Raptor research

RAPTOR

RESEARCH

Raptor Research Foundation, Inc.
Provo, Uuh, UJS.A.

Summer 1983

RAPTOR RESEARCH

Volume 1 7, Number 2, Pages 33-64

CONTENTS

SCIENTIFIC PAPERS

Flights of Nesting Peregrine Falcons

Recorded by Telemetry — James H. Enderson

and Monte N. Kirven 33

Post-Release Flight and Foraging Behavior
of a Bald Eagle Hacked in Western Kentucky —

Robert L, Altman 37

Effects of Weather on Accipiter Migration
in Southern Nevada — Brian A. Millsap

and James R. Zook 43

Activity Patterns of Bald Eagles Wintering

in South Dakota — Karen Steenhof 37

Mouse Trap Recovered in Harrier Nest

— DaleGawlik 62

Precocious Nest Defense Behavior by

a Sharp-shinned Hawk — Robert N. Rosenfield

and Andrew Kanvik 62

BOOK REVIEWS 63

ANNOUNCEMENTS 42,56

RAPTOR RESEARCH

Published Quarterly by the Raptor Research Foundation, Inc.

Editor Dr. Clayton M. White, Dept, of Zoology, 161 WIDB, Brigham Young Univer-
sity, Provo, Utah 84602 (801) 378-4860

Editorial Assistant Sandra Fristensky, 159 WIDB, Brigham Young University,
Provo, Utah 84602

Editorial Staff Dr. Fredrick N. Hamerstrom, Jr. (Principal Referee)

Dr. Byron E. Harrell (Editor of Special Publications)

International Correspondent Dr. Richard Clark, York College of Pennsylvania,
Country Club Road, York, PA 17405

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series for lengthy and significant contributions containing new knowledge about birds
or new interpretations of existing knowledge (e.g., review articles). However, authors
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r

FLIGHTS OF NESTING PEREGRINE FALCONS RECORDED BY
TELEMETRY

by

James H. Enderson
Department of Biology
The Colorado College
Colorado Springs, CO 80903
and

Monte N. Kirven
5375 Linda Ln.

Santa Rosa, CA 95404

Abstract

Both adult Peregrine Falcons (Falco peregrinus) were radio-tagged to determine foraging
behavior and other movements at an eyrie containing young in Sonoma County, California,
April-June 1979. Equipment failure and terrain prevented a complete record of hunting
flights by triangulation, but tracking data coupled with visual observations made near the eyrie
permitted analysis of 139 flights by the female and 40 by the male. The adults tended to use
corridors along ridges when arriving or departing the eyrie. When the female remained
within 1 km of the eyrie 74% of her flights were along the ridge behind the eyrie, and often
included perching. About 47% of all the female’s flights were known to be farther than 1 km
from the eyrie and these were generally made in all directions. The male seldom remained
near the eyrie; at least 65% of his flights were farther than 1 km, and about one-third of these
were along a single ridge extending several kilometers towards a broad valley. In 20 cases the
adults were tracked to distances between about 3 and 8 km from the eyrie, and the average was
about 5 km. Prey was apparently taken fairly uniformly in most directions from the eyrie.

Introduction

Despite the potential of radio telemetry for revealing movements of wide-ranging birds
there are no published reports of telemetry studies on peregrines. The present work sought to
describe the extent and direction of foraging flights of a pair of peregrines with young. The
plan was to obtain bearings on the transmitter signal simultaneously from two receiver stations
so that the position of the bird could be plotted by triangulation. Sets of bearings obtained at
short intervals would allow the tracking of the flying bird. These data, combined with those
from a full-time observer near the eyrie were to reveal the pattern of habitat use.

A good deal of information was gathered. However, equipment difficulties and problems of
interpretation encountered are also of interest to those who may be considering similar studies
on raptors.

Methods

Telemetry receivers were AVM Instrument RB-4 single-channel units with three sub-channels. The receiver antennas
were four-element yagis mounted on a three-meter- long rotating mast equipped with a bearing disc. Transmitters were
AVM SM-1 single stage units operated at 148.1 MHz and weighed about 12 g when potted in dental acrylic. The
transmitter antenna was 0.28 mm diameter guitar wire. The transmitter was sewn to the underside of a center tail feather
and the antenna tied with nylon thread to the feather four places along its length (Craighead and Dunstan, 1976).

Several tests established bearing error on a test transmitter 8 km from a receiver. The error averaged ± 5°, but one 8°
error was obtained. Maximum line-of-sight range exceeded 15 km, but intervening terrain interrupted transmission.

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Raptor Research 17(2):33-37

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RAPTOR RESEARCH

Vol. 17, No. 2

When possible, bearings of instrumented birds were taken simultaneously by both tracking stations at 30 second intervals.
The stations were in contact by two-way radio.

The two adults were trapped, hooded, instrumented, and immediately released. The female was trapped on 21 April
and her transmitter operated until 1 June. The male was trapped on 2 June and his transmitter failed on 6 June. Both
birds behaved normally after release and eventually fledged a brood of young in late June.

The field data, consisting of synchronous bearings taken by tracking stations, bearings taken by a single station, and
notes taken by station operators and a full-time observer in view of the eyrie were collated in the following way. First,
bearings were drawn for each day from the tracking stations on an overlay of a USGS 7.5 minute topographical map, and
the time of the bearing was noted. Remarks from notes of station operators were included on the overlays. The notes of
the eyrie observer, who kept detailed accounts of falcon movements, were included on the overlays for flights for which
telemetry data were available. Last, the general routes of flights made each day were traced on overlays integrating
telemetry bearings by triangulation, direction of transmitter signal when only one station received a signal, observer notes
relating to signal strength, and departure and arrival at the eyrie as seen by the eyrie observer. The resulting routes did
not represent the exact track of each flight by the two adults, but only its general course and distance. Often only a portion
of a flight could be followed.

The routes taken by the adults were assigned one of seven corridors around the eyrie (Fig. 1). These corridors were
used on nearly all flights to or from the eyrie and correspond to topographical features and landmarks often referenced
in the field notes. If abird departed on one corridor and returned on another, a flight was shown for each. All flights were
placed in one of three groups: those that 1) did not range beyond 1 km of the eyrie, 2) exceeded 1 km, and 3) flights of
uncertain distance.

Results

Figure 1 shows the distribution of flights along the corridors by the adults. Data plotted near
the focus are for round-trip flights, simetimes interrupted by perching, which did not exceed 1
km from the eyrie. Some of these included hunting or defense. Data plotted away from the
focus represent round-trip flights, or separate arrivals and departures, where the flight
exceeded 1 km from the eyrie. These flights were presumably foraging flights. When a falcon
returned to a corridor left earlier in the same flight another datum was recorded. Numbers
along corridors show how many flights were of uncertain distance, they may or may not have
exceeded 1 km.

The beacon of the adult female provided useful information on her position for 14 days in
the period27 April-31 May 1979. Flights less than 1 km centered on corridors Cl and C7, both
included favored perches in view of the nest-cliff. Corridor C5 passes an apparent perching
area southeast of the eyrie, but often the female’s position there was uncertain because a ridge
blocked radio reception. Flights exceeding 1 km are distributed asymetrically bo corridor. Of 64
such flights for the female on Fig. 1, 36% were on C5 to the southeast and only one flight was
eastward from the eyrie over the deepest part of the east valley . Flights of uncertain distance also
predominate on C5,

The adult male was instrumented in the period 3-6 June 1979. Of the 29 flights recorded,
only 3 were less than 1 km. The remaining 26 flights were generally along all corridors except
he made 9 flights on C2 southwest from the eyrie over the deepest part of the west valley (Fig.
1 ).

Twenty long flights by the adults were tracked beyond about 3 km from the eyrie (Fig. 2).
Most of these flights were southward and four, including a 7 km flight, were substantiated by
triangulation. The others were inferred by signal strength and flight duration. The average
distance of these 20 flights was about 5 km and the most distant was about 8 km.

Foraging

In 92 instances, 20 for the female and 72 for the male, the observer near the eyrie saw
inbound flights with prey. These flights suggest the regions of hunting success because adults
carrying prey probably return directly to the eyrie from the site of the kill. The sightings, by
corridor Cl to C7 were 16, 21, 13, 15, 8, 3, and 16 when the data for the adults are combined.
Except for C5 and C6, inbound flights with prey used all corridors generally and prey was
apparently obtained in most directions from the eyrie.

Summer 1983

Enderson and Kirven — Peregrine Telemetry

35

Figure 1. Distribution of flights about the eyrie by adult peregrines. The eyrie is at the focus of lines representing
generalized corridors used by the birds. Data points near the focus are for flights shorter than 1 km, distal points are for
flights exceeding 1 km, and numbers indicate flights that may or may not have exceeded 1 km.

Telemetry Problems

We experienced several problems with the telemetry system used that greatly curtailed the
amount of information we obtained and reduced its precision:

1) Both transmitters failed long before the batteries would have been exhausted. The
guitar-wire antenna on the female broke after about one month and thereafter only a
weak, useless, signal could be obtained. The antenna on the male was sharply bent on the
second day and no signal could be obtained after five days. Guitar- wire antennas may not
be satisfactory on tail-mounted transmitters on active raptors.

2) A ± 5° error in bearing determination may be excessive for accurate tracking. In this
study the receiver stations were about 3 km apart. Such an error could lead to a 2 km
mis-location of the transmitter if it were lateral to a line between the receiver stations. A
mis-location of up to about 5 km is possible if the transmitter were far away but near a line
passing through the stations. Double yagi receiver antennas would reduce this error.

