in this
population due to biocide use has been well documented, as has its recovery following the U.S. ban on
DDT in 1972. A general increase in the nesting distribution and abundance of Ospreys was reported in
the U.S. in 1981 but there was limited dispersal into states with low or extirpated populations. We
conducted a nationwide nesting survey of nesting Ospreys in 1994, updating the 1981 data. Our data
indicate a dramatic increase in the U.S. Osprey population from —8000 nesting pairs in 1981 to —14
200 in 1994. The most dramatic increases were seen in traditional nesting areas, with some new nesting
in the interior U.S. Hacking projects, construction of reservoirs, nest platform management and in-
creased public relations have contributed to the growth of this nesting population.

Key Words: Pandion haliaetus; Ospreys ; population status ; limiting factors', dispersal ; management.

Resumen. — Aguila pescadoras (Pandion haliaetus), anidaba coraunmente en la mayor parte de los
EE.UU. hasta que sus poblaciones fueron afectadas severamente debido al uso de insecticidas. Sin
embargo, despues de la prohibicion del uso de DDT en 1972, la especie parece estar recuperandose.
En 1981, se reporto un incremento general en la distribution de sitios de nidacion y abundancia de la
especie. Sin embargo, la recuperacion parece ser menor en los estados en los que las poblaciones fueron
mas afectadas o extirpadas. En este estudio, reportamos el resultado de un muestreo nacional de nidos
de Aguila Pescadora, que realizamos en 1994 y que reemplaza los datos de 1981. Nuestros datos indican
un incremento dramatico en las poblaciones del Aquila de —8000 pares anidando en 1981 a —14 200
in 1994. Los incrementos mas dramaticos fueron observados en areas tradicionales de nidacion, y en
algunos sitios neuvos en el interior de los EE.UU. Proyectos de reintroduction, construction de presas,
implementation de platformas de nidacion, y un mejor entendimiento del problema por parte del
publico, han sido factores importantes que han contribuido a la recuperacion de la especie.

[Traduction de Jorge Vega Rivera]

Ospreys generally occur along rivers, lakes, sea
coast bays and estuaries, reservoirs, small streams
and ponds, or any body of water where fish, their
principal food, are available (Poole 1989a). Histor-
ical data, though limited, indicate that Ospreys
once nested in suitable habitats throughout most
of the contiguous U.S., but their numbers were
never equally distributed throughout the country.
Records suggest that the Central States Region
(Fig. 1) had the smallest population, which was
nearly extirpated by the early 1900s. The entire
U.S. Osprey population declined precipitously
throughout the 1950s, 60s and early 70s, a result
of widespread use of chlorinated hydrocarbon in-
secticides and habitat destruction (Ames and Mer-

1 Present address: 134 Cheatham Hall, Virginia Polytech-
nic Institute and State University, Blacksburg, VA 24061-
0321 U.S.A.

sereau 1964, Ames 1966, Peterson 1969, Postupal-
sky 1969, Henny and Ogden 1970). The popula-
tion was “Redbook listed” under “rare and endan-
gered fish and wildlife of the U.S.” and classified
as “status undetermined” by the U.S. Fish and
Wildlife Service in 1966 and 1968 (Henny 1977).

Henny (1983) conducted a comprehensive sur-
vey of the distribution and abundance of the entire
U.S. Osprey population in 1981. His research in-
dicated a general increase in the overall popula-
tion, with limited dispersal into states with low or
extirpated populations. This population enhance-
ment was primarily the result of greater reproduc-
tive output after the U.S. ban on DDT in 1972. The
slow dispersal rate was principally a consequence
of high natal site fidelity, especially for males
(Spitzer et al. 1983). Such slow, limited dispersal
forced hacking methods as a means of restoration
in areas with low or extirpated populations. In the

March 1997

1994 Osprey Population

Wmtcni Region

Central States

GttffStaiatflorida MkHSoutli Atlantic

Gnat Lakes Region

Eastern Interior

Northeast Region

1981 1476 pain
1994 2970 pain

1981 Opain
1994 2 pain

1981 1793 pain
1994 2846 pain

1981 2263 pain
1994 4750 pain

1981 579 pain
1994 1066 pairs

1981 1291 pain
1994 2505 pain

Figure 1. U.S. Osprey regions (1994).

late 1970s and early 1980s, Pennsylvania, New York
and Tennessee began programs to restore low or
extirpated populations (Hatcher and Hammer
1983, Schaadt and Rymon 1983).

Our objectives in this study were: 1 ) to survey 48
states for information concerning the general
trend of U.S. Osprey nesting populations, 2) to
compare the 1994 and 1981 distribution and abun-
dance of U.S. nesting Ospreys, 3) to examine any
changes in population numbers and range expan-
sion that may have occurred since 1981, and 4) to
suggest continued management options.

Methods and Materials

Data on the U.S. Osprey population were obtained
from professional sources in each of the lower 48 states.
Biologists from state and federal agencies and individual
Osprey researchers were contacted by telephone or in
person. A follow up questionnaire, state distribution map
and completed sample questionnaire was sent to each of
the contacted individuals. Survey questions addressed the
distribution, abundance, historical data, nesting prefer-
ences, reproductive success and hacking status of the
U.S. Osprey population. State breeding bird adases and
other published and unpublished sources were also re-
viewed, as were state bird books, for information on the
historical population and broad population and dispersal

patterns and trends. To provide an estimate of the com-
pleteness of nesting data, we completed a list of nesting
survey methods used by each state (some states had no
recent surveys) (Table 1).

For purposes of evaluation, Henny (1983) divided the
entire U.S. Osprey population into five regional popula-
tions: Pacific Northwest, Western Interior, Great Lakes
Region, Atlantic Coast, and Gulf States and Florida. An
alternate approach was taken in our study. Comparable
regional populations (Western Region, Great Lakes,
Northeast, Mid and South Adantic Coast, and Gulf States
and Florida) were established, but additional regions
(Eastern Interior and Central States) were added to ex-
amine dispersal patterns, recent population fluctuations
and migratory movements between wintering and breed-
ing areas (Fig. 1)

Results and Discussion

Mid and South Atlantic Coastal Region. This re-
gion begins in Delaware Bay and encompasses all
of Delaware, Maryland, the coastal plain of Virgin-
ia, North and South Carolina and Georgia (Fig. 2).
It supports one of the largest concentrations of
nesting Ospreys in the world (Chesapeake Bay)
(Henny 1983, Poole 1989a). It is within this major
estuarine system that most Ospreys in Virginia and
Maryland breed. The number of nests in the Ches-

Houghton and Rymon

Vol. 31, No. 1

Table 1. Approximate number of Osprey nesting pairs in the U.S. (1981 vs. 1994).

State

Nesting pairs
1981 1994

Source

Alabama

23 2

R. Clay, Alabama Game and Fish

Arizona

25-35 3

G. Beatty, Bald Eagle Management Coordinator

Arkansas

0 4

K Yaich, Arkansas

California

359 (1975)

500-700 5

Ron Jurek, California Dept, of Fish and Game

Colorado

17 2

J. Craig, Colorado Division of Wildlife

Connecticut

95 2

J. Victoria, CT Supervisor of Wildlife Research

Delaware

75-85 2

L. Galvin-Innvaeer, DE Div. of Fish and Wildlife

Florida

1750

2500-3000 4

Mark Westall, President TIOF

Georgia

225-275 3

J. Ozier, GA Dept, of Natural Resources

Idaho

323 (1974-80)

400-425 4

W. Melquist, Idaho Fish and Game Department

Illinois

O 4

V. Kleen, Department of Conservation

Indiana

I s

J. Castrale, Indiana Nongame Biologist

Iowa

0 4

B. Harrisman, IA Dept, of Natural Resources

Kansas

0 4

J. Zimmerman, Div. of Biology, KSU

Kentucky

16 1

D. Yancy, KY Dept, of Fish and Wildlife Resource

Louisiana

10 3

S. Shively, LA Dept, of Wildlife and Fisheries

Maine

1000

1 300-1 800 5

D. Hudson, ME Fish and Game

Maryland

847 (1973-75)

1 000-1 400 4

S. Cardano, MD Dept of Natural Resources

Massachusetts

32 (1980)

260 2

B. Davis, MA Division of Fisheries and Wildlife

Michigan

123

223 1

S. Postupalsky pers. comm.

Minnesota

160

350-450 2

M. Martell, UMN Raptor Center

Mississippi

55-65 2

M. Woodrie, MS State Ornithologist

Missouri

0 4

W. Crawford, Raptor Res. Tyson Research Center

Montana

149

500-600 4

D. Flath, Montana Fish, Wildlife and Parks, MSU

Nebraska

O 4

J. Dinan, NE Game and Parks Commission

Nevada

4 3

G. Herron, NV Department of Wildlife

New Hampshire

29 2

C. Martin, Audubon Society of New Hampshire

New Jersey

200 2 (1993)

C. Clark, Endangered + Nongame Species Program

New Mexico

2 3

S. O. Williams III, NM Dept, of Game and Fish

New York

120

315 2

B. Loucks, NY Endangered Species Unit

North Carolina

450 (1974)

800-1 200 4

R. Wilson, NC Wild Resource Commission

North Dakota

0 4

C. Grondahl, ND Game and Fish Department

Ohio

l 3

D. Case, Ohio Division of Wildlife

Oklahoma

0 4

S. Sherrod, Dir. G.M. Sutton Avian Res. Center

Oregon

308 (1976)

675-700 2

C.J. Henny, NBS Leader NW Research Station

Pennsylvania

20 2

L.M. Rymon, Environmental Studies Dir., ESU

Rhode Island

442

L. Suprock, Div. Fish, Wildl. and Estuarine Res.

South Carolina

151 (1979)

800-1 000 4

T. Murphy, South Carolina

South Dakota

2 3

D. Backlandi, SD Game and Fish Department

Tennessee

66 2

B. Hatcher, TN Nongame and Endangered Species

Texas

3 3

B. Ortego, Biologist, TX

Utah

30 3 (1995)

S. Cranney, UT Division of Wildlife Resources

Vermont

12 2

S. Parren, VT Fish and Wildlife

Virginia

722 (1973-75)

1300-1500 4 (1987)

Westall 1990

Washington

229

350-400 2

K. McCallister, WA Department of Wildlife

West Virginia

3 3

S. Butterworth, WVA Div. of Wildlife Resources

Wisconsin

176

39 1 1

D. Flaspohler, WN Bureau of Endangered Res.

Wyoming
Estimated Total

82 (1974-81)
8000

150-200 4

12,769-15,603

B. Oakleaf, WY Game and Fish Department

Completeness of data

1 Both aerial and ground surveys, this percent was established by Henny et al. (1974) for the efficiency of combined aerial and ground

March 1997

1994 Osprey Population

Table 1 . Continued.

surveys and based on visibility rates.

2 Less intense aerial, ground, or boat surveys (Osprey nests recorded during Bald Eagle aerial surveys, or incomplete aerial and
ground surveys).

3 Intense local surveys, or only surveying/monitoring of suspected and/or traditional nesting areas.

4 No statewide surveys, information acquired from local biologist and/ or other individuals aware of current nesting and population
trends.

5 No statewide data available, therefore, a percent increase for regional data was determined and this percent increase was applied
to the most recent statewide nesting data in order to provide a current estimate.

6 Very low number of nesting pairs (<15), no statewide survey (* states with <15 nests are listed under other categories where
appropriate) .

apeake Bay area has increased slowly over the past
20 years. Apparendy, there has been a large in-
crease in occupied nests along the Patuxent River
from 22 in 1973 to 72 in 1994 (S. Cardano pers.
comm.). Overall, the number of Ospreys nesdng
in the Bay area appears to be leveling off and Spitz-
er (1989) suggests that the population is nearing
carrying capacity. Spitzer (1989) estimated the
mean age at first breeding in part of the Chesa-
peake Bay region to be about two years higher than
that of the region between New York City and Bos-
ton (5.7 vs. 3.7 yr), apparently the result of limited

nest-site availability. This delay should slow popu-
lation growth rate by bringing mortality into bal-
ance with natality (Spitzer 1989, Poole 1989b). The
number of Ospreys nesting in Virginia, the coastal
Carolinas and Georgia has more than doubled
since 1981. Several new interior sites have contrib-
uted to this growth (T. Murphy and J. Ozier pers.
comm., We stall 1990).

Despite fluctuations, the overall number of nest-
ing pairs in Delaware increased from 56 pairs in
1981 to 75—85 in 1994 (L. Gelvin-Innaer pers.
comm.). In Delaware Bay, however, where breed-

• Netting concentration!

Approiimatc taanber of netting pain In
• a pacific area (eg. cxnaaty, reaervolr.etc.)

Figure 2. U.S. Osprey nesting distribution and abundance (1994).

Houghton and Rymon

Vol. 31, No. 1

ing pairs declined drastically in the 1960s and early
1970s apparently due to use of biocides, recovery
has not occurred. On the Delaware side of the Bay,
a region which historically supported numerous
nests, only three nests were occupied in 1987
(Spitzer 1989) and approximately the same num-
ber were reported in 1994 (L. Gelvin-Innaer pers.
comm.). Spitzer (1989) suggests increased water
turbidity may be one limiting factor.

The general trend in traditional areas of these
states has been rapid population growth but very
limited range expansion.

Northeast Region. This region has both coastal
and interior populations. Maine is the only state in
this region which shows a contiguous population
linking the coast with the interior. New Hampshire
has an interior nesting population contiguous with
that of Maine. Other northeast interior popula-
tions are located in northeastern Massachusetts,
western Vermont, western New York and the Adi-
rondack Mountain region of New York (Spitzer
1989).

Expansion of the breeding range of Ospreys in
this region has been slow. From 1975—87, there was
only a gradual spread of breeders along the coast
of Connecticut, New York (Long Island), Rhode
Island and Massachusetts, and relatively few Os-
preys dispersed 25 km or more into substantially
different and unoccupied habitats (Poole 1989a,
Spitzer 1989). Spitzer (1989) recorded eight pairs
that made moderate dispersals to the interior zone
and this number has increased slightly (2-5 addi-
tional pairs) within the past 5 yr.

Most new nests in the Northeast region are lo-
cated in the vicinity of previously established Os-
prey breeding habitats. In the coastal region, this
trend has been due in large part to intensive man-
agement, including nest-site protection and nest
platform construction during the last two decades
(1967-87) (Poole 1989a, Spitzer 1989).

The area from Cape May, New Jersey to Cape
Cod, Massachusetts was most heavily affected by bi-
ocide use. The number of nesting pairs in this vi-
cinity declined from over 1000 nesting pairs in the
early to mid 1900s to less than 200 nesting pairs in
the mid 1970s (Henny 1977, Spitzer et al. 1983).
Following the 1972 ban on DDT, nesting pairs in
this region have gradually increased (Poole 1989a,
Spitzer 1989).

In Massachusetts, nesting has increased dramat-
ically as a result of nesting platform construction.
Over 90% of the Ospreys nesting in Massachusetts

now use such platforms (B. Davis pers. comm.). In
1994, seven out of the 12 Vermont nests were on
platforms and active management should continue
to play a major role in the expansion of this pop-
ulation (S. Parren pers. comm.). Nest manage-
ment, particularly predator guarding of natural
nests, is credited for recent increases in the Osprey
breeding population in New Hampshire (C. Martin
pers. comm.).

The number of Ospreys nesting between New
York City and Boston has grown approximately
10% annually since 1975. If this trend continues,
the number of Ospreys nesting there in the year
2000 should equal or exceed historical records
(Spitzer 1989). New Jersey has experienced an in-
crease of approximately 6% annually (Clark and
Jenkins 1993). New limiting factors (loss of suitable
habitat and decreased suitability of nest sites) , how-
ever, may prevent the number of nesting pairs
from reaching historical or pre DDT numbers
(Clark and Jenkins 1993).

Western Region. Statewide aerial and ground
surveys have been limited in this region so avail-
able data are less conclusive. Despite this, the Os-
prey population is considered to be expanding and
increasing in the region.

In Montana, Ospreys nest primarily in western
portions of the state, mostly at Flathead Lake. In
1974, approximately 23 nesting pairs were located
along the Northern Valley of Flathead Lake. In
1986, there were 66 (Henny and Anthony 1989)
and, in 1994, there were close to 100 nesting pairs
(D. Flath pers. comm.). Overall, statewide nesting
increased from 149 nesting pairs in 1981 to 500-
600 in 1994 (D. Flath pers. comm.).

Wyoming Ospreys are concentrated near the
Montana border, in the northwestern part of the
state (Yellowstone and Grand Teton National
Parks). Henny and .Anthony (1989) indicated that
new nesting occurred in Johnson, Sheridan, Crook
and Carbon Counties and it is estimated that the
number of nesting pairs in Wyoming at least dou-
bled between 1981-94 from 80 to —160 pairs (B.
Oakleaf pers. comm.).

In Idaho, nesting concentrations occur at Lake
Coeur d’Alene, Lake Pend Oreille, Palisades Res-
ervoir and Cascade Lake (W. Melquist pers.
comm.). Henny and Anthony (1989) reported
nesting productivity at Lake Coeur d’Alene in the
mid to late 1980s to be among the highest report-
ed in the literature. W. Melquist (pers. comm.) es-
timated the 1994 population to be over 400 nesting

March 1997

1994 Osprey Population

pairs, a number which greatly exceeds historical
records (Larrison et al. 1967).

Washington state conducted aerial and ground
surveys in 1984 and 1989. Nesting data from these
surveys indicated msyor nesting concentrations in
both eastern and western portions of the state.
These surveys showed an increasing population,
with 275 nesting pairs in 1984 and 346 in 1989
(Watson and K. McCallister unpubl. data) . The av-
erage annual percent increase from 1981-89 was
approximately 5%. If the rate of growth remained
constant between 1989-94, the number of nesting
pairs in 1994 should have been approximately 450-
500. However, because of potential limiting factors
such as a lack of suitable nest sites and habitat
availability, a more conservative estimate for this
population in 1994 is 350—400 pairs.

In Oregon, the largest concentration of nesting
Ospreys is located in the Central Cascade Moun-
tains. The overall Osprey population in Oregon ap-
pears to be expanding. Henny and Kaiser (1996)
reported that the nesting population along the
Willamette River (between Portland and Eugene)
increased from 13 pairs in 1976 to 78 pairs in 1993.
Sixty-six of the pairs were nesting on utility struc-
tures in 1993, while none were nesting on them in
1976. The number of Osprey nesting within the
state has increased from 308 in 1976 (Henny et al.
1978) to >700 in 1994 (C. Henny pers. comm.)

California nesting populations are concentrated
in northern coastal and mountain regions (P.
Bloom pers. comm.). Henny and Anthony (1989)
identified four major populations at Klamath-Trin-
ity system, Shasta Lake, Eagle Lake and Lake Al-
man, but an estimate for the state’s nesting popu-
lation in 1994 was not available, R. Jurek (pers.
comm.) indicated that there were 21-23 nests on
Tamales Bay, 15-20 nests along the Russian River,
35 pairs along the upper Sacramento River and 52
occupied and 30 successful nests in Marin County.
He also noted that the overall number of Ospreys
nesting in California has risen dramatically over
the past 20 yr. Numbers of nesting pairs at Eagle
Lake do not appear to be increasing (Bloom pers.
comm.). Henny and Anthony (1989) indicated a
substantial range expansion and population in-
crease on small reservoirs in extreme northeastern
California (Modoc County) where the population
increased from three pairs in 1980 to 10 pairs in
1987. They also noted an increase in nesting pairs
at Kent Lake (Marin County) from seven pairs in
1975 to 22 in 1986, and an increase in the number

of nesting pairs located within the Sierra Nevada
region. Since 1975, newly reported Osprey nesting
areas have included: Lake Tahoe (El Dorado
County), Lake Oroville (Butte County), Basse Lake
(Madera County), New Melones Reservoir (Tuol-
umne County) and New Bullards Bar Reservoir
(Yuba County) (Henny and Anthony 1989).

The number of Ospreys nesting in Nevada re-
mains low. Two of the four existing pairs are locat-
ed at Lake Tahoe and the other two pairs are nest-
ing along the Huntington Valley (Gary Herron
pers. comm.).

In Arizona, a sizable increase in nesting pairs has
taken place within the past 10 yr. Most of the Os-
preys nest at the White River east/west fork and
the main stem of the Black River in southeastern
Arizona. However, three nests are located near
Flagstaff, there w r as a new breeding attempt on
Lynx Lake near Prescott in 1994 and, in 1996, a
pair nested for the first time in over 30 yr at the
confluence of the Salt and Verde Rivers, east of
Mesa (G. Beatty and R. Vahle pers. comm.).

New Mexico had its only two pairs of Osprey
(1994) nesting on reservoirs in the northern por-
tion of the state. Both pairs began nesting in the
1990s (S. Williams pers. comm.).

The Colorado Osprey population (1994) is small
and concentrated in the northcentral portion of
the state. Hacking has been undertaken to en-
hance the already existing population and to ex-
tend breeding to the front range of the Rockies.
Currendy, three nesting pairs are located far from
the hacking areas; two are located in La Plata
County and one in Pueblo County (J. Craig and K.
Luft pers. comm.).

