Raptor Research

A Quarterly Publication of The Raptor Research Foundation, Inc,

Volume 20, Number 2, Summer 1986

(ISSN 0099-9059)

Contents

Notes on the Breeding Cycle of Cafe Vultures (Gyps r opr other esj
Ahst.ur S. , . . . . . .... Si

Attacks by Black Eagles on Vultures

P, J. Hundy, A.5 Rjl^Lk. 1 i is* i ,1 . KDmen.and TC, O t Counor 61

The Behavioral Response or a Rld- tailed Hawk to Military Training Activity
David E. Andersen, Orrin |. ELmgsuud and W3B»n R Myimn 65

Short Communication

] J.ihiijil t’se by WinlfTin^ CnJdc-n s and Elnugh-l.rj(gf , d

E. H. Craig. T. H. Craig and Lain R. Piiwer^i , 69

Diitribuuon and Dauby oi Prairie F&kttrti Nesting; in Calfomia During [hi- IWK

Douglas A. Boyce, Jr,* Ruiuld L. Laired anti Brian ) Walton . . . 71

Bald Eagle Pursue-. and 1 njurtA SotHv Guos*- ml Ri^ht

Richard S, Bennett, Jr., and Erwin E. Klu^s 75

Osprey Captures Griiy Squired. Ptirr Taylor ....... 76

Adoprian of a Juvenile by Brn-ding Sp.-inish Impcn.il K.igk<i During

The P<wtflecl;gi]Lg PeritHi. Jose L. Gon/alez. Borja Heredia r Luis M. (iim/aiL? and Juan C. Alonso 77

Harder Kills MuMttmy Wlllct. Marilyn E. En inland 78

News and Reviews (Resolutions) + , + , + , w + 4 „ + » 79

Thesis Abstracts * ,,,..,. * , . . . , * + + + + + + + + + + + + + 89

The Raptor Research Foundation. Inc,

Provo, Utah

THE RAPTOR RESEARCH FOUNDATION, INC.

(Founded 1966)

OFFICERS

PRESIDENT: Jeffrey L. Lincer, Office of the Scientific Advisor, 4718 Dunn Drive, Sarasota, Florida 33583

VICE-PRESIDENT: Richard Clark, York College of Pennsylvania, Country Club Road, York, Pennsylvania 17403-
3426

SECRETARY: James D Fraser, Virginia Polytechnic Institute and State University, Cheatham Hall, Blacksburg,
Virginia 24061

TREASURER: Jim Fitzpatrick, Carpenter Nature Center, 12805 St. Croix Trail, Hastings, Minnesota 55033

BOARD OF DIRECTORS

EASTERN DIRECTOR: James A. Mosher, Savage River Consulting, P.O. Box 71, Frostburg, Maryland 21532

CENTRAL DIRECTOR: Patrick T. Redig, Department of Veterinary Biology, 295 Animal Science/ Veterinary
Medicine Building, University of Minnesota, St. Paul, Minnesota 55108

MOUNTAIN & PACIFIC DIRECTOR: Al Harmata, Department of Biology, Montana State University, Bozeman,
Montana 59717

EAST CANADA DIRECTOR: David M. Bird, Macdonald Raptor Research Centre, Macdonald Campus of McGill
University 21,111 Lakeshore Road, Ste. Anne de Bellevue, Quebec H9X ICO

WEST CANADA DIRECTOR: Lynn Oliphant, Department of Veterinary Anatomy, University of Saskatchewan,
Saskatoon, Saskatchewan S7M 0W0

INTERNATIONAL DIRECTOR: Martin Bottcher, Postfach 2164, Blankenheimner Strass 3, 5372 SCHLEIDEN,
Federal Republic of Germany, GERMANY

DIRECTOR AT LARGE # 1 : Michael Collopy, University of Florida, School of Forest Resources and Conservation,
118 Newins-Ziegler Hall, Gainesville, Florida 32601

DIRECTOR AT LARGE #2: Gary Duke, Department of Veterinary Biology, 295/K Veterinary Medicine Building,
University of Minnesota, St. Paul, Minnesota 55108

DIRECTOR AT LARGE #3: Richard P. Howard, U.S. Fish and Wildlife Service, 4696 Overland Road, Room 566,
Boise, Idaho 83705

s|c :jc sf: sjc ;jc sf: ^ % He

Persons interested in predatory birds are invited to join The Raptor Research Foundation, m Inc. Dues are $15 per
year in the U.S., $17 per year outside the U.S., $13 per year for U.S. students, and $15 per year for students outside the
U.S. Add $2 to dues if membership is received after 15 February. The Foundation’s journal Raptor Research is
distributed quarterly to all current members. Subscription price to institutions and nonmembers is the same as regular
membership. Single copies and back issues are available from the Treasurer. A Contributing Membership if $25, a
Sustaining Membership is $100, and a Life Membership is $500. All contributions to The Raptor Research Foundation,
Inc., are tax-deductible.

Send requests for information concerning membership, subscriptions, special publications, or change of address to
the Treasurer. Other communications may be routed through the appropriate Officer or Board member. All inquiries
concerning the journal should be addressed to Clayton M. White, Editor , Raptor Research, Department of Zoology, 161
WIDB, Brigham Young University, Provo, Utah 84602, U.S.A

Published quarterly by The Raptor Research Foundation, Inc. Business Office: Jim Fitzpatrick, Carpenter Nature
Center, 12805 St. Croix Trail, Hastings, Minnesota 55033, U.S.A.

RAPTOR RESEARCH

A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC.

VOL. 20

Summer 1986

No. 2

NOTES ON THE BREEDING CYCLE OF CAPE VULTURES ( Gyps coprotheres)

Alistair S. Robertson

Abstract - Observations were made of the pre-laying, incubation and nestling periods of the Cape Vulture ( Gyps
coprotheres) at a colony in the southwestern Cape Province, South Africa. Some members of the colony were colour-ringed
as nestlings; this allowed the sex of breeding partners to be determined by subsequent observation of copulation.
Information on occupancy of nest sites, nest-building, a sex- related behavioural difference, dates of egg-laying, incuba-
tion and nestling periods, parasite infestation (Prosimuliium spp,), and associated blood parasites ( Leucocytozoon ) of
nestlings is presented.

The Cape Vulture ( Gyps coprotheres) is the heaviest
endemic accipitrid in southern Africa and is the
only vulture in the region to' breed in colonies on
cliff faces (Mundy 1982). Approximately 3500
Cape Vulture nestlings have been colour-ringed
(banded) in southern Africa since 1974; studies of
marked birds, however, are mostly confined to as-
pects of dispersal and survival (Houston 1974;
Piper et al. 1981; Mundy 1982).

Here I present results of observations made
during a case study of a particular colony, to sup-
plement Mundy (1982) and provide information on
the behaviour of colour- and metal-ringed breed-
ing adults. Sexes of Gyps are morphologically indis-
tinguishable in the field (Houston 1976; Mundy
1982), therefore marked birds provided an op-
portunity to investigate behavioural differences
between sexes, as well as nest-site fidelity
(Robertson 1984). The pre-laying, incubation and
nestling periods (Newton 1979) are considered
separately; observations on the post-fledging de-
pendence period and copulatory behaviour are de-
scribed elsewhere (Robertson 1983, 1985).

Study Area and Methods

Observations were made at a colony in the Potberg (34° 22' S;
2 (f 33' E), approximately 10 km from the Indian Ocean coast and
approximately 50 km northeast of the southern tip of Africa.
About 50 vultures inhabit a ravine of the mountain, and use cliffs
facing southeast and southwest for nest and roost sites. In addi-
tion, a smaller colony of about 20 at Aasvogelvlei, 120 km from
Potberg, was monitored for numbers and breeding success.

The region has a temperate Mediterranean climate and receives
most of its rainfall (« 530 mm/y) from about May to September
(l e., winter).

All active nest sites and roosting ledges were visible from the
southeast side of the ravine, 300-400 m away. Observations were
made with binoculars and a 15-60x telescope. Observations began
in May 1981, continued for 12 d each month regardless of weather
conditions, and ended in May 1 982. One “day” represents the time
period 0730 H - 1630 H. The concluding stages of the 1980
breeding season, all stages of the 198 1 season and the initiation of
the 1982 season were covered. Presence of birds at sites and
roosting ledges was noted every 30 min during each observation
day; the proportion of each class (i.e., one, both or neither occup-
ant present at each count during an observation day) was deter-
mined and converted to an arcsine transformation (Schefler
1969). Although most of the breeding pairs were not individually
recognizable, some ringed breeders were present. At a nest where
both birds were colour-ringed, strangers alone at the nest for
longer than 5 min were seen only 3 times. Unless shown to the
contrary, vultures observed at a site were regarded as the occup-
ants. Individual vultures were age-estimated using the characters
given by Mundy (1982), At nests where at least one partner was
ringed, sex was determined by observation of copulation attempts.

Laying dates for 1981 and 1982 were determined by checking
the colony regularly, although precise dates for 3 nests were not
obtained in 1982. Dates for 1977, 1979 and 1980 as well as the
dates of laying in both 1981 and 1982 at Aasvogelvlei were esti-
mated using an average incubation period of 57 d and a known-
age/wing length curve (Mundy 1982). Wing lengths of nestlings
measured before 1981 were obtained from unpublished records;
thus, only laying dates of sites that produced large nestlings were
obtained for years other than 1981 and 1982.

Nestlings at both colonies were colour-ringed, weighed and
measured as part of the colour ringing scheme on Cape Vultures
(Ledger 1974). Nests were visited on 5 other occasions in order to
collect addled eggs or shell fragments (measurements in
Robertson 1984) and to replace an egg that an adult had knocked
from the nest cup. In 1981, nestling blood samples were obtained
from both colonies. Smears were prepared following Greiner &
Mundy (1979) and scanned for blood parasites by M.B. Markus.