3) Where temporary receiver stations are set up and dismantled daily, equipment is subject
to great wear. Transceivers for station-to-station communication, battery packs, and
antennas, and their fittings, are especially prone to failure.

4) Transmissions in the telemetry bands normally used are useful only on a line-of- sight
basis. Telemetry is not practical in hilly or mountainous country.

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RAPTOR RESEARCH

Vol. 17, No. 2

Figure 2. General orientations of 20 flights by adult peregrines that exceeded about 3 km from the eyrie. Dashed lines
indicate uncertain flight paths.

Discussion

About 25% of the adult female’s flights were within 1 km of the eyrie and centered on
perching areas on the ridge behind the eyrie. When she flew over about 1 km from the eyrie,
she favored corridors along ridges, especially one to the southeast. The adult male made very
few short flights and hunted in nearly all directions from the eyrie, favoring a deep valley to
the southwest. Of the 64 flights by the female definitely exceeding 1 km 12 exceeded about 3
km from the eyrie and two were about 8 km distant. Of 26 flights beyond 1 km for the male, 7
were beyond 3 km and two were about 7 km distant.

The pattern of use at this territory was one of foraging flights up to 7 km, and probably
beyond, along most of the corridors around the eyrie. The female made proportionately fewer
long foraging flights than the male, but when she left the vicinity of the eyrie she appeared to
go as far as the male. In an earlier study, an instrumented adult female in Colorado showed a
similar pattern of flights in all directions from an eyrie, but two long flights extended about 19
km from the eyrie (J. Enderson, unpublished data).

Long flights are harder to track than shorter flights and the equipment and system we used
is inadequate for thorough tracking of such a wide-ranging species, especially in hilly terrain.
Where there is a question of the use by peregrines of a specific area near an eyrie, we
recommend a more direct approach: place radio-beacons on the adults and monitor the
approaches of these birds with a receiver station at the specific area.

Summer 1983

Altman — Bald Eagle Post-release Flight

37

Acknowledgements

We thank R. Olendorff, T. Cordill, P. Anderson, B. Braker, and B. Bainbridge for their
valuable assistance in this project. Funding was provided by the Bureau of Land Management.

Literature Cited

Craighead, F.C. and T.C. Dunstan. 1976. Progress toward tracking migrating raptors by
satellite, Raptor Res. 10:112-120.

POST-RELEASE FLIGHT AND FORAGING BEHAVIOR OF A BALD
EAGLE HACKED IN WESTERN KENTUCKY

by

Robert L. Altman 1
Department of Biological Sciences
Eastern Kentucky University
Richmond, Kentucky 40475

Abstract

A Bald Eagle (. Haliaeetus leucocephalus) hacked at Land Between the Lakes in the summer of
1981, was observed for 1 13 h from its release until its dispersal from the area. Eighty-three
major flights were timed, with an average of one flight per 1 .4 h. Longest flight time was nearly
25 minutes, and longest straight line distance covered during any single flight was approxi-
mately 3.0 km. Foraging success showed an improvement through time. The eagle exhibited
many behaviors similar to other birds of the same age, but appeared to be advanced in the
onset of soaring flight and capturing of live fish.

Introduction

Hacking is a technique of placing raptors on artifical nesting platforms remote from where
they were hatched. They are fed and monitored with a minimum of human contact until
capable of flight, when they are released into the wild. The biological premise is that when the
birds are sexually mature they will return to the general area from which they were released to
nest and raise young (Milburn 1979).

Bald Eagle (. Haliaeetus leucocephalus) hacking was based on a successful Peregrine Falcon
(Falco peregrinus) hacking program at Cornell University (Sherrod and Cade 1978). The state
of New York pioneered Bald Eagle hacking in 1976 at Montezuma National Wildlife Refuge
and has continued the program each year since. In 1980, the first two New York hacked eagles
nested and successfully reared two eaglets (Nye 1980). This demonstrated that hacking is a
promising means of reestablisng Bald Eagles in their former range.

The Tennessee Valley Authority (TVA) and the Tennessee Wildlife Resources Agency
(TWRA) initiated a cooperative Bald Eagle hacking program during the summer of 1980. The
goal was to reestablish a population of breeding Bald Eagles in western Kentucky and
Tennessee. Bald Eagles formerly nested in this area, but the last documented successful

‘Present address: 550 Defense Highway, Crownsville, Maryland 21032.

Raptor Research 17(2):37-42

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RAPTOR RESEARCH

Vol. 17, No. 2

nesting at Land Between the Lakes (LBL) occurred in the 1940’s (Peterson 1973). Five Bald
Eagles have been successfully hacked at LBL during the first 2 years. The eagle in this study
was produced and parent-reared in captivity at the Columbus Ohio Zoo.

Study Area and Methods

Land Between the Lakes is a 68,000 hectare (170,000 acres) peninsula located between Kentucky Lake and Lake
Barkley in western Kentucky and Tennessee (Fig. 1). There are many bays and coves along the 480 km of relatively
undeveloped shoreline which offer seclusion from the main bodies of water and potential Bald Eagle nesting habitat. The
hacking site is located along the Prior Bay shoreline of Lake Barkley (Lowe et al. 1981).

Radio telemetry equipment was utilized for short term monitoring of the eagle. The bird was banded with a U.S. Fish
and Wildlife Service rivet band and a red plastic band for long term identification.

A small flat boat with an 85 hp motor was utilized for following the eagle. The bird’s general location was established
with telemetry equipment and pinpointed with 10X binoculars. Once located, a minimum distance of approximately 70
m at a right angle to the shoreline was maintained between the eagle and the boat to avoid forcing any movements and
direction of movement. When tracking the eagle in flight a minimum distance of about 0.4 km was maintained for the
same reasons. All time periods between dawn and dusk were similarily represented avoiding any time of day bias.

Flights that were observed were timed with a stopwatch; those that lasted more than 15 seconds were considered major
flights. A flight was defined as the interval from one perch to another or from the time the eagle was seen in the air until it
went out of view. Distance of a flight was determined by plotting perch locations on a topographic map and measuring
straight line distance from perch to perch. Flight altitude was estimated. The term range refers to the maximum distance
traversed during a particular period.

Foraging methods were observed and the frequency of foraging attempts and successes were quantified. Only those
times when the eagle swooped and actually struck the water surface were considered foraging attempts. Foraging success
was the percentage of foraging attempts in which a fish was secured.

Results

The eagle was observed for 1 1 3 h during which time 83 major flights were timed. The study
was divided into four periods based on the eagle’s movements: release and the first day, early,
intermediate, and late periods.

Release and First Day

On Tuesday 7 July, the 14.5 week old eagle made its first flight at 0645 (CST), only a few
seconds after biologists had removed one side panel from the hacking enclosure. The eagle
alternated flapping and gliding without losing altitude and ascended twice. It banked and
made several circular patterns as it flew in a southward direction. It landed about 9 m up in a
tree with dense foliage that was slightly less than 0.4 km southeast of the hack site in a swampy
subimpoundment. Total flight time was 70 seconds, and the altitude varied from 9 to 18 m.
The eagle remained on this perch for 2.5 h before taking a second flight, which was similar to
the first and lasted 1 min.

In late afternoon, the eagle soared above the tree tops for 4.5 min. and reached an altitude
of 80 m. At sunset the bird was in the main section of Prior Bay, 0.8 km from the hack site.

Early period

This period lasted 3.5 days and was characterized by random movements about the main
section of Prior Bay (Fig. 1). The eagle’s range was less than 0.8 km, and it was never observed
to approach within 0.5 km of the hacking tower. Most flights were short (less than 200 m) along
the southern shoreline of Prior Bay or across the mouth of a small cove. All were under 18 m in
height and no soaring was observed.

On 9 July, the eagle was observed capturing a live fish. The bird was perched in a shoreline
tree about 10.5 m above the water when it suddenly left the perch flying directly towards the
water. It struck the water surface about 4.5 m from the shoreline, submerging all but its wings
and upper body. It immediately began moving towards the shore by using its wings in a
paddling motion. When the eagle reached the shore it hopped onto a fallen log and a fish was
observed in its talons.

Summer 1983

Altman — Bald Eagle Post-release Flight

39

Figure 1. Movements of a fledgling Bald Eagle hacked at LBI,.

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RAPTOR RESEARCH

Vol. 16, No. 2

Although no other prey capture was observed during this period, observation of certain
behaviors indicate that the eagle was feeding. These included low altitude foraging searches
along the shoreline and walking along the shoreline which could have been scavenging
behavior. Foraging success for the period was 33% (1-3).

Intermediate Period

This period lasted 4.5 days and was characterized by consistent northward movements until
the eagle reached the Crab Creek area of LBL (Fig. 1), some 40 km from the hack site.
Although the eagle’s circling flights sometimes took it a short distance south, when it finally
landed it was always perched north of the previous perch. It’s flight path followed the LBL
shoreline of Lake Barkley, and use of the many bays along the route was minimal. Flight height
usually varied between 3-18 m, and the longest distance was approximately 3.0 km. These
movements resulted in an average range of 8.2 km per day.

During this period the eagle was first observed picking up dead fish off the water surface.
All successful forages were at the end of lengthy flights of more than 2 min. Foraging success
was 50% (8-16).