Most Ospreys in Utah nest along the Green River
and Flaming Gorge Reservoir area in the northeast
corner of the state (S. Cranney unpubl. data). C.
Monson (unpubl. data) reports that nesting also
occurs at Fish Lake (six pairs), Panguitch Lake
Navajo (two pairs), and, in 1995, one pair nested
on Deer Creek Reservoir and another pair nested
in Highland. Construction of reservoirs appears to
have increased growth of Utah’s Osprey popula-
tion.

Many western states continue to show an expan-
sion of nesting pairs eastward, partially due to
changes in inland habitat, particularly the con-
struction of reservoirs (Swenson 1981, Henny
1983). Reservoirs often provide foraging advantag-
es over rivers and lakes because their still, shallow,
open, water areas and reduced turbidity result in

Houghton and Rymon

Vol. 31, No. 1

increased water clarity and higher visibility of fish
(Swenson 1981, Henny 1983).

A comparison of foraging times and nesting den-
sities between free-flowing river habitat and three
river impoundments on the upper Missouri River
in Montana showed food sources and Osprey nest-
ing densities to be higher at impoundments (Grov-
er 1984). This indicates that additional impound-
ments could benefit Osprey populations and en-
courage future range expansion.

Florida and the Gulf Coast. Florida has the high-
est number of nesting Ospreys in this region, with
distinct concentrations from the St. Johns River
south to Lake Okeechobee (Westall 1990). Ospreys
also nest along the east and west coasts and across
Florida Bay, including the Ten Thousand Islands
area (southwest Florida). Ospreys nesting in pen-
insular Florida south of the 29th parallel are non-
migratory or resident birds and, therefore, may be
subject to different biological limiting factors than
Ospreys nesting further north (Poole 1989a).

Food stress may be affecting the once healthy
Florida Bay population (Poole 1989). Declines
there have prompted Florida to designate Osprey
as a species of concern in Monroe, County. Al-
though Florida Ospreys are currently adapting to
an exploding human population, further land de-
velopment could limit food supply and nesting
habitat and thus should be carefully monitored
(Ogden 1978, Westall 1990).

Nesting in the Gulf Coast has been extremely
limited and sporadic (Lowery 1974, Imhof 1976,
Henny 1983, Reinman 1984). The number of Os-
preys nesting in this region has fluctuated through-
out this century and only limited nesting has been
documented (Henny 1983). J. and B. Jackson (un-
publ. data) note that Ospreys were historically
more abundant along the lower Mississippi than
now. A decrease in the number of nesting pairs
there has most likely resulted from human distur-
bance and manipulation (change in water flow, in-
dustry and pollution, and loss of nesting habitat).

In 1 994, Gulf Coast Ospreys were most abundant
on the gulf islands at the southern tip of Mississippi
(50-55 pairs) (M. Woodrie pers. comm.). Only
three nesting pairs were recorded in Texas in 1994,
and though no statewide survey was conducted, it
is unlikely that many more Ospreys nested there.
However, many Ospreys migrate through Texas
and several have been recorded during winter
months (B. Ortego pers. comm.). Alabama and
Louisiana have increasing populations (Table 1),

but inland nests remain sparse and irregular. The
low number of documented nests in these states
may be partially related to a lack of survey cover-
age. Why nesting remains low is unclear, but it mer-
its further attention (Westall 1990). This region is
an important study area for future productivity,
range expansion, nesting and predation research.

The Great Lakes Region. The total number of
breeding pairs in the Great Lakes region has al-
most doubled from an estimated 579 pairs in 1981
to approximately 1014 in 1994. Today’s distribu-
tion of nests is similar to that reported for the pe-
riod of 1963-71 (Postupalsky 1969, 1977); however,
the aggregations are larger, additional adjunct
nests exist between traditional clusters and some
range expansion has occurred. Nests still remain
concentrated in northcentral and northeastern
Minnesota and in northern portions of Michigan
and Wisconsin (M. Martell pers. comm.).

The growth of the statewide population in Mich-
igan observed during 1977—92 has apparently
stopped and some local declines have been noted,
despite high availability of nest sites. The statewide
total remained near 225 pairs from 1992-94 (S.
Postupalsky pers comm.). Wisconsin Osprey nest-
ing data indicate that the number of nesting pairs
there increased steadily during 1983-93, however,
S. Postupalsky (pers. comm.) suggests that present
numbers may be leveling off. Minnesota’s nesting
data was inconclusive but >200 nests were unoc-
cupied in 1994 (M. Martell pers. comm.).

Wisconsin, Minnesota and Michigan all show
signs of range expansion to the south. The expan-
sion in Minnesota has been enhanced by hacking
efforts initiated in Hennepin County in 1984 (M.
Martell pers. comm.). Further expansion and dis-
persal is expected as nesting continues and more
hacked birds return. A recent surge of nesting on
artificial structures (~68% of nests in Wisconsin),
could affect future Osprey numbers and status in
this region. This region should continue to be eval-
uated for factors limiting population growth (e.g.,
measurements of aquatic productivity, fish popu-
lation dynamics, prey accessibility to Ospreys, pred-
ators and competitors, and land use) (S. Postupal-
sky pers. comm.).

Eastern Interior. Major nesting concentrations
such as those in Florida and Chesapeake Bay may
never be realized in this region, but with the ad-
vent of large water-management projects such as
the TAW reservoir and waterway system, and the
hacking projects that have been implemented

March 1997

1994 Osprey Population

throughout this region, a sizable increase in the
number of nesting pairs and distribution may be
expected (Westall 1990).

Freshwater reservoirs have been most beneficial
to the nesting success here, particularly in Ken-
tucky, Tennessee, West Virginia and Pennsylvania
where impoundments have benefited recent Os-
prey reintroductions. Overall, the Osprey popula-
tion in these states surged from five nesting pairs
in 1981 to 105 in 1994. Many of these breeding
pairs are known to be the result of several intensive
hacking programs. For example, in Pennsylvania
1 7 of the 20 statewide nesting pairs include hacked
birds, four of which were hacked in West Virginia
(L. Rymon unpubl. data) . All three of West Virgi-
nia’s pairs are the result of hacking in West Virgin-
ia. Most of the Ospreys now nesting in Tennessee
are either the direct result of hacking or were at-
tracted by recruits from local hacking projects (B.
Hatcher pers. comm.). Kentucky has had similar
results but the birds in both Tennessee and Ken-
tucky have not been well monitored.

Central States. Both the Mississippi and Missouri
Rivers seem ideal as conduits for interior nesting,
yet they remain virtually unoccupied by Ospreys.
Historical data suggest that Ospreys once nested in
small numbers along parts of the Mississippi (J.
and B. Jackson unpubl. data) . Other historical re-
cords indicate that very few Ospreys nested within
the Central States Region (Hicks 1935, Black 1992,
Robbins and Easterla 1992). Reasons why Ospreys
are not presently nesting in most of this region
remain unclear but the Osprey’s slow pioneering
rate may be responsible. Many of these states ap-
pear to have some suitable habitat and most are
adjacent to states where Ospreys currently nest.
Habitat Suitability Indices could be used to deter-
mine if habitat is a major limiting factor (Vana-
Miller 1987). However, recent dispersals into South
Dakota, New Mexico, Texas, Ohio and Indiana, all
states with no known nesting pairs in 1981, indicate
that it may only be a matter of time before Ospreys
expand their range into this region also. If time is
inhibitory, hacking may be one option for accel-
erating the process (Rymon 1989a).

Conclusion

1994 U.S. Osprey Distribution and Abundance.

There has been a significant increase in the num-
ber of nesting pairs in the U.S. from 1981-94. The
overall estimate for the population in 1981 (Henny
1983) was approximately 8000 nesting pairs. Our

1994 population estimate was —14 186 ± 1417
(SD) indicating that the U.S. Osprey population
has increased —75% in just over the past decade.

There were many similarities in the distribution
and abundance of Osprey nesting pairs between
1981 and 1994. Largest increases in numbers of
nesting pairs took place in areas of traditional nest-
ing: the Atlantic Coast, Pacific Northwest and the
Great Lakes Region. A large increase in nesting
pairs also occurred in the Eastern Interior Region
where hacking has taken place. A1 though the re-
cent interior population expansion can be attrib-
uted to hacking efforts, hacking has only expanded
the range and played a very modest role in the
growth of the entire U.S. Osprey population.

Overall, population growth has resulted from: 1)
increased production rates following the 1972 ban
on DDT, most prominent in the Northeast and
Great Lakes Regions, 2) construction of numerous
new impoundments, especially in the Western and
Eastern Interior Regions, 3) artificial nest con-
struction in nearly all regions, particularly the rel-
atively recent use of utility structures and other
man-made structures in the west (Henny and Kai-
ser 1996), 4) hacking projects in the interior, and
5) increased public awareness and support. These
dramatic changes stress the importance for region-
al Osprey management while monitoring the en-
tire U.S. population.

Growth and Expansion of the U.S. Osprey Pop-
ulation. The future growth of regional Osprey pop-
ulations depends, in part, on the rate at which new
breeders are recruited. Regional differences in the
dispersal distances of young may be a reflection of
the differences in the density and availability of
nest sites (Poole 1989b). In New England, where
nesting pairs were severely reduced by pesticides
during the 1950s and 60s, artificial nest sites are
now clustered, abundant and widely available so
most new recruits find nests quickly and breed
soon after arrival (Poole 1989b). In New England,
dispersal distances >50 km are rare (Poole unpubl.
data) .

Ospreys in western and midwestern North Amer-
ica seem to be dispersing much greater distances.
There the breeding range may be restricted by lack
of suitable nest sites and large expanses of unsuit-
able habitat (Poole 1989a). Ospreys in the western
U.S. have traditionally nested in trees or snags near
lakes and rivers (Henny 1983); however, nesting
areas are becoming saturated (C. Henny pers.
comm.) For this reason, they are slowly dispersing

Houghton and Rymon

Vol. 31, No. 1

into new breeding areas around reservoirs where
breeding densities may largely exceed those along
nearby free-flowing rivers (Henny 1983, Swenson
1981, Grover 1984). Lack of natural nest sites at
traditional nesting areas in the western states may
have caused longer dispersal and colonization at
newly constructed impoundments and along utility
structures (Henny and Kaiser 1996).

In the Chesapeake Bay area where nest sites are
saturated, Ospreys have begun to delay breeding
rather than disperse to new areas (Spitzer 1989).
Perhaps this is due to their high natal-site fidelity.
Changes in breeding rates, proportions of non-
breeders in different populations, choices of nest
sites, competition for nest sites, natal dispersal dis-
tances, age at first breeding and nesting dispersion
should be monitored in future seasons (Spitzer
1989).

An increase in the use of artificial nest structures
has played an important role in the overall in-
crease in the number of nesting pairs in the U.S.
(Poole 1989a). Regional data on nesting structures
indicate that approximately 64% of Ospreys in the
U.S. nest on artificial structures, particularly artifi-
cial platforms erected specifically for them (ap-
proximately 50%). Excessive construction of nest-
ing platforms may have drawbacks in the long run,
including habituation to humans, necessary main-
tenance of platforms and higher predation rates
(Poole 1989a).

The construction and addition of artificial nest
structures on public lands has played a critical role
in increasing public awareness and support for Os-
prey. Several states have had volunteer Osprey nest
platform projects, and some have set aside viewing
areas for aesthetic and educational purposes. Nest
platform management is just one example of ex-
panding public awareness. Support from public
utilities and media coverage appear to be enhanc-
ing efforts in public relations which will continue
to be important for the preservation of this species
(Rymon 1989b).

Acknowledgments

We would like to thank all of the dedicated osprey re-
searchers who contributed their statewide nesting data
for the compilation of the 1994 U.S. Osprey nesting data.
We are especially grateful to Charles J. Henny for his re-
search efforts and his encouraging remarks toward our
research. We would also like to thank A.F. Poole and S.
Postupalsky for their valuable comments toward the re-
vision of our original manuscript.

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Received 6 February 1996; accepted 31 October 1996

J. Raptor Res. 31 (1 ) : 54—58
© 1997 The Raptor Research Foundation, Inc.

THE DELAWARE BAYSHORE OF NEW JERSEY: A RAPTOR
MIGRATION AND WINTERING SITE OF
HEMISPHERIC SIGNIFICANCE

Clay Sutton 1 and Paul Kerlinger 2

Herpetological Associates, Inc., 129 Buck Ave.,

Cape May Court House, NJ 08210 U.S.A.

Abstract. — The Delaware Bayshore of New Jersey, including the lower Maurice and Cohansey Rivers
and the coastal zone of Cumberland County, is a major migration and wintering area for raptors. Over
89 days of autumn migration for three years, almost 12 000 raptors of 17 species were counted at East
Point at the mouth of the Maurice River (x = 30.7 raptors/hr). In addition, the lower drainage areas
of the Maurice and Cohansey Rivers and the Cumberland County coastal zone were found to support
high densities of Black ( Coragyps atratus ) and Turkey Vultures ( Cathartes aura). Bald Eagles ( Haliaeetus
leucocephalus) , Red-tailed Hawks (Buteo jamaicensis) , Northern Harriers ( Circus cyaneus), Cooper’s Hawks
( Accipiter cooperii), Sharp-shinned Hawks (A. striatus), Rough-legged Hawks ( Buteo lagopus) and American
Kestrels ( Falco sparverius) in winter. Such high numbers and diversity of migrating and wintering raptors
make the area exceptional and, perhaps, unique in eastern North America. The importance of this area
for raptors emphasizes the need for its conservation, especially in light of rapid development of nearby
farmlands and forests for homes and industry.

KeyWords: migration ; winter population, Delaware Bayshore, conservation.

El Delaware Bayshore de New Jersey: Una migracion de rapaces y invernada con importancia hemisfer-
ico.

Resumen. — El Delaware Bayshore de New Jersey, incluyendo los rios abajo de Maurice y Cohansey y la
zona costera de el condado Cumberland, es el mayor lugar de migracion invernada para rapaces. Arriba
de 89 dias de migracion en el otono por tres anos, casi 12000 rapaces de 17 especie fueron contados
en el Punto Este en la boca del Rio Maurice (x = 30.7 rapaces/hr). En suma, en la boca baja del
desagiie del Maurice y Cohansey Rios y la zona costera del condado Cumberland se encontro apoyo de
densidad alto de Coragyps atratus y Cathartes aura, Haliaeetus leucocephalus, Buteo jamaicensis, Circus cyaneus,
Accipiter cooperii, A. striatus, Buteo lagopus, y Falco sparverius en invierno. Tanta cantidad y diversidad de
rapaces de migracion y sidos de invierno hace la area excepcional y, quiz.as, unico en el este de Norte
America. La importancia de este area para rapaces enfasisa la necesidad para su conservacion, espe-
cialmente con el rapido desarrollo de tierras agricolas y bosques para casas e industria.

[Traduccion de Raul De La Garza, Jr.]

The Delaware Bayshore of New Jersey, especially
Cape May, has long been known as a critical area
for a diversity of migrating birds (Allen and Peter-
son 1936, Stone 1937, Dunne and Sutton 1986,
Wiedner et al. 1992), Although much of the region
has been threatened or altered by human devel-
opment, areas north of Cape May along the Dela-
ware Bayshore including Cumberland County,
have been largely spared. Recent proposals for

1 Present Address: 129 Bucks Ave., Cape May Court
House, NJ 08210 U.S.A.

2 Present Address: 31 Jane St. 14D, New York, NY 10014
U.S.A.

barge port development, industrial sand mines and
other projects along the Maurice and Cohansey riv-
ers in Cumberland County threaten to alter aquat-
ic habitats, wetlands and uplands, rendering them
less suitable for wildlife.

Because of these threats, we conducted an 8-yr
study of raptor use during autumn migration and
winter along the Delaware Bayshore of Cumber-
land County, New Jersey including the Maurice
and Cohansey river drainages. In this report we
document a large and previously unknown aggre-
gation of migrating and wintering raptors in this
area that is of hemispheric significance and worthy
of major conservation effort.

March 1997

Raptor Migration at Delaware Bay

Figure 1. Maps of New Jersey and Cumberland County Coastal Zone study area including the lower Maurice and
Cohansey Rivers, and the Delaware Bayshore. Closed circles indicate observation sites along the rivers. East Point
hawk watch is denoted by a star at the mouth of the Maurice River.

Methods

Migrating raptors were counted at East Point, Cum-
berland County, New Jersey (Fig. 1) from 9 September-
30 November 1989-91, by a single observer during pre-
determined peak flight hours (mid-morning through
mid-afternoon) for an average of 4.4 hr per day (range
= 1-9 hr) from the wooded edge of a salt marsh near
the mouth of the Maurice River at Delaware Bay. On
about one-third of all days, the observation site was
moved 3 km inland from the Bay at about midday, as the
flight became higher and moved farther inland. East
Point is a peninsula that juts out into Delaware Bay. To
assess the relative importance of this site, counts from
East Point were compared with same day hawk migration
counts from Cape May Point, New Jersey, 32 km to the
south. Counters at both sites used standard Hawk Migra-
tion Association of North America methods and record-
ing forms (Bednarz and Kerlinger 1990, Kerlinger 1989).

We sampled wintering raptors along tidal portions of
the Maurice River during eight winters, 1987-88 through
1994—95; the Cohansey River during five winters, 1990-
91 and 1994-95; and in the Cumberland County Coastal
Zone (CCCZ) in five winters 1989-94. Transport among
sites was by automobile. Surveys were done between 22
November— 21 March, mostly between 0800—1600 EST. A
total of 69 surveys were done on the Maurice River and
16 on the Cohansey River. On the Maurice River, a survey

consisted of seven observation stations from a point 22 4
km upstream from the river mouth (Fig. 1 ) to East Point
and at five stations on the Cohansey River from Bridge-
ton downstream to within 2 km of the mouth of the river,
a distance of 17.6 km. At each observation site, all flying
and perched raptors within view were counted for 50
min. Care was taken to avoid recounting individuals by
noting plumage characteristics and flight direction. If a
raptor was observed flying into the adjacent sampling
area it was not counted if resighted in that area. Raptors
seen perched between sampling sites were included in
the nearest sampling site if an individual of that species
was not sighted from the adjacent stations. Ten power
binoculars and a 20 power spotting scope were used. On
about 96% of the surveys, two observers were present and
on the remaining surveys only one observer. The same
observer was present for 65 of the 69 Maurice River sur-
veys and all of the others.

Counts were conducted mostly in sunny weather with
northwest winds. Ninety percent of the counts were done
with these conditions, since they promoted soaring and
hunting of raptors. Habitat along the Maurice and Co-
hansey rivers and the Cumberland County coastal zone
includes tidal marsh and swamp forest, as well as adjoin-
ing upland forests and farms. Wetlands in these areas
range from marshes typical of tidally inundated areas to
tidally influenced freshwater marshes farther upstream.

Sutton and Kerlinger

Vol. 31, No. 1

Saltmarsh cordgrass ( Spartina alterniflora) dominates the
marshes at the mouth of the rivers, with saltmarsh hay
(5. patens ) present. Farther upstream, although tidally in-
fluenced, the marshes are comprised of freshwater and
brackish wetland plants such as wild rice ( Zizania aquati-
ca), common reed ( Phragtnit.es communis), pickerel weed
( Pontederia cordata ) , arrow arum ( Peltandra virginica) , spat-
terdock ( Nuphar advena), waterlily ( Nyrnphaea odorata)
and cattail ( Typha latifolia ) . Common reed is more prev-
alent on the Cohansey River. The banks along portions
of the Maurice River are quite steep and often heavily
wooded, although cropland is near both rivers. Devel-
oped areas are limited to single family homes and a few
recreational boat yards. Both rivers average less than one-
half mile wide and both are about one mile wide near
the mouth.

In addition to surveys on the Maurice and Cohansey
rivers, one road survey per year was made in the first two
weeks of January 1990-94, during which raptors were
counted in the entire Cumberland County coastal zone
(Fig. 1). This latter survey consisted of 12 primary obser-
vation sites along a 48 km course. Standard raptor road
survey methods were used between observation sites. The
coastal zone count included regular counts (nearest cal-
endar date) from the Maurice and Cohansey rivers, all
conducted in a 3-d-period. This method was an attempt
to determine roughly the total number of raptors that
winter in the Cumberland coastal region.

Results and Discussion

Over 89 d (389.25 hr), we observed 11 944
hawks (30.7/hr) of 17 species at East Point (Table
1). Sharp-shinned Hawks ( Accipiter stnatus ) were
most numerous, accounting for 39.7% of all hawks
counted. This species, plus Turkey Vultures (Ca-
thartes aura). Northern Harriers ( Circus cyaneus ),
Red-tailed Hawks (Buteo jamaicensis) and American
Kestrels ( Falco sparverius ) accounted for 84.5% of
all raptors observed. In 1990, the year most obser-
vations were made, a total of 9042 raptors was re-
corded during 308.5 hr of observation on 60 d (9
September-7 December). This was 34.6% of the 26
164 raptors seen at Cape May Point during the
same 60-d-period.