51

Raptor Res arch 20 ( 2 ): 5 1-60

52

Alistair S. Robertson

Vol. 20, No. 2

PRE-LAYING

INCUBATION

NESTLING

POST-FLEDGING

VULTURES AT SITE

Figure 1 : The average level of occupancy of nests during different periods of the breeding cycle. The number of nests
used in the calculations for each period is indicated.

Results

Pre-laying period. — Because of the length of the
breeding cycle, certain pairs displayed an overlap of
post-fledging and pre-laying periods (Robertson
1985). Other pairs that failed in breeding retained
their nest site (by aggressively chasing off visitors)
from the date of failure.

Figure 1 depicts the average occupancy of 10
nests for the 1981 pre-laying period. On average,
nest sites were left vacant for 5% of the observation
time.

(a) Activation of nest sites. — Of 17 breeding
sites active in 1981, 14 were active again in 1982. Of
4 additional nests active in 1982, one was active in
1980. Thus only 3 sites were initiated during the
observation period.

At one site on 29 October 1 98 1 , 2 birds were seen
to stand very close together, facing inwards and
picking at sticks on the surface. Seven min later, the
ringed female occupant landed at the site and re-
pulsed both, which then repeated this behaviour at
another (vacant) site. The impression was of a pair
“prospecting” for a possible future breeding site,
about 7 months before the next season’s average
laying date.

At another site, an immature male (est. 3rd y) and
female (est. 4th y) were observed to copulate 36
times between 30 June and 30 December 1981. On
most occasions after the copulation attempt, both

birds bent over and picked at the nest contents. The
nest was examined on 8 October and consisted of a
few sticks and green sprays — an “intermediate”
nest structure. The first intruder repulsion at this
site was observed on 20 December. This site pro-
duced a fledgling in 1982/3, thus it was effectively
activated in mid- 1 98 1 , if the parents are assumed to
be the same individuals.

A probable mate replacement (certainly the initi-
ation of breeding) occurred at one site in 1983,
where a colour-ringed, 6 y old female was observed
from late December 1982. This site was active in
1980 and 1981 (both partners adult and unringed).
The 6 y old’s sex was determined by observation of a
copulation attempt with an unknown adult at a
perch on 18 May 1982.

(b) Nest-building. — No sign of nest-building at
any site was observed before the end of March
1982, and at only 2 sites, successful in the previous
cycle, were there any remains of the nest structure.
By the end of December 1981, nest material on the
sites of all failed breeding attempts had disap-
peared. Collection of nest material in earnest was
first observed on 25 April 1982, about 10 wk before
the average laying date. Branches comprising the
nest structure were collected in most cases from 2
separate areas on the ravine slopes and carried back
to the nest in the bill.

At 6 nests where male and female were distin-
guished, frequency of collecting forays by either sex

Summer 1986

Breeding Cycle of Cape Vultures

53

was determined. Collecting forays were mainly de-
termined for 3 nests only — those situated on the
cliff directly opposite the observation point where
the collector’s sex was determined with greater
certainty. Of 98 collections, males collected 75 times
(76%), far more than did females, although at one
nest the difference was not so marked. The female
generally packed the material into the structure,
although both sexes were often seen bending over
and rearranging material.

Incubation period. — In 1981, 356 nest-d of
observation of 16 nests with eggs were made, and I
made a further 99 nest-d of observation during an
8-d consecutive period in 1982.

No double clutches (Mundy 8c Ledger 1975) or
replacement clutches were recorded, although the
egg at nest #55 was laid 40 d later than the esti-
mated 5 June mean for that year and may thus have
been a replacement clutch.

For 16 incubation periods in 1981, which in-
cludes one egg lost after 41 d, one adult was at the
site at 97.7% of the counts (range 91.2% - 100%).

1980 1981 1982

Figure 3: Rainfall in the six months preceding egg-lay-
ing, expressed as a percentage deviation above
and below the monthly mean.

These sites were unoccupied only 0.09% of this
observation period, where an average of 359
thirty-min counts were made per nest. (Fig. 1). On
average, 2 birds were at the site 2.0% of the time
(range 0% - 8.8%). Birds generally incubated the

potberg

13

1982 1979 X ,1980 X ,1981

16

YY

1981 ' 1 = All POTBERG YEARS

Q AW1982 X

o

1980 ^ ~ = 1982 X ,1981 XX

9

1979 | 1982 X , 1981 XX

12

1977 ■ | = 1981 XX

aasvogelvtei

5

1982 ' | AW1981 X ,1981 X

6 ^

1981 — " 1 = AW1982 X ,1977 X

1979 X

1 1 1 1 1 1 1 1 1 1 1

may 18 24 31 june 14 21 28 iuly 12 19

Figure 2: Dates of egg-laying at Potberg and Aasvogelvlei. The mean (± s.d.) and range are shown, as well as significant
differences in dates of laying between the various years (0.01 level:year xx ; 0.05 level:year x ).

54

Alistair S. Robertson

Vol. 20, No. 2

egg very tightly and on only one occasion did an
adult leave an egg unoccupied (for 182 min) for no
apparent reason.

(a) Date of laying. — Only in 1977 did the corre-
lation of higher rainfall and later mean date not
occur, although the differences are very slight
(compare Figs. 2 and 3). The 1981 mean laying date
was significantly later than any other year at Pot-
berg, and similar only to the mean for the same year
at Aasvogelvlei (t-Test, 20d.f., P = 0.09). The over-
all average laying date for the 5 y is 17 June (s.d. =
17.5 d, n eggs = 58), and this is clearly later than the
early-May average for the Skeerpoort, Magalies-
berg colony (Mundy 1982) and the similar average
for the Collywobbles, Transkei colony (Vernon et
al. 1982).

All 1981 eggs were laid considerably later than in
1980, except the likely replacement clutch men-
tioned. Considering the 3 long, post-fledging de-
pendence periods observed in 1981/2 (Robertson
1985), an egg was laid at the same site before the
mean 1981 laying date on one occasion and after it
on another; the third site was not active in the
subsequent season. After the post-fledging depen-
dence periods observed in 1981/2, eggs were laid at
the same sites before the mean 1982 date on 4 of 5
occasions and significantly later by the same adults
(colour-ringed) in one instance. At the site with the
longest 1981 post-fledging dependence period, the
following year’s egg was laid before the colony’s
mean egg-laying date for that year.

The most accurate times of laying were obtained
for 3 eggs in 1981, where eggs were laid between
1630 H and 0730 H the next day. At one site (nest
#63), the same colour-ringed female laid an egg in
1982 between 1730 H and 1500 H, on exactly the
same date as the previous year.

(b) Changeovers and attentive periods. —
Changeovers (nest relief) occurred within about 3
min of the mate’s arrival, although certain partners
gave the impression of being “reluctant” to leave. In
such cases, changeovers occurred after the in-
cubator had been gently nudged off the nest; this
was more frequent during the nestling period.
During what was most likely the first changeover at
one nest, the incoming adult allowed the egg to roll
out of the cup by standing on the rim. After a few
feeble attempts at replacing the egg with its bill, it
continued to “incubate” the empty nest. During the
next 90 min, the bird tried 3 times to replace the egg
which lay some 8 cm from the rim. I replaced the

egg at 1540 H, after it had been exposed for 430
min (clear weather, 9-13 knot breeze). This egg
subsequently produced a juvenile that fledged. A
total of 186 changeovers of birds incubating eggs
were noted in 1981, averaging 11.6/site, or 0.5/
nest-d of observation. A number of changeovers at
outlying sites were probably missed. If the ledge
directly opposite the observation point is con-
sidered alone, 61 changeovers were documented
for 4 incubation periods during 88 nest-d (0.7
changeovers/nest/day). Rarely up to 3 changeovers
might occur on the same day. Changeovers occur-
red in the mornings, before a day’s foraging, or in
the afternoon after foraging. The earliest time
where the incoming partner had fed that morning
(bulging crop observed) was 1 130 H; if changeovers
are then divided into pre-1 130 H (61, or 33%) and
post-1130 H groups (125, or 67%), then incoming
adults had obtained food on 64% of the latter (8.6%
cases unknown). The overall average changeover
time was 12 hr 54 min, 50 min earlier than that
documented for the Skeerpoort, Magaliesberg col-
ony (Mundy 1982:161).

Incubating birds occasionally stood up to stretch,
flap their wings, repulse intruders or move the egg
(along its long axis, Mundy 1982), and the longest
period in such cases that the egg was exposed was 4
min (n = 18, X — 1.2 min). Attentive periods have
been divided into those where 2 changeovers oc-
curred within one observation day (“within-day”),
and longer stints. In 1981, the mean “within-day”
attentive period was 3 hr 15 min (n = 44), which was
not significantly different from the 3 hr 31 min
period in 1982 (t-Test, 60d.f., P = 0.30). The mean
of 198 1 “long” attentive periods was 26 hr 2 1 min (n
= 72), significantly longer than the 1982meanof22
hr 35 min (t-Test, 98d.f., P - 0.01). If 11 abnor-
mally long attentive periods (mean 44 hr 3 min,
range 40 hr 45 min - 48 hr 8 min) are excluded, the
1981 mean is not significantly longer than that ob-
tained for 1982 (t-Test, 86d.f., P = 0.37). In 5 of
these extra-long cases, the individual concerned
was ringed, confirming that no changeover had
been missed.