Late period

This period was characterized by a “settling in” as the eagle remained in the Crab Creek area
for 29 days. The bird left the area once, when it spent 1 day, 9 August, in the Cypress Creek
area (Fig. 1). The eagle’s overall range for this period was approximately 2.0 km.

The majority of flights were low altitude foraging searches that involved a great deal of
circling as the eagle scanned the water surface below. These were usually under 18 m in height,
and covered a distance of less than 0.4 km from perch to perch. Soaring flights were also
observed and they were usually along the shoreline where winds sweeping across the lake
created an updraft. One in particular, on 21 July, lasted nearly 25 min.

The eagle became very adept at finding and picking up fish on the water’s surface as
evidence by a foraging success of 76% ( 1 6-2 1 ). Most feeding perches were just a few feet off the
ground on low stumps or snags.

Dispersal

It is believed the eagle dispersed from the study area on 15 August. It was last seen on 1 1
August, but transmitter signals through the 14th indicated that it was still in the Crab Creek
area. On 15 August there was no signal in the Crab Creek area or 2 km east or west of there.
Several days later surface and aerial searches of both Lake Barkley and Kentucky Lake
revealed no transmitter signals.

Discussion

The strength of the eagle’s first day flights may be related to the age at which the bird was
released. In wild nests, when most birds fledge at 1 1 or 12 weeks of age, first flight is usually a
glide onto or near the ground (Harper 1974; Kussman 1977). Milburn (1979) observed similar
flights in hacked fledglings and several times had to retrieve them from the ground because
they could not attain lift. This problem seems to be avoided by keeping eaglets on the hacking
tower an extra 2 or 3 weeks and allowing them to develop greater strength in the flight
muscles (Lowe, R.L., per. comm.). The first-day flights of 5 hacked eagles at LBL support this
contention. Milburn (1979) first observed soaring in hacked eagles at 3 or 4 weeks after release
(15-16 weeks of age). Kussman (1977) intensively studied 8 fledgling Bald Eagles from wild
nests and found an average of 32.8 days off the nest (16 weeks of age) before the onset of
soaring activity.

Summer 1983

Altman — Bald Eagle Post-release Flight

41

It was unusual that the subject eagle was observed successfully hunting on the second day
after release. Milburn (1979) observed 7 hacked eagles and did not witness it until 7 or 8 weeks
after release (19-20 weeks of age). Harper (1974) never observed hunting behavior in 3 eagles
for 20 weeks after they fledged. Kussman’s (1977) earliest observation of scavenging was 6.5
weeks after fledging (18.5 weeks of age), and most birds were 5 months old before they
exhibited this behavior.

Jaffe (1980) studied the foraging behavior of immature Bald Eagles in mid-summer and
found that foraging success in immature eagles increased through time with an overall success
rate of 80%. This compares favorably with the 76% foraging success here during the late
period.

The condition of fish that the eagle captured was difficult to ascertain. Bald Eagles often
take live fish, but being opportunistic feeders they frequently take dead or dying fish if
available (Southern 1963; Bent 1961; Herrick 1933; Brown and Amadon 1968; Wright 1953;
Broley 1958). Immature Bald Eagles tend to rely more heavily on dead fish than adults
(Sherrod et al. 1976). I frequently saw dead fish floating on the surface of the water, and the
eagle took these several times. The only instance when the eagle was observed to actually strike
beneath the water surface for a fish was on the second day following release.

Movements of juvenile Bald Eagles are not well documented. Only Kussman (1977) and
Harper (1974) have dealt with this subject in detail. Bald Eagles usually follow shorelines
because of perch sites and fish availability. The methodical northward movement of the eagle
in this study ended abruptly when it reached the northern boundary of Lake Barkley. Gerrard
et al. (1974) correlated movements of juvenile Bald Eagles with wind direction, but subjective
observations by the author indicated that winds were variable throughout this period.

This eagle and the other four hacked at LBL were never observed to return to the hacking
tower after being released. All seven of the hacked eagles that Millburn (1979) observed
returned regularly to the tower, but two birds hacked in Georgia never returned to the tower
(Odum 1980). In wild nests, recently fledged Bald Eagles often return to the nest (Gerrard et
al. 1974; Harper 1974), although some do not (Weeks 1975).

The eagle remained in the study area for 39 days after release. This is similar to the
observations of Milburn (1979) who recorded variability in the dispersal times of hacked eagles
in New York from 3.5 weeks to 14 weeks after release; and to Gerrard et al. (1974) who
observed seven immature eagles in Saskatchewan and found that dispersal began at 20-21
weeks of age.

Acknowledgements

This research was funded by TVA. I thank John L. Mechler, Marcus E. Cope, and Robert M.
Hatcher for supervision throughout the study. Dr. Branley A. Branson, Carol A. Schuler, and
an anonymous reviewer commented on the paper. Rick Lowe provided guidance during the
research, attached the transmitter, and critically reviewed an earlier manuscript.

Literature Cited

Bent, A.C. 1961. Life histories of the North American birds of prey, Part 2. Dover Publica-
tions, New York. 482 pp.

Broley, C.L. 1958. Plight of the American Bald Eagle. Audubon Mag. 60:162-163, 284-286.
Brown, L.H., and D. Amadon. 1968. Eagles, hawks, and falcons of the world. Volume II.
McGraw-Hill Book Co., New York, 414 pp.

Gerrard, P., J.M. Gerrard, D.W.A. Whitfield, and W.J. Maher. 1974. Post-fledging move-
ments of juvenile Bald Eagles. Blue Jay 32:218-226.

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Vol. 17, No. 2

Harper, J.F. 1974. Activities of fledgling Bald Eagles in north-central Minnesota. M.S. Thesis,
Western Illinois Univ., Macomb. 68 pp.

Herrick, F.H. 1933. Daily life of the American eagle: Early phase (concluded). Auk 50:35-53.

Jaffee, N.B. 1980. Nest site selection and foraging behavior of the Bald Eagle (Haliaeetus
leucocephalus) in Virginia. M.S. Thesis, William and Mary College. 112 pp.

Kussman, J.V. 1977. Post-fledging behavior of the northern Bald Eagle Haliaeetus
leucocephalus alascanus Townsend, in the Chippewa National Forest, Minnesota. Ph.D.
Thesis, Univ. of Minn., St. Paul.

Lowe, R.L., R.L. Altman, and R.M. Hatcher. 1981. Behavioral patterns of Bald Eagles utilized
in an experimental hacking project. Pro. Ann. Conf. S.E. Assoc. Fish and Wildl. Agencies
35: In press.

Milburn, E.H. 1979. An evaluation of the hacking technique for establishing Bald Eagles
(. Haliaeetus leucocephalus). M.S. Thesis, Cornell University. 184 pp.

Nye, P.E. 1980. Successful establishment of nesting Bald Eagles by hacking. Proceedings of the
Raptor Research Found. Ann. Meeting. October 10-13, 1980, Duluth, Minnesota.

Odum, R.R. 1980. Current status and reintroduction of the Bald Eagle in Georgia. Oriole
45:1-14.

Peterson, C.T. 1973. Bald Eagles in Land Between The Lakes. TVA, LBL, Golden Pond,
Kentucky. 5 pp.

Sherrod, B.K. and T.J. Cade. 1978. Release of Peregrine Falcons by hacking. Pages 12U136
in T.G. Geer, ed. Birds of Prey Management Techniques. British Falconers’ Club.

Sherrod, B.K., C.M. White, and F.S.L. Williamson. 1976. Biology of the Bald Eagle on
Amchitka Island, Alaska. Living Bird, 143-182.

Southern, W.E. 1963. Winter populations, behavior, and seasonal dispersion of Bald Eagles in
northwestern Illinois. Wilson Bull. 75:42-55.

Weeks, F.M. 1975. Behavior of a young Bald Eagle at a southern Ontario nest. Canadian Field
Nat. 89:35-40.

Wright, B.S. 1953. The relationship of Bald Eagles to breeding ducks in New Brunswick./.
Wildl. Manage. 17:55-62.

ANNOUNCEMENT

A NEW INFORMATION SYSTEM FOR RAPTORS

The Raptor Management Information System (RMIS) is a collection of published and unpublished
papers, reports, and other works on raptor management and human impacts on raptors and their
habitats. It currently consists of nearly 2,500 original papers, 178 keyworded notecard decks comprised of
15,000 key paragraphs from the original papers, and a computer program to retrieve partially annotated
bibliographies by species, by keyword, or by any combination of keywords and/or species. A geographical
index is under development, and new papers are added as they are received.

Originally designed to facilitate land-use planning and decision-making by government agencies and
industry, the RMIS has since grown into a powerful research and environmental assessment tool for
scholars, students, consultants, as well as land managers and their staff biologists. For more information
write Dr. Richard R. Olendorff, U.S. Bureau of Land Management, 2800 Cottage Way, Sacramento,
California U.S. A. 95825, or phone commercial (916) 484-4541 or through the Federal Telephone System
468-4541.

EFFECTS OF WEATHER ON ACCIPITER MIGRATION IN
SOUTHERN NEVADA

by

Brian A. Millsap 1 and
Janies R. Zook. 2

U.S. Bureau of Land Management
Phoenix District Office
2929 W. Clarendon Ave.