Species composition differed between East Point
and Cape May Point which may be attributable, in
part, to differences among species in their tenden-
cy to cross water. Greater numbers of vultures, Bald
Eagles ( Haliaeetus leucocephalus) , Northern Harri-
ers, Rough-legged Hawks ( Buteo lagopus), Red-
tailed Hawks and relatively fewer Ospreys, Sharp-
shinned Hawks and falcons were counted at East
Point (Table 1). Kerlinger (1985) documented dif-
ferences in water crossing tendency among most
of the species reported here in a study at Cape May
Point. Upon reaching the end of the peninsula,
only some species crossed without hesitation. At

Table 1. Summary of autumn migrants recorded at East
Point and Cape May, New Jersey, on the same 89 days
during 1989, 1990 and 1991. Total numbers of migrants
counted and percentage of total hawks counted at each
site are given. One Swainson’s Hawk ( Buteo swainsoni) was
also seen at East Point.

East Point Cape May

Per- Per-

Species

Count

CENT

Count

CENT

Black Vulture

<1%

Turkey Vulture

1219

10%

Osprey

233

2012

Bald Eagle

<1%

Northern Harrier

888

1110

Sharp-shinned Hawk

4744

40%

16 853

47%

Cooper’s Hawk

754

2312

Northern Goshawk

<1%

Red-shouldered Hawk

234

<1%

Broad-winged Hawk

188

1168

Red-tailed Hawk

1398

12%

1453

Rough-legged Hawk

<1%

Golden Eagle

<1%

American Kestrel

1854

16%

7400

21%

Merlin

297

1551

Peregrine Falcon

855

Total Hawks

11 944

35 968

Total Hours

389.25

798.5

Hawks Per Hour

30.7

45.0

East Point, the vast majority of most species moved
west, whereas Osprey ( Pandion haliaetus), Merlins
( Falco columbarius) and Peregrine Falcons (Falco per-
egrinus ) usually flew to the southeast toward Cape
May. The latter species are not reluctant to cross
Delaware Bay (Kerlinger 1985, 1989). It is likely
that upon seeing the end of the Cape May penin-
sula, many soaring birds turn westward to follow
the Delaware Bayshore, rather than proceeding to
the tip of the Cape May peninsula where they
would be counted. Others fly through Cape May
and are counted near the Point before flying north
and west along the Bayshore toward East Point.

During January road surveys of the coastal zone,
14 species were observed totaling between 472 and
932 birds/survey (Table 2). This year to year vari-
ation was attributable largely to variation in the
number of roosting vultures and, perhaps, to
weather. Species composition and numbers of most
species observed along the Maurice and Cohansey
rivers were similar (Table 2). The numbers of
American Kestrels and Sharp-shinned Hawks along

March 1997

Rartor Migration at Delaware Bay

Table 2. Summary of wintering diurnal raptors along Maurice and Cohansey Rivers and the Cumberland County
Coastal zone area. Letters following species names indicate status of the species (T = federally threatened, t = New
Jersey threatened, e = New Jersey endangered, u = unknown status in New Jersey; first of state letters indicates
breeding season, second indicates nonbreeding season — migration and winter) . Totals are the mean of yearly mean
totals and min-max of yearly average totals.

Species

Maurice River
(1987-1995)

Cohansey River
(1990-1995)

Cumberland Coastal, Zone
(1989-1994)

Mean

Range

Mean

Range

Mean

Range

Black Vulture

9.9

1-45

7.0

3-10

35.8

9-77

Turkey Vulture

73.4

49-111

54.8

41-68

272.8

165-501

Bald Eagle (T,e,e)

5.5

3-10

2.4

2-4

21.2

17-27

Northern Harrier (e,u)

20.0

15-24

19.0

16-22

122

79-171

Sharp-shinned Hawk

2.6

2-4

4.1

3-6

19.8

6-37

Cooper’s Hawk (e,e)

1.3

1-2

1.9

1-3

8.2

3-14

Northern Goshawk

0.1

0-1

0.2

0-1

1.4

0-2

Red-shouldered Hawk (e,t)

0.2

0-1

0.4

0-1

3.6

2-5

Red-tailed Hawk

37.9

33-42

37.3

34-42

134.6

86-159

Rough-legged Hawk

1.0

0-2

0.4

0-1

10.2

4-17

Golden Eagle

0.2

0-1

0.1

0-1

0.8

0-3

American Kestrel

2.2

1-3

7.2

6-11

30.6

18-43

Merlin

0.1

0-1

0.2

0-1

1.4

0-4

Peregrine Falcon (T,e,e)

0.1

0-1

0.2

0-1

1.2

0-2

Totals

154.7

117-204

134.7

115-151

663.8

72-932

Years of sureys

Mean number of surveys

per year (range)

8.6

7-14

3.2

2-4

the Cohansey River were slightly greater than
along the Maurice River because there is more
farmland and forest edge along the Cohansey.
Conversely, more Bald Eagles and Turkey Vultures
were reported from the Maurice River, a function
of greater waterfowl numbers, waterfowl hunting
and carrion present at a livestock farm. Determin-
ing absolute abundance for statistical comparison
is extremely difficult in such a heterogeneous land-
scape.

Turkey Vultures were the most numerous of all
species, followed by Red-tailed Hawks and North-
ern Harriers. Bald Eagles were an important com-
ponent of the winter raptor community of the Del-
aware Bayshore of New Jersey; as many as 16 were
seen on a survey of the Maurice River. Among the
wintering raptors observed, five were on the fed-
eral or New Jersey threatened or endangered lists
(Table 2). Single Merlins, typically a Neotropical
migrant, wintered on the study site in most years.

Probably a combination of factors is attributable
to the great diversity and abundance of raptors mi-
grating and wintering along the Delaware Bay-
shore. Delaware Bay is a topographic barrier to mi-

gration. Raptors reluctant to cross water seem to
either fly around the Bay or terminate their migra-
tion along the Bayshore. The importance of migra-
tion stopovers among songbirds, shorebirds and
waterfowl has long been known, but few biologists
have recognized its importance among raptors
(Newton 1979, Kerlinger 1989). Many of these
birds are compelled to make migratory stopovers
in the area. The abundance of open space, high-
quality habitat and abundant prey make the Bay-
shore very attractive to both migrating and winter-
ing hawks.

The Bayshore has frequently been recognized as
an ecosystem of hemispheric or global significance.
For example, the wetlands of the Delaware Bay are
a RAMSAR site, in part because they support glob-
ally important populations of shorebirds and wa-
terfowl. In addition, the EPA National Estuary Pro-
gram has recognized the area as biotically impor-
tant and has funded m*yor studies of the area (Zap-
palorti et al. 1993) . Much of the Maurice River and
three tributaries, the Manumuskin River, Menan-
tico Creek and Muskee Creek, are now included in
the U.S. National Park Service Wild and Scenic Riv-

Sutton and Kerlinger

Vol. 31, No. 1

er program. The data presented herein support
these designations.

Another factor that influences the winter abun-
dance of Bald Eagles, vultures and to a lesser ex-
tent Red-tailed Hawks, Golden Eagles and Pere-
grine Falcons are the vast concentrations of water-
fowl and other birds that migrate and winter along
the Delaware Bayshore. Winter waterfowl counts
conducted simultaneous to these raptor counts
show that more than 25 species of waterfowl in
numbers totaling greater than 10 000 to 15 000
occur regularly on the Maurice River portion of
the study site alone (Sutton 1988). Likewise, huge
numbers of migrating songbirds, prey for some
hawks, are known to frequent the Delaware Bay-
shore (Wiedner et al. 1992). These birds search for
stopover habitat as they move along the Bayshore.

Our data indicate that the Delaware Bayshore,
and Maurice and Cohansey river drainages, sup-
port one of the largest and most diverse concen-
trations of migrating and wintering raptors report-
ed in eastern North America. We make this claim
understanding that there is little comparative in-
formation on abundance and winter concentration
areas for raptors in eastern North America (other
than for eagles and vultures). The Delaware Bay-
shore is all the more important because it serves
hawks during both winter and migration. We know
of no other migration sites that also serve as im-
portant wintering areas for raptors. It seems that
many migrants end their southbound migration on
the Delaware Bayshore. The conservation value of
this area is obvious.

Acknowledgments

Financial support for this research came from Citizens
United to Protect the Maurice River and its Tributaries,

as well as Cohansey Area River Protection, Cumberland
County Department of Planning and Economic Devel-
opment and New Jersey Audubon Society’s Cape May
Bird Observatory. We also thank J. Galetto, G. Ewan, D.
Fauerbach, C. Zirkle, E. Zirkle, R. Zappalorti and J. Lau-
bengeyer for assistance during this project. C. Shultz, V
Elia and J. Dowdell assisted with the fieldwork. K. Bild-
stein made helpful comments on the manuscript.

Literature Cited

Allen, R.P. and R.T. Peterson. 1936. The hawk migra-
tions at Cape May, New Jersey. Auk 53:393—404.
Bednarz, J. and P. Kerlinger. 1990. Monitoring hawk
populations by counting migrants. Pages 328-342. in
Proc. Northeastern Raptor Symposium, Rochester,
NY. Natl. Wildl. Fed. Sci. and Tech. Ser. #13.

Dunne, P-J- and C.C. Sutton. 1986. Population trends
in coastal raptor migrants over ten years of Cape May
Point autumn counts. Rec. N.J. Birds 12:39-43.
Kerlinger, P. 1985. Water-crossing behavior of raptors
during migration. Wilson Bull. 97:109-113.

. 1989. Flight strategies of migrating hawks. Univ.

Chicago Press, Chicago, IL U.S.A.

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

Stone, W. 1937. Bird studies at Old Cape May. Delaware
Valley Ornithological Club, Philadelphia, PA U.S.A.
SUTTON, C.C. 1988. Wintering raptors and waterfowl on
the Maurice River. Rec. N.J. Birds 14:42-51.

Wiedner, D.S., P. Kerlinger, D.A. Sibley, P. Holt, J.
Hough and R. Crossley. 1992. Visible morning
flights of Neotropical landbird migrants at Cape May,
New Jersey. Auk 109:500-510.

Zappalorti, R., C.C. Sutton and R. Radis. 1993. Cum-
berland County Delaware estuary study; Vol. 1. Rare,
threatened and endangered species study; Vol. 2: Ap-
pendices and mapping. USEPA Delaware Estuary
Study, Cumberland County Department of Planning
and Development, Bridgeton, NJ U.S.A.

Received 9 October 1995; accepted 31 October 1996

J Raptor Res. 31(1) : 59-64
© 1997 The Raptor Research Foundation, Inc.

FOOD HABITS OF COMMON BARN-OWLS ALONG AN
ELEVATIONAL GRADIENT IN ANDEAN
ARGENTINE PATAGONIA

Alejandro Travaini, Jose A. Donazar

Estacion Biologica de Donana, Consejo Superior de Investigaciones Cientificas,
Apartado 1056, 41080 Sevilla, Spain

Olga Ceballos

Grupo de Estudios Biologicos Ugarra, Carlos III 1 9,

31002 Pamplona, Spain

Alejandro Rodriguez, Fernando Hiraldo, Miguel Delibes

Estacion Biologica de Donana, Consejo Superior de Investigaciones Cientificas,
Apartado 1056, 41080 Sevilla, Spain

Abstract. — We evaluated the diet of Common Barn-owls (Tyto alba ) along an elevational gradient in
Argentine Patagonia. Small mammals (mainly rodents) were the main prey accounting for 93.2% of
total prey items. Consumption of rodents appeared to be dependent on their availability. Sizes of mam-
malian prey were variable but most ranged from 10-100 g in body mass. We concluded that the diet of
these barn owls could be used as an index of cricetid rodent populations along the gradient.

Key Words: Common Barn-owl, Tyto alba; prey ; Patagonia ; selectivity, gradient.

Dieta de la lechuza ( Tyto alba ) a lo largo de un gradiente altitudinal en la Patagonia Andina Argentina.

Resumen. — Se estudio la dieta de la lechuza {Tyto alba ) y se la contrasto con la composicion especifica
de la comunidad de micromamiferos a lo largo de un gradiente altitudinal en la Patagonia Argentina.
La principal presa la constituyeron pequenos mamiferos (fundamentalmente roedores) alcanzando un
93.2% del total de presas consumidas. La masa corporal media de los mamiferos presa se concentro
fuertemente en un rango comprendido entre 10 y 100 g. La dieta de la lechuza resulto ser un buen
indicador de la composicion de Cricetidos a lo largo del gradiente estudiado. El consumo de cada uno
de los roedores presa dependio de su disponibilidad en el terreno.

[Traduccion Autores]

Few of the many studies of Common Barn-owl
{Tyto alba) food habits have examined dietary re-
sponses to elevational distribution of prey species
(Herrera 1974, Brunet-Lecomte and Delibes
1984). In northwestern Patagonia, a steep eleva-
tional gradient (600-3000 m) occurs over just a few
km. The abrupt change in elevation and the asso-
ciated change in precipitation (300-3600 mm)
causes a distinct shift in the vegetation from shrub-
steppe to montaneous forest habitat within a few
km. Pearson and Pearson (1982) qualitatively de-
scribed the small mammal species composition
along this gradient. They found six species of ro-
dents {Aconnaemys fuscus, Dromiciops australis, No-
tiomys macronyx, Notiomys valdivianus, Irenomys tar-
salis and Akodon olivaceous) occurred in the humid

forest, while another seven rodent species {Cteno-
mys haigi, Akodon xanthorrinus, Reithrodon auritus,
Eligmodontia typus, Phyllotis darwini, Euneomys sp,
and a marsupial, Lestodelphis hally) were in the dry
shrubsteppe. They also analyzed the diets of Com-
mon Barn-owls at three different localities at the
forest-steppe transition and found that the propor-
tions of species captured by traps and by owls were
different.

Besides providing new information on the tro-
phic niche of Common Barn-owls in Argentina,
here we test the feasibility of using barn owl food
habits to describe changes in composition and
abundance of small mammals along an elevational-
precipitation gradient. We considered the data
provided by Pearson and Pearson (1982) as the ac-

Travaini et al.

Vol. 31, No. 1

tual representation of the small mammal commu-
nity along this gradient and the barn owl prey as
its descriptor. Additionally, Pearson (1986) provid-
ed data on relative abundance of cricetid species
in eight different habitats ranging from steppe to
forest over an 8-yr-period.

We made two comparisons between estimates of
rodent availability and their occurrence in the diet
of common barn owls: (1) on a broad scale, along
the elevational-precipitation gradient, using data
from Pearson and Pearson (1982), and (2) on a
fine scale, along a segment of the complete gra-
dient, using abundance estimates for rodent spe-
cies given by Pearson (1986) . We predicted that the
barn owl diet would reflect gradient changes in ro-
dent community composition (Herrera 1974, Pear-
son and Pearson 1982, Taylor 1994), but it would
be less accurate in reflecting availability of prey at
a fine scale (Jaksic and Yanez 1979).

Study Area and Methods

Located in northwestern Patagonia (70°30'-71°30'W;
39 o 30-40 o 20'S), the study area constitutes a portion of
the Precordillera gradient and partially overlaps with the
area studied by Pearson and Pearson (1982) and Pearson
(1986). The greatest distance between our site and that
of Pearson was under 150 km. The climate of the area is
dry and cold with frost throughout most of the year and
frequent snowfall in winter. Topographically, the area
consists of plains from 800-900 m above sea level that
are dissected by steep rugged valleys and large rivers. In
general, the study area consists of lowland which coin-
cides with river valleys and highland Piedmont with an
intermediate area between them. Pearson (1986, 1987,
1988) has described five different habitats in intermedi-
ate and highland Piedmont: steppe or scattered bushes,
usually <1 m tall, usually mixed with bunch grass, much
of the ground is devoid of vegetation; bunchgrass or hab-
itats with relatively pure stands of one or more species of
bunchgrass; weeds or areas with dense weeds and grasses,
usually growing in moist places; rocks or cliffs with tum-
bled rocks large enough to provide refuge for rodents;
and bare ground or large areas not vegetated with a sub-
strate of fine scree or rock and very few rocky shelters.

Barn owl pellets were collected from September-Feb-
ruary 1991, 1992 and 1994—95. Pellets were collected at
32 isolated localities more than 2 km apart. On this basis,
we assumed that barn owls at each locality were different
individuals. Only fresh pellets were included in our sam-
ples in order to restrict our study to the spring season
and avoid seasonal variation. All pellets were dissected
using standard techniques (Yalden 1977). Small mammal
remains were identified using taxonomic keys (Pearson
1986), reference specimens collected in the study area
and museum collections. Small mammal biomass was de-
rived from Pearson (1983, 1984) and Redford and Eisen-
berg (1992).

Following Flerrera and Jaksic (1980), we characterized
barn owl food habits by the following parameters: mean

mass of all small mammals (MWSM) in the diet; H'NGG,
trophic diversity in relation to the number of individuals
contributed by each higher taxonomic unit (mammals,
birds, amphibians, invertebrates); H'NM, trophic diver-
sity in relation to the small mammal component of the
diet (rodents, lagomorphs, marsupials); and H'NR, tro-
phic diversity in relation to the number of individuals
contributed by each rodent species. The latter three pa-
rameters were computed by means of Shannon’s infor-
mation function (Ludwig and Reynolds 1988). Corre-
sponding values of evenness (J = H'/H'max) were also
calculated.

The use of the barn owl food habits to detect and de-
scribe changes in the composition and abundance of
small mammals along a gradient was explored using cor-
respondence analysis (SAS 1987). This is a multivariate
ordination method (Digby and Kempton 1987, Pielou
1984), applied on a data matrix that included frequen-
cies of appearance (%) of several prey categories for each
barn owl locality. This type of analysis permits one to plot
points for both rows and columns (here localities and
small mammal prey categories) on the same plane. Cor-
respondence analysis is especially appropriate for matri-
ces with numerical frequencies (Cuadras 1980) and does
not normally require previous transformation of data
(Digby and Kempton 1987). We used this analysis to ver-
ify if the pattern in rodent species distribution derived
from barn owl diets was similar to that described by Pear-
son and Pearson (1982) along both the elevational and
precipitation gradients. For this analysis, we restricted
ourselves to those localities (N = 23) with >30 identified
prey items.

Selection among potential prey species was studied
only in intermediate and highland Piedmont. Barn owl
selectivity was evaluated by comparing the species’ rank
for each cricetid rodent in the barn owl diet with the
species’ abundance in the field as estimated by Pearson
(1986). We used a Spearman Rank Correlation Coeffi-
cient (Siegel and Castellan 1988) for this analysis. It was
calculated from the sum of the ranks obtained for each
species in each of the five habitats considered. Compar-
ison was restricted to cricetids because these were the
rodent species for which Pearson (1983) estimated rela-
tive abundance.

Results

A total of 2447 prey items were identified from
barn owl pellets. Small mammals were the main
prey in northwestern Patagonia, accounting for
93.2% of the total. Birds, amphibians and inverte-
brates made up the remaining 6.8% (Table 1). Cal-
culated from a random sample of 71 pellets, the
mean number of prey/pellet was 1.85 (SD = 1.09,
range 1-6).

Among mammals, rodents most frequently oc-
curred in the diet, representing the 98.9% of the
total (Table 1). The MWSM was 54.2 g (SD - 36.7,
range 17.5 g for Calomys musculinus, 286.1 g for
Microcavia australis) but most small mammals were
between 10-100 g (Fig. 1).

March 1997

Barn owl diet in Patagonia

Table 1. Composition of the Common Barn-owl in the lowland, midland, and highland piedmont, northwestern
Argentine Patagonia.

Prey Type

Lowland Midland

Piedmont (%) Piedmont (%)

Highland
Piedmont (%)

Total

MAMMALS

Rodents

Hystricognath

Ctenomys haigi

13.1

3.1

3.0

134

5.5

Galea musteloides

1.7

0.3

0.0

0.5

Microcavia australis

0.3

0.0

0.1

Cricetids

Akodon spp

9.6

21.4

25.0

484

19.8

Auliscomys micropus

0.2

6.3

18.7

226

9.2

Chelemys macronix

0.0

1.0

3.1

1.5

Eligmodontia typus

31.6

13.1

6.7

374

15.3

Euneomys sp

0.0

0.1

2.8

1.0

Geoxus valdivianus

0.2

0.3

1.0

0.5

Irenomys tarsalis

0.0

0.1

0.5

0.2

Oryzomys longicaudatus

6.6

8.2

6.0

171

7.0

Phyllotis darwini

4.0

12.8

5.6

198

8.1

Reithrodon auritus

23.0

16.9

24.4

522

21.3

Calomys musculinus

5.4

1.9

0.0

2.1

Marsupials

Marmosa pusilla

1.8

1.1

0.0

1.0

Lagomorphs

Oryctolagus cuniculus

0.0

0.2

0.1

Birds

1.3

0.8

0.3

0.8

AMPHIBIANS

0.0

0.5

0.0

0.2

INSECTS

0.3

11.9

2.8

143

5.8

TOTAL PREY

H’NGG

JNGG

H’NM

JNM

H’NR

JNR

(595)

(980)

(872)

(2447)

0.28

0.20

0.06

0.05

2.06

0.78

These barn owls had a relatively narrow diet as
shown by the H'NGG value (Table 1). The low even-
ness index was largely due to their concentration
on mammal prey (Table 1). The diversity and even-
ness of the small mammal component (H'NM)
showed that the diet was based upon a small num-
ber of small mammal species, most of which were
rodents. Among rodents, H'NR (2.06) and evenness
(0.78) reached the highest values, denoting both
the high number of rodent prey species consumed
(N = 14, 82% of total present in the area) and their
overall even representation in the diet.