Sexes could be distinguished at 5 nests by the
presence of either a metal-ringed (2 nests) or col-
our-ringed partner. Only when a ring was clearly
resighted at both changeovers, were the corres-
ponding attentive periods used in analysis of any
sex-related differences. The mean for males (1981
and 1982 values combined) was not significantly

Summer 1986

Breeding Cycle of Cape Vultures

55

longer than that for females (t-Test, 30d.f., P =
0.32). The frequency of attentive periods by either
sex was not significantly different (XI 2 “=” ex-
pected values ets, P = 0.72).

Five eggs failed to hatch in 1981 and 1982. In 2
cases no change in incubation behaviour after the
expected hatching date was noted. For example,
adults at nest #29 continued to incubate normally
for 18 d after the expected hatching date, until I
collected the egg. At one nest, however, the in-
cubating adult tended to stand over the egg, rather
than sit on it from about 14 d after the expected
hatching date. Twenty-four days after the expected
hatching date, the incumbent adult deserted this
egg for 152 min, returned to stand near it for 355
min, to leave it unattended that night (I collected
the egg the next day).

At 6 sites in 1981 where both laying and hatching
dates were accurately obtained, the average incu-
bation period was 57 d (95% C.L.’s 55.6 - 58.4).

Nestling period. — At 10 sites with nestlings, 377
d of observation were made in 1981, and 15 nest-d
at 4 nests in 1982. In 1981, 7 nestlings lived longer
than lid and 6 fledged, and of 10 nestlings col-
our-ringed in 1982, 7 fledged (Robertson 1984).

Only 11% of copulation attempts observed at
successful sites through the 1981 cycle occurred
during the nestling period (Robertson 1983). The
mean hatching date for the years 1977 and 1979-
1982 was 14 August (using mean laying date of 17
June and a 57-d incubation period).

(a) Parental care of nestling. — In 1981, nest-
lings at 7 nests were left unattended for 1.2% of the
counts (range 0.1% - 4.5%). One nestling was unat-
tended for significantly more time than 3 others
(Sheffe’s tests, P < 0.05). Six nestlings were unat-
tended at least once during the period (i.e., one
never), and the average age at the first such instance
was 71.7 d (range 41-93 d). Nestlings were accom-
panied by 1 adult on 76.4% of the counts (Fig. 1).
Both adults were at the nest on 22.4% of the counts;
in all cases attendance by both parents was greater
than during incubation.

In 1981 sexes were distinguishable at only one
active nest and both partners attended the nestling
for similar lengths of time (t-Test, 2 ld.f., P = 0.43).
Both male and female attendance periods were sig-
nificantly shorter than those obtained during the
incubation period (t-Tests, P < 0.01).

The mean “long” attendance period of 2 1 hr 56
min (n = 71) was not significantly shorter than the
mean for incubation, if the 1 1 “extra-long” stints
are excluded (t-Test, 129d.f., P = 0.09). If these
values are included, the difference becomes sig-
nificant. The mean “within day” attentive period of
3 hr 39 min (n = 53) was not significantly longer
than the value for the incubation period (t-Test,
95d.f.,P= 0.17). The average changeover time was
1229 H, significantly earlier than that documented
for the incubation period (t-Test, 37 5d.f., P = 0.49).
Of 193 changeovers, 65.8% occurred after 1 130 H
and on 86% of these, the incoming adult had ob-
tained food.

For its first few weeks, the nestling was brooded
closely by the adult, making the precise time of
hatching difficult to discern. There was sub-
sequently a gradation in attendance behaviour of
parents, particularly after the nestling was dorsally
feathered. A 4-month-old nestling wandered 2 m to
an adjacent site containing a 5-month-old nestling
on 3 times; both pecked gently at each other’s
necks before being aggressively jabbed at by an
attendant adult. Similar behaviour was never ob-
served for adults of adjacent sites. Parents con-
tinued to bring sprays of fresh green leaves to the
nest during this period.

Most feeding was bill-to-bill, with the nestling
actively calling and tapping the parent’s bill. On at
least 7 occasions, parents that had not fed were
observed to go through the motions of regurgi-
tating seemingly without any food appearing.
Nestling development has been described by
Mundy (1982), and correlations of wing with body
mass and age are presented in Robertson
andBoshoff (in press).

(b) Parasite infestation. — All nestlings visited
on 8 October 1981 were infested with black flies
Prosimuliium spp. (identified by J.S. Paterson). The
average age of these nestlings was 43 d (range 22-55
d). The heaviest infestation was on a 47-d-old
nestling, where one-half of the head was fully
covered (Fig. 4). The main area affected was the
crown, but one nestling showed 2 areas of infesta-
tion under the wing. The infestation on one nest-
ling, that was unattended while adjacent nestlings
were ringed, increased on its head and back, i.e., the
presence of the adult (that had flown because of my
presence) affected the degree of infestation. This
was confirmed later by observing attendant adults
removing flies from nestlings with their bills. When

56

Alistair S. Robertson

Vol. 20, No. 2

Figure 4: Simuliid infestation (dark area on the otherwise
white head) of a 47-day old Cape Vulture
nestling.

all nestlings were visited 34 d later, only one fly was
observed on the head of an 83-d-oId nestling. One
55-d-old nestling had a very low Leucocytozoon in-
fection (2 gametocytes in a blood smear); no infec-
tion was noted for the same individual in 2 smears
taken 34 d earlier. Plasmodium spp., microfilariae

500-

?

w 400-

— i
_j

2 300-
!

200 -

H H H

M M

A A X

I I I I I I I

76 77 78 79 80 81 82

YEAR

Figure 5 : Simuliid presence related to the amount of rain
in the six months preceding September. The
degree of infestation is classed as heavy (H)
(i e.g ., Fig. 4), medium (M) or absent (A), and the
mean amount of rainfall for these months is
shown (x).

and other protozoa were searched for but not
found (M.B. Markus in litt.).

No simuliids were observed at Aasvogelvlei in
either year, and examination of samples from 4
nestlings did not reveal the presence of any blood
parasites.

In 1982, less than 10 black flies were present on
the crowns of only 2 of 10 nestlings examined.

Figure 5 records the degrees of simuliid infesta-
tion for the years 1976-1982, relative to the rainfall
for the 6 months before September (data from
Boshoff and Currie 1981, unpubl. records).

(c) Length of nestling period. — Where no days
of observation preceded the day the fledgling was
first seen away from the nest, the fledging date was
not accurately known (4 cases known to within 5 d).
Where at least 2 observation days preceded the date
of the first observed flight, that day was taken as the
fledging date. In 1981, mean nestling periods were
148.0-150.3 d (range 139-171 d). Both these means
were significantly longer than the 1 982 mean of 1 37
d (range 124-147 d) (t-Testformeanof 148, lld.f.,
P = 0.04). The individual with the 125-d nestling
period was found dead an estimated 3 wk after
fledging. The next shortest nestling period was 136
d, and this bird survived at least 5 mo.

The mean for all available nestling periods was
143 d, very similar to Mundy’s (1982) estimate of
140 d. The mean 1981 fledging date was 21-23
January 1982, and this contrasts with 28 December
for 1982 fledglings.

Discussion

The Pre-laying period. — Breeding sites are
seemingly occupied about 6 mo or more before
egg-laying. 1 can only conclude that pair formation
preceded occupancy of a site: single vultures were
not seen consistently at the few sites initiated during
the study period. I discerned few courtship proce-
dures except, perhaps, mutual preening and
scapular action (Berruti 1981). Obviously, many
signals perceived by either sex may not be perceived
easily by human observers. In the Cape Vulture,
pair formation occurs in association with the
breeding colony, and specifically the nesting ledges,
where 96% of all copulation attempts occurred
(Robertson 1983). In contrast to the feeding area
which, certainly in this colony’s case, is totally
man-influenced (Boshoff et al. 1984, Robertson

Summer 1986

Breeding Cycle of Cape Vultures

57

Table 1: Copulation attempts observed at the Potberg Cape Vulture colony during the study period.

Site

(A)

No. Before
Egg Laid

(B)

Total Observed
May 81 - Dec 81

A/B

Total Observed
Jan 82 - May 82

Egg Laid
(E)

7

112

113

99.1

38

E

13

31

35

88.6

18

E

16

42

53

79.2

54

E

6

30

39

76.9

34

E

26

49

61

80.3

53

E

4

44

55

80.0

64

E

63

59

59

100.0

22

E

57

44

45

97.8

25

E

17

48

57

84.2

51

E

61

17

30

56.7

54

E

30

38

49

77.5

5

EE

18

39

49

79.6

48

E

57

31

31

1Q0.0

46

E

29

39

57

68.4

4

-

45

21

21

100.0

0

-

3

54

54

100.0

1

-

2

-

32

-

30

E

50

-

8

-

25

E

25

-

24

-

71

E

64

-

0

-

19

E

8

-

117

-

21

_

57 a

-

34

-

-

-

Roost

Points

56

21

Total

1079

T otal

704

and Boshoff in press), the breeding area is unaf-
fected by man. Aspects of breeding behaviour that I
observed that are not directly linked to feeding,
e.g., nestling attendance and foraging times, might
then be representative of the species throughout its
range. Because nest material disappeared from
some sites before the next season’s cycle, the site
rather than the neststructure (Newton 1979) would
appear to be the more valuable resource.