Phoenix, AZ 85017

Abstract

Migrating Sharp-shinned Hawks (Accipiter striatus) and Cooper’s Hawks (A. cooperii) were
observed along a forested ridge surrounded by desert in the Spring Mountains of southern
Nevada from 31 August until 17 October 1980. Greatest numbers of accipiters were counted
on days cold fronts passed through our study area; however, fronts typically separated
relatively homogeneous air masses, and passage produced no perceptible or consistent
changes in surface weather variables (as measured at our study site). Analysis indicated that
perceived migrant abundance, although strongly associated with cold front passage, was not
related to surface weather conditions as many other studies have suggested. The “extra”
accipiters observed on front days appeared between mid-morning and late afternoon. This is
the period of the day when accipiters and other hawks frequently migrate at high altitudes
riding thermal updrafts. We believe post-frontal atmospheric stability and strong winds aloft
confined thermal activity to a narrow zone in the lower atmosphere on front days, which
resulted in more accipiters migrating at lower altitudes. Increased counts probably resulted
because: (1) a higher proportion of the daily flight occurred within visible range; (2) more
accipiters may have sought lift from updrafts along mountain ridges as an alternative to
thermal updrafts; and (3) migrating accipiters may have become reluctant to cross inhospita-
ble deserts at lower altitudes, and instead directed flights over boreal forests along ridgetops.
We suggest post-frontal atmospheric conditions may similarly affect raptor migration
elsewhere, and future studies should more thoroughly investigate the role of weather in
influencing the height of migration.

Introduction

Autumn raptor migration has been studied in few parts of North America, notably several
localities in the east and midwest where large numbers of raptors concentrate under certain
conditions (Heintzelman 1975). At these sites raptor counts are typically greatest following the
passage of a cold front when surface winds switch to a westerly or northerly direction,
barometric pressure rises, temperature falls, the sky clears, and often, wind speed increases
(Mueller and Berger 1961, Haugh 1972). Many researchers have postulated a direct relation-
ship between frontal changes in these weather variables, either singly or additively, and the
magnitude of hawk migration (Mueller and Berger 1961, Haugh 1972, Hoffman 1981).

Strong cold front activity is not universal throughout North America in autumn. For

*Current address: Raptor Information Center, National Wildlife Federation, 1412 Sixteenth St. N.W., Washington,
D.C. 20036.

2 Current address: P.O. Box 1327, Tempe, AZ 85281.

43

Raptor Research l7(2):43-56

44

RAPTOR RESEARCH

Vol. 17, No. 2

For example, at low latitudes in the western United States autumn is a fairly stable
meteorological period (Brown 1974, Sellers and Hill 1974) and most cold fronts separate
relatively homogeneous air masses (hence are “weak”). Accordingly, many of the surface
weather effects noted with front passage further north are absent. In an attempt to determine
how weather influences raptor migration in this region, we observed migrating Sharp-shinned
Hawks and Cooper’s Hawks in southern Nevada for the bulk of the autumn migration period
in 1980. This paper summarizes data collected and presents findings which, we believe, help
explain the relationship between raptor migration and cold front passage.

Study Area

Observations were made from Potosi Mountain (Potosi), located 48 km west of Las Vegas in Clark County, Nevada (Fig.
1). Potosi is the southernmost peak in the Spring Mountains (Springs) and rises sharply out of a pass to an elevation of
2592 m, forming a narrow north-south ridge for about 6 km.

Like other mountain ranges in southern Nevada, the Springs rise abruptly from low elevation (900 m) valleys. Annual
precipitation ranges from about 1 1 cm in valleys to 50 cm in mountains (Brown 1974). Vegetation typical of Transition,
Upper Sonoran, and Lower Sonoran Life-zones occur in the area in broadly overlapping altitudinal zones. Boreal and
Rocky Mountain conifer forests of bristlecone pine ( Pinus aristata), limber pine (P. flexilis), and ponderosa pine (P.
ponderosa) occur along ridgetops above 2430 m elevation. Cold temperate Great Basin conifer woodlands of pinyon pine
(P. Monophylla) and juniper (Juniperus spp.) dominate at elevations between 2740 m and 1830 m. Below 1830 m warm
temperate Mohave desert scrub associations of joshuatree ( Yucca brevifolia ) and creosotebush ( Larrea tridentata) predomi-
nate (vegetation formation follow Brown et al. 1979, plant names follow Lehr 1978). Boreal forests in the Springs and
nearby Sheep Mountains are isolated from other tracts of similar vegetation by at least 160 km of Upper and Lower
Sonoran Life-zone vegetation (Fig. 1).

Methods and Data Treatment

Raptors were counted, captured, and banded from a blind in a clearing atop Potosi, Counts included all accipiters
caught or enticed into the area as well as nonresponsive individuals. We initiated observations on 31 August 1980 and
continued daily counts until 17 October 1980. Raptors were identified to species as conditions allowed and tallied by hour
on daily count forms. Weather conditions were recorded at the start and close of each observation day and at least once
each 2 h between start and close. Temperature, percent cloud cover, wind speed, wind direction, and barometric
pressure were determined at each reading. Raptor counts and weather data were obtained for 34 complete days (i.e.
beginning at 0800 h and continuing until 1700 h).

Using these and other data available to us we calculated three variables describing the accipiter migration and 14
variables describing weather conditions for each complete observation day (Table 1). We then placed each day into one of
four groups according to prevailing wind direction (i.e. days dominated by northerly winds in one group, easterly winds
in another, southerly in another, and westerly in another) and searched for bivariate and multivariate correlations
between count and weather variables within groups. We also compared average daily counts between groups. Sample
sizes were sufficient to yield meaningful conclusions for only two groups; days with southerly (n= 17 days) and westerly
(n= 12 days) winds. Accordingly, we confined analysis of migration/weather relationships to this 29 day sample.

All analyses were performed on a Honeywell 6680 computer using STATPAC statistical packages with probability
levels of o< =0.05. Relations between two sets of variables were tested using product-moment correlation coefficients.
Comparisons between means of two populations were conducted using the t-test (Sokal and Rohlf 1969) which requires
no assumption of homogenity of variance nor equal sample sizes. Multivariate trends in weather data were determined
using Principle Component Analysis (PCA). Care was taken to scale variables properly for PCA, PCA reduces a set of n
raw variables (in our case, weather variables) to n components; each component consisting of a unique set of intercorre-
lated raw variables. In a PCA components are ranked so that each successive component accounts for a smaller
proportion of total variance in the original data set. In most cases the first three components cumulatively account for 60
to 80 percent of the variance and additional components can be ignored (Levins 1968, Green 1974, Johnson 1977,
Rotenbury 1978, Rotenbury and Weins 1 980). In our analysis the first three components defined multifactorial gradients
in total weather condition (as limited by the scope of our measurements). Component scores were calculated for each
south and west wind day, and days were plotted along component axes. By comparing TAC on days falling in different
positions along component axes (i.e. ordinating in different regions of the three-dimensional space), it was possible to
assess the relationship between accipiter counts and general weather conditions.

Summer 1983 Millsap and Zook — Accipiter Migration

45

KILOMETERS \o

; : O

Fig. 1. Map of southern Nevada showing position of Potosi in relation to other physiographic features.

anges with peaks over 2400 m are outlined (high peaks are marked for reference), and stippled areas
delineate boreal islands of montane conifer forest vegetation. r

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Vol. 17, No. 2

Table 1 Description of accipiter count and weather variables calculated for each complete observa-
tion day.

No.

Code

Code

1 . Mean number total accipiters observed per h

between 0800h and 1700

TAC

2.

Mean number Sharp-Shinned Hawks observed per h
between 0800h and I700h

TACST

3.

Mean number Cooper’s Hawks observed per h
between 0800h and 1700h

TACCO

4.

Maximum temperature (°F)

MXTEMP

5.

Temperature diversity 3

HTEMP

6.

Average barometric pressure (cmHg)

AVBAR

7.

Barometric pressure diversity 3

HBAR

8.

Average cloud cover (percent)

AVCC

9.

Cloud cover diversity 3

HCC

10.

Equitability of cloud cover* 5

ECC

11.

Average wind speed (km/h)

AVWS

12.

Wind speed diversity 3

HWS

13.

Equitability of wind speed* 5

EWS

14.

Prevailing wind direction

-

15.

24h barometric pressure change (cmHg)

-

16.

24th change in TAC

-

17.

Cold front passage c

-

Calculated using the formula given in Shannon and Weaver (1949);

H= 2 PilnP-

i=l

where Pi= proportion of readings in the ith measurement subdivision, s= the total number of
subdivisions occupied, and H= the diversity index (HTEMP, HBAR, HCC, or HWS). For
HTEMP each subdivision was 5°F. For HBAR each subdivision was 0.13 cmHg. For HCC each
subdivision was 20 percent. For HWS each subdivision was 16 km/h.

Calculated using the formula given in Power (1971);

E=H/Hmax,

where H= HCC or HWS, and Hmax= Ins, the maximum possible diversity index for s occupied
subdivisions.

Determined from NOAA Daily Weather Charts.

Summer 1983

Millsap and Zook — Accipiter Migration

47

Results

Count Totals and Chronology of Migration

A total of 359 Sharp-shinned Hawks, 215 Cooper’s Hawks, and 67 unidentified accipiters
were observed. Accipiter counts varied from day to day in a series of peaks and troughs (Fig.
2). The mean interval between peaks for Sharp-shinned Hawks was 3.00 ± 1.04 days (1 SD).
The mean interval for Cooper’s Hawks was 2.91 ± 0.94 days. Intervals did not differ
significantly between species (p > 0.05).