Two axes generated by the correspondence anal-

ysis accounted for 49.5% of the variance in the diet
(Fig. 2) . Representation of localities and prey cat-
egories on the plane defined by the two axes clear-
ly segregated the dry, lowland Piedmont localities
(which tended to the positive zone of Axis I and
negative zone of Axis II) from the rest of localities
(which occupied all the space defined by Axis I
and the positive zone of Axis II) . Prey in the low-
land Piedmont localities included Ctenomys haigi,
Microcavia australis, Galea musteloides, Eligmodontia
typus and Calomys musculinus. Prey in most of the
other localities, included in mid- and high Pied-
monts were Akodon sp, Auliscomys micropus, Chelemys

Travaini et al.

Vol. 31, No. 1

Figure 1. Relative frequencies (%) of small mammal prey in the diet of Common Barn-owls in northwestern Ar-
gentine Patagonia, ordered along a logarithmic axis of body weights.

macronix and Geoxus valdivianus. In a few localities,
Phyllotis darwini and Euneomys sp. also occurred.

The similarity between the expected and ob-
served rank order in cricetid species in the diet

Figure 2. Variation in the taxonomic composition of the
diet of the Common Barn-owl in northwestern Argentine
Patagonia. Plot of the Correspondence Analysis, areas of
each delimited by minimum poligon. See Table 2 for
most species abbreviations. MAR, Marmosa pusilla ; CAL,
Calomys musculinus; GAL, Galea musteloides; CTE, Ctenomys
haigi. Circles correspond to highland Piedmont, quadrats
to lowland Piedmont and triangles to intermediate Pied-
mont.

(Table 2) indicated that barn owls fed on prey ac-
cording to their availability in intermediate and
highland Piedmont areas ( r s = 0.91, P< 0.01). The
most important difference was the greater impor-
tance of Reithrodon auritus and the overall absence
of Akodon sp. in the diet.

Discussion

Barn owls in our study preyed almost exclusively on
rodents, as found in most other studies (Smith and Cole
1989, Bellocq 1990, Iriarte et al. 1990, De Santis et al.
1994, Taylor 1994). This suggests that these barns owls
do not behave as opportunistic, nonselective predators as
suggested by Mikkola (1983). Taking this into account,
care should be taken when interpreting the unusually
high predation on birds reported by Noriega et al. (1993)
in the Patagonian zoogeographic domain (Ringuelet
1961). Their data may reflect individual differences of
individual barn owls.

Our mean body mass estimate of small mammals in
the diet (56.2 g) was intermediate between that reported
in Spain (21.2 g) and Chile (70.7 g) (Herrera and Jaksic
1980). Because the largest prey taken in the three areas
was the European rabbit, ( Oryctolagus cuniculus), the
smaller MWSM in Argentina when compared with Chile
was a consequence of both the high concentration of
smaller prey than in Chile (30-500), and the greater con-
sumption of smaller rodents than in Chile. In Argentine
Patagonia, only one of the three available rodent species
weighing more than 150 g ( Ctenomys haigi ) was readily
consumed by owls, while in Chile three of four such spe-
cies were consumed in amounts similar to that of our C.
haigi (Herrera and Jaksic 1980).

H'NGG diversity and evenness indexes were similar to

March 1997

Barn owl diet in Patagonia

Table 2. Expected composition of Common Barn-owl diet expressed as a percentage for five habitats, based on
Pearson (1983, 1986).

Species 1

Steppe

Bunch

Grass

Weeds

Rocks

Bare

Owl Diet

Field Rank

AKO

42.0

76.0

49.0

41.0

36.0

27.0

AKO

AUL

2.0

12.0

16.0

2.0

23.0

12.0

REI

ELI

CHE

0.0

1.0

0.0

10.0

1.9

ELI

AUL

ELI

43.0

12.0

2.0

3.0

13.0

AUL

ORY

EUN

0.0

1.0

15.0

2.8

PHY

REI

GEO

0.0

2.0

0.0

2.0

1.0

ORY

PHY

IRE

0.0

1.0

0.0

0.3

EUN

ORY

4.0

0.0

19.0

2.0

8.0

10.0

CHE

PHY

4.0

0.0

48.0

1.0

11.0

GEO

REI

5.0

0.0

11.0

2.0

21.0

IRE

1 AKO, Akodon sp.; AUL, Auliscomys micropus ; CHE, Chelemys macronix; ELI, Eligmodontia typus\ EUN, Euneomys sp.; GEO, Geoxus valdi-
vianus, IRE, Irenomys tarsalis; ORY, Oryzomys longicaudatus; PHY, Phyllotis darwini; REI, Reithrodon auritus.

those found by Herrera and Jaksic (1980) for Chilean
barn owls. The inclusion of amphibians and the absence
of reptiles by the Argentine owls were the main reasons
for these differences.

Among mammals, the preponderance of rodents
among the diets of Argentine owls made the H'NM di-
versity and evenness indices very low compared to those
obtained for Chilean owls (Herrera and Jaksic 1980),
which were more equally represented. In Chile, Spain
and Argentina, the low consumption of the European
rabbit is likely related to its large body size.

The correspondence analysis results concurred with re-
sults obtained by Pearson and Pearson (1982) and our
prediction with respect to the gradient with which cri-
cetid rodents are associated. Both species and localities
were segregated by an aridity gradient.

Irenomys tarsalis occurred in the barn owl diet only near
the Nothophagus forest. Auliscomys micropus was eaten only
in forest or dense cover habitats but Oryzomys longicau-
datus was eaten in all the habitats considered for this
analysis. Eligmodontia typus occurred in the diet of owls
only in open habitats with bare soil and scattered desert
shrubs, Ctenomys haigi occurred in the diet in open areas
with sandy soils, and Calomys musculinus and Reithrodon
auritus occurred in the diet of birds associated with the
arid portion of our gradient.

Observed consumption of Reithrodon auritus was, both
in percent frequency and rank order, higher than ex-
pected. A similar situation was observed by Jaksic and
Rau (1986) for the Great Horned Owl ( Bubo virginianus)
in a Chilean Patagonian steppe with about the same
mammalian composition that we report. As in Europe
and North America, comparison of the abundance of
small mammal species is often susceptible to problems of
differential trapability. On the other hand, Akodon sp. was
consumed less than expected by chance in all five habi-
tats. Perhaps their more diurnal activity period and small
body weight contributed to these results (Taylor 1994).
Based on our knowledge, the ecology of Reithrodon auritus
and Akodon sp. resembles that of voles and mice de-
scribed in North America and Europe (Marti 1974, Col-

vin 1984). There, barn owls frequently select voles in the
presence of other prey, probably because the former are
heavier and easier to catch (Taylor 1994). Three factors
provide a plausible explanation. First, K auritus (80.4 g)
is 3 times heavier than Akodon sp. (28.0 g), even a bigger
difference to that found between voles and mice (Marti
1974, Colvin 1984). Second, Akodon sp. shows greater ac-
tivity during the daytime (Rau et al. 1981). The opposite
is true for R, auritus which is a typical nocturnal species
(Pearson 1988), thus overlapping its activity period with
that of the barn owl. Finally, R. auritus feeds in open
grassy habitat where it may be more exposed to owl pre-
dation, while Akodon sp. prefers dense bushes in the
steppe (Pearson and Pearson 1982), perhaps gaining pro-
tection against owl predation.

Acknowledgments

We are very grateful to Martin Funes, Obdulio Mon-
salvo and Miguel Angel Pineda for help with the field
work. Our thanks to Gerardo Alena, Gabriel and Miguel
Anz, Raul Cordero, Victor Soleno, the administrators of
the Estancias Cerro los Pinos, Serranias de Lolen, Collon-
Cura and Chimehuin, who permitted us to work on their
land. Logistic support was provided by the Centro de
Ecologia Aplicada del Neuquen (Argentina); we thank
Alejandro del Valle and Antonio Guinazu for their con-
stant kind assistance. We also want to thank Marta Pian-
tanida from the Museo de Ciencias Naturales Bernardino
Rivadavia for providing us with a complete small mammal
reference collection. D.W. Holt, C. Marti and two anon-
ymous referees improved the manuscript. Financial sup-
port was provided by Instituto de Cooperacion Iberoam-
ericana and the Ministerio de Asuntos Exteriores (Spain)
throughout the Programa de Cooperacion Cientifica con
Iberoamerica.

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Received 23 April 1996; accepted 26 November 1996

INDICATORS OF MALE QUALITY IN THE HOOTS OF

TAWNY OWLS (STRIX ALUCO)

Bridget M. Appleby

Wildlife Conservation Research Unit, Department of Zoology, University of Oxford,

South Parks Road, Oxford 0X1 3PS UK

Stephen M. Redpath

I. T.E. Monks Wood, Abbots Ripton, Cambridgeshire PEI 7 2LS UK

Abstract. — The number of songs in a male’s repertoire, and the amount of time he spends singing,
have been shown to correlate with territory size and quality, reproductive success, parental care and
parasite load in some passerine species. In addition, females of some species use song rate and com-
plexity as a cue to mate choice and are more responsive to more frequent and complex songs. Few
studies, however, have examined the influence of body size and parasitic infections on the sound fre-
quency (pitch) and structure of vocalizations of birds. The Tawny Owl ( Strix aluco ) hoot is important in
communication between birds at night when visual information is limited, and is simple enough to allow
a quantitative analysis of its structure. Six temporal and four frequency measures of the hoots of 50
Tawny Owls were taken, and compared to body mass, wing length, breeding success and number and
intensity of parasitic infections of the singers. There was a decrease in call frequency with increasing
body mass and the vibrato tail of the last note was longer in larger birds, but there was no part of the
call that correlated with breeding success. There was an increase in call frequency as the number of
parasitic infections increased, and there was a decrease in the length of calls as the intensity of parasitic
infections increased.

Key WORDS: Strix aluco; hoot structure, body weight, body size, breeding success; blood parasites; female choice,

assessment.

Indicadores de calidad del macho en ululatos de Strix aluco.

Resumen. — La cantidad de canciones en el repertorio del macho, y la cantidad de tiempo que dedica
cantando, han mostrado una correlacion entre el tamano del territorio y la calidad del exito de re-
production, preocupacion paterna, y carga parasitica en especies passerinus. En suma, hembras usan la
velocidad y complication de la cancion como serial para escoger su pareja estan mas interesadas en
canciones complicadas y de mas frecuencia. Pocos estudios, sin embargo, han examinado la influencia
del tamano corporal y infeccion parasitica en la frecuencia modulada del sonido (tono) y estructura de
vocalization en pajaros. El ululato de Strix aluco es importante en comunicacion entre los pajaros por
la noche cuando information visual es limitada, y es lo suficientemente como simple para dejar un
analisis cualitativo de su estructura. Seis temporal y cuatro frecuencias moduladas de los ululatos de 50
S. aluco fueron tornados, y comparados al tamano corporal, largo de ala, exito de cria, y numeros e
intensidad de infeccion parasitica de los cantadores. Hubo reduction en frecuencia de llamadas y au-
mento de el tamano corporal y la cola vibratus de la ultima nota fue mas larga en los pajaros grandes,
pero no hubo ninguna parte de la llamada que hizo correlacion con el exito de cria. Hubo un aumento
en la frecuencia de llamadas cuando la cantidad de infeccion parasitica aumentaron, y hubo una re-
duccion en la duration de llamadas cuando la intensidad de infeccion parasitica aumento.

[Traduction de Raul De La Garza, Jr.]

Some bird species can recognize their offspring
(Tschanz 1968, Beer 1969, Beecher et al. 1981),
mates (White 1971, Brooke 1978, Jouven tin et al.
1979) and territorial neighbors (Falls 1992, Gal-
eotti and Pavan 1993) through their vocalizations.

Song can also give information on the position and
orientation of the signaler (Richards 1981, Mcgre-
gor and Falls 1984, McGregor and Krebs 1984).

Song appears to also be important in sexual se-
lection in many species and females have been

Appleby and Red p ath

Vol, 31, No. 1

shown to be more responsive to males with higher
song output (Houtman 1992, Payne and Payne
1977) whether song output was increased by
lengthening songs or reducing intersong intervals
(Wasserman 8c Cigliano 1991). In Willow Warbler
( Phylloscopus trochilus) males, there was a link be-
tween high song output and good territories (Ra-
desater et al. 1987, Radesater 8c Jakobsson 1989),
and Houtman (1992) showed that Zebra Finch
(Poephila guttata ) males with higher song output
produced heavier offspring. Females of some spe-
cies have also been shown to prefer males with
more complex repertoires (Searcy 8c Marler 1984,
Baker et al. 1986, Catchpole et al. 1984, Eens et al.
1991a). The complexity of song has been found to
correlate with territory size, survival and reproduc-
tive success (Hiebert et al. 1989, McGregor et al.
1981, Catchpole 1986).

As song is used by males to attract females, it has
been hypothesized that song may give information
on the parasite infections of male singers. Hamil-
ton 8c Zuk (1982) found bird species with more
complex songs were more likely to have blood par-
asite infections, but this correlation disappeared
when phylogeny was taken into account (Read 8c
Weary 1990). Moller (1991) studied the effect of
parasites within a population of Barn Swallows (Hi-
rundo rustica ) and found that males infected with
mites produced less song.

Despite the implications for mate choice, litde is
known about how the size and weight of a bird
affects its vocalizations, and whether parasite infec-
tions affect the structure or frequency of a bird’s
call. This is particularly relevant for nocturnal
birds, which may not have detailed visual infor-
mation about a potential mate or rival. In this pa-
per, we present an analysis of the information con-
tained in the “hoot” vocalization of the Tawny Owl
(Strix aluco). Tawny Owls are nocturnal woodland
birds, so visual information transferred between
territorial rivals or potential mates at night is prob-
ably very limited. The hoot is individually distinct
and constant with time and is thought to function
in communication between the sexes as well as ter-
ritorial defense (Galeotti 8c Pavan 1991).

Methods

Recordings of Tawny Owls were made at Wytham
Woods, Oxfordshire (51.46° N 1.2° W), Monks Wood,
Cambridgeshire (52.24° N 0.14° W), a farmland area,
(The ‘Fens’) Cambridgeshire (52.29° N 0.1° W) and
Kielder Forest, Northumberland (55° 15'N 2° 35'W). Re-
cordings were made using a Uher or Sony Walkman Pro-

fessional tape recorder (TC-D5 PRO) with a Sennheiser
MZW 816 microphone. Recordings were made of male
birds on calm, dry nights from October-December 1992-
93 in Wytham Woods, and in March 1993 in Monks Wood
and the Fens. Recordings of birds at Kielder Forest were
made in November 1994. Birds were stimulated to hoot
using playback of an unfamiliar male owl. The hoots of
male owls can be distinguished from hoots of female owls
by the squeaky grating quality of the female hoot. In the
case of the Wytham owls, the sex of the birds was con-
firmed using radiotags. Recordings were made as near to
birds as possible, at distances between 5-50 m.

Sonograms were produced on a Macintosh LCII com-
puter. Tawny Owl hoots have a basic structure of three
notes (Fig. 1) that can be clearly determined in owls from
all areas. Sonograms of the whole hoot were made using
Soundedit Pro software (Macromind Paracomp, Inc., 600
Townsend, San Francisco, GA 94103 U.S.A. 1992) and
temporal measures were recorded. Soundedit Pro did
not give accurate frequency measures, so sonograms of
the first note of each hoot were made with Canary soft-
ware (Cornell Laboratory of Ornithology, 159 Sapsucker
Woods Road, Ithaca, NY 14850 U.S.A.), and the frequen-
cy measures were recorded.

Temporal measures used were similar to those de-
scribed by Galeotti and Pavan (1991). Six temporal mea-
sures were recorded for each call. These were: note 1
(Dl), internote interval one (14), note two (D2), inter-
note interval two (15) and note 3 was split up into fre-
quency modulated length (FML) and tail (Fig. 1). Fre-
quency measures used were the highest and two lowest
frequencies of the first note (HIGH, LOW1 and LOW2)
and the middle of the highest part of the first note
(MED) . All time measures were recorded in milliseconds
(ms) and all frequency measures were recorded in KHz,
Only clear recordings with little background noise were
used for making sonograms. Any sonograms where all
call parameters could not be determined were discarded.
An “average” sonogram was calculated for each owl by
taking the mean of a minimum of 3 calls from each owl.

Breeding success of owls was measured at Wytham
Woods, Monks Wood and the Fens in 1993. Breeding suc-
cess was defined as the total number of owlets fledged/-
breeding area and was found by monitoring nests or by
locating calling chicks in June and July before mortality
of fledged chicks has occurred. Fledged young can be
located in June and July when they call loudly and con-
tinuously for food (Muir 1954, Southern 1970).

Males were caught in spring 1993 and 1994 using nest-
box traps (Petty et al. 1994). Males were weighed, mea-
sured for wing length and blood slides were prepared by
placing a drop of blood from the branchial vein directly
onto a glass slide and smearing it with a second slide to
produce a blood layer one cell thick. The smear was then
air dried and fixed in absolute ethanol and stained with
Giemsa’s stain. Parasite species were identified by M. An-
war at Oxford University and the number of each species
was quantified by counting the number of parasites in 10
000 blood cells. The two measures of parasite infection
used were the number of parasite species present and
the total number of blood cells containing parasites (all
parasite species pooled) . For some owls, the blood para-
site counts and body measurements were taken at differ-

March 1997

Information in Tawny Owl Hoots

Figure 1. Sonogram of Tawny Owl hoot showing time and frequency measures.

ent times of year than recordings of their hoots. In this
case, the identity of the owl was confirmed using radi-
otags or by confirming that the same male was present
the following breeding season.

The data were examined visually using histograms and
no evidence was found for a significant deviation from
the normal distribution. Parametric tests were therefore
used.

Results

The hoots of 50 male Tawny Owls were recorded
at the four study sites (Table 1). Although none of
the temporal measures of call varied significandy
among the four sites (ANOVA, P > 0.05), there was
a significant difference between the sites in all of

Table 1. The number of Tawny Owls from the four
study sites for which data on both call parameters and
body measurements, breeding success and parasite loads
were available.

Fledg- # Para-

Study # Body Wing ing Infec- site
Site Owls Mass Length Success tions Load

Wytham

Monks Wood

Fens

Kielder

the frequency measures (LOW1 P 3>46 = 1 7.03, P <
0.0001; LOW2 P 3>46 = 12.58, P< 0.0001; MED P 3f46
= 4.63, P = 0.007; HIGH P 3)46 = 6.08, P = 0.001).
The Kielder owls were lighter than those of the
other populations (jF 3>15 — 3.25, P — 0.051), but this
was based on only 4 birds from Kielder and there
was no significant difference among the sites for
wing length and the breeding success of pairs (P
> 0.05).

Pearson Rank Correlations were not significant
for body mass, wing length, breeding success and
parasite number and load for any of the owls used
in the analysis; however, there was a significant cor-
relation between number of parasites and breed-
ing success (r = 1, P < 0.001) which was based on
only four Wytham owls.

There was no significant correlation between the
body mass of a bird and the length of any of the
notes or the internote intervals of its call, but there
was a negative correlation between the highest fre-
quency of the song and body mass (Fig. 2, r =
—0.48, P <0.05). There was no significant corre-
lation between wing length and any of the fre-
quency measures of the call. The only time mea-
sure of the call that correlated significantly with
wing length was the tail of the third note (r = 0.71,
P < 0.003). Likewise, there was no significant cor-

Appleby and Redpath

Vol. 31, No. 1

+ LOW 1ST weight

▲ HIGH
• LOW 2ND
B MEDIUM

Figure 2. Effect of body mass of Tawny Owls on the four
frequency measures of their hoots.

relation between average breeding success in 1993
and any of the call parameters. All four frequencies
were entered into a regression model as indepen-
dent variables to try to predict breeding success as
the dependent variable, however the model did
not significantly predict breeding success (iq^o =
0.29, P = 0.88).

The number of parasite species present in an
individual ranged between one and four. The par-
asites found were Leucozytozoon ziemanni, Haemopro-
teus syrnni, H. noctuae and Trypanosoma. All fre-
quency measures increased with increasing num-

X Wytham
□ Kielder

Figure 4. Effect of parasite load on the total length of
Tawny Owl hoots.

bers of parasites (Fig. 3) , but this was only signifi-
cant for the lowest frequency at the start of the first
note. The Kielder birds had higher numbers of
parasites and higher frequency calls, but due to the
small sample size it was not possible to analyze the
data by site to see if the relationship held in each
case.

There was a significant negative correlation be-
tween the length of the internote interval (r =
— 0.75, P < 0.05), gaps in the song (r = —0.76, P
< 0.05), and the total length of the song (Fig. 4, r
= -0.87, P ^ 0.05) and the parasite load, but there
was no significant correlation with any of the fre-
quencies measured (P > 0.05).