Houston’s (1976) observation of a sex-related dif-
ference in behaviour during this period is con-
firmed; males collected nest material more fre-
quently than females. Such behaviour would re-
duce the female’s energy expenditure; is it then
related to production of the egg? Several female

raptors increase in mass before egg production
(Newton 1979). However, in the Griffon Vulture
that share parental duties, mass increase occurs in
both sexes (Houston 1976). Thus energy is ac-
cumulated not merely to enable the female to pro-
duce eggs, but to supplement the energy budgets of
both sexes during the breeding cycle (Newton
1979). Also, the Griffon Vulture clutch of one egg
weighs 3-5% of the female’s body weight (Mundy
1982) and, in contrast to smaller raptors, makes a
relatively small demand on her reserves. Greater
male participation in collecting forays may be re-
lated to some other factor, such as courtship (akin to
gannet behaviour?, see Nelson 1976). In a review of
dimorphism in predatory birds, Andersson and

58

Alistair S. Robertson

Vol. 20, No. 2

Table 2: Copulation attempts observed per period of breeding cycle. The figures for nest no. 61 are in parentheses as
the nestling disappeared before fledging, and the total for successful sites excludes this site.

Nest

Number

Pre-Laying

Period

A

A/E

%

Incubation

Period

B

B/E

%

Nestling

Period

C

C/E

%

Post-Fledging
Dependence
Period (D)

D/E

%

Total No.
Copulation
Observed (E)

6

31

75.6

8

19.5

0

0.0

2

4.9

41

17

47

62.6

8

10.6

20

26.6

0

0.0

75

16

42

38.8

2

1.8

15

13.8

49

45.4

108

18

37

-

1

-

-

-

-

-

38

29

38

-

5

-

-

-

-

-

43

4

44

73.3

0

0.0

13

21.7

3

5.0

60

26

47

-

6

-

-

-

-

-

53

61

16

(59.2)

0

0.0

(11)

(41.7)

-

-

27

30

38

-

0

-

-

-

-

-

38

13

27

-

2

-

-

-

-

-

29

57

30

-

0

-

-

-

-

-

30

55

46

-

0

-

-

-

-

-

46

45

21

-

0

-

-

-

-

-

21

3

45

-

0

-

-

-

-

-

45

63

49

69.0

0

0.0

2

2.9

20

28.2

71

7

112

96.5

0

0.0

3

2.6

1

0.8

116

MEAN

67.8

4.5

15.4

14.0

52.5

Norberg (1981) present hypotheses to account for
the lack of (reversed) size dimorphism in colonial
carrion feeders; the general lack of sexual differ-
ences recorded in this study accords with their pre-
dictions.

Incubation period. — The timing of egg-laying is
apparently not related to success or failure of the
previous season’s breeding attempt, as indicated at
the few sites where juveniles continued to solicit
food from parents near the following season’s lay-
ing date. This suggests that lengthy post-fledging
dependence periods may not drain the female’s
reserves. Whether or not both parents contribute
equally to supplying the juvenile with food is un-
known, and deserves investigation.

Raptors in general lay earlier in good than in
poor food conditions, and food therefore has both a
proximate and an ultimate influence (Newton
1979). Long periods of rain or heavy cloud before
laying inhibits vultures’ foraging and presumably
reduces the rate of body reserve accumulation. This
could delay laying, and thereby account for the
rain-correlated variations in mean laying date. The

markedly regular laying date of particular pairs
(identifiable partners at nest #63 had laying dates
exactly the same in both study years), however, may
point to the operation of other proximate factors.
Throughout the Cape Vulture’s range there is a
gradation in laying dates, from earliest in the
Transvaal (early May) through to latest in the
southwestern Cape (mid-June).

One should bear in mind that the food source of
the Potberg colony is an “artifact” of stock-farming
practices. Because the food source has changed
markedly within the past century, it is unrealistic to
relate overall timing of the colony’s breeding cycle
to its current food source. Houston (1976) con-
cluded that the breeding season of G. rueppellii in
East Africa was so timed that the fledging date
coincided with a relatively abundant period of food.
This link certainly does not hold at Potberg, where
nestlings fledge during a time of relative decrease
in, but still adequate, food abundance (Robertson
and Boshoff in press).

The accurately determined incubation period
length of 57 d compares favourably with the 56-d

Summer 1986

Breeding Cycle of Cape Vultures

59

estimate of Mundy (1982) and the 57.6 d period of
Boshoff and Currie (1981). It is clearly longer than
the figure given by Brown and Amadon (1968) and
McLachlan and Liversidge (1978). Regression equ-
ations that compare female weight with incubation
periods in a range of raptors (Newton 1979) allow
for comparison here: using weights of 8 and 8.5 kg
(Mundy 1982), incubation periods of 55 and 56 d
are predicted.

Nestling period. — Immediately after hatching,
both parents usually remained at the nest together
for longer periods than during incubation. Nest-
lings were guarded most of the time by at least one
parent (Mundy 1982; this study), unlike other vul-
ture species (Pennycuick 1976; Munday 1982).
Predation may account for a substantial portion of
egg and nestling loss in colonies, e.g., gulls, re-
viewed by Hunt and Hunt (1976). The high paren-
tal attendance by Cape Vultures, following the
period after the nestling is dorsally feathered (and
capable of thermoregulation), could reflect an
anti-predator response. Mundy (1982) suggested it
was an effect of coloniality and the traffic of stran-
gers to the nest. The ability of parents to repulse
Black Eagle (Aquila verreauxi) predatory attempts is
enhanced by their colonial nesting habit (Mundy et
al. in press).

That attentive periods of both sexes were shorter
than during the incubation period reflects the in-
creased attendance of both parents, as well as less
intensive attendance by any one parent. This made
changeovers more difficult to discern and resulted
in a greater proportion of “within-day” attentive
periods being recorded. The earlier changeover
time probably reflects the earlier sunrise of ap-
proaching summer.

Nestlings of the Lappet-faced Vulture ( Torgos
tracheliotus) are paratized by hippoboscid flies
(Icosta meda ) (Anthony 1976), and in 1981 Potberg
nestlings were heavily infested with simuliid flies.
Because parents remove flies, the number of flies
that I observed was probably greater than it would
have been had I not disturbed the parents. The
degree of nestling infestation is related to preced-
ing rain as indicated in Fig. 5, although other fac-
tors may influence their presence.

Simuliids are known to be vectors of protozoan
parasites (Fallis et al. 1973), and are likely vectors of
avian haematozoa (Boshoff 1981; Boshoff and
Currie 1981). Blood smears of 1979 nestlings (a
year of heavy infestation) contained one or more

species of Leucocytozoon, the degree of infection var-
ying (Boshoff and Currie 1981). The early blood
film taken from a 22-d-old nestling revealed no
Leucocytozoon presence probably because of aspects
of the fly’s life cycle: gametocytes ofL. neavei were
first seen in peripheral blood of Helmeted
Guineafowl ( Numida meleagris) nestlings 14 d after
infection from sporozoites (Fallis et al. 1973). The
subsequent fledging success of those Potberg
nestlings is unknown, thus, whether Leucocytozoon
contributes to Cape Vulture mortality or morbidity
remains undetermined. A nestling that exhibited
low Leucocytozoon infection (and which was fed on by
a large number of simuliids) was last seen flying 6 d
after it fledged, and it did not have a post-fledging
dependence period. Given the necessity of at least
some supplementary aid from parents during this
period (Robertson 1985), even if it dispersed, the
chances are that it did not live long. However, no
link between its probable death and parasitaemia is
suggested; the nestling certainly lived for about 3
mo with this infection. More observations of the
fate of nestlings infected with blood parasites are
needed before effects of either parasite can be de-
termined. Adults remove flies with their bills,
suggesting that they are behaviourally adapted to
their parasites (Markus 1974). In a previous study
of haematozoa from southern African vultures, the
Cape Vulture was the only species not infected with
blood parasites (Greiner & Mundy 1979). This was
ascribed to its unique nesting habit as well as the
timing of its breeding season. In their first few
weeks of life, nestlings have a relatively soft skin and
are either naked or sparsely covered with feathers;
consequently it is at this stage they are most vulner-
able to biting insects (Markus 1974). Greiner and
Mundy (1979:152) sampled nestlings from sum-
mer-rainfall colonies, during which time “approp-
riate ornithophilic vectors are evidently scarce or
absent . . . precluding transmission of potential
pathogens . . . .” Simuliid flies require water in
which to breed (Ledger 1979), and the stream in the
ravine provides suitable habitat. The Potberg col-
ony is the only one I know of with a suitable breed-
ing ground for these flies within 400m of the
breeding ledges (see Mundy 1982; Tarboton and
Allan 1984).

The length of the nestling period, although vari-
able between years, is also closely predicted by
Newton’s (1979) regression equation: 141-148 d
for body masses of 8 and 8.5 kg.

60

Alistair S. Robertson

Vol. 20, No. 2

Acknowledgments

I thank Miles Markus for analysing the blood smears, Andre
Boshoff, Peter Mundy and Hugh Paterson for criticizing the dis-
sertation from which this paper is drawn, and Gordon Maclean,
Dave Ward and a referee for comments on drafts. During the
fieldwork, I was employed by the Cape Department of Nature and
Environmental Conservation and, in 1983, received bursaries
from the CSIR and the University of the Witwatersrand.

Literature Cited

Andersson, M. and R.A. Norberg. 1981. Evolution of
reversed sexual size dimorphism and role partitioning
among predatory birds, with a size scaling of flight
performance. Biol.J. Linn. Soc. 15:105-130.

Anthony, A J. 1976. The Lappet-faced vultures of the
Gonarezhou. Bokmakierie 28:54-57.

Berruti, A. 1981. Displays of the sooty albatrosses
Phoebetriafusca and P. palpebrata. Ostrich 52:98-103.

Boshoff, A.F. 1981. Notes on two Cape Vulture col-
onies in the southwestern Cape Province, South Af-
rica. Vulture News 5:3-10.

Boshoff, A.F. and M.H. Currie. 1981. Notes on the
Cape Vulture colony at Potberg, Bredasdorp. Ostrich
52:1-8.