Seventy-two percent of Cooper’s Hawks were observed during the first three weeks of
September, with a noticeable peak between 10 and 19 September. Sharp-shinned Hawk
migration appeared to increase during the first 10 days of September and remained relatively
constant thereafter. Although observations did not cover the entire migration period for
either species, Sharp-shinned Hawks appeared to migrate over a longer period of time than
Cooper’s Hawks.

Weather and Intensity of Observed Migration

Component patterns resulting from PCA for west wind days are summarized in Table 2,
and the ordination of observation days is shown in Fig. 3. The first component described a
gradient (from positive to negative in Fig. 3) from warm, wide ranging temperature; steady
pressure; mostly clear skies; and light winds to low steady temperature; unsteady pressure;
mostly cloudy skies; and strong winds. The second component described a gradient (from
positive to negative in Fig. 3) from gusty to steady winds. The third component described a
gradient (from positive to negative in Fig. 3) from steady sky conditions (i.e. completely
overcast to completely clear) to variable sky conditions. Seven of eight west wind days with high
TAC received positive scores on the first component, and six received negative scores on the
third component. With one exception, days with low TAC received negative scores on the first
component. This suggests that high TAC on west wind days was associated with warm, fair to
partly cloudy weather, and light to moderate winds. Bivariate analyses supported this conclu-
sion. TAC was negatively correlated with HBAR (r=-0.53 p < 0.05), AVCC (r=-0.65 p <
0.05) and HCC (r=-0.72 p < 0.01).

Component patterns resulting from PCA for south wind days are summarized in Table 3,
and the ordination of observation days is shown in Fig. 4. The first component described a
gradient (from positive to negative in Fig. 4) from warm temperature; high pressure; mostly
clear skies; and steady winds to cool temperature; low pressure; mostly cloudy skies; and gusty
winds. The second component described a gradient (from positive to negative in Fig. 4) from
variable sky conditions and light winds to steady sky conditions and strong winds. The third
component described a gradient (from positive to negative in Fig. 4) from wide ranging to
steady temperature. Days with high TAC were relatively evenly distributed along all compo-
nent axes. Bivariate analyses indicated there were no significant correlations between TAC,
TACST, or TACCO and any of the weather variables used in PCA (p > 0.05 for all).

The direction and magnitude of 24 h changes in barometric pressure were not significantly
correlated with changes in TAC regardless of wind direction (n=34 days) (r=-0.06, p > 0.05).
Accordingly, a falling or rising barometer did not appear to influence count totals. There was,
however, a significant difference in mean TAC and TACST between south and west wind
days; both variables were greater with south winds (p < 0.01). Mean TACCO was also greater
with south winds, but the difference was not statistically significant (p > 0.05).

48

RAPTOR RESEARCH

Vol. 17, No. 2

<r

UJ

CD

o

»-

o

o

IT

UJ

CD

2

UJ

H

Q.

UJ

(S>

Fig. 2. Total accipiter counts (Sharp-shinned Hawks + Cooper’s Hawks + unidentified accipiters) for complete observation days
(0800h to I700h). Vertical lines marked F indicate passage of a cold front. Dashed lines indicate front days not analyzed due to
incomplete weather measurements.

Summer 1983

Millsap and Zook — Accipiter Migration

49

Table 2„ Factor loadings of weather principal components for days with west winds. Only significantly
correlated (p < 0.05) values shown.

Component

I

II

III

Eigenvalue :

5.09

1.48

1.26

% Variance

50.89

14.80

12.64

£ % Variance :

50.89

65.69

78.34

Variable

Maximum Temperature (MXTEMP)

.92

Temperature Diversity (HTEMP)

.79

Average Barometric Pressure (AVBAR)

Barometric Pressure Diversity (HBAR)

-.76

Average Cloud Cover (AVCC)

-.92

Cloud Cover Diversity (HCC)

-.88

Equitability of Cloud Cover (ECC)

.76

Average Wind Speed (AVWS)

Wind Speed Diversity (HWS)

-.73

.62

Equitability of Wind Speed (EWS)

-.81

There is strong evidence that differences in accipiter counts between south and west wind
days were not related to wind direction per se, but resulted from a strong positive relationship
between count totals and cold front passage (Fig. 2). Days of cold front passage at Potosi were
always dominated by southerly winds, and mean TAC was significantly greater on frontal
compared with nonfrontal south wind days (p < 0.05).

Most cold fronts which, according to daily weather charts, passed Potosi were weak and
produced no perceptible change in weather conditions on Potosi. Furthermore, weather
conditions on front days were highly variable. For example, front days were evenly distributed
along all three south wind PCA components; of seven front days, three received positive scores
and four negative scores on the first component, three were positive and four negative on the
second, and three were positive and four negative on the third (see Fig. 4). This suggests
accipiter counts were positively influenced by front passage regardless of weather conditions
at our study site.

50

RAPTOR RESEARCH

Vol. 17, No. 2

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Summer 1983

Millsap and Zook — Accipiter Migration

51

Table 3. Factor loadings of weather principal components for days with south winds. Only signific-
antly correlated (p < 0.05) values shown.

Component :

I

II

III

Eigenvalue :

3.57

2.21

1.26

% Variance :

35.75

22.06

15.20

2 % Variance :

35.75

57.81

73.01

Variable

Maximum Temperature (MXTEMP)

.84

Temperature Diversity (HTEMP)
Average Barometric Pressure (AVBAR)
Barometric Pressure Diversity (HBAR)

.66

.92

Average Cloud Cover (AVCC)

-.63

.60

Cloud Cover Diversity (HCC)
Equitability of Cloud Cover (ECC)

-.71

.72

Average Wind Speed (AVWS)

-.67

-.64

Diversity of Wind Speed (HWS)

-.68

-.64

Equitability of Wind Speed (EWS)

Although accipiter counts showed a consistent daily rhythm, the distribution of observations
differed between front and nonfront south wind days (Fig. 5). On non-front days most
Cooper’s Hawks appeared in the morning, and a moderate proportion of Sharp-shinned
Hawk observations occurred prior to 0930 h. On front days, however, most Cooper’s Hawks
were observed after mid-day and a relatively small proportion of Sharp-shinned Hawks were
seen in the early morning. This indicates that for both species, increased counts on front days
were the result of more individuals appearing during the late morning and afternoon hours
rather than an overall increase in migrant numbers throughout the day.

Discussion

Accipiter migration occurred on all days with generally fair weather. High counts, however,
were strongly associated with cold front passage and did not appear related to weather
conditions on Potosi. Although it is possible that accipiters responded to changes in barometric
pressure or temperature too small to be detected on our instruments, Mueller and Berger
(1961) present evidence that such perception is unlikely in raptors. This leads us to conclude
other factors associated with cold fronts affected migration. The most logical alternatives are
that: (1) cold front passage caused changes in surface weather conditions to the north of

Potosi, and these changes produced an increase in the volume of movement into our study
area (i.e. more hawks were aloft over Potosi); or (2) front passage produced changes in
atmospheric weather conditions (rather than changes in weather on Potosi) which altered
flight conditions and resulted in more accipiters passing within our range of vision.

>2.3 TAC

52

RAPTOR RESEARCH

Vol. 17, No. 2

Fig. 4. Graphic ordination of south wind observation days on weather principle component axes. Each pin represents one
observation day, and dates are marked at the base of each pin where S = September and O = October. Pinhead size indicates
relative TAC (Total Accipiter Abundance, or mean number of total accipiters observed per hour) as defined in the key above
graph. Small dashed lineson pins indicate positive scores on component II. Large dashed lines separate quadrates. Front days are
marked F. See text and T able 3 for interpretation of the axes.

Summer 1983

Millsap and Zook — Accipiter Migration

53

TIME (HOURS)

u.

O

TIME (HOURS)

Fig. 5. Proportion of total number of Sharp-shinned Hawks (ACST) and Cooper’s Hawks (ACCO)
observed by hour on south wind days when cold fronts passed Potosi (S wind front days) and non-frontal
south wind days (S wind non-front days).

We doubt the former factor was responsible. If the volume of accipiter movement increased
further north it is unlikely all migrants from all affected latitudes would reach Potosi on the
same day as the front; our counts would have been higher than normal not only on front days,
but following days as well (see Fig. 2). Furthermore, if more hawks were aloft over Potosi on
front days, counts for all periods of the day (rather than only specific hourly periods) should

54

RAPTOR RESEARCH

Vol. 17, No. 2

have been greater than normal (Fig. 5). On the other hand, the latter observation is perhaps
the best evidence that local atmospheric conditions were responsible. Migrant raptors, in-
cluding accipiters, are known to travel at high altitudes (above the range of visual detection by
ground observers), and such flights may be particularly common in areas like southern
Nevada where inhospitable expanses (deserts) must be crossed (Allen and Peterson 1936,
Deelder and Tinbergen 1947, Evans and Lathbury 1973, Richardson 1975). High altitude
flights by soaring birds are typically associated with (or initially depend upon) thermal
updrafts and usually occur between mid-morning and late afternoon when thermal activity is
greatest (Pennycuick 1979, Heintzelman 1975, Miller 1976, Thiollay 1980). The “extra”
accipiters observed on front days at Potosi appeared almost exclusively during this period of
the day, which suggests atmospheric conditions behind fronts resulted in a greater proportion
of a normally high and/or dispersed mid-day flight occurring within visible range. We believe
atmospheric conditions behind fronts forced accipiters to migrate at lower than normal
altitudes.