NUMBER OF PARASITIC INFECTIONS

+ Low 1st
A High
O Low 2nd
A Medium

Figure 3. Effect of parasitic infections of Tawny Owls on the four frequency measures of their hoots.

March 1997

Information in Tawny Owl Hoots

Discussion

For a bird embarking on a territorial dispute, assessing
the size of the opponent is important in predicting the
probable outcome of a fight. This is especially true for
an aggressive bird like a Tawny Owl, where vocal disputes
are often fierce and prolonged. For a female assessing a
potential mate, the body mass of a male might give in-
formation on resources available in his territory. The size
of the syrinx is likely to affect the frequency of the call
and could vary with the size of a bird. We found that the
highest frequency of Tawny Owl calls to be more closely
correlated with body mass than the lowest frequency.
This decrease in highest note frequency with size implies
that birds might be emitting their highest note to adver-
tise their small size. Newton (1988) found that smaller
sparrowhawk males had a higher lifetime reproductive
success than large males. Although the selective pressures
favoring reduced size in Tawny Owl males are probably
different to those in sparrowhawks, reversed sexual di-
morphism implies there might be selection for smaller
males in Tawny Owls. Females might, therefore, favor
small males and be using male hoots to assess this.

The only call parameter that correlated significantly
with wing length was the length of the vibrato tail of the
third note, which increased with wing length. Wing
length is an indication of body size of birds. It is possible
that a larger bird might be able to sustain the last note
for a longer time, perhaps due to increased lung volume.
Data on more birds would be needed to ascertain wheth-
er the length of the last note was a reliable signal of body
size.

None of the measures of the call were significantly cor-
related with breeding success, so there is no detectable
signal that females could reliably use to assess the breed-
ing capability of a potential partner. As there was only
one year’s breeding data available, and this varies with
food supply and site (Petty 1992), it is possible that a
correlation does exist between reproductive ability and
call parameters, but it was obscured by other factors.

All measures of frequency increased with increasing
number of parasitic infections and this was significant for
the lowest frequency at the start of the first note. Males
that are able to give low frequency calls might therefore
be indicating a resistance to parasites. Birds with higher
parasite loads gave significantly shorter calls. Physiologi-
cal reasons that might cause this are obscure, but it might
imply that long calls are costly to give and are therefore
an indication of good health. In frogs, heavily infested
males have also been shown to have below average call
durations (Read 1988), and long calls in frogs are more
effective in attracting females (Rand and Ryan 1981,
Wells and Schwartz 1984, Ryan 1985). Lengths of owl
hoots are fairly constant with time (Hirons 1976) so it is
possible that call length could be utilized to signal para-
site burdens and potential breeding capabilities.

Acknowledgments

We are grateful to David Macdonald, Paul Johnson, Ian
Newton and Nick Davies for commenting on earlier
drafts of manuscripts. Parasite analysis was performed by
All Anwar, and Steve Petty kindly provided information
on the weights and breeding success of Kielder birds.

This work was completed while B.M.A was in receipt of

a NERC studentship.

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Received 8 April 1996; accepted 24 November 1996

ROLE OF REFUSE AS FOOD FOR MIGRANT,
FLOATER AND BREEDING BLACK KITES
(. MILVUS MIGRANS)

Guillermo Blanco

Departamento de Biologta Animal, Universidad de Alcala de Henares,

Alcala de Henares, E-28871 Madrid, Spain

Abstract. — The use of refuse by breeding, floating, and migrating Black Kites ( Milvus migrans) was
studied near a large garbage dump in Madrid, Spain. Refuse was an important food resource for non-
breeding Black Kites, especially migrants that fed almost exclusively at the garbage dump. The dump
was only of secondary and variable importance for kites during the breeding season. Pairs of breeding
kites nesting in nearby wooded areas foraged mainly on a wide variety of wild prey and only sporadically
ate refuse at the dump. Floaters roosted in nearby wooded areas but, unlike breeding pairs, they foraged
mainly by scavenging at the dump. Exploitation of food resources other than refuse by breeding kites
may be due to their need for large amounts of prey biomass for brood provisioning. Perhaps dumps
augment populations of Black Kites by providing rich foraging areas where large numbers of nonbreed-
ing and migratory kites can scavenge for food.

Key Words: Black Kite, Milvus migrans; garbage dump ; foraging, refuse.

Importancia de la basura en la dieta de Milanos Negros ( Milvus migrans) migrantes, flotantes y reprod-
uctores.

Resumen. — La importancia de la basura en la dieta de Milanos Negros ( Milvus migrans) reproductores,
migradores e integrantes de la poblacion flotante fue estudiada en las cercanias del basurero de Madrid,
Espana. La basura fue un recurso importante para los milanos no reproductores, especialmente para
los migrantes, pero su papel fue secundario aunque variable para los milanos que se reprodujeron cerca
del basurero. Los individuos flotantes forrajearon en el basurero pero tambien consumieron una amplia
variedad de presas salvajes. La abundancia de alimento en el basurero no provoco que los milanos
nidificaran en sus cercanias, ya que la parejas que asi lo hicieron consumieron basura solo esporadica-
mente. Por el contrario, los individuos flotantes percnoctaron a diario en la zona arbolada mas cercana
al basurero. El uso preferencial de presas salvajes frente a la basura por los milanos reproductores
puede ser explicado como consecuencia de la necesidad de presas de gran biomasa para su consumo
por los polios en crecimiento. Durante la estacion reproductora, el vertedero de Madrid podria tener
una importancia indirecta para el mantenimiento y conservation de la poblacion reproductora a traves
del reemplazo de las perdidas en los reproductores por los individuos flotantes.

[Traduction del Autor]

Breeding and nonbreeding segments of raptor
populations often share the same foraging habitats
(Newton 1979). Floaters, nonbreeding, nonterri-
torial adults and subadults, have larger home rang-
es and greater mobility than breeders, and usually
concentrate in areas rich in food (Newton 1979,
Ceballos and Donazar 1990). Interactions between
floaters and breeding conspecifics has been little
studied despite its potential to influence raptor
population dynamics (Newton 1979). Both breed-
ers and floaters of social species such as the Milvus
kites coexist in breeding areas when food is abun-
dant (Espina 1986, Koga et al. 1989, Heredia et al.

1991), especially where large waste accumulations
occur at garbage dumps (Ceballos and Donazar
1988, Blanco 1994). Rubbish dumps and dung-
heaps are frequently used by scavenging birds that
normally exploit temporary and unpredictable
food sources (Pomeroy 1975, Donazar 1992). The
importance of refuse dumps for breeding popula-
tions of scavenging birds has been repeatedly em-
phasized (Pomeroy 1975, Coulson et al. 1987, Don-
azar 1992) but, to date, it is not known whether
these unnatural foraging places are influencing the
population dynamics of scavenging species.

The Black Kite ( Milvus migrans ) is a widespread

Blanco

Vol. 31, No. 1

species that opportunistically exploits a wide array
of food sources (Delibes 1975, Shiraishi et aL
1990). Floaters form an unknown proportion of
populations, and usually share habitat with the
breeding segment (Espina 1986, Koga et al. 1989,
Blanco 1994). At present, little is known about the
relationship between these two sectors of Black
Kite populations. This study deals with the role of
refuse in the diet of breeding, floating and mi-
grating Black Kites living in the vicinity of a large
garbage dump near Madrid, Spain.

Study Area and Methods

The study area was located at an elevation of 500-700
m in the flood plain of the Jarama River at its confluence
with River Manzanares (40° 19'N, 3° 31'W) in southeast-
ern Madrid Province, central Spain. It included a com-
plex of riverine gypsum cliffs, riparian forests dominated
by poplars ( Populus alba, P. nigra, P. X cultivar), willows
( Salix spp.) and elms ( Ulmus minor), but most of the area
was converted to agriculture (mainly cereal crops, sun-
flower, alfalfa and vegetables), cattle grazing and gravel
extraction. The Madrid garbage dump was located in the
northeastern portion of the study area. From 100 to
>1000 Black Kites routinely gathered there to feed on
the refuse (Blanco 1994).

During the breeding season of 1994, the Black Kite
population consisted of about 50 pairs nesting on trees,
cliffs and electric pylons, and about 80-300 nonbreeding
individuals forming a floating population of both adults
and subadults (Blanco 1994). Each day, resident floaters
roosted communally about 4 km southeast of the dump,
in a portion of riverine elm forest where 6-10 pairs nest-
ed Numbers of Black Kites in the area increased consid-
erably after breeding and especially during migration
(Blanco 1994).

I determined the diet of nonbreeding kites by analyz-
ing food remains collected beneath roosting and diurnal
communal perching trees during the period when Black
Kites are resident in the area (Blanco 1994). All prey
remains (bones, fur and feathers) were collected on each
visit to the roost, from the final stage of the nestling pe-
riod (June) to the end of August. Prey collections were
grouped into three time periods (22 July, 31 July and
August) to assess possible temporal variations in the con-
sumption of refuse. The first period (Period A: until 22
July) included the spring migration, breeding season,
fledgling period and the start of migrant arrivals; Period
B (23-31 July) coincided with an increase in the influx
of migrants; and Period C (from 1 August to the end of
that month) included the peak abundance of migrants
and the remainder of the residency period of Black Kites
in the area (Blanco 1994). Because all food remains
found below the roosting trees were collected in each
visit to the roost, I am confident that they were accurately
grouped on a temporal basis. Because breeding Black
Kites usually roosted close to their nesting sites during
the breeding period, prey remains found at the com-
munal roost site were mosdy from food eaten by local
nonbreeders. Prey remains found at the roost after the
breeding season corresponded to food of both local

(breeding and floating) and migrant kites. After the
breeding season, field observations suggested that breed-
ing Black Kites and their fledglings joined communal
roosts together with floaters and northern migrants (J.
Vinuela pers. comm, and pers. obs.). The first juveniles
appeared in the roost soon after the breeding season
(Blanco 1994) when breeders left their territories. There-
fore, some remains collected at this time might have orig-
inated from prey of local breeders which represented a
high proportion of the birds roosting communally from
23-31 July. Afterwards, the collecting period coincided
with the major influx of migrants (>1000 kites in early
August); at this time local breeders constituted a small
proportion of the birds.

The diet of breeding kites and their nestlings was de-
termined by analysis of food remains found in and below
the nests of 18 pairs. The material was collected after the
breeding season to avoid disturbance at the nests. Pairs
studied nested in trees and cliffs about 4-12.5 km from
the dump.

Food remains were identified by macroscopic compar-
ison with reference collections and quantified assuming
the smallest possible number of individuals (Marti 1987).
Two main categories of food were considered in the anal-
ysis: wild prey obtained by hunting and scavenging (e.g.,
wild birds, mammals, fish), and refuse (e.g., food items
intentionally discarded by humans such as domestic re-
fuse, offal from slaughterhouses and butcher shops, and
marine fish) . To detect general differences in food habits
of breeding and nonbreeding Black Kates, prey items
were classified into nine major groups (Table 1). The
data were likely biased in favor of the most durable prey
remains and did not reflect the importance of arthro-
pods and other prey (Marti 1987). However, the study
was not designed to provide detailed descriptions of the
Black Kite diet (Delibes 1975, Veiga and Hiraldo 1989,
Shiraishi et al. 1990) but to detect broad differences in
the consumption of refuse.

Biomass of each prey species was estimated using mean
weights of each prey taxon obtained from the literature.
Weights of prey taxon identified at nests could not ex-
ceed 300 g even when the mean weight of the taxon
exceeded that weight because Black Kites usually do not
transport heavier prey to the nest (Espina 1986, pers.
obs.). In this case, I assumed that prey heavier than 300
g were not delivered to the nest whole. Weights of large
prey from the roost were estimated based on the daily
food requirements of adult Black Kites (100 g, Espina
1986; see also Heredia et al. 1991 for Red Kites [Milvus
milvus]). Therefore, I assumed that the maximum inges-
tion capacity per feeding bout to be equivalent to the
daily food requirements (Blanco et al. 1990). Although
Black Kites may tear off small pieces of the prey and
bring them to the nest one by one (J. Vinuela pers
comm.), and nonbreeding kites may eat prey over several
days, I felt my criteria avoided an overestimation of the
biomass of large prey up to the size of a White Stork
( Ciconia ciconia, weight = 3.5 kg).

Refuse consumed both by breeding and nonbreeding
kites typically consisted of small pieces of food, including
parts of fish and fowl (mostly heads, wings and legs) and
livestock bones, but no large fragments of food, which
was estimated to yield 50 g of biomass on average. Be-

March 1997

Refuse Use by Black Kites

Table 1 . Diets of breeding and nonbreeding Black Kites in the southeast of Madrid during and after the breeding
season of 1994. Results are expressed as percentages of number of prey (NP) and biomass (B) for each prey class.

Prey Items

Breeding
Black Kites

Non-Breeding Black Kites

Breeding Season

Nonbreeding Season

% NP

% B

% NP

% B

% NP

% B

Wild prey

76.4

93.1

40.6

57.4

21.4

29.4

Rabbits and Hares a

22.5

39.7

18.8

26.9

8.7

16.3

Other mammals b

6.0

4.9

3.1

4.5

1.0

1.8

Pigeons c

8.8

15.5

3.1

4.5

2.9

5.4

Other birds d

18.7

21.1

12.5

17.0

2.9

5.4

Fish e

13.2

11.0

3.1

4.5

0.0

Other prey f

7.1

0.9

0.0

5.8

0.4

Refuse

23.6

6.9

59.4

42.6

78.6

70.6

Livestock carrion

17.0

5.0

37.5

26.9

48.5

42.6

Chicken

5.5

1.6

18.8

13.5

22.3

20.8

Marine fish

1.1

0.3

3.1

2.2

7.8

7.2

Number of prey

G'T

103

0.87

0.73

0.66

a Oryctolagus cuniculus, Lepus granatensis.

b Erinaceus europaeus, Rattus norvegicus, Apodemus sylvaticus, unidentified rodents.
c Columba livia, Columba livia var. domestica, Columba palumbus, unidentified pigeons.

d Ciconia ciconia, Anas platyrhynchos, unidentifeid Anatidae, Milvus migrans, Alectoris rufa, Gallinula chloropus, Larus ridibundus, Clamator
glandarius, Otus scops, Athene noctua, Picus viridis, Sturnus unicolor. Pica pica, Corvus monedula, unidentified corvid, Turdus merula, Passer
sp., unidentified bird.

e Cyprinus carpio, Ictalurus melas, unidentified fish.

1 Lacerta lepida, unidentified Colubridae, unidentified Coleopterans, bird eggs.

cause of the difficulty in accurately estimating biomass
values from prey remains of scavengers and facultative
predators (Marti 1987), the analysis conducted in this
respect should be considered as a simple approximation
of the diet. The aim of the biomass analysis was provide
a comparative assessment of the energetic role of refuse
with the natural prey for breeding and nonbreeding
kites.

Dietary diversity was calculated with the Shannon-
Weaver index (H') considering the major prey classes
(Table 1). Dietary overlap between breeding and non-
breeding kites was calculated using Pianka’s index (Pian-
ka 1973). Chi-square tests with Yates’ correction when df
= 1, were used to test for differences in the number of
prey items (NP) consumed by breeding and nonbreeding
kites.

Results

General Food Habits. The diet of Black Kites in-
cluded a wide range of wild prey and refuse (Table
1). Up to 300 nonbreeding kites returned to the
roost every day from the garbage dump through-
out the breeding season. Most of the food they ob-
tained from the dump was small pieces of domestic
refuse and slaughter offal ( Ovis, Sus, Bovis, Gallus)
which was spread fairly evenly over large amounts
of inorganic materials because of the daily treat-

ment measures at the dump. Unidentified marine
fishes, available only at the dump, were also con-
sumed. Wild prey consumed by nonbreeders in-
cluded mostly European rabbits ( Oryctolagus cuni-
culus, NP = 34.3%, B = 41.4%), but hares ( Lepus
granatensis) and pigeons ( Columba spp.) also were
of some importance.

Breeding kites ate mainly wild prey (Table 1),
the main prey species being European rabbits (NP
= 18.7%, B — 32.9%). Altogether, birds accounted
for 27.5% by NP and 36.6% by B, and they were
mosdy medium-sized corvids and pigeons.

Spatial Variation in the Consumption of Refuse
by Breeding Kites. The roost site used by non-
breeding kites was 4 km from the dump. Pairs nest-
ing near the roost site had a higher proportion of
refuse in their diets than did pairs that nested 5.1-
12.5 km from the dump (x ± SE = 6.6 ±0.6 km;
X 2 = 12.53, df = 1, P< 0.001 for NP; Fig. 1). How-
ever, there was variation in the consumption of re-
fuse among pairs that nested close to the dump
(18.2-72.7% of NP, Fig. 2), with somewhat lower
values for biomass (5.4-30.8%, Fig. 1).

Although some kites probably obtained refuse at

Blanco

Vol. 31, No. 1

BIOMASS
PREY ITEMS

BREEDING FAR
FROM THE DUMP

BREEDING CLOSE
TO THE DUMP

RESIDENT

NON-BREEDING

Figure 1. Percentage of refuse in the diet of Black Kites. Data are presented for resident floaters and for kites
nesting either far or close to the garbage dump of Madrid, respectively.

sites other than the dump, the proportion of re-
fuse in the diet of breeding pairs was negatively
related to distance to the dump (Spearman rank
correlation coefficient r s = —0.64, P = 0,0087 and
r s = —0.70, P = 0.0038 for %NP and %B, respec-
tively, N = 18, Fig. 2). Thus, the proportion of re-
fuse in the diet of breeding Black Kites was variable
for those pairs breeding in the vicinity of the dump
but almost insignificant in terms of biomass for the
remainder of the breeding population.

Diet Differences and Overlap within the Local
Population. The proportion of prey remains in-
cluded in the refuse class was significandy higher

for nonbreeding than for breeding kites (100 of
135 vs. 44 of 182, x 2 = 75.84, df = 1, P < 0.001).
A similar conclusion was reached by comparing
food remains from the breeding season only, both
for all nests (x 2 = 14.58, df = 1, P< 0.001 for NP)
and when nests located close to and far from the
roosting area were considered separately (P < 0.05
in all cases, Fig. 1). I found a higher overlap in the
diets of breeding Black Kites for prey biomass than
for prey type (Table 2). Nonbreeding floaters
showed a higher overlap with kites breeding close
to the dump than with those breeding farther away,
both in type and biomass of the prey they con-

Figure 2. Proportion of prey from refuse in the diet of breeding Black Kites ( N = 18 pairs) in relation to the
distance of their nest from the garbage dump of Madrid. Large dots represent two nests.

March 1997

Refuse Use by Black Kites

Table 2. Percentage of diet overlap (Pianka’s index) be-
tween breeding and nonbreeding Black Kites.

% of Diet

Overlap

Number

Comparisons

of Prey Biomass

Breeding close vs. far from the dump

69.8 86.0

Breeding close vs. nonbreeding

91.0 80.9

Breeding far vs. nonbreeding

55.6 70.8

sumed. Nevertheless, dietary overlap was high in
all the comparisons as expected for individuals of
the same species.

Seasonal Variation in the Consumption of Re-
fuse by Non-breeding Kites. Refuse was the main
food for nonbreeding kites (Table 3) . However, the
proportion of refuse increased as the season pro-
gressed from the breeding season, when most kites
gathering at the roost belonged to the local non-
breeding population, toward the end of the col-
lecting period when most kites were migrants (x 2
= 7.75, df = 2, P — 0.02 for NP; Table 3). As a
result, dietary diversity declined as the season pro-
gressed.

Discussion

In this study, refuse was a more important food
resource for Black Kites, especially nonbreeding
and migrant individuals. Floaters roosted in wood-
ed areas near the dump and consumed refuse al-
most extensively and very seldom taking wild prey.
Apparently they were able to exploit this abundant
and predictable food source because their move-
ments were not restrained by breeding. Contrast-
ingly, breeding Black Kites fed mosdy on wild prey
items but there was a significant correlation be-
tween the distance of nests from the dump and the
consumption of refuse with pairs nearest the dump
increasing their consumption of refuse. Preferen-
tial use of natural food sources by breeding Black
Kites is probably related to constraints placed on
breeding kites by the need to provision broods of
developing young with adequate prey biomass
(Donazar 1988, Vinuela and Veiga 1992). In addi-
tion, high percentages of rubbish/carrion in the
diet could negatively affect the growth of young
(Hiraldo et al. 1990, Vinuela 1991).

After the breeding season, refuse is also used by
local breeders and their fledglings. Inexperienced
juveniles benefit greatly by using the dump and by

Table 3. Seasonal variation in the consumption of re-
fuse by nonbreeding Black Kites, expressed as percentage
of number of prey items (NP) and biomass (B) .

Period

% B

% NP

A (to July 22)

42.6

59.4

0.73

B (23-31 July)

64.7

69.8

0.66

C (August)

75.0

85.0

0.60

group foraging at a place where several hundreds
of kites feed together.

The presence of floaters may enhance the breed-
ing population of a species by providing a mecha-
nism for quick replacement of lost mates at nest
sites. Through their support of juvenile and sub-
adult, nonbreeders, garbage dumps may provide a
mechanism of increasing the survival and recruit-
ment of breeding individuals into the populations
of scavenging birds. Indeed, garbage dumps have
been highlighted for their potential importance in
the conservation of Black Kite populations, es-
pecially during migration (Donazar 1992, Blanco
1994).