Boshoff, A.F., A.S. Robertson and P.M. Nor-
ton. 1984. A radio-tracking study of an adult Cape
Griffon Vulture Gyps coprotheres in the southwestern
Cape Province. S. Afr. J. Wildl. Res. 14:73-78.

Brown, L.H. and D. Amadon. 1968. Eagles, hawks and
falcons of the world. Feltham: Country Life.

Drent, R.H. and S. Daan. 1980. The prudent pa-
rent: energetic adjustments in avian breeding. Ardea
68:225-252.

Fallis, A.M., R.L. Jacobson and J.N. Raybold.
1973. Haematozoa in domestic chickens and Guinea
Fowl in Tanzania and transmission of Leucocytozoon
neavei and Leucocytozoon schoutedeni. J. Protozool.
20:438-442.

Green, R. 1976. Breeding behaviour of Ospreys Pandion
haliatetus in Scotland. Ibis 118:475-490.

Greiner, E.C. and P.J. Mundy. 1979. Haematozoa from
southern African vultures, with a description of
Haemoproteus janovyi sp. n.J. Parasitol. 65:147-153.

Hinde, R.A. 1970. Animal Behaviour. 2nd ed. New
York: McGraw-Hill.

Houston, D.C. 1976. Breeding of the White-backed
and Ruppell’s Griffon Vultures, Gyps africanus and
Gyps rueppellii. Ibis 118: 14-40.

Houston, D.C. 1974. Mortality of the Cape Vulture.
Ostrich 45:57-62.

Hunt, G.L. and M.W. Hunt. 1976. Gull chick survi-
val: the significance of growth rates, timing of breed-
ing and territory size. Ecology 57:62-75.

Lack, D. 1954. Ecological adaptations for breeding in
birds. London: Methuen.

Ledger, J. A., 1974. Colour rings for vultures. Safring News
3:23-28.

1979. African insect life, rev. end. Cape

Town: Struik.

Markus, M.B. 1974. Arthopod-borne disease as a pos-
sible factor limiting the distribution of birds. J.
Parasitol. 4:609-612.

McLachlan, G.R. and R. Liversidge. 1978. Roberts
birds of South Africa, 4th edition. Cape Town: John
Voelker Bird Book Fund.

Mundy, P.J. 1982. The comparative biology of southern
African vultures. Johannesburg: Vulture Study
Group.

Mundy, P.J. and J. A. Ledger. 1975. Notes on the Cape
Vulture. Honeyguide 83:22-28.

Nelson, J.B. 1976. The breeding biology of Frigate
birds. Living Bird 14:113-155.

Newton, I. 1979. Population ecology of raptors. Ber-
khamsted: Poyser.

Pennycuick, C.J. 1976. Breeding of the Lappet-faced
and Whiteheaded Vultures ( Torgos tracheliotus Forster
and Trigonoceps occipitalis Burchell) on the Serengeti
Plains. £. Afr. Wildl. J. 15:67-84.

Piper, S.E., P.J. Mundy and J.A. Ledger. 1981. Esti-
mates of survival in the Cape Vulture Gyps coprotheres.
J. Anim. Ecol. 50:815-825.

Robertson, A.S., 1983. The feeding ecology and breed-
ing success of a Cape Vulture colony in the southwest-
ern Cape Province. Unpubl. M.SC. Thesis. Johannes-
burg, University of the Witwatersrand.

1984. Aspects of the population dynamics

of Cape Vultures. Ostrich 55:196-206.

1985. Observations on the post-fledging

dependence period of Cape Vultures. Ostrich
56:58-66.

Robertson, A.S. and A.F. Boshoff (in press). The
feeding ecology of Cape Vultures Gyps coprotheres in a
stock-farming area. Biol. Conserv.

Schefler, W.C. 1969. Statistics for the Biological Sci-
ences. London: Addison-Wesley Publishing Com-
pany.

Steyn, P. 1982. Birds of prey of southern Africa. Cape
Town: David Philip.

Tarboton, W.R. and D.G. Allan. 1984. The status and
conservation of Birds of Prey in the Transvaal. Pre-
toria: Transvaal Museum Monographs no. 3.

Vernon, C.J., S.E. Piper and D.M. Schultz. 1982. The
breeding success of Cape Vultures at Collywobbles,
Transkei, 1981. Vulture News 8:26-29.

Department of Zoology, University of Witwatersrand, Johan-
nesburg 2001, South Africa. Present address: Department
of Zoology, University of Natal, Pietermaritzburg 3200,
South Africa.

Received 20 May 1985; Accepted 15 September 1985.

ATTACKS BY BLACK EAGLES ON VULTURES

PJ- Mundy, A.S. Robertson, J. Komen and T.G. O’Connor

Abstract - Numerous attacks by the Black Eagle ( Aquila verreauxi) on vultures were observed. While only once was an
eagle seen eating a vulture nestling, many other attacks by eagles on the nestlings and eggs of the Cape Griffon Vulture
(Gyps coprotheres) were interpreted as predation attempts. The vulture recognizes the eagle as a predator, and the plumage
and behavior of feathered nestlings possibly have a camouflage function. Eagles also attack flying vultures, but these
instances are probably territory defense.

The Black (Verreaux’s) Eagle (Aquila verreauxi) is
a cliff and mountain dwelling raptor that preys
largely on the Dassie (Order Hyracoidea), which
are compact tail-less mammals up to 5 kg in wt. In
the Matopos area of Zimbabwe, for example, where
the eagle has been intensively studied for 20 yrs,
98.5% of more than 2,000 prey items were of two
species of dassie (Gargett 1980). In other areas a
greater variety of prey has been recorded including
birds, and particularly gamebirds, but Brown and
Amadon (1968) suggested that all prey is “taken on
the ground.” The Black Eagle also readily feeds on
carrion (Steyn 1982).

Having studied vultures around southern Africa
for some years, we are impressed by the number of
interactions that we have seen between Black Eagles
and vultures. Always the eagle has initiated an en-
counter; sometimes the vulture has reacted to the
eagle (a true interaction) and at other times it has
not. Because the Black Eagle inhabits the same
mountainous areas as does the Cape Griffon Vul-
ture ( Gyps coprotheres), all but one of the encounters
we have seen were between these species. We detail
our observations here, and review others from the
literature.

Observations

All nine colonies of Cape Griffon Vultures that
we have studied are contained within the territories
of Black Eagles. Thus the paired eagles have daily
access to vultures, and on many occasions we have
seen one or both eagles, and occasionally the
juvenile, fly into the vulture colony and attack birds
at a nest. Thirteen of these incidents are listed in
Table 1, and 11 were directed at a nest with a
nestling; five attacks were successful, in that the
nestlings were removed from their nests. Six attacks
on nestlings were unsuccessful, as were two di-
rected at a nest with an egg. In some instances, the
nestlings were described as downy or feathered; the
eagle flew off with a downy chick in its foot on three

occasions, but knocked the feathered nestling out
of its nest and thus onto the ground. In this latter
instance the eagle flew down with the falling nest-
ling and remained with it on the ground, but did
not actually feed on it. The two adults listed in
incident no. 13 (Table 1) possibly worked in coop-
eration, for one eagle harassed the vulture which, in
rearing up in threat posture, kicked its downy
nestling out of the nest, whereupon the second
eagle flew in and grabbed it. Such cooperation has
also been suggested by Bowen (1970).

Incident no. 14 in Table 1 deserves separate
comment. The Gonarezhou National Park in the
southeast corner of Zimbabwe is a savanna wiidlife
area. In the whole Park there is only one strecth of
cliffs, and one pair of Black Eagles nests on them.
At 0700 H one morning, an adult was seen in the
nest of a pair of White-headed Vultures (Trigono-
ceps occipitalis ) eating the nestling, which was just
becoming feathered (but had been plucked) and
was estimated to be 50 d of age from its wing length
(Mundy 1982). The eagle was presumed to have
killed the nestling, as the carcass was very fresh and
there were signs of hemorrhaging on the downy
feathers and on some of the bones; unfortunately it
was not noticed if there was any subcutaneous
hemorrhage. The distance between the eagle’s nest
on the cliffs and the vulture’s nest in the savanna
country was 10 km. This is the only incident when
we have seen the eagle actually eating a vulture
nestling.

In addition to the 14 attacks on nestlings and eggs
that are listed in Table 1, we have also collected
three “contacts” by Black Eagle on Cape Griffon
Vultures and one on a Bearded Vulture (Gypaetus
barbatus) (Table 2). The eagle may or may not harass
a vulture on the cliff, but then flies after it and
strikes it in mid-air. For incident no. 1 (Table 2), the
vulture was knocked to the ground and it died
(Pitman 1960). For incident no. 4, the eagle flew
after a group of vultures for an estimated 2 km, and
for no apparent reason struck one in mid-air and

61

Raptor Research 20 ( 2 ) : 6 1-64

62

Mundy et al.

Vol. 20, No. 2

Table 1. Attacks (presumed predatory attempts) by Black Eagles on vulture eggs and nestlings.

Date

Place 3

TARGET b

Black Eagles

Result c

Observer

1 . —

Magaliesberg, SA

CGV eyas

2 adults

S

Bowen (1970)

2. —

Mannyelanong, B

CGV eyas (d) d

—

S

Steyn (1973)

3. June 1974

Magaliesberg, SA

CGV egg

1 juvenile

U

PJM

4. September 1980

Langeberg, SA

CGV eyas

2 adults

u

Boshoff (1981).

5. September 1980

Langeberg, SA

CGV eyas

1 adult

u

Boshoff (1981).

6. September 1980

Langeberg, SA

CGV eyas

1 adult

u

Boshoff (1981).