Although we are uncertain how flight conditions changed, at least two meteorological
factors may have been involved. Thermals form when a parcel of air near the surface becomes
warmer than the surrounding air and begins to rise. The parcel continues to rise until it is
sheered and disseminated by winds or, through radiation and intermixing, it reaches the same
temperature as the air around it (Miller 1976). Accordingly, thermals are particularly preva-
lent and attain greatest heights on days when the atmosphere is unstable (i.e. when tempera-
ture decreases steadily with altitude) and winds aloft are light (Miller 1976). At the leading
edge of a cold front, however, warm air is displaced up and over cool air near the surface; the
actual front slopes back over the cool air mass. Behind the leading edge of the front, where the
cool air is overlain by warm air, rising thermal parcels probably cease vertical motion upon
penetrating the warm air layer. The resultant decrease in vertical motion produces stronger
winds aloft (although not necessarily at the surface) because frictional drag with the surface is
reduced (Miller 1976). These conditions could act to confine thermal activity to (and hence,
force raptors to travel in) a relatively narrow altitudinal zone near the surface following front
passage.

A simple lowering of flight height for any reason would increase the proportion of migrants
visible to observers on the ground. In addition, however, it could increase use of ridges by
migrating accipiters. Horizontal winds striking the sides of a ridge are deflected upward
(declivity currents), and raptors make use of these currents to remain aloft and expedite
passage on migration (Heintzelman 1975). On days when thermal activity was unrestricted
migrants probably traveled directly across deserts around Potosi by gliding from thermal to
thermal after mid-morning. With reduced high altitude thermal activity on front days,
accipiters may have been forced to rely upon declivity currents to remain aloft throughout the
day. It is also possible accipiters were reluctant to cross deserts at low altitudes; birds are often
hesitant to cross inhospitable terrain at other than great heights (Deelder and Tinbergen
1947). This might further increase migrant use of ridges, which supported forest and wood-
land vegetation typical of accipiter habitat in the west (Reynolds 1982).

Although increased wind speeds have long been known to cause birds to fly at lower
altitudes (Deelder and Tinbergen 1947), we know of no studies which suggest that a decrease
in the altitude of migration behind fronts may be the initial factor contributing to high raptor
counts at various autumn lookouts. Many researchers have implied that maximal numbers of
hawks migrate behind fronts, and post-frontal surface weather conditions and geography act
to concentrate migrants at particular locations (Mueller and Berger 1961 and 1967, Heintzel-
man 1975). Although radar studies have confirmed that large numbers of hawks are aloft

Summer 1983

Millsap and Zook — Accipiter Migration

55

following passage of many cold fronts, comparable or larger flights of some species occur
unassociated with typical post-frontal weather and at altitudes and/or locations where they are
indiscernible from usual lookouts (Robbins 1956, Evans and Lathbury 1973, Richardson
1975). We suggest that strong raptor movements probably occur during fair weather regard-
less of cold front activity in autumn, and typically at high altitudes where mountain updrafts
are not influential and short water and desert crossings are not prohibitive. The volume of
movement is probably more closely associated with the direction of winds aloft (Richardson
1975) and/or, as our findings indicate, thermal activity. Behind cold fronts, however, our data
suggests flight may be restricted to lower altitudes. Under these conditions migration probably
becomes more visible and concentrations appear because: (1) updrafts along ridges are sought
out as an alternative to thermals; and (2) hawks become reluctant to cross expanses of atypical
or unsuitable habitat. Although speculative, our findings point out the possibility that autumn
raptor concentrations may be merely temporary glimpses of a nearly continuous and largely
invisible movement. Analysis of raptor migration data should be conducted with this possibil-
ity in mind, and more intensive study of the affects of atmospheric weather conditions on both
raptor and bird migration in general is warranted.

Acknowledgements

This study was supported in part by the U.S. Bureau of Land Management, Las Vegas and
Phoenix District Offices. We thank Arizona State University Center for Environmental
Studies and the U.S. Forest Service, Forest and Range Experiment Station, Tempe, Arizona
for personnel assistance; the Phoenix Zoo for various supplies; and Gary Herron and Bob
Turner of the Nevada Department of Wildlife for their constant assistance in planning and
execution of the study. Robert Hall, Steve Hoffman, Patricia Millsap and Kent Woodruff
contributed considerable amounts of time and personal funds toward the study and without
their help the project would not have been possible. Scott Belfit, Richard Glinski, Bob
Goodman, Michele Hall, Carole Hamilton, Jim Harrison, Denny Haywood, Rick Hibbard,
Marti Jackie, Bruce Jones, William Kepner, Paul Makela, Mark Maley, Rebecca Peck, Lauren
Porzer, and Dave Pulliam also provided services and assistance at various times during the
study. Albert Bammann, Erik Campbell, William Clark, Keith Cline, Maurice LeFranc, and
Douglas Miller reviewed various drafts of this manuscript and provided much helpful criti-
cism. We wish to extend our appreciation to all involved.

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Sokal, R. and F. Rohlf. 1969. Introduction to Biostatistics. W. H. Freeman and Co., San
Francisco, CA.

Thiollay, J. 1980. Spring hawk migration in eastern Mexico. Raptor Res. 14: 13-20.

HAWK MOUNTAIN RESEARCH AWARD. The Board of Directors of the Hawk Moun-
tain Sanctuary Association announces its annual award for raptor research. Students wishing to
apply for the $500 award should submit a description of their research program, a curriculum
vita, and two letters of recommendation by 30 September 1 983 to JamesJ. Brett, Curator, Hawk
Mountain Sanctuary, Route 2, Kempton, Pennsylvania 19529. The Final decision will be made
by the Board of Directors late in 1983.

Only students enrolled in a degree-granting institution are eligible. Both undergraduate and
graduate students are invited to apply. The award will be granted on the basis of a project’s
potential to improve understanding of raptor biology and their ultimate relevance to conserva-
tion of North American hawk populations.

ACTIVITY PATTERNS OF BALD EAGLES WINTERING IN SOUTH
DAKOTA 1

b y

Karen Steenhof 2

Gaylord Memorial Laboratory

School of Forestry, Fisheries and Wildlife

University of Missouri - Columbia

Puxico, MO 63960

Abstract

Observations of Bald Eagles ( Haliaeetus leucocephalus) wintering along the Missouri River
floodplain in South Dakota showed that weather strongly influenced eagle activity patterns.
Feeding activity peaked at -5° to 0° C and dropped significantly when wind speeds exceeded
20 km/h. Reduced feeding activity during unfavorable weather conditions apparently pro-
vided energy savings for eagles. Findings are consistent with theories of optimal time and
energy allocation.

Introduction

The influence of weather on avian foraging activity is an important component of avian
energetics (Schoener 1971). Rough-legged Hawk (Buteo lagopus ) activity, for example, is
strongly linked to weather conditions (Schnell 1967). Under certain weather conditions, both
the success and frequency of Osprey (Pandion haliaetus ) fishing efforts decline (Grubb 1977),
and American Kestrel (Falco sparverius ) activity apparently decreases with high winds and low
temperatures (Enderson 1960). Similar effects would be expected for the Bald Eagle, and the
impact should be especially critical during cold winter months when shorter days decrease the
amount of foraging time available. Energetic considerations are important in the ecology of
wintering Bald Eagles. Stalmaster (1981) has argued that eagles are “time minimizers”
(Schoener 1971), restricting their flight and feeding time to optimize fitness. This paper
examines the proportion of a wintering population engaged in feeding and foraging under
different weather conditions and provides additional evidence that Bald Eagle foraging
strategies minimize energy expenditure during winter.

Methods

Daily activity patterns of Bald Eagles were observed from November to March in 1974-75
and 1975-76 from three observation points on a 30 km 2 section of the Missouri River
floodplain below Fort Randall Dam, South Dakota. In all, 8848 eagle sightings were recorded
and categorized by activity. Weather conditions within 3 h of each observation were obtained
from the Pickstown, South Dakota weather station, 1 km from the Fort Randall Dam. Eagles
were considered “feeding” if they were observed consuming food or actively foraging from a

'Contribution from the Gaylord Memorial Laboratory (University of Missouri-Columbia and Missouri Department of
Conservation cooperating), Missouri Cooperative Wildlife Research Unit (U.S. Fish and Wildlife Service, Wildlife
Management Institute, Missouri Department of Conservation, and University of Missouri-Columbia cooperating),
Lake Andes National Wildlife Refuge, the National Wildlife Federation, the Office of Biological Services, U.S. Fish
and Wildlife Service, and the Omaha District, U.S. Army Corps of Engineers; and from the Missouri Agricultural
Experiment Station, Journal Series 9160.

2 Present Address: Snake River Birds of Prey Research Project, Boise District, Bureau of Land Management, 3948
Development Avenue, Boise, Idaho 83705.

57

Raptor Research l7(2):57-62

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RAPTOR RESEARCH

Vol. 17, No. 2

perch. I defined “food-searching” eagles as those that were not actively feeding or foraging
but were associated with a feeding situation or a potential food source that was being used by
other eagles. This category included eagles that were apparently “waiting” for a feeding
opportunity (Stalmaster 1981). Eagles on the floodplain fed primarily on gizzard shad
{Dorosoma cepedianum), goldeye (Hiodon alosoides), white bass {Roccus chrysops), and carp (Cyp-
rinus carpio) (Steenhof 1976). Based on population counts throughout both winters, I esti-
mated that at least 500 different individuals were observed during the study. I was unable to
observe roosting activity of eagles in the floodplain communal roost, but on 20 days, I watched
eagles departing from a communa roost near Lake Andes, approximately 10 km from the
floodplain.