Acknowledgments

I thank A. Acha, L. Blanco, M.C. Blanco, J.A. Fargallo,
F. Gomez, F. Martinez and E. Soto-Largo for help with
the field work. J.L. Telia discussed some aspects of the
work and helped me with the analysis of data, and J. Vih-
uela, C.D. Marti, J. Bustamante and an anonymous ref-
eree provided many useful comments on the manuscript.

Literature Cited

Blanco, J.C., J.L. Gonz al ez and F. Hiraldo. 1990. Tro-
phic and spatial relationships between wintering Red
Kites ( Milvus milvus ) and Marsh Harriers ( Circus aeru-
ginosus) in the Guadalquivir Marshes. Miscelanea Zool-
ogica 14:161-166.

Blanco, G. 1994. Seasonal abundance of Black Kites as-
sociated with the rubbish dump of Madrid, Spain. /.
Raptor Res. 28:242-245.

Ceballos, O. and J.A. DonAzar. 1988. Actividad, uso del
espacio y cuidado parental en una pareja de Alimoch-
es ( Neophron percnopterus) durante el perfodo de de-
pendence de los polios. Ecologia 2:275-291.

. 1990. Roost-tree characteristics, food habits and

seasonal abundance of roosting Egyptian Vultures in
northern Spain./. Raptor Res. 24:19-25.

Coulson, J.C., J. Butterfield, N. Duncan and C. Tho-
mas. 1987. Use of refuse tips by adult British Herring
gulls Larus argentatus during the week. J. Appl. Ecol.
24:789-800.

Delibes, M. 1975. Alimentation del Milano Negro (Mil-

Blanco

Vol. 31, No. 1

vus migrans) en Donana (Huelva, Espana). Ardeola 21:
183-207.

DonAzar, J.A. 1988. Variaciones en la alimentacion en-
tre adultos reproductores y polios en el Buho Real
{Bubo bubo). Ardeola 35:278-284.

. 1992. Muladares y basureros en la biologia y

conservation de las aves en Espana. Ardeola 39:29-40.

Espina, J. 1986. Variaciones en la alimentacion de una
poblacion de Milanos Negros Milvus migrans. Tesina
de Licenciatura, Universidad Complutense de Ma-
drid, Madrid, Spain.

Heredia, B., J.C. Alonso and F. Hiraldo. 1991. Space
and habitat use by Red Kites Milvus milvus during win-
ter in the Guadalquivir marshes: a comparison be-
tween resident and wintering populations. Ibis 133:
374-381.

Hiraldo, F.,J.P. Veiga and M. Manez. 1990. Growth of
nestling Black Kites Milvus migrans : effects of hatching
order, weather and time season. J. Zool. London 222:
197-214.

Koga, K., S. Siraishi and T.A. Uchida. 1989. Breeding
ecology of the Black-eared Kite Milvus migrans lineatus
in the Nagasaki Peninsula, Kyushu. Jap. J. Ornith. 38:
57-66.

Marti, C.D. 1987. Raptor food habits studies. Pages 67-

80 in B.A. Giron-Pendleton, B.A. Millsap, K.W. Cline
and D.M. Bird [Eds.], Raptor management tech-
niques manual. Natl. Wild. Fed., Washington, DC
U.S.A.

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

Pomeroy, D.E. 1975. Birds as scavengers of refuse. Ibis
117:69-81.

Shiraishi, S., K. Koga and N. Kawaji. 1990. Food habits
of the Black-eared Kite, Milvus migrans lineatus, in Na-
gasaki airport and its adjacent areas./. Fac. Agric. Kyu-
shu Univ. 34:247-254.

Veiga, J.P. and F. Hiraldo. 1989. Food habits and the
survival and growth of nestlings in two sympatric kites
( Milvus milvus and Milvus migrans). Holarct. Ecol. 13:
62-71.

Vinuela, J. 1991. Ecologia reproductiva del Milano Ne-
gro en el Parque Nacional de Donana. Ph.D. disser-
tation, Universidad Complutense de Madrid, Madrid,
Spain.

and J.P. Veiga. 1992. Importance of rabbits in

the diet and reproductive success of Black Kites in
southwestern Spain. Ornis Scand. 23:132-138.

Received 10 April 1996; accepted 16 November 1996

Short Communications

Interspecific and Intraspecific Aggression Among
Griffon and Cinereous Vultures at Nesting and

Foraging Sites

Guillermo Blanco

Departamento de Biologia Animal, Universidad de Alcala de Henares,
28871 Alcala de Henares, Madrid, Spain

Jose M. Traverso

Jardines 18, 28610 Villamanta, Madrid, Spain

Javier Marchamalo
Tomas Borras 12, 28045 Madrid, Spain

Felix Martinez

Puerto Canfranc 22, 28038 Madrid, Spain

Key Words: Aegypius monachus; Cinereous Vulture, Gyps

fulvus; Griffon vulture, foraging, nesting aggression.

Food partitioning within vulture guilds reduces the in-
tensity of competition even when several species share
the same carcass (Konig 1983, Hiraldo 1977, Mundy
1982). Apparently, aggressive encounters are minimized
by the relative sizes and ages of individuals in these feed-
ing groups (Alvarez et al. 1976, Grubh 1978, Anderson
and Horwitz 1979, Mundy 1982, Blanco and Martinez
1996). Interspecific and intraspecific aggression is fre-
quent when large numbers gather at large carcasses
(Mundy 1982). Most interactions involve dominance dis-
plays that either do not result in direct contact or, at
most, result in slight disputes reduced to minor pecking
and kicking. Overall, severe aggression seems to be avoid-
ed.

In addition to carcasses, vultures can at times also show
intra- and interspecific competitive behaviors at their nest
sites. Here too, competition may be passive and regulated
by the abundance, spacing and timing of breeding of the
different species, or it may result in active aggression and
displacement among individuals of the same species
(Donazar et al. 1989, Fernandez and Donazar 1991).

Griffon Vultures ( Gyps fulvus) and Cinereous Vultures
{Aegypius monachus ) share feeding and nesting situations
across wide areas of the Iberian peninsula, but little is
known of the frequency, intensity and context of agonis-
tic interactions that result from competition between
them (Konig 1974, Alvarez et al. 1976, Fernandez and
Donazar 1991, Hiraldo et al. 1991). In this note we report

two cases of intense aggression between Griffon and Ci-
nereous Vultures.

The first case occurred while we observed Cinereous
and Griffon Vultures feeding on a cattle carcass near the
Natural Park of Monfragiie, Caceres Province (western
Spain) in November 1992. A maximum of 94 Griffon Vul-
tures and 48 Cinereous Vultures were observed gathered
to feed on the carcass. Most of them were feeding on the
back of the cow, but two subadult Cinereous Vultures
were feeding on the head by putting their heads inside
the mouth of the cow. A first-year Griffon Vulture ap-
proached and began to feed together with the two Ci-
nereous Vultures. When the Griffon Vulture had its head
inside the mouth of the cow, an adult Cinereous Vulture
approached and attacked the Griffon Vulture by grasping
its neck with its bill for >30 sec. When the Cinereous
Vulture released the Griffon Vulture, it left a deep wound
that bled profusely. Immediately the injured vulture
abandoned the carcass, leaving a clear blood trail and
disappeared into the adjacent vegetation. The wound ap-
peared serious enough that it might have been lethal.

The second case involved of a conflict between two
adult (probably female) Griffon Vultures at a nesting site
traditionally used in the gorges of the Riaza River, Sego-
via Province (central Spain) in November 1995. We had
observed an adult pair at this nest two months previously
showing pair-bonding behavior and even copulating. The
mates were sexed when copulation occurred and subse-
quently were easily recognized by individual plumage fea-
tures. The third bird was probably a female, based on the
size and shape of the bill and head. The aggressive en-

Short Communications

Vol. 31, No. 1

counter began when the two presumed female Griffon
Vultures began to make rasping calls and to fight at the
nest site. During the fight, one of the vultures laid on its
back with its wings extended and defended itself with its
legs and bill from pecking attacks by the other vulture.
The two vultures continued to fight, alternating positions
and pecking at each other’s head and neck for >20 min.
At the end of the fight, both females remained motion-
less on their backs and face to face with their wings ex-
tended for about 5 min. Finally, after another 6 min of
violent aggression, one of the vultures displaced the oth-
er from the nest site. As a result of die fight, both vultures
sustained numerous bleeding wounds on their heads and
necks, especially around their eyes, mandibles, and
throats.

Initially, the male w r atehed the conflict from the edge
of the nest site and only pecked a few times at the tips
of the primaries of the fighting females. The male later
flew to a nearby rock from which he observed the fight
for the remainder of its duration.

During many years of research, we have observed
several hundred vulture gatherings at carcasses where
numerous agonistic interactions have occurred. Most
aggressive behaviors have been between individuals of
the same species and especially between the more so-
cial and abundant Griffon Vulture (Hiraldo et al.
1979). These encounters never generated wounds and
usually resulted in the loss of a few neck or rarely pri-
mary feathers. The more violent encounters have in-
volved first-year birds assaulted by older individuals in-
dicating that immature vultures may not have yet es-
tablished dominance relationships or developed ap-
propriate social behavior. Our first observation
indicates that Cinereous Vultures are capable of oc-
casionally injuring Griffon Vultures seriously enough
to result in death. Undoubtedly, their larger body size
and stronger bill enables them to dominate in aggres-
sive encounters (Konig 1983). Nevertheless, this be-
havior is exceptional and probably occurs only when
large numbers of vultures, especially Cinereous Vul-
tures, gather at a carcass.

Competition for nest sites may be a cause of aggression
between conspecifics under conditions of high popula-
tion density (Martinez and Cobo 1993). The availability
of nest sites for Griffon Vultures has not been docu-
mented to be a factor limiting the population size in
northern Spain where high densities are reached (Arroyo
et al. 1990, Donazar and Fernandez 1990). Nevertheless,
we observed an increasing number of aggressive inter-
actions over the past 10 years (Martinez and Cobo 1993).
These encounters have mainly been brief and have
amounted to nothing more than attempts to steal nest
material and food delivered to nestling. Our observation
of actual fighting between two female Griffon Vultures
was unusual but it may indicate that as die density of the
nesting population of Griffon Vultures continues to in-

crease in the Riaza River gorge, aggression within this
species will become more violent.

Resumen. — Se describe un caso de agresion violenta de
un buitre negro (Aegypius monachus) adulto a un juvenil
de buitre leonado (Gyps fulvus) en una carrona de vaca
que congrego un numero inusual de individuos de la
primera especie. El buitre leonado pudo haber muerto
como consecuencia de la gravedad de la herida produ-
cida en el cuello por el buitre negro, Se documenta tam-
bien la observation de una pelea muy violenta entre dos
hembras de buitre leonado en un lugar de nidificacion
ocupado por una de ellas y su pareja, lo cual se interpreta
como resultado de una fuerte competencia por el lugar
de nidificacion en una colonia de muy alta densidad pob-
lacional en Espana.

[Traduction de los Autores]

Acknowledgments

The manuscript was written during a short stay by the
first author in the Centro de Investigaciones Riologicas
del Noroeste (Mexico) and benefited from the com-
ments of P. Mundy, R. Rodriguez-Estrella, J.L. Telia and
S.R. Wilbur. The stay was financed by a F.P.U grant from
the Spanish Ministerio de Education y Ciencia.

Literature Cited

Alvarez, F., L. Arias De Reyna and F. Hiraldo. 1976.
Interactions among avian scavengers in southern
Spain. Ornis Scand. 7:215-226.

Anderson, J. and P. Horwitz. 1979, Competitive inter-
actions among vultures and their avian competitors.
Ibis 121:505-509.

Arroyo, B., E. Ferreiro and V. Garza. 1990. II Censo
national de Buitre Leonado ( Gyps fulvus ): poblacion,
distribution, demografia y conservation. ICONA, Ma-
drid, Spain.

Blanco, G. and F. MartInez. 1996. Sex difference in
breeding age of Griffon Vultures (Gyps fuhrus). Auk
113:247-248.

DonAzar, J.A., O. Ceballos and C. FernAndez. 1989.
Factors influencing the distribution and abundance
of seven cliff-nesting raptors: a multivariate study.
Pages 545-552 in B.U. Meyburg and R.D. Chancellor
[Eds.], Raptors in the modern world. WWGBP, Berlin,
Germany.

and C. FernAndez. 1990. Population trends of

the Griffon Vulture Gyps fulvus in northern Spain be-
tween 1969 and 1989 in relation to conservation mea-
sures. Biol. Conserv. 53:83-91.

FernAndez, C. andJ.A. DonAzar. 1991. Griffon Vultures
( Gyps fulvus) occupying eyries of other cliff-nesting
raptors. Bird Study 38:42-44.

Grubh, R.B. 1978. Competition and co-existence in Grif-
fon Vultures: Gyps bengalensis, G. indicus and G. fulvus
in Gir forest./. Bom. Nat. Hist. Soc. 75:810-814.
Hiraldo, F. 1977. Relaciones entre morfologia, ecologia

March 1997

Short Communications

y distribucion de los buitres del Viejo Mundo. Actas I
Reunion Iberoamer. Zool. Vert., La Rabida: 753—757.

, M. Delibes and J. Calderon. 1979. El Quebran-

tahuesos Gypaetus barbatus (L). Monografias 22.
ICONA, Madrid, Spain.

, J.G. Blanco and J. Bustamante. 1991. Special-
ized exploitation of small carcasses by birds. Bird Study
38:200-207.

KOnig, C. 1974. Zum verhalten spanischer Geier an Ka-
davern . J. Orn. 115:289-320.

. 1983. Interspecific and intraspecific competition

for food among Old World vultures. Pages 153-171 in
S.R. Wilbur and J. A. Jackson [Eds.], Vulture biology
and management. Univ. of California Press, Berkeley
and Los Angeles, CA USA.

MartInez, F. and J. Cobo. 1993. Gestion actual de AD-
ENA/WWF Espana en el Refugio de rapaces de Mon-
tejo de la Vega (Segovia). Alytes 6:507-521.

Mundy, r-J. 1982. The comparative biology of southern
African Vultures. Vulture Study Group, Johannesburg,
South Africa.

Received 1 April 1996; accepted 14 November 1996

J. Raptor Res. 31(1):79-81

Hunting Synchrony in White-Tailed Kites
Molly F. Skonieczny

Department of Biology, Humboldt State University,
Areata, CA 95521 U.S.A.

Jeffrey R. Dunk

Department of Wildlife, Humboldt State University,
Areata, CA 95521 U.S.A.

Key Words: White-tailed Kite, hunting, Elanus leucurus;

California.

Hunting behavior of White-tailed Kites {Elanus leucu-
rus) has been extensively studied (Bammann 1975, War-
ner and Rudd 1975, Mendelsohn andjaksic 1989). Kites
primarily use open to semi-open habitats for hunting
(Waian 1973, Bammann 1975, Dunk and Cooper 1994).
In California, kites almost exclusively hover while they
hunt (Mendelsohn andjaksic 1989, Dunk 1995) and prey
on small mammals, primarily voles ( Microtus spp.) (Haw-
becker 1940, 1942, Stendell 1972).

In previous studies of kites, we observed that groups
of kites (2-20 individuals) appeared to hunt relatively
synchronously and the probability of an individual kite
hunting appeared to be related to whether other kites
were hunting. Hunting synchrony could result from kites
advertising their presence to conspecific territory holders
to potentially decrease subsequent interactions, or to
more easily patrol and defend a territory, or from kites
responding to variability in prey availability as a function
of prey activity rhythms. Shields (1976, cited in Madison
1985) found California voles (M. californicus) exhibited
ultradian rhythms in activity varying from 2-6 hr. Daan
et al. (1982) reported positive correlations between vole
activity and timing of hunting by raptors in Europe. We

examined whether kites hunted independent of other in-
dividuals hunting.

Study Area and Methods

This study was conducted at the Mad River Slough
Wildlife Area, Areata, California. The area consists of ap-
proximately 185 ha of ungrazed grassland with little to-
pographic relief. It contains very few trees and shrubs,
and fence posts and T-bars (ca. 3-m tall) provide most of
the perches for raptors. The climate is maritime with
mild winters and cool summers.

The study took place from 20 November 1994-30 Jan-
uary 1995. We made observations during seven 1-2 hr
periods. Random points within the study area were estab-
lished from which observations were made. Scan sam-
pling (Altmann 1974) was used to record number of kites
hunting. Using a landmark on the horizon as a starting
point, we slowly scanned (x time per scan = 90 sec, range
= 45-135 sec) 360° and recorded the number of kites
hunting (hovering). Scans were made using binoculars.
All kites were within approximately 400 m of the observ-
er. We waited 5 min between scans based on Bammann’s
(1975) findings that mean hunting time for kites was 5.04
min (N = 674 hunts) in this area.

To determine whether kites hunted independent of
other hunting kites, we compared observed numbers of
kites hunting during each scan to expected numbers un-
der the Poisson distribution using Chi-square analysis. We
used this analysis because comparison of observed events

Short Communications

Vol. 31, No. 1

Table 1. Number of White-tailed Kites observed vs. ex-
pected (using the Poisson distribution) to be hunting si-
multaneously at Areata, GA 1994-95,

Number
Hunting
per Scan

Number of Times
Observed

Number of Times
Expected

to the Poisson distribution allows a test of the indepen-
dence of the individual events (Zar 1984). Groups of 4,
5, 6 and 7 kites hunting simultaneously had expected
frequencies less than 5, so they were combined into one
group (>4 individuals).

Results

We observed 168 kites hunting during 119 scans. Mean
number of kites hunting per scan was 1.41 (range = 0-
7) Individual kites hunted in a nonrandom fashion with
respect to the presence of other hunting kites (x 2 =
19.36, df = 3, P = 0.0002). Few kites (1 or 2) hunted less
often than expected whereas many kites (>4) hunted
simultaneously more often than expected (Table 1).

Discussion

We found that kites hunted in a nonrandom way with
respect to the presence of other hunting kites. Our statistical
test was probably conservative because data were analyzed
as if there were always ^4 kites on the study area that could
have hunted simultaneously. On some occasions there were
only a couple of kites in the sampling area. Thus, our anal-
ysis was probably biased against observing ^4 kites hunting
simultaneously more than expected. It appears that kites
responded to either other individuals hunting or, very rap-
idly, to some other factor. It was beyond the scope of this
study to examine whether kites were ultimately responding
to vole activity rhythms, to other individual kites for terri-
torial reasons, or both. Proximately, however, kites appeared
to respond to other hunting kites.

Species such as Common Ravens ( Corvus corax), Bald
Eagles ( Haliaeetus leucocephalus) , and many vultures are
known to forage together and “obtain information” about
food resources from one another (Knight and Knight
1983, Heinrich 1988), often at communal roosts, but also
through local enhancement (Knight and Knight 1983)
where searchers are attracted to actively foraging individ-
uals, or by actually recruiting other individuals (Heinrich
1988). Although kites are also communal roosters during
the nonbreeding season (Bolander and Arnold 1965,
Waian 1973, Clark and Wheeler 1989), their diurnal be-
havior during the nonbreeding season contrasts with other
species in that kites hold hunting territories (Dunk and
Cooper 1994). Moreover, amongst Common Ravens, Bald
Eagles, vultures and kites, kite prey items are relatively
more predictable spatially because they are not nearly as

patchily distributed and are of much smaller size. We pro-
pose that the information that nonhunting kites obtain
from hunting individuals is not where to forage, as has
been found for other species, but when to forage.

If kites were ultimately responding to variations in vole
activity, nonhunting kites probably benefit by observing
whether a hunting kite makes an attempt at prey (wheth-
er successful or not because any attempt is presumably
in response to a prey detection) . If no prey attempts are
made, then nonhunting individuals should remain
perched and thus conserve energy. If attempts at prey
are made, then nonhunting kites should use this infor-
mation to decide whether to hunt. If this “vole rhythm”
hypothesis is correct, then voles should exhibit synchro-
nous activity rhythms over relatively large areas, at least
the size of our study area. Subsequent studies should ex-
amine the relationship between vole activity and the
number of kites (and other raptors) hunting.

Resumen. — Nosotros examinamos si Elanus leucurus en el
norte de California cazaba en sincronizacion mas que es-
peraban bajo el pronostico Poisson. Nosotros encontra-
mos que pocos E. leucurus cazaban con menos frecuencia
de lo que esperaban y muchos (>4) E. leucurus cazaban
con mas frecuencia de lo que esperaban segun parecen
responder con la presencia de otros E, leucurus cazando.
Finalmente, E. leucurus podia estar respondiendo a activ-
idad ritmica de presa, a otros E. leucurus por razones ter-
ritorial, o los dos.

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

We thank P.H. Bloom and J.B. Buchanan for helpful
comments on an earlier draft of the manuscript and the
Department of Wildlife, Humboldt State University, for
logistical support during manuscript preparation.