7. October 1980

Kransberg, SA

CGV eyas (p)

1 subadult

s

TO’C

8. July 1981

Potberg, SA

CGV egg

1 subadult

u

A S R

9. December 1981

Potberg, SA

CGV eyas (p)

2 adults

u

ASR

10. December 1981

Potberg, SA

CGV eyas (p)

1 adult

u

ASR

11. January 1982

Potberg, SA

CGV eyas (p)

1 juvenile

u

ASR

12. July 1982

Magaliesberg, SA

CGV eyas (d)

1 adult

s

JK

13. July 1982

Magaliesberg, SA

CGV eyas (d)

2 adults

s

JK

14. October 1982

Gonarezhou, Z

WHV eyas (p)

1 adult

s

PJM

a

b

c

d

Places are Botswana (B), South Africa (SA), and Zimbabwe (Z).

Target species are Cape Griffon Vulture (CGV), and White-headed Vulture (WHV).
Results are successful (S) for the eagle, or unsuccessful (U).

Vulture nestlings were sometimes recorded as feathered (p) or downy (d).

caused it to tumble; the vulture continued flying,
and the eagle flew away from it. For the incident
with the Bearded Vulture, where both adult eagles
were involved, it seems that the eagles may have
been defending their nesting area.

Discussion

The Black Eagle is said to be an “opportunist”
hunter that takes whatever may be available to it (V.
Gargettm litt.). Those eagles that live with a colony

Table 2. Attacks by Black Eagles on flying vultures (with abbreviations as for Table 1).

Date

Place 3

Target 13

Black

Eagles

Result

Observer

1. December 1958

Mannyelanong, B

CGV

1 adult

CGV killed

Pitman ( 1 960)

2. - 1980

Drakensberg, SA

BV immature

2 adults

BV “limping”

Damp (1980)

3. December 1981

Potberg, SA

CGV adult

1 adult

no contact

ASR

4. May 1983

Magaliesberg, SA

CGV

1 adult

CGV unharmed

PJM

a Places are Botswana (B) and South Africa (SA).

k Target species are Cape Griffon Vulture (CGV) and Bearded Vulture (VB).

Summer 1986

Attacks by Black Eagles on Vultures

63

of vultures certainly attack nestlings (and eggs) and
flying vultures. While we have seen an eagle actu-
ally eat a vulture nestling only once, we have seen
eagles remove nestlings several times and have seen
the birds thwarted at their attempts at other times.
We regard these attacks on nestlings (and eggs) as
attempted predation. In contrast, the attacks by the
Black Eagle on flying vultures are likely to be ter-
ritorial encounters, even though in one instance the
vulture was knocked to the ground. Pitman (1960)
called this incident “predation,” but we do not re-
gard it as such as the eagle did not follow the vulture
to the ground.

Downy vulture nestlings weigh less than 3 kg. and
can be carried by an eagle, but feathered ones are
likely to be too heavy and certainly too unwieldy;
these can be knocked to the ground. Eagles pluck
their prey, and the White-headed Vulture nestling
was also plucked in the nest. In addition, at the foot
of cliffs where Cape Griffon Vultures breed we
have often found piles of feathers plucked from
nestlings; we suggest that these are the result of
eagle attacks and that they represent true preda-
tion. Vulture nestlings may fall out of their nests
and be scavenged by mammals such as Jackals
(i Cams spp.) and the Porcupine ( Hystrix africaeaus-
tralis), but these do not pluck prey or carrion.

Black Eagles breed from April to June, and the
young fledge in August to October (Steyn 1982);
the Cape Griffon Vulture breeds at the same time
but its young fledge in October to December. Thus
the eagle has a potential food supply throughout its
breeding season, and Table 1 shows that the eagle
takes the opportunity throughout that period. (The
lack of observations from February through April
are due both to there being no vulture nestlings at
this time, and also because these months are in the
rains when observations are more difficult to
make). We have visited an eagle nest in a vulture
colony only once, in the Magaliesberg in September
1976, and no vulture remains were found. Nor do
we know of any instance where vulture remains
have been found in Black Eagle nests, though
eagles in vulture colonies have not been studied in
this respect (Allan 1984).

The Black Eagle and the White-headed Vulture
do not overlap in their nesting niches, and do not
compete for any resources as far as is known. We
regard the predation incident as described above
(no. 14 in Table 1) as most unusual, and do not
think that the eagle is of any significance in the life

of this vulture. By contrast, the Black Eagle must be
regarded as a capable and perhaps frequent pre-
dator of the nestlings of the Cape Griffon Vulture
simply because the two species occupy the same
nesting niche. The observations of unsuccessful
attacks by the eagle, however, where it has been
repelled by the vultures, highlight one of the ad-
vantages associated with colonial breeding. That is,
a clumped group of nesting vultures with each pa-
rent involved in chasing away a potential predator
from its nest, results in an increased repulsion ef-
fect. The Black Eagle is certainly seen by the Cape
Griffon Vulture as a threat, and the vulture has a
characteristic and hoarse call that is directed at an
eagle flying past the nest. Vulture nestlings of all
ages are vulnerable to predation, which is perhaps
why there is usually a parent in attendance
throughout the nestling period (Mundy 1982). In
addition, we suggest that the nestling plumage has a
camouflage function, which is enhanced by the
typical raptor reaction to disturbance or to intrud-
ers of crouching motionless and facing into the cliff
wall (Fig. 1). The plumage of the nestling is
streaked. In color it is creamy-white dorsally, the
flight feathers are a dark slate, and the last row of
upper- wing coverts (i.e., the greater coverts) are
pointed with a conspicuous white tip (Fig. 1). When
the feathered nestling crouches the visual impres-
sion is one of an accumulation of white mutes trick-
ling down a dark rock. Feathered nestlings of
theRuppell’s Griffon (G. rueppellii ) have the same

Figure 1 . A well-feathered Cape Vulture nestling, aged
about 1 1 weeks.

64

Mundy et al.

Vol. 20, No. 2

coloration and patterning as do those of the Cape
Griffon Vulture (pers. obs.), and this species may
also live with the Black Eagle.

The vulnerability of vulture eggs and nestlings to
predation by the Black Eagle, incidentally, is in-
creased if the parent birds are disturbed on their
ledges, and man-induced disturbance should
therefore be kept to a minimum.

Acknowledgments

We thank the landowners for allowing us access to the vulture
colonies on their properties, and the Department of National
Parks (Zimbabwe) for permission to study the vultures in the
Gonarezhou, where Clive Stockil and Peter Taylor helped us. Val
Gargett gave us information on Black Eagles. During these studies
ASR was employed by the Cape Department of Nature and En-
vironmental Conservation, JK was fmanically supported in part by
the Witwatersrand Bird Club, and both were supervised by the
Zoology Department, University of the Witwatersrand. We thank
A.F. Boshoff, M.P.S. Irwin, J.A. Ledger and three referees for
useful comments on an earlier draft, and Val and Eric Gargett for
particularly critical comments. However, we cannot agree with
Eric that the noble eagle takes exception to the vultures in its
territory and attacks them out of exasperation!

Literature Cited

Allan, D. 1984. Black Eagles in the Magaliesberg. Bok-
makierie 35:55-57.

Boshoff, A. 1981. Notes on two Cape Vulture colonies
in the south-western Cape Province, South Africa
Vulture News 5:3-10.

Bowen, P. 1970. Some observations of the Cape Vulture.
African Wild Life 24:125-128.

Brown, L. and D. Amadon. 1968. Eagles, hawks and
falcons of the world. Country Life Books, Feltham. 2
volumes.

Damp, M. 1980. Giants Castle Lammergrier hide Alba-
tross 261:5.

Gargett, V. 1980. The Black Eagles of the Matopos.
Zimbabwe Science News 14: 155-159.

Mundy, P.J, 1982. The comparative biology of sou thern
African vultures. Vulture Study Group, Johannes-
burg.

Pitman, C.R.S. 1960. An unusual case of predation by
Aquila verreauxi. Bull. Brit. Ornithol. Club 80:67.

Steyn, P., 1973. Eagle days. Purnell, Johannesburg.

1982. Birds of prey of southern Africa.

David Philip, Cape Town.

Vulture Study Group. Endangered Wildlife Trust, P.O. Box
1941, Harare, Zimbabwe. Present address of first author:
Department of National Parks and Wildlife Management,
P.O. Box 8365, Causeway, ZIMBABWE.

Received 30 May 1985; Accepted 27 February 1986.

THE BEHAVIORAL RESPONSE OF A RED-TAILED HAWK
TO MILITARY TRAINING ACTIVITY

David E. Andersen, Orrin J. Rongstad and William R. Mytton

Abstract — From 13 August through 9 November 1982, we systematically radio-tracked an adult male Red-tailed Hawk
(Buteo jamaicensis) on the Fort Carson Military Reservation in east-central Colorado. Harmonic mean analysis of telemetry
data indicated that the bird shifted its center of activity away from areas of high human activity, but returned to those
areas after human activity ceased.

Both short and long term effects of human dis-
turbance on birds of prey have been the subject of
recent investigation. Changes in density and species
composition (Voous 1977; Craighead and Mindell
1981) and in population size and nesting distribu-
tion (Swenson 1979) have all been attributed to
increased human activity. Nesting failures (Boeker
and Ray 1971), lowered nesting success (Wiley
1975), and impacts on wintering distribution and
behavior (Stalmaster and Newman 1978) have also
occurred as a result of increased human activity.
Activities of short duration, however, have not had

as great an impact. Helicopter overflights (Platt
1977), frequent low level jet overflights (Snyder et
al. 1978), and jet flights with associated sonic booms
(Ellis 1977) did not appear to limit productivity or
adversely affect behavior in a variety of North
American raptors. Jackson and McDaniel (1977)
even observed a Northern Harrier (Circus cyaneus )
they believed exploited military activity to increase
hunting success. Here, we report on behavioral re-
sponses of an adult Red-tailed Hawk (Buteo
jamaicensis) to periodic military training activity.