Results

Most eagles left the communal night roost in the half hour immediately before sunrise,
although some stayed in the vicinity of the roost during the day. Times of earliest observed
departures from the roost ranged from 13 to 38 minutes before sunrise (x = 27 minutes before
sunrise, s.d. = 5.2). In general, eagles moved directly from the roost to feeding areas.

The percent of birds observed feeding and food-searching was significantly higher (X 2 =
239, P < .05) in the first 6 h after sunrise than later in the day (Figure 1). As in Stalmaster’s

70-

Ul
— I

60-

o

<

Ul

50-

Ik

o

t-

40-

z

Ul

u

30-

Ul

0.

20-

FEEDING

FOOD SEARCHING

123456789 1011
HOURS AFTER SUNRISE

Figure 1. Percent of observed Bald Eagles feeding and food-searching in South Dakota in
relation to time of day, 1974-76.

Summer 1983

Steenhof — Bald Eagle Activity Patterns

59

(1981) study, the bimodal pattern in feeding schedules described by Grewe (1966) and
Servheen (1975) was not apparent. Increased morning feeding was probably due to daily
variations in food availability (Steenhof 1976) as well as increased hunger in the morning
(Stalmaster 1981).

The proportion of feeding and food-searching eagles peaked when temperatures were -5°
to 0° C. and decreased with both higher and lower temperatures (Figure 2). Although this
relationship was confounded by typically cold temperatures at preferred morning feeding
times, the pattern persisted when morning and afternoon periods were considered separately
(Figure 2). Warner and Rudd (1975) observed that hunting by Black-shouldered Kites ( Elanus
caeruleus) increased with decreasing ambient temperatures, and Fevold and Craighead (1958)
showed that food consumption by a captive Golden Eagle ( Aquila chrysaetos) increased with
decreasing air temperatures. The ambient temperatures during this study, however, were
colder than during the Golden Eagle and kite studies. Foraging at extremely cold tempera-
tures may yield a net energy loss. Hayes and Gessaman (1980) calculated that American
Kestrels could conserve up to 15% of their winter daily energy requirement by restricting
activity at cold temperatures. Although this savings would be much less in the larger eagle, it
may explain the observed foraging patterns.

TEMPERATURE ( C )

Figure 2. Percent of observed Bald Eagles feeding and food-searching in South Dakota in
relation to temperature and time of day, 1974-76. The top line represents eagles observed less
than 6 h after sunrise; the middle line represents all eagles observed; and the bottom line shows
eagles seen more than 6 h after sunrise.

60

RAPTOR RESEARCH

Vol. l7,No.2

Wind velocity also influenced eagle feeding activity (Figure 3). The proportion of feeding
and food-searching eagles was highest when wind speeds were 15-20 km/h, and the propor-
tion dropped significantly (X 2 = 45.2, P < 0.05) when winds exceeded 20 km/h. Ueoka (1974)
suggested that wind speeds of 8 to 15 km/h are optimal for Osprey maneuverability, and
Grubb (1977) noted decreased fishing efficiency by Ospreys above 15 km/h. Wind speeds
probably affect Bald Eagles similarly, and eagles apparently can save energy by not foraging
when wind conditions reduce fishing efficiency. Kites apparently use this strategy, because
Bammann (1975) noted that they did not hunt when winds exceeded 40 km/h. On the South
Dakota study area, Bald Eagles did not leave the communal roost during a severe 2-day
windstorm when winds gusted to 80 km/h. The roost was protected from the wind and afforded
shelter to the eagles (Steenhof et al. 1 980).

WIND VELOCITIES (KM/H)

Figure 3. Percent of observed Bald Eagles feeding and food-searching in South Dakota in
relation to wind velocity, 1974-76.

Summer 1983

Steenhof — Bald Eagle Activity Patterns

61

Soaring activity was also clearly influenced by weather. Soaring by eagles was recorded 22
times in 1975 and 1976. As in Preston’s (1981) study of Red-tailed Hawks (Buteo jamaicensis) ,
the incidence of soaring appeared to be related more to wind velocities than time of day,
season or wind direction. Eagles soared during all months of the study, at all times of day, and
during most prevailing wind directions. Wind velocities during soaring observations ranged
from 7.4 to 25.9 km/h, with 82% of all soaring activity occurring in velocities between 12.9 and
22.2 km/h. Although these velocities are the same as those apparently preferred for foraging,
the optimal conditions for these two activities apparently are not identical. Stalmaster (1981)
noted that soaring by eagles in Washington was most common during warm periods. In this
study, more than 70% of all soaring occurred when temperatures exceeded 0° C., the temper-
ature above which foraging activity declined.

The data indicate that weather conditions strongly influence Bald Eagle feeding activity,
and the findings are consistent with theories of optimal time and energy allocation (Schoener
1971). As colder temperatures raise energy demands, eagle foraging increases. At approxi-
mately -5° C., however, the benefit/cost ratio apparently does not favor foraging, and eagles
begin to restrict feeding activity. Eagles also apparently reduce energy expenditures by not
foraging when wind reduces foraging efficiency. Stalmaster (1981) estimated that eagles could
survive for 2-3 days during winter without feeding. Thus, only unusually persistent severe
storms would make this strategy of restricted feeding ineffective.

A ckno wledgments

I thank L.H. Fredrickson and S.S. Berlinger for advice and guidance. S. Hoffman and two
anonymous reviewers offered valuable criticisms and suggestions. G.F. Krause and S. Ward
provided assistance in computer summarization of data. T. Box allowed me to use Utah State
University computer facilities for further summarization, and T.L. Thomason typed the
manuscript.

Literature Cited

Bammann, A.R. 1975. Ecology of predation and social interactions of wintering White-tailed
Kites. M.S. Thesis. Humboldt State Univ., Areata, California. 81 pp.

Enderson, J.H. 1960. A population study of the Sparrow Hawk in east-central Illinois. Wilson
Bull. 72:222-231

Fevold, H.R. andJ.J. Craighead. 1958. Food requirements of the Golden Eagle. Auk 75:312-
317.

Grewe, A. A. Jr. 1966. Some aspects in the natural history of the Bald Eagle ( Haliaeetus
leucocephalus ) in Minnesota and South Dakota. Ph.D. Thesis. Univ. South Dakota, Ver-
million. 68 pp.

Grubb, T.C. Jr. 1977. Weather-dependent foraging in Ospreys. Auk 94:146-149.

Hayes, S.R. and J.A. Gessaman. 1980. The combined effects of air temperature, wind and
radiation on the resting metabolism of avian raptors./. Therm. Biol. 5:1 19-125.

Preston, C.R. 1981. Environmental influence on soaring in wintering Red-tailed Hawks.
Wilson Bull. 93:350-356.

Schnell, G.D. 1967. Environmental influence on the incidence of flight in the Rough-legged
Hawk. Auk 84:173-182.

Schoener, T.W. 1971. Theory of feeding strategies. Ann. Rev. Ecol. Syst. 2:369-404.
Servheen, C.W. 1975. Ecology of the wintering Bald Eagles on the Skagit River, Washington.

M.S. Thesis. Univ. of Washington, Seattle. 96 pp.

Stalmaster, M.V. 1981. Ecological energetics and foraging behavior of wintering Bald Eagles.

Ph.D. Thesis. Utah State Univ., Logan. 157 pp.

Steenhof, K. 1976. The ecology of wintering Bald Eagles in southeastern South Dakota. M.S.
Thesis. Univ. of Missouri, Columbia. 146 pp.

62

RAPTOR RESEARCH

Vol. 17, No. 2

Steenhof, K., S.S. Berlinger and L.H. Fredrickson. 1980. Habitat use by wintering Bald Eagles
in South Dakota./. Wildl. Manage. 44:798-805.

Ueoka, M.L. 1974. Feeding behavior of Ospreys at Humboldt Bay, California. M.S. Thesis.

Humboldt State Univ., Areata, California. 75 pp.

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

MOUSE TRAP RECOVERED IN HARRIER NEST
by

Dale Gawlik
3218 Post Road
Stevens Point
Wisconsin 54481

An annual vole ( Microtus sp.) index is an important part of Hamerstrom’s study of the Northern Harrier
(Circus cyaneus) in central Wisconsin (Hamerstrom, F., Auk 96:370-374, 1979). Vole trapping on her study
area began in 1964 and 28,91 1 trap nights have been accumulated by Hamerstrom and her coworkers
through 1981. On 4 July 1981 I found evidence that a harrier had stolen a trap.

On 1 July, 120 traps were put out at about 2000 hours. When they were picked up at about 1200 hours 2
July, 1 trap was missing. Tufts of vole hair were found within 10 cm of the missing trap. On 4 July at 0945
hours I visited a harrier nest about 2.2 km from the trap-line. The nest has been deserted within the past 2
days, and an empty sprung trap lay upside down near the center of the nest. I believe it unlikely that the
harrier carried an empty trap. It seems reasonable to conclude that the harrier was attracted to the trap by
the presence of a vole in it. The vole may have been dead at the time it was taken since in a few instances
harriers have been known to feed on carrion (Bent, U.S. Natl. Mus. Bull. No. 167, 1937:86; Randall,
Wilson Bull. 52: 165-172, 1940; and Errington and Breckenridge, Am. Midland Nat. 17: 831-848, 1936).
It is also possible that the vole may have been alive when the trap was taken because a few live voles have
been found in sprung traps in previous years (Hamerstrom pers. comm.).