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Clark, W.S. and B.K. Wheeler. 1989. Unusual roost site
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March 1997

Short Communications

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Stendell, R.C. 1972. The occurrence, food habits, and
nesting strategy of White-tailed Kites in relation to a
fluctuating vole population. Ph.D. dissertation, Univ.
California, Berkeley, CA U.S.A.

Waian, L.B. 1973. The behavioral ecology of the North
American White-tailed Kite ( Elanus leucurus majusculus )
of the Santa Barbara coastal plain. Ph.D. dissertation,
University of California, Santa Barbara, CA U.S.A.

Warner, J.S. and R.L. Rudd. 1975. Hunting by the
White-tailed Kite. Condor 77:226-230.

Zar, J.H. 1984. Biostatistical analysis. Prentice Hall, En-
glewood Cliffs, NJ U.S.A.

Received 23 April 1996; accepted 29 November 1996

Blood Parasites of Nestling Goshawks

E.P. Toyne 1

Department of Biology, Imperial College, London SW7 2BB UK

R.W. Ashford

Department of Parasitology, School of Tropical Medicine,
Pembroke Place, Liverpool L3 5QA UK

Key Words: Northern Goshawk ; Accipiter gentilis; nestling

survival ; blood parasites.

There are a few studies which have investigated patho-
genic effects of haematozoa on wild raptors. Ashford et al.
(1990, 1991) were unable to demonstrate any effect of Leu-
cocytozoon toddi on the mortality of nestling or adult Euro-
pean Sparrowhawks ( Accipiter nisus) in England and Korpi-
maki et al. (1993) showed no effect of L. ziemanni on body
mass or molting progress in almost 200 Tengmalm’s Owls
(, Aegolius Junereus) in Finland. They did, however, find that
four of six females which laid unusually small clutches had
relatively heavy infections. In a second large study, Korpi-
maki et al. (1995) found that mates of male European Kes-
trels (Falco tinnunculus) infected with Haemoproteus produced
smaller clutches earlier than mates of uninfected males. It

1 Present address: WWF-UK, Panda House, Weyside Park,
Catteshall lane, Godaiming, Surrey GU7 1XR, UK.

is unfortunate that the precise ages of these birds were un-
known as this was likely to be the confounding variable.

The occurrence of blood parasites in nestling North-
ern Goshawks (Accipiter gentilis) is unknown and, if they
do occur, their role as a factor of regulation of goshawk
reproduction is unclear. The aim of this preliminary
study was to investigate occurrences of blood parasites
and to assess whether they are a significant mortality fac-
tor in nestling goshawks.

Methods

Nests of Northern Goshawks were studied from March
through late July 1994. Clutch sizes were determined by
viewing nest contents in late April and early May 1994 with
a mirror attached to a telescopic pole. We climbed into nests
between 11-14 June to sex, band, weigh and measure wing
lengths (standard B.T.O. maximum chord: Spencer 1984)
of nesdings. Body mass was adjusted for crop contents by
subtracting 60 g if nesdings had full crops and 15 g if they
had crops that were half-full or less. Wing-length measure-
ments were used to age nesdings (±4 d) from growth equa-
tions of Swedish goshawks (Kenward et al. 1993). The first

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Vol. 31, No. 1

Table 1. Distribution of infections of Leucocytozoon toddi
in nestling Northern Goshawks by brood size and sex.

Brood Size
1 2 3

Total

Nests

# Infected broods

# Uninfected broods

# Infected males

# Infected females

egg-laying date, if not known from direct observations, was
calculated by backdating from the age of the oldest nestling
in a brood. The incubation period used was 38 d (36 d +
2d), as incubation does not start until at least the second
egg is laid, two days after the first egg is laid (Cramp &
Simmons 1980). We are not aware of any instances of gos-
hawks removing unhatched eggs from their nests, so any
eggs that were unaccounted for, after thorough searches of
nest material, were assumed to have hatched and the re-
sulting young to have died. In late July, nesting territories
were revisited to check for occupancy and fledgling mortal-
ities. Juveniles were classified as having dispersed when they
were >400 m from the nesting territory and its immediate
vicinity.

Blood samples were taken at the same time as nestlings
were banded. This was done by clipping the tip of the
talon on the inner toe. All nestlings were handled in the
same manner by the same observers. Blood smears were
prepared in the field, immediately air-dried, fixed with ab-
solute methanol and later stained with Giemsa stain in a
laboratory. Slides were examined under a microscope and
the parasite load was estimated on a logarithmic scale of
0 to 4, where 0 = no parasites seen in the entire film
examined; 1 = fewer than 1 parasite per 100 high-power
fields (X400); 2 = 1-10 parasites per 100 high-power
fields; 3 = 11-100 parasites per 100 high-powered fields;
4 = more than 100 parasites per 100 high-power fields.

Results and Discussion

We examined the blood of 48 nestlings from 23 nests.
Five female (26% of females) and four male (14% of
males) nestlings from eight nests were infected with Leu-
cocytozoon toddi (Table 1). One male was infected with a
trypanosome, presumably Trypanosoma avium, but not
with L. toddi. The single trypanosome infection suggested
that trypanosomes had a negligible impact on nestling
mortality. In Britain, avian trypanosomes are also found
in blackflies (Simulium sp.) and European Sparrowhawks
(Pierce and Marquiss 1983, Dirie et al. 1990).

The median date for first egg laying of infected broods
(median =13 April, range = 7-9 April, N = 8) was sim-
ilar to that of uninfected broods (median = 9 April,
range = 31 March-18 April, N = 14; Mann-Whitney
U-test, U 8 14 = 35.0, P > 0.05) suggesting parasite infec-
tion and date of egg laying were not related. L. toddi in-
fections also did not appear to be associated with body
mass or sex of nestlings, as male and female nestlings of
differing body masses and ages were infected and, in gen-

Figure 1. Effect of Leucocytozoon toddi infection on
growth of male and female nestling Northern Goshawks
in Wales in 1994. Males: Y= —805.9 + 1010.01og(x), fe-
males: Y= -1434.5 + 1632.81og(x).

eral, the mass of infected individuals of both sexes were
within ranges of uninfected nestlings (Fig. 1 ) .

There was no apparent association between infections
and brood size (Likelihood ratio test, x 2 2 = 1-182, P >
0.05, Table 1), nor was there any apparent clustering of
infections by sex with brood size (x 2 2 = 6.352, P> 0.05).
Four male nestlings had infection loads of 1 and 2 and
none had loads of 3. Conversely, only two female nest-
lings had infection loads of 1 and 2 and three had loads
of 3. Although higher infection loads were higher in fe-
males this was not significant (x 2 2 = 5.094, P> 0.05).

The mean clutch size, brood size at banding and fledg-
lings at dispersal in infected and uninfected broods were
similar (Table 2). The mortality of young goshawks up to
time of banding (>13 d) in infected broods (5 from 19
broods) was not significantly higher than in uninfected
broods (4 from 28 broods; Fisher’s exact 1-tail test, P =
0.255, Table 2). Parasites take around 14 d to appear in the
blood (Pierce and Marquiss 1983), but infected nestlings

Table 2. Productivity and survival of Northern Goshawk
eggs and nestlings in nests infected (N = 6) and unin-
fected (N= 10) with the blood parasite Leucocytozoon toddi
in Wales in 1994.

Uninfected

Nests

(Number)

Infected

Nests

(Number)

Eggs laid

Unhatched eggs

Nestlings hatching

Nestlings dying

Nestlings fledging

Fledglings dying

Fledglings dispersing

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Short Communications

may be ill and possibly die during this prepatent period.
Further studies involving a larger sample size are needed to
examine if there is a higher mortality of young goshawks in
infected broods. If so, this might explain why only light in-
fections were found since heavy infections, if they occurred,
would have resulted in death before 13 d of age.

There was no association between infection and mor-
tality in a brood between banding (13-39 d) and fledging
as nestlings from both infected and uninfected broods
died (x 2 i = 0.166, P > 0.05). This suggested that infec-
tions did not reduce the survival of nestlings past the
young nestling stage (>14 d).

The geographic distribution and physical characteristics
of nesting territories of infected goshawks were compared
to those of uninfected goshawks (Mann-Whitney U-test).
There were no statistically significant differences (P > 0.05)
between distances to nest trees in other nesting territories
and running water, or between nesting territory elevations
of infected and uninfected goshawks. The distribution of
infected nestlings within the study area did not suggest any
clustering. Physical characteristics of nesting territories of
infected and uninfected territories were also similar. This
was not surprising because most nests were built in larch
(Larix sp.) trees >25 yr old and larch was the most common
tree providing a suitable nesting substrate for goshawks.

Our results suggest that parasitic infections are likely to
cause no short-term mortalities in goshawks in the post-dis-
persal period. Infection loads were light compared with
those of sparrowhawks (Ashford et al. 1990, 1991).

Resumen. — Nosotros encontramos nueve pajaritos de Ac-
cipiter gentilis (cinco hembras y cuatro machos) en ocho
nidos de 48 pajaritos en 23 nidos en Wales que estaban
infectados con Leucocytozoon toddi y un macho que estaba
mfectado con Trypanosoma. La mortalidad de los pajaritos
hasta el tiempo de ser marcados (>13 d) en crias infec-
tados no fue considerablemente mas alto que en crias
que no estaban infectados, ni hubo ninguna asociacion
entre infecciones y mortalidad en una cria entre los mar-
cados (13-39 d) y los pajaros jovenes porque los pajaritos
de los infectados y sin infeccion murieron.

[Traduccion de Raul De La Garza, Jr.]

Acknowledgments

We wish to thank the Countryside Council for Wales
for the provision of the necessary licenses (SB:6:94) and

Herman Ostroznik, Herman Ostroznik, Jr., Steve Binney
and staff of a Forest Enterprise district for help with field-
work and logistics. We are also grateful to the British Eco-
logical Society (SEPG No. 1119) and the Liverpool
School of Tropical Medicine for funding the project.

Literature Cited

Ashford, R.W., I. Wyllie and I. Newton. 1990. Leuco-
cytozoon toddi in British Sparrowhawks Accipiter nisus.
observations on the dynamics of infections. J. Nat.
Hist. 24:1091-1100.

, E.E. Green, P.R. Holmes and AJ. Lucas. 1991. Leu-
cocytozoon toddi in British Sparrowhawks Accipiter nisus. pat-
terns of infection in nestlings. J. Nat. Hist. 25:269-277.
Bennet, G.F., M.A. Pierce and R.W. Ashford. 1993. Avi-
an haematozoa: mortality and pathogenicity. J. Nat.
Hist. 27: 993-1001.

Dirie, M.F., R.W. Ashford, L.M. Mungomba, D.H. Mo-
lyneux and E.E. Green. 1990. Avian trypanosomes
in Simulium and Sparrowhawks ( Accipiter nisus) . Para-
sitology 101:243-247.

Kenward, R.E., M. Marquiss, and I. Newton. 1981.
What happens to goshawks trained for falconry. J.
Wildl. Manage. 45:802-806.

, V. Marcstrom and M. Karlbom. 1993. Post-nest-
ling behaviour in Goshawks ( Accipiter gentilis) I. The
cause of dispersal. Anim. Behav. 46:365—370,
Korpimaki, E., H. Hakkarainen and G.F. Bennet. 1993.
Blood parasites and reproductive success of Teng-
malm’s Owls: detrimental effects on females but not
on males. Fund. Ecol. 7:420-426.

, P. Tolonen and G.F. Bennet. 1995. Blood par-
asites, sexual selection and reproductive success of
European Kestrels. Ecosdence 2:335—343.

Marquiss, M. and I. Newton. 1982. The goshawk in
Britain. Brit. Birds 75:243—260.

Pierce, M.A. and M. Marquiss. 1983. Haematozoa of Brit-
ish birds. VII. Haematozoa of raptors in Scotland with
a description of Haemoproteus nissi sp. nov from the
Sparrowhawk (Acdpiter nisus) . J. Nat. Hist. 17:813-821.
Spencer, R. 1984. The Ringer’s manual, 3rd ed. B.T.O.,
Norfolk, UK

Received 16 March 1996; accepted 30 November 1996

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Vol. 31 , No. 1

Migration of Flocks of Honey Buzzards in
Southern Italy and Malta

Nicolantonio Agostini

Via Carlo Alberto n.4, 89046 Marina di Gioiosa Jonica ( RC '), Italy

Daniela Logozzo

Via A. Oramsci n.26, 89046 Marina di Gioiosa Jonica (RC), Italy

Charles Colero

Birdlife Malta (MOS), P.0. Box 498 Valletta, CMR 01 Malta

Key WORDS: Honey Buzzards; Pernis apivorus; migration;

flocking behavior, orientation; navigation.

Honey Buzzards ( Pernis apivorus) frequently migrate in
flocks that become concentrated in narrow coastal areas
(Cramp and Simmons 1980, Kerlinger 1989). During mi-
gration over the central Mediterranean, they use mainly
two routes. One route is from Sicily to Tunisia and the
second route goes across a larger stretch of sea via the
islands of Malta (Moreau 1953, Beaman and Galea 1974,
Brown et al. 1982, Agostini et al. 1994b). In autumn, a
major concentration of buzzards occurs in the Calabrian
Apennines (southern Italy), where the distance between
the Tyrrhenian and Jonian coasts is narrowest (Agostini
and Logozzo 1995a, 1995b). As at the Strait of Gibraltar
and the Bosphorus Porter and Willis 1968, Bernis 1973),

peak numbers of buzzards are observed between the end
of August and the beginning of September.

Adult Honey Buzzards migrate using this route and
probably use the same route in spring, crossing the cen-
tral Mediterranean between Sicily and Tunisia (Agostini
et al. 1994a, Agostini and Logozzo 1995b). This assump-
tion is supported by observations on the island of Mar-
ettimo (western Sicily), where large flocks of buzzards are
seen between the end of August and the beginning of
September (Zangirolami pers. comm.). Unlike adults,
young buzzards appear to concentrate on the island of
Malta (Agostini and Logozzo 1995b) after the second
week of September (Beaman and Galea 1974) . This study
was undertaken to determine if flocks of immature buz-
zards on Malta result because young birds stopover on
the island during periods of bad weather or if they con-

Figure 1. Seasonal occurrence of migrating Honey Buzzards in the Calabrian Apennines in summer and autumn
of 1995.

March 1997

Short Communications

September 1993

□ M. Covello ■ Malta

Figure 2. Variations in migration of Honey Buzzards from 14-30 September 1993 on Mount Covello and Malta.

gregate until adult buzzards can show them the shortest
route across the Mediterranean.

Study Area and Methods

Migrating Honey Buzzards were counted in the Cala-
brian Apennines and from a post on the slopes of Mount
Covello (elevation 700 m) from 24 August-5 October
1993 and 1995. Buzzards were also counted on the Island
of Malta. There, the observation post was situated on the
highest point on the island (approximately 250 m eleva-
tion) from 6 September-5 October 1993 and 1995. A
total of 334 hr of observations were tallied at Mount Cov-
ello using 10 X 50 binoculars and a total of 188 and 177
hr of observation were tallied using 10 X 40 binoculars
on the Island of Malta in 1993 and 1995, respectively.

Results

A total of 1095 Honey Buzzards were counted in the
Calabrian Apennines and more than 90% were observed
between 24 August-12 September. The number of mi-
grating buzzards showed a bimodal distribution with
peaks occurring in late August and after the first week of
September (Fig. 1). In both years, there was overlap in
the migration periods of adult and juvenile buzzards but
over twice as many adults were observed in both years
(adults = 346, juveniles = 136). In 1995, the number of
adults observed was also significantly greater than in 1993
(X 2 = 6.38, P< 0.05).

A total of 483 Honey Buzzards were counted on the
Island of Malta in 1993 but only 88 were counted in 1995.
More than 97% of them were observed in September and
this corresponded with counts in the Calabrian Apen-
nines (Fig. 2).

Discussion

Some authors have shown that migrating birds orient
better when they fly in groups (Keeton 1970, Rabol and
Noer 1973, Wallraff 1978, Von Helbig and Laske 1986),
especially when groups contain adults that can show to
correct migration route (Kerlinger 1989). Our observa-
tions suggest that juvenile Honey Buzzards learn the
shortest route to cross the central Mediterranean by mi-

grating in flocks of adults. This would explain why the
Honey Buzzard is commonly seen in Malta, where the
African coasts are more than 400 km away.

Flocking behavior might have another function. Stud-
ies carried out on the Cap Bon promontory (Tunisia)
during the spring migration of Honey Buzzards and
Black Kites ( Milvus migrans) have shown that these rap-
tors cross the Channel of Sicily more frequently when
migrating in large flocks (Agostini and Duchi 1994, Agos-
tini et al. 1994a). This behavior suggests that flocking is
important for water crossing because an increase in flock
size increases the probability the journey over water will
be successfully completed (Agostini and Duchi 1994,
Agostini et al. 1994b).

Resumen. — Observaciones de Pernis apivorus emigrando
a traves de el centro Mediterranean fueron hechos en los
Calabrian Apennines (el sur de Italy) del 24 de agosto-
5 de octubre 1995, y en Malta del 6 de septiembre-5 de
octubre 1993 y 1995. En los Calabrian Apennines, 1095
P. apivorus fueron observados, con un maximo de 237
pajaros emigrando e el 7 de septiembre. Casi todos los
P. apivorus fueron observados entre el 24 de agosto-12
de septiembre. En Malta, 483 y 83 P. apivorus fueron con-
tados en 1993 y 1995, respectivamente, con casi todos
contados despues del 15 de septiembre. Estos resultados
proponen que P. apivorus jovenes aprenden la ruta mas
corta para cruzar el centro Mediterranian durante el
primer ano de migracion cuando van en bandadas con
adultos que han hecho la migracion antes.

[Traduccion de Raul De La Garza, Jr.]

Acknowledgments

We are greatly indebted to Paul Portelli, Francesco
Cecere and Alberto Zangirolami for their valuable collab-
oration. Our particular gratitude goes to Allen Fish,
Keith Bildstein and Javier Bustamante for their useful
comments on the manuscript.

Short Communications

Vol. 31, No. 1

Literature cited

Agostini, N. and A. Duchi. 1994. Water-crossing behav-
ior of Black Kites ( Milvus migrans) during migration.
Bird Behav. 10:45-48.

Agostini, N. and D. Logozzo. 1995a. Observations on
the autumn migration of raptors over the Calabrian
Apennines. Riv. Ital. Om. 64:117-120.

and . 1995b. Autumn migration of Honey

Buzzards in southern Italy. J. Raptor Res. 29:275-277.

Agostini, N., G. Malara, F. Neri, D. Mollicone and S.
Melotto. 1994a. La migrazione primaverile del Fal-
co pecchiaiolo ( Pernis apivorus) a Cap Bon (Tunisia)
e sullo Stretto di Messina. Atti VI Convegno Ital. di
Ornit.:451-452.

Agostini, N., G. Malara, F. Neri, D. Mollicone and S.
Melotto. 1994b. Flight strategies of Honey Buzzards
during spring migration across the central Mediter-
ranean. Avocetta 18:73-76.

Beaman, M. and C. G al ea. 1974. The visible migration
of raptors over the Maltese Islands. Ibis 116:419-431.

Bernis, F. 1973. Migracion de Falconiformes y Ciconia
spp. por Gibraltar, verano otono 1972-1973: Primera
parte. Ardeola 19:152-224.

Brown, L. H., E.K. Urban and K. Newman. 1982. The
birds of Africa, Vol. I. Academic Press, London, UK.

Cramp, S. and K.E.L. Simmons. 1980. The birds of the
western Palearctic, Vol. II. Oxford Univ. Press, Oxford,
UK.

Keeton, W.J. 1970. Comparative orientational and hom-
ing performances of single pigeons and small flocks.
Auk 87:797-799.

Kerlinger, P. 1989. Flight strategies of migrating hawks.
Univ. Chicago Press, Chicago, IL U.S.A.

Moreau, R.E. 1953. Migration in the Mediterranean
area. Ibis 95:329-364.

Porter, R.F. and I. Willis. 1968. The autumn migration
of soaring birds at the Bosphorous. Ibis 110:520—536.

Rabol, J. and H. Noer. 1973. Spring migration in the
Skylark ( Alauda arvensis ) in Denmark: influence of
environmental factors on the flocksize and the cor-
relation between flocksize and migratory direction.
Vogelwarte 27:50-65.

Von Helbig, A. and V. Laske. 1986. Zeitlicher Verlaf und
Zugrichtungen beim Wegzug des Stars ( Sturnus vul-
garis) in nordwestdeutschen Binnenland. Vogelwarte
33:169-191.

W at .i .r aff, H.G. 1978. Social interrelations involved in
migration orientation of birds: possible contributions
of field studies. Oikos 30:401-404.

Received 18 June 1996; 30 November 1996

Nesting-Tree Preference and Nesting Success of
Japanese Lesser Sparrowhawks in Japan

Mutsuyuki Ueta

Research Center, Wild Bird Society of Japan,

2-35-2 Minamidaira, Hino, Tokyo 1 91, Japan

Key Words: Accipiter gularis; Pinus densiflora; Japanese
Lesser Sparrowhawk; Japanese red pine, nesting success', nest tree
preference.