Figure 1. Activity areas of an adult male Red-tailed Hawk on the Fort Carson Military Reservation, Colorado, from 13
August through 9 November 1982. Activity contours represent isopleths that enclose 25, 50, 75 and 90% of
all relocations. The larger area was calculated from 74 relocations obtained when there was no military
activity in the training area, and the smaller area was calculated from 31 relocations obtained when the
training area was used for tank maneuvers and heavy weapons firing.

65

Raptor Research 20(2):65-68

66

Andersen et al.

Vol. 20, No. 2

Table 1 . Tracking periods on an adult male Red-tailed Hawk at Fort Carson Military Reservation, Colorado, from 13
August to 9 November 1982.

Date

Number of
Relocations

Hours

Tracked

Military

Activity 1

13 August

9

4.0

none

25 August

6

3.0

none

26 August

3

1.0

none

31 August

9

4.0

present

13 September

7

3.0

none

14 September

8

3.5

none

20 September

9

4.0

none

26 September

7

3.5

none

3 October

8

3.5

none

12 October

9

4.0

present

20 October

8

3.5

none

27 October

7

3.0

none

2 November

2

2.0

none

5 November

5

2.0

present

9 November

8

3.5

present

Totals

105

47.5

‘Refers to the activity level in the tank training area (see text). Military personnel and equipment were either training in (“present”) or
absent from (“none”) the area.

Study Area and Methods

The Fort Carson Military Reservation is in east-central Col-
orado along the eastern edge of the Front Range south of Col-
orado Springs, Colorado. Climate there is classified as dry conti-
nental and major plant associations include pinyon pine ( Pinus
edulis) - juniper (Juniperus monosperma), blue grama ( Bouteloua
gracilis) - western wheatgrass ( Agropyron smithii), and cottonwood
( Populus spp.) - willow ( Salix spp.) (Harrington 1954; U.S. Army

1980) . Elevation is approximately 1700 m and topography is
characterized by moderately rolling high plains whose surfaces are
often interrupted by scattered rocky escarpments and low foot-
hills. The study area is used by the U.S. Army for the training,
housing and support of the 4th Infantry Division (Mechanized).

On 31 May 1982, we captured an adult male (sex was deter-
mined by behavior and relative size) Red-tailed Hawk and fitted it
with a radio transmitter attached as a backpack (Smith and Gilbert

1981) . The bird’s nest was situated on a cliff immediately adjacent
to a military training area (Fig. 1) in a 1-3 km wide valley. One
young fledged from the nest in late June. The training area was
used periodically during the study for tank maneuvers and heavy
artillery firing. When in use, approximately 50 tanks were in the
area and periodic firing occurred at stationary and moving
targets. Vehicles and military personnel used the entire valley, and
a major vehicle trail passed 250 m in front of the nest cliff. The
primary firing area was 500 m south of the nest site and tanks fired
from this area past the nest cliff at targets either in the valley or in
an impact area 1 1 km to the north.

We monitored the bird for 47.5 hrs between 13 August and 9
November 1982. Relocations were obtained every 30 min during 2
to 4 hr tracking periods and plotted on 1 :24,000 U.S. Geol. Survey
topographic maps. Tracking was conducted from a vehicle and on
foot. Military acdvity was assessed visually throughout each
tracking period and categorized on the basis of presence or ab-
sence of military personnel and equipment. Four tracking periods
occurred when the training area was being used by the military,
and 11 periods occurred when it was not (Table 1).

We calculated minimum perimeter polygons (Mohr 1947;
Jennrich and Turner 1969) for periods during military activity
and for periods when the training area was not in use. Harmonic
mean measures of activity areas (Dixon and Chapman 1980) were
used to assess behavioral responses to military activity. Arbitrary
10x10 grids were selected for calculating harmonic means in a
computer program developed by J.R. Cary at the University of
Wisconsin.

Results and Discussion

During periods when there was no military activ-
ity in the tank training area, the adult male Red-
tailed Hawk included much of the training area
within its activity area (Fig. 1). Twenty- five of 74
(34%) total relocations were obtained within the
boundaries of the training area and 37% of the 90%

Summer 1986

Red-Tailed Hawk Behavior

67

harmonic mean activity area was included in the
training area. In contrast, during periods of mili-
tary activity, none of the 31 relocations obtained
were inside the training area and the bird shifted its
center of activity approximately 1.75 km southeast
(Fig. 1). During these periods, the bird was never
located west of the ridge which defined the east
boundary of the training area. This north-south
oriented ridge supported the nest and visually iso-
lated the training area from the bird’s southeast
activity area.

Minimum perimeter polygons also showed a shift
to the southeast during periods of military training.
The minimum perimeter polygon calculated for
periods of army activity defined an area of 2 1 9 ha in
the southeast portion of the bird’s range. Thirty
percent (75 ha) of this area was outside of the
minimum perimeter polygon (790 ha) defined by
relocations obtained when there was no military
activity in the training area. Harmonic mean
analysis of all relocations obtained for the bird
(combining data collected during periods of army
activity and periods of no activity) showed the same
2 activity centers as those generated when data were
separated on the basis of military activity.

The adult male hawk apparently altered its be-
havior to accommodate periods of military activity.
It was never relocated in the training area when
military personnel and equipment were present,
although it returned to the training area after
training ceased. The bird’s mate was not marked,
and we did not determine whether it exhibited
similar behavior. Since the pair nested within the
confines of the training area, and since most Red-
tailed Hawks are year-round residents on the study
area (Andersen 1984), the male had probably been
exposed previously to periods of intensive military
activity. We do not know how frequently this short
term displacement/relocation behavior may occur
in Red-tailed Hawks or whether it is also exhibited
by other birds of prey.

Acknowledgments

We thank T. Prior and S. Emmons for administrative help; J.R.
Cary for statistical assistance and computer programming; T.
Aydelott and J. Tonies for field help and L.L. Kinkel, R.L. Knight,
E. Santana C., K. Steenhof, J.W. Lish, J.J. Dinsmore, and F.L.
Knopf for critical review of previous drafts of the manuscript.
Funding was provided by the U.S. Army [Fort Carson, 4th Infan-
try Division (Mech.)l through the U.S. Fish and Wildlife Service
and the University of Wisconsin Cooperative Wildlife Research
Unit.

Literature Cited

Andersen, D.E. 1984. Military training and the ecology
of raptor populations at Fort Carson, Colorado. M.S.
Thesis, Univ. of Wisconsin, Madison.

Boeker, E.L. and T.D. Ray. 1971. Golden Eagle popu-
lation studies in the southwest. Condor 73:463-467.

Craighead, F.C. Jr. and D.P. Mindell. 1981. Nesting
raptors in western Wyoming, 1947 and 1975./. Wildl.
Manage. 45:865-872.

Dixon, K.R. and J. A. Chapman. 1980. Harmonic mean
measures of animal activity areas. Ecology 61:1040-
1044.

Ellis, D.H. 1981. Rsponses of raptorial birds to low level
military jets and sonic booms. Inst, for Raptor Studies.
Unpubl. report.

Harrington, H.D. 1954. Manual of the plants of Col-
orado. Sage Books, Denver.

Jackson, J. A. and T.H. McDaniel. 1977. Opportunistic
hunting of a Marsh Hawk on a bombing range. Raptor
Res. 11:86.

Jennrich, R.I. and F.B. Turner. 1969. Measurements
of noncircular home range./. Them . Biol. 22:227-287.

Mohr, C.O. 1947. Table of equivalent populations of
North American small mammals. Am. Midi. Nat.
37:233-249.

Platt, J.B. 1977. The breeding behavior of wild and
captive Gyrfalcons in relation to their environment
and human disturbance. Ph.D. Thesis, Cornell Univ.
Ithaca, N.Y.

Smith, D.G. and R, Gilbert. 1981. Backpack radio
transmitter attachment success in Screech Owls (Otus
asio). N. Am. Bird Band. 6:142-143.

Snyder, N.F.R., H.W. Kale II and P.W. Sykfs,
Jr. 1978. An evaluation of some potential impacts of
the proposed Dade county training jetport on the en-
dangered Everglade Kite. Patuxent Wildl. Res. Cent.,
U.S.F. & W.S.

Stalmaster, M.V. and J.R. Newman. 1978. Behavioral
responses of wintering Bald Eagles to human activity.
J. Wildl. Manage. 42:506-513.

Swenson, J.E. 1979. Factors affecting status and repro-
duction of Ospreys in Yellowstone National Park./.
Wildl. Manage. 43 : 595-60 1 .

U.S. Dept, of Army. 1980. Draft: Environmental im-
pact statement for the continuing operation of Fort
Carson. Fort Carson, Colorado.

Voous, K.H. 1977. Three lines of thought for consider-
ation and eventual action. Pp. 343-347, In Proc. Int.
Counc. for Bird Preserv. World Conf. on Birds of
Prey, Vienna, Austria 1975.

Wiley, J.W. 1975. The nesting and reproductive success
of Red-tailed and Red-shouldered Hawks in Orange
County, California: 1973. Condor 77:133-139.

68

Andersen et al.

Vol. 20, No. 2

Address of first and second authors: Department of Wildlife
Ecology, University of Wisconsin, Madison, Wisconsin
53706. Address of third author: U.S. Fish and Wildlife Ser-
vice, Colorado Field Office, 730 Simms St., #292, Golden,
Colorado 80401.