PRECOCIOUS NEST DEFENSE BEHAVIOR BY A SHARP-SHINNED HAWK
by

Robert N. Rosenfield
College of Natural Resources
University of Wisconsin-Stevens Point
Stevens Point, WI 54481
and

Andrew Kanvik
House 10161 Highway 10
Amherst, WI 54406

On 22 July 1981 we observed 3 fledged Sharp-shinned Hawks (Accipiter striatus) in trees within 20 m of
their nest in Door County, Wisconsin. They were food-calling (for a description of calls, see Beebe, F.L.,
Occas. Pap. B.C. Prov. Mus. 17. 163 pp., 1974) and we anticipated the return of an adult with prey for
them. To capture adults, we placed a mist net within 3 m of the nest tree and 1 m of a tethered live Great
Horned Owl (Bubo virginianus) (Hamerstrom F., Proc. Int. Ornithol. Congr. 13: 866-869, 1963). We

Raptor Research l7(2):62-63

Summer 1983

Book Reviews

63

moved about 20 m away and waited. Approximately 30 min later one of the young’s food-call changed to a
nest defense call and then it stooped at the owl, hit the net, but escaped. This behavior by the same
fledgling occurred 4 times within the next 15 min before it was captured. Its weight (159 g) indicated a
female and all her flight feathers had blood in quill; we estimated her age at 30-32 days. After banding and
releasing, she immediately perched and uttered a nest alarm call (we believe at us for she could not see the
owl from her position) before flying from view. The other 2 young had continued food-calling but they
never uttered a nest alarm call.

F. Hamerstrom (pers. comm.) observed 2 similar occurrences where 2 recently fledged Northern
Harriers (Circus cyaneus) were caught after stooping at decoy live Great Horned Owls. Acker (Auk
94; 374-375, 1977) reported an immature (65-70 days old) female Red-shouldered Hawk (Buteo lineatus),
at hack, attempting to build a nest and feed 2 captive Northern Harrir chicks. These observations suggest
that some behavior patterns commonly associated with breeding adults, are present soon after fledging in
some raptors.

We would like to thank D. Amadon, D. Evans, M. Fuller, M. Gratson, and F. and F. Hamerstrom for
their review of this note.

Book Reviews

Recent Advances in the Study of Raptor Diseases. Proceedings of the International Sym-
posium on Diseases of Birds of Prey, J.E. Cooper and A.G. Greenwood, eds., 1981. Chiron
Publications, Ltd., West Yorkshire, England. 165 pp. $25.00. (obtainable through CHIRON
PUBLICATIONS, P.O. Box 25, Keighley, West Yorkshire BD22 VBA, United Kingdom.

This publication contains the edited proceedings of the First International Symposium on
Diseases of Birds of Prey held in London, July 1 -3, 1980. The text provides excellent clinical
and surgical information for veterinarians treating raptors. The volume is divided into three
parts: Part I - Pathology and Microbiology; Part II - Surgery and Anesthesia; and Part III -
Medicine and Therapeutics. Two additional workshops are incorporated which contain topics
on mortality factors in wild populations and captive breeding that will appeal to the raptor
biologist, aviculturalist, and individuals involved with rehabilitation of raptors.

Highlights of Part I include discussion on bacterial flora and haematozoa of raptors, effects
of chronic lead ingestion, causes of death in trained raptors and infectious diseases of birds of
prey. Part II deals with anesthesia, surgical treatment of bumblefoot and diagnostic laparos-
copy. Significant information is presented on the ossification of long bones in raptors,
thermaplastic coating material in fracture repair and the use of external fixation is de-
monstrated with several illustrated case reports. The section on medicine and therapeutics
contains discussion on avian malaria, serum chemistry profiles, aspergillosis, tuberculosis,
management of bumblefoot and visual defects in raptors.

Topics on captive breeding include the influence of cross-fostering on mate selection in
captive kestrels, microbiological aspects of egg hatchability in captive American Kestrels,
breeding of condors at the New York Zoological Park, hand rearing of vultures and abnormal
and maladaptive behavior in captive raptors.

The section on mortality factors in the wild included studies on the causes of mortality in
British kestrels, problems of rehabilitation, maintenance energy requirements and rate of
weight loss during starvation in birds of prey and the relationship of body weight, fat deposit,
and moult to the reproductive cycles in wild Tawny and Barn Owls.

64

RAPTOR RESEARCH

Vol. 17, No. 2

In summary, a program of well-respected speakers from several countries presented well
illustrated material covering a wide range of selected topics based upon their experience and
investigative studies in addition to reviewing applicable literature. It contains useful informa-
tion for the veterinarian and avicultural personnel involved with breeding and rehabilitation
of raptors.

Philip K. Ensley, D.V.M.

The Barn Owl. D.S. Bunn, A.B. Warburton, R.D.S. Wilson. 1982. buteo Books, Vermillion,
South Dakota ($32.50). 264 pages, 1 color frontispiece, and 32 black and white plates.

In the preface, the authors state their main reason for producing this monograph on the
Barn Owl ( Tyto alba) was “the very fact that so little was known about the species. . .”, and they
set out to improve our understanding of this strigiform by drawing upon their combined 38
years experience with it in Britain and from both published and unpublished data from
Britain, Europe, and elsewhere. Perhaps the most impressive feature of the monograph is its
scope — chapters include topics such as Description and Adaptations, Voice, General Be-
havior, Food, Breeding, Movements, Factors Controlling Population. . ., and Distribution in
the British Isles. Also included is a chapter on Folklore, as well as Appendices on development
of young and techniques for observing Barn Owls. The sheer volume of information pre-
sented certainly leaves one with a better understanding of this interesting raptor, and in this
sense the author’s objective is attained.

Despite its good points, the professional is apt to be a bit disappointed. There is little hard
data presented from the authors’ own studies, and their most valuable contributions in the
sections on territory and hunting methods are based primarily upon observations of diurnally
active and unmarked individuals. One cannot help but wonder if the conclusions would differ
had the subjects been marked and diurnal observations supplemented with radio-tracking at
night. Chapters upon which the authors place considerable emphasis, particularly Voice,
General Behavior, and Breeding, tend to be overly anthropomorphic and many of the
conclusions the authors arrive at are not supported by compelling or even highly persuasive
data. A shortcoming which I found particularly evident was a dearth of information from
North America; many pertinent findings of comparative value concerning T.a. pratincola were
not mentioned. This is particularly true in the section on possible conservation measures
where nest boxes are discussed. Reference to the highly successful work in this area by Carl
Marti and Phil Wagner in Utah (Marti el al. 1979. Nest boxes for the management of Barn
Owls. Wildl. Soc. Bull., 7:145-148) would have greatly strengthened this section.

These faults are not likely to keep the nonprofessional from enjoying the monograph, and
persons with an avid interest in owls will certainly want to obtain a copy if they can afford the
rather steep price. The book should be especially interesting to those who have an occasional
opportunity to observe Barn Owls and want to learn more about this intriguing species.

Brian A. Millsap

THE RAPTOR RESEARCH FOUNDATION, INC.

OFFICERS

President Dr. Jeffrey L. Lincer, Office of Environmental Management, 2086 Main
Street, Sarasota, Florida 33477

Vice-President Dr. Richard Clark, York College of Pennsylvania, Country Club
Road, York, PA 17405

Secretary Ed Henckel, RD 1 , Box 1 380, Mt. Bethel, PA 1 8343

Treasurer Dr. Gary E. Duke, Department of Veterinary Biology, College of Veteri-
nary Medicine, University of Minnesota, St. Paul, Minnesota 55108

Address all matters dealing with membership status, dues, publication sales, or other
financial transactions to the Treasurer. See inside front cover.

Send changes of address to the Treasurer.

Address all general inquiries to the Secretary.

See inside front cover for suggestions to contributors of manuscripts for Raptor
Research, Raptor Research Reports, and special Raptor Research Foundation publica-
tions.

BOARD OF DIRECTORS

Eastern Dr. James Mosher, RT 2, Box 572-D, Frostburg, Maryland 21532

Central Dr. Patrick Redig, Department of Veterinary Medicine, 295K AnSci/
Veterinary Medicine Bldg., University of Minnesota, St. Paul, MN 55108

Pacific and Mountain Dr. Joseph R. Murphy, Department of Zoology, 167 WIDB,
Brigham Young University, Provo, Utah 84602

Canadian Eastern Dr. David Bird, Macdonald Raptor Research Center, Macdonald
College. Quebec, H9X ICO, Canada

Canadian Western Dr. R. Wayne Nelson, 42 1 8-63rd St., Camrose, Alberta T4V 2W6,
Canada

At Large #1 - Dr. Lynn Oliphant, Universty of Saskatchewan, Veterinary Anatomy,
Saskatoon, SA Canada S7N OWO

At Large #2 - Dr. Tom Dunstan, Biology Science, Western Illinois University,
Macomb, Illinois 61455

At Large #3 - Dr. Mark R. Fuller, Migratory Bird Lb, U.S.F.W.S., Patuxent Research
Center, Laurel, Maryland 2081 1