Japanese Lesser Sparrowhawks ( Accipiter gularis) breed
throughout northeastern Asia (Brown and Amadon
1968). It has been shown that tree plantings in lowland
areas are important breeding sites for this hawk in Japan
(Endo et al. 1991) and these hawks mainly nest in Japa-
nese red pines ( Pinus densiflora) (Endo and Hirano 1990,
Hirano and Kimizima 1992). Because prey abundance
does not seem to be limiting (Hirano and Kimizima 1992,
Ueta 1992), the availability of nest sites may be an im-
portant factor limiting the population of Japanese Lesser
Sparrowhawks (Ueta 1996). Because nest sites of Japa-

nese Lesser Sparrowhawks are important in predicting
future populations of this hawk in Japan, I examined the
nest-tree preferences of Japanese Lesser Sparrowhawks
and determined whether such preferences influence
nesting success.

Methods

The study was conducted from 1987-94 at 16 groves
of trees in suburban areas of Tokyo. The groves were
isolated and ranged from 1-4 ha in area. They were
mainly coppices composed primarily of Japanese chest-
nut oak ( Quercus acutissima), Storax ( Sty rax japonica) and
Sawara cypress ( Chamaecyparis pisifera) .

To determine nest-site preference, use by sparrow-
hawks and the availability of different tree species were
compared. I excluded nests in which the hawks did not

March 1997

Short Communications

50
40
30
20
10
0

Pinus densiflora Other Tree Species

r—t

T — (

Figure 1. Nest-site preference of Japanese Lesser Spar-
rowhawks. Values for expected use were calculated based
on the total number of sparrowhawk nests found and the
percentage of Japanese red pines and other tree species
in study groves.

lay eggs. Because the sparrowhawks nested mosdy near
the periphery of groves (3.89 ± 5.4 m inward from the
edge, ± SD, N = 16; Ueta unpubl. data), I counted the
number of each tree species on the edges of all nesting
groves. To determine the abundance of different tree
species in the groves, I used three 10 X 20 m rectangles
that were randomly placed along the edges of groves and
counted the number of each tree species larger than 25
cm in DBH (diameter at breast height, the ideal tree size
for nesting Japanese Lesser Sparrowhawks, Hirano and
Kimizima 1992) within each rectangle.

Nesting success of sparrowhawks in Japanese red pines
and in other tree species was compared to determine the
effect, if any, of nest-tree use on nesting success. Nesting
success was calculated as the number of successful nests
per total occupied nests and a successful nest is defined
as one in which at least one fledgling was raised.

To determine the availability of preferred nest sites
and to predict their future availability, I measured DBH
of all Japanese red pines in all nesting groves.

Results

I found a total of 43 Japanese Lesser Sparrowhawk nest
structures from 1987-94. More than half of the nests
were in Japanese red pines (53.5%) despite the fact that
pines were the least available of all nest tree species (Fig.
1 ) . Because I could not identify individual hawks, I ana-
lyzed nest-site preference, using only the first nesting rec-
ord in each grove to avoid the affects of any individual
variation in preference. Again, 10 of 1.6 occupied nests
were in Japanese red pines (62.5%) despite that fact that
pines were not the most abundant tree species in the
nesting groves (5.8%, N = 774) indicating that sparrow-
hawks significandy preferred pine trees as nest sites over
other nesting substrates (Binomial test, P < 0.001).

Eight nests in Japanese red pines successfully fledged
young for a nesting success of 80% (N = 10 nests). Nest-
ing success was significantly lower (Fisher Exact Proba-
bility Test, P = 0.025) for other tree species at only 16.7%
( N = 6). Causes of nesting failures were confirmed for
15 of 18 failed nests. Most failures resulted when nests
fell from branches (93.3%, N = 15) and 6.7% were due
to predation by Japanese rat snakes ( Elaphe climacophora ) .

Class of DBH (cm)

Figure 2. Frequency distribution of DBH of Japanese
red pines in 16 groves in suburban areas of Tokyo.

Most pine trees in the study area had DBHs larger than
25 cm (95.0%, N = 635, Fig. 2). There were on average
37.7 pine trees within each grove ( N = 16) . No new pine
trees had recently been planted in the area.

Discussion

Many studies show the importance of nest-site prefer-
ence in reducing nest predation (Martin and Roper
1988, Martin 1993a, Kelly 1993, Pieman et al. 1993) in-
dicating that nest predation is a common source of nest-
ing failure for many bird species (Nilsson 1986, Martin
1988, 1993b). In this study, nest predation was minimal
and 93.3% of nest failures of Japanese Lesser Sparrow-
hawks were caused when nests fell from nest trees. There-
fore, it may be more important for sparrowhawks to se-
lect nesting substrates which best hold their nests rather
than to avoid nest predation.

The stature of Japanese red pines seems to prevent
nests from falling from branches. Sparrowhawk nests are
built on thick, stable branches. Japanese red pines re-
quire abundant sunshine for growth. There were few
young Japanese red pines in the study area probably be-
cause the crowns of groves had become closed by Japa-
nese chestnut oaks. Numbers of Japanese red pines have
decreased in some areas of Japan (Da and Osawa 1992,
Fujiwara et al. 1992) and the lack of young pine trees
indicates that there will be further decreases in the num-
bers of pine trees in the future. Because the nesting suc-
cess of Japanese Lesser Sparrowhawks appears to rely on
the availability of Japanese red pines, the breeding pop-
ulation of the sparrowhawks in Japan could correspond-
ingly decline if the abundance of pine trees continues to
decline.

Resumen. — Yo estudie la preferencia del arbol para el
nido de Accipiter gularis y sus afectos en el exito del nido.
A. gularis prefirio hacer nido en Pinus densiflora, y el exito
del nido fue significantemente mas alto en P. densiflora
que en otras especies de arboles. exitos de razon altos
resultaron porque nidos casi nunca se cayeron de los P
densiflora pero frecuentemente se cayeron o estaban cai-
dos cuando construidos en otras especies de arboles. Los
resultados indicaron que la preferencia de nido en P. den-
siflora por A. gularis evitaba el desalojamiento del nido.

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Vol. 31, No. 1

La estructura por anos del P. densijlora en estudios de
arboleda fueron medidos para pronosticar el futuro dis-
ponibilidad de nidos. Pinos jovenes mas bajos que 25 cm
DBH hicieron no mas de 5% de todos los arboles y nin-
gun pino nuevo fue recientemente plantado en la area.
Desde entonces una falta posible de arboles de nido con-
veniente puede ocurrir en el futuro, yo a concluido que
la poblacion de A. gularis tambien se va ir disminuyendo
mientras la disponibilidad de pinos tambien disminuye si
no tomamos pasos para otra vez continuar plantacion de
pinos en Japan.

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

I thank Navjot Sodhi, Hiroyoshi Higuchi, Reiko Kuro-
sawa, Jason Minton, Go Fujita and Toshiaki Hirano for
discussion of ideas that led to this work and for helpful
comments on earlier drafts.

Literature Cited

Brown, L. and D. Amadon. 1968. Eagles, Hawks & Fal-
cons of the World. Country Life Books, London, UK
Da, L. and M. Osawa. 1992. Abandoned pine-plantation
succession and the influence of pine mass-dieback in
the urban landscape of Chiba, Central Japan. Jpn. J.
Ecol 42:81-93.

Endo, K. and T. Hirano. 1990. Breeding records and
nesting habitats of the Japanese Lesser Sparrowhawk
Accipiter gularis in residential areas of Tochigi Prefec-
ture. Jap, J. Ornithol. 30:35-39.

Endo, K, T. Hirano and M. Ueta. 1991. Breeding re-
cords of the Japanese Lesser Sparrowhawk A ccipiter gu-
laris in Japan. Strix 10:171—179.

Fujiwara, M., G. Toyohara, Y. Hada and Z. Iwatsuki.
1992. Successional stages and degree of damage of
secondary pine forests in Hiroshima city, western Ja-
pan. ]pn. J. Ecol. 42:71-79.

Hirano, T. and M. Kimizima. 1992. Breeding status and
foods of the Japanese Lesser Sparrowhawks Accipiter
gularis in residential areas of Utsunomiya city, central
Japan. Strix 11:119—129.

Kelly, P. 1993. The effect of nest predation on habitat
selection by Dusky Flycatchers in limber pine-juniper
woodland. Condor 95:83—93.

Martin, T.E. 1988. Process organizing open-nesting bird
assemblages: competition or nest predation? Evol.
Ecol 2:37-50.

. 1993a. Nest predation and nest sites — new per-
spectives on old patterns. BioScience 43:523-532.

. 1993b. Nest predation among vegetation layers

and habitats: revising the dogmas. Am. Nat. 141:897-
913.

Martin, T.E. and J.J. Roper. 1988. Nest predation and
nest-site selection of a western population of the Her-
mit Thrush. Condor 90:51-57.

Nilsson, S.G. 1986. Evolution of hole-nesting in birds:
on balancing selection pressures. Auk 103:432-435.

Picman, J., M.L. Milks and M. Leptich. 1993. Patterns
of predation on passerine nests in marshes: effect of
depth and distance from edge. Auk 110:89-94.

Ueta, M. 1992. Comparison of the prey abundance for
Japanese Lesser Sparrowhawks Accipiter gularis in sub-
urban and mountainous areas. Strix, 11:137-141.

. 1996. Causes for decrease in breeding success

of Japanese Lesser Sparrowhawks. Strix 14: 65-71.

Received 25 January 1996; accepted 12 November 1996

Letter

J. Raptor Res. 31(1):89

Notes on Northern Goshawks Nesting in an
Abandoned Heronry in Wales

In Britain, Northern Goshawks ( Accipiter gentilis) nest in a variety of wooded habitats including plantations (S.J.
Petty 1989, Forestry Commission Bulletin 81, Her Majesty’s Stationery Office, London, UK; E.P. Toyne 1994, Ph.D.
dissertation. Imperial College, London, UK). In Wales, they usually build their own nests or build on old Common
Buzzard ( Buteo buteo) and European Sparrowhawk ( Accipiter nisus) nests. This paper presents the first account of
goshawks using the nests of Grey Herons ( Ardea cinerea). We are aware of only one other instance of an accipiter
using a heron nest. This was of a European Sparrowhawk that successfully nested on a Litde Egret’s ( Egretta garzetta)
nest while the egrets occupied the heronry (H. Hafner 1978, La Terre at La Vie 32:279-289).

The heronry was situated in Wales in an area comprised of conifer plantations, hill sheep farms, moorland and
watercourses. The area was semi-upland (80-500 m elevation) and the majority of goshawk nesting territories were
around 250 m (E.P. Toyne 1994, Ph.D. dissertation. Imperial College, London, UK). The heronry was in a 3-ha stand
of Sitka Spruce ( Picea sitchensis) planted in 1950 in a managed forest (>1000 ha). The stand was approximately 800
m from a reservoir and 5 km from the nearest town.

In 1989, the heronry was used by approximately 25 pairs of breeding herons. The herons began to use the heronry
again in April 1990 but, shortly after their eggs were laid, strong winds blew them from the nests and heron eggs
were found littering the ground. After the herons had left the heronry, a goshawk was found nesting in one of the
abandoned heron nests. Over the next seven breeding seasons, goshawks bred in a total of six abandoned heron
nests laying an average of 3.3 eggs (range = 2-4) and fledging an average of 2.5 young (range = 1-4).

All nest trees were Sitka Spruce and all heron nests were made of larch ( Larix sp .) branches. Goshawks refurbished
these nests with larch branches and lined the nest cups with foliage from larch, spruce and Douglas-fir ( Pseudotsuga
menziesii). Nest measurements were similar to other goshawk nests built on whorls of larch and conifer trees (length:
0.94—1.20 m, breadth: 0.83-1.00 m, depth: 0.25-0.50 m, N = 3). Five of the six nests that were used were well
concealed in trees with dense canopies. The other nest was in an open, thin-crowned tree.

It was unclear why the herons moved from the heronry and where they moved. Herons often relay after egg loss
(C. Voisin 1991, The herons of Europe, T. & A.D. Poyser, London, UK) so it was unlikely they deserted their colony
after losing their clutches during the wind storm. Although there is no record of goshawks in Britain killing herons
(M. Marquiss and I. Newton 1982, British Birds 75:243-260; E.P. Toyne 1994, Ph.D. dissertation, Imperial College,
London, UK), it is plausible that the pair of goshawks disturbed the herons enough to make them move elsewhere.
At another site within the study area, herons moved after goshawks first nested within 500 m of a heronry indicating
that the presence of goshawks may cause herons to desert their nesting areas.

Goshawks are known to use existing nests, including artificial ones (P. Saurola 1978, Pages 72-80 in T.A. Geer
[Ed.] , Bird of prey management techniques, British Falconer’s Club, Oxford, UK; S .J. Petty 1989, Forestry Commission
Bulletin, Her Majesty’s Stationery Office, London, UK). In our study, larch appeared to be the preferred nest tree
of goshawks with 61.2% of 116 nesting attempts in larch as opposed to 15.5% in Sitka spruce (E.P. Toyne 1994, Ph.D.
dissertation, Imperial College, London, UK) . While larch trees were nearby the heronry, the presence of heron nests
appears to have attracted goshawks to the spruce site.

Data for this paper were collected during a larger study of goshawk ecology supported by the Science and Engi-
neering Research Council, Imperial College, Forest Enterprise, The British Ecological Society and The Hawk and
Owl Trust. We are grateful to the Countryside Council for Wales for providing the necessary licences. We thank Steve
Petty, Thomas Bosakowski and an anonymous referee for useful comments to an earlier draft. We thank Herman
Ostroznik Jr., Mike Coleman, Steve Binney, Steve Galleozzie and staff of the Forest Enterprise for aid with this
fieldwork. — E.P. Toyne, 1 Department of Biology, Imperial College, London SW7 2BB, UK and H. Ostroznik, % The
Hawk and Owl Trust, Zoological Society of London, Regent’s Park, London NW1 4RY, UK

1 Present address: WWF-UK, Panda House, Weyside Park, Catteshall Lane, Godaiming, Surrey GU7 1XR, UK.

BOOK REVIEW

Edited by Jeffrey S. Marks

J. Raptor Res. 31(1):90

Handbook of the Birds of the World, Volume 2.
New World Vultures to Guineafowl. Edited by Jo-
sep del Hoyo, Andrew Elliott and Jordi Sargatal.
1994. Lynx Edicions, Barcelona. 638 pp., 60 color
plates, 302 color photographs, 590 distribution
maps. ISBN 84-87334-15-6. Cloth, $175.00.— The
volumes in this series are magnificent in both ap-
pearance and content. This volume covers the Fal-
coniformes and Galliformes, and volume 3 (Hoat-
zin to alcids) will appear before this review is pub-
lished. Twelve volumes are planned in all. The
price tag might seem excessive until one puts
things in proper perspective. Volume 2 weighs in
at 8 pounds, for a cost of about $1.37 per ounce.
By comparison, Cody’s Habitat Selection in Birds
(1985) now costs $136.00 in cloth, or about $4.00
per ounce (and even in 1985, it cost more than
$2.00 per ounce). Given the shear quantity of in-
formation and quality of presentation, I believe
that The Handbook is an exceptional deal in today’s
market of inflated book prices.

The book is organized by family, each of which
contains a general review of the topics “Systemat-
ics,” “Morphological Aspects,’’ “Habitat,” “Gen-
eral Habits,” “Voice,” “Food and Feeding,”
“Breeding,” “Movements,” “Relationships with
Man” and “Status and Conservation.” Following
the family introductions are the species accounts,
typically two to four per page. Virtually every ex-
tant species is covered, with each account contain-
ing notes on taxonomy, subspecies and distribution
(including a range map), habitat, food, breeding
biology, movements and conservation status. Each
account concludes with a list of recent references.
In addition, each species is depicted in a color

plate (averaging more than 20 individuals per
plate), often with multiple paintings to show dif-
ferences in sex, color morph and subspecies. The
color plates appear to be excellent, although I am
not qualified to evaluate all of them. The color
photographs scattered throughout the text are ab-
solutely outstanding. A tremendous range of spe-
cies is presented, and each photograph is sharply
focused and pleasingly composed. Moreover, many
of the photographs depict individuals that are ac-
tually doing something besides posing for portraits.
For example, a Verreaux’s Eagle ( Aquila verreauxii)
is seen capturing a hyrax (p. 78), an African Har-
rier-Hawk ( Polyboroides typus ) hanging from a weav-
er nest (with a nestling in its bill; p. 81), and a
female Montagu’s Harrier ( Circus pygargus ) deliv-
ering a prey item to its nestlings (p. 101).

I should mention that the family overviews and
species accounts were written by well-qualified rap-
tor biologists, including Richard Bierregaard, Wil-
liam Clark, David Houston, Alan Kemp, Lloyd Kiff,
Bernd-Ulrich Meyburg, Penny Olsen, Alan Poole,
Jean Marc Thiollay and Clayton White. The depth
of treatment in the species accounts does not sur-
pass Brown and Amadon’s Eagles, Hawks and Fal-
cons of the World (1968), but the updated informa-
tion and excellent plates and photographs provid-
ed in The Handbook make it an excellent compan-
ion piece to Brown and Amadon. As a sound,
general reference, The Handbook will be indispen-
sable. I cannot imagine that anyone with a keen
interest in raptors could be disappointed with this
book. As a bonus, the material on the Galliformes
is just as impressive as that on the raptors. I urge
you to obtain your own copy, or at the very least,
to prod your library into acquiring the entire se-
ries. — Jeff Marks, Montana Cooperative Wildlife
Research Unit, University of Montana, Missoula,
MT 59812 U.SA.

J. Raptor Res. 31(1):91

Manuscript Referees

The following people reviewed manuscripts for the Journal of raptor Research in 1996, Peer review plays a vital role
in the publishing process and in improving the quality of the Journal. The names of individuals who reviewed two or
more manuscripts are followed with an asterisk.

Dean Amadon, David J. Anderson*, James C. Bednarz, Maria Isabel Bellocq*, James R. Belthoff*, Keith L. Bild-
stein*, David M. Bird*, William Block, Peter H. Bloom, Clint B. Boal, Gary R. Bortolotti*, Thomas Bosakowski,
Buchanon, Javier Bustamente*, Tom Cade*, Richard J. Cannings, Carss, Richard J. Clark, Stephen DeStefano, Gary
E. Duke, David H. Ellis, Susan Ellis, James H, Enderson*, Sidney England, Allen Fish*, Eric Foresman*, Frederick
Gehlbach*, James A. Gessaman, Teryll Grubb*, Ralph J. Gutierrez, Patricia A. Hall, Alan H. Harmata, Elizabeth Haug,
Gregory D. Hayward, Julie Heath, Charles J. Henny*, Denver Holt*, David Houston*, C. Stuart Houston, C. Hubert,
Fabian M. Jaksic*, Paul C. James, Alan Kemp, Brian Kimsey, Jon S. Kirkley, Michael N. Kochert*, Robert Lehman,
Carl Marti*, Riley McClelland, Michael McGrady*, Kenneth D. Meyer*, Heimo Mikkola*, Brian A. Milsap, Peter
Mundy*, Robert Nero, Ian Newton*, Torgeir Nygard, Susan M. Patla, Steve Petty, Alan F. Poole, Sergej Postupalsky,
Richard Reynolds, Gary Ritchison*, Robert J. Ritchie, Robert N. Rosenfield*, Josef K Schmutz*, Dwight Smith*,
Karen Steenhof, Ted Swem, Jean Marc Thiollay, Sergio Tiranti, Kimberly Titus*, Mutsuyuki Ueta, Helen Ulmschnei-
der, Daniel Varland*, Ian G. Warkentin*, David Whitacre*, Clayton M. White, Sanford Wilbur*, Christian Wiklund,
Joseph W. Witt, Petra Bohall Wood.

THE RAPTOR RESEARCH FOUNDATION, INC.

(Founded 1966)

OFFICERS

PRESIDENT: David M. Bird SECRETARY: Betsy Hancock

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

BOARD OF DIRECTORS

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

Karen Steenhof

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

Jemima ParryJones

INTERNATIONAL DIRECTOR #2:

Michael McGrady

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

jjc

EDITORIAL STAFF

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

ASSOCIATE EDITORS

Keith L. Bildstein Fabian Jaksic

Gary R. Bortolotti Daniel E. Varland

Charles J. Henny Javier Bustamente

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

Missoula, MT 59812 U.S.A.

The Journal of Raptor Research is distributed quarterly to all current members. Original manuscripts
dealing with the biology and conservation of diurnal and nocturnal birds of prey are welcomed from
throughout the world, but must be written in English. Submissions can be in the form of research articles,
letters to the editor, thesis abstracts and book reviews. Contributors should submit a typewritten original
and three copies to the Editor. All submissions must be typewritten and double-spaced on one side of 216
X 278 mm (8 Vi X 11 in.) or standard international, white, bond paper, with 25 mm (1 in.) margins. The
cover page should contain a tide, 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 tide, 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.

1997 ANNUAL MEETING

The Raptor Research Foundation, Inc. 1997 annual meeting will be hosted by Georgia Southern
University and will be held October BO 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.SA. 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.SA. 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, Kempton, 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, Sweetwater Environmental Biologists, Inc., 3838 Camino del Rio North, Suite 270, San Diego,
CA 92108 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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