Received 18 October 1985; Accepted 31 March 1986

Summer 1986

Short Communications

69

Habitat Use by Wintering Golden Eagles and Rough-Legged Hawks
In Southeastern Idaho

E.H. Craig, T.H. Craig and Leon R. Powers

Little information is available on winter habitat prefer-
ences of raptors, even though they spend nearly as much
time in wintering areas as nesting areas. Wilkinson and
Debban (1980) identified a need for determining habitat
preferences for wintering raptors so that effects of habitat
alterations on raptor populations might be predicted.
This may be especially important where native habitat is
being altered to increase grazing, or for agricultural lands
in the intermountain west. The purpose of this study was

to identify wintering habitats used by the Golden Eagle
(Aquila chrysaetos ) and Rough-legged Hawk (. Buteo lagopus )
on and around the Idaho National Engineering Labora-
tory (INEL) in southeastern Idaho.

During 1981-1982, we conducted a daytime winter in-
ventory (30 November - 23 February) of raptors on INEL.
During the recent inventory (7 surveys), we plotted the
specific location of 626 Rough-legged Hawks and 283
Golden Eagles on vegetation maps of the INEL (McBride

Table 1. Habitat comparisons for wintering Golden Eagles and Rough-legged Hawks observed in various habitat
types on a 1981-1982 car survey in southeastern Idaho. Chi-square values are indicated in parentheses.

Habitat s

Golden Eagle
Observed 13

Expected 13

Rough-Legged Hawk
Observed 13

Artemesia tridentata - grass

32.1

29.1

10.9

(1.048) h

(71.67) e

A. tridentata - grass (no power poles)

9.9

22.9

4,2

(21.03) e

(96.07) e

A. tridentata - Certoides lanata

29.0

13.6

11.3

(49.28) e

(2.59)8

Juniperus osteosperma - A. tridentata - grass

5.3

1.0

1.6

(53.29) e

(2.59)8

Agricultural Land

4.2

9.4

63.1

(8.14) f

(1924. 90) e

Chrysothamnus viscidiflorus - grass - Opuntia polycantha

3.5

3.1

1.9

(0.82)'

(2.59)8

Tetradymia canescens - C. viscidiflorus - A. tridentata

(no power poles)

2.5

7.3

3.8

(9.19) f

(12.89) e

Agropyron cristatum (no power poles)

2.1

5.2

0.5

(5.28) c

(29.46) e

Atriplex nuttalii - C. lanata - grass

1.8

2.1

2.0

(0.82)'

(2.59)8

Artemesia arbuscula - Atriplex confertifolia - grass

1.4

1.0

0.2

(0.82)'

(7.05) d

A. tridentata - A. arbuscula - Atriplex confertifolia

8.1

3.1

0.5

(23.04) e

(16.45) e

a Power poles were present in all sections unless noted otherwise.

^Percent of total birds seen rounded to nearest tenth of a percent.

C P < 0.025; d P < 0.001; e P < 0.001; f P < 0.005; SP > 0.05; h >P 0.25; 1 >P 0.75.

70

Short Communications

Vol. 20, No. 2

et al. 1978). The frequency of raptor occurrences per
habitat was compared to expected frequencies of both
species in each habitat type (Chi-square (x 2 ) test). The
expected frequencies were determined by multiplying the
total number of each species observed per survey by per-
cent occurrence of each habitat along the 187 km survey
route. The percent habitat coverage was determined by
selecting 95 random points on a map of the survey route
and calculating the percent of points which fell in each
habitat type (after Marcum and Loftsgaarden 1980). All
agricultural lands, usually alfalfa (Medicago sativa) and
grain fields, were considered one habitat. Areas where
few or no power poles were available for perches were
listed separately, since Craig (1978) reported that both
Golden Eagles and Rough-legged Hawks showed a pre-
ference for power poles as perch sites on INEL.

Golden Eagles occurred in agricultural areas signifi-
cantly less often than expected (P < 0.005; x 2 = 8.1422;
Table 1) and were found most frequently in certain
habitats dominated by native vegetation (Table 1). How-
ever, 3 areas covered with native vegetation or Agropyron
cristatum, but containing few or no powerlines were used
significantly less often (P <T 0.01, x 2 = 1.0482; P < 0.005,
x 2 = 9.194; P < 0.025, x 2 = 5.2776; Table 1) than ex-
pected by Golden Eagles. Birds either selected preferred
habitats where perches were available, or birds were more
visible when perched. These findings are similar to those
of Fischer et al. (1984) in Utah. Golden Eagles occurred
most often in areas where winterfat ( Ceratoides lanata),
grass and shrub species such as Utah juniper (Juniperus
osteosperma) and big sagebrush ( Artemesia tridentata) pre-
dominated. Johnson (1982) observed that shrubby
habitats are used for cover during the daytime by Black-
tailed Jackrabbit ( Lepus calif ornicus) and that winterfat and
some species of grass comprised most of the hares’ diet.
Gross et al. (1974) found higher densities of jackrabbits in
sagebrush habitats than in croplands, and Johnson (1982)
seldom found them in open areas of native habitat during
the day. Therefore, the areas where eagles were most
frequently observed may have contained higher numbers
of jackrabbits, an important prey of Golden Eagles (Smith
and Murphy 1979; Thompson etal. 1982).

Rough-legged Hawks were observed in farmland areas
significantly more often (P < 0.001;x 2= 1924.9;Table 1)
than expected in most other areas (see also Fischer et al.
1984). Since these hawks feed largely on small mammals
(Craighead and Craighead 1956), they would likely occur
in areas where such animals are abundant. Watson (1983)
found that the major prey of Rough-legged Hawks in the
agricultural land of our study areas was Microtus montanus.
In southwestern Idaho, Microtus spp. are more numerous
near agricultural lands than in desert areas (Marks and
Yensen 1980). This is probably also true on our study area
and would thus account for the distribution of these
hawks. As with Golden Eagles, significantly fewer (P <
0.001; x 2 = 96.0174; P < 0.001, x 2 = 12.8900; P < 0.001;

X 2 = 29.4622; Table 1) Rough-legged Hawks were ob-
served in areas lacking power poles; however, other native
habitats also hosted fewer hawks than expected.

In conclusion, Golden Eagles were observed most fre-
quently in certain parts of the INEL study area covered
with native shrub-steppe vegetation and significantly less
often (P < 0.005; x 2 = 8.1422; Table 1) in farmland. In
contrast, Rough-legged Hawks were sighted more often
in agricultural areas. The distribution of these raptors
may have resulted because of the prey species associated
with the 2 different habitats. Fleming (1981) also noted
that Rough-legged Hawks occurred primarily in agricul-
tural areas and Golden Eagles in areas of native vegetation
in southeastern Washington. Our results suggest that the
native shrub-steppe habitats in Idaho are important as
wintering habitat for Golden Eagles. Alterations of this
habitat may adversely affect the wintering Golden Eagle
population but may favor wintering Rough-legged
Hawks, if the habitat alterations benefit small mammal
populations.

Acknowledgments

This research was funded by the Office of Health and
Environmental Research of the U.S. Department of
Energy and is a contribution from the INEL Ecology
Program. We thank O.D. Markham for comments on the
manuscript.

Literature Cited

Craig, T.H. 1978. A car survey of raptors in southeast-
ern Idaho, 1974-1976. Raptor Res. 12:40-45.

Craighead, J.J. and F.C. Craighead. 1956. Hawks,
owls and wildlife. Stackpole Co., Harrisburg, PA.

Fischer, D.L., K.L. Ellis and R.J. Meese. 1984. Winter
habitat selection of diurnal raptors in central Utah.
Raptor Res. 18:98-102.

Fleming, T. 1981. A three-winter raptor survey of the
Columbia Basin; 19 December 1978 through 27 Feb-
ruary 1981. Unpubl. report to U.S. Army Corps of
Engineers. Portland District, Portland, OR.

Gross, J.E., L.C. Stoddart and F.H. Wagner.
1974. Demographic analysis of a northern Utah jack-
rabbit population. Wildl. Monog. No. 40.

Johnson, R.D. 1982. Relationships of black-tailed jack-
rabbit diets to population density and vegetal compo-
nents of habitat. M.S. thesis, Idaho State Univ.,
Pocatello, ID.

Marcum, C.L. and D.O. Loftsgaarden. 1980’ A non-
mapping technique for studying habitat preferences.
/. Wildl. Manag. 44:463-468.

Marks, J.S. and E, Yensen. 1980. Nest sites and food
habits of Long-eared Owls in southwestern Idaho.
Murrelet 61:86-91.

Summer 1986

Short Communications

71

McBride, R., N.R. French, A.H. Dahl and J.E. Detmer.
1978. Vegetation types and surface soils of the Idaho
National Engineering Laboratory Site. DOE/I DO-
12084. Natl. Tech. Inf. Serv., Springfield, VA.

Smith, D.G. and J.R. Murphy. 1979. Breeding re-
sponses of raptors to jackrabbit density in the eastern
Great Basin desert of Utah. Raptor Res. 13: 1-14.

Thompson, S.P., R.S. Johnstone and C.D.
Littlefield. 1982. Nesting history of Golden Eagles
in Malheur-Harney Lakes Basin, southeastern Ore-
gon. Raptor Res. 16:116-122.

Watson, J.W. 1983. An ecological study of the Ameri-
can Rough-legged Hawk on the Idaho National En-
gineering Laboratory. In: O.D. Markham, Ed., 1983

progress report, Idaho National Engineering
Laboratory Radioecology and Ecology Programs,
DOE/IDO- 12098. Natl. Tech. Inf. Serv., Springfield,
VA.

Wilkinson, G.S. and K.R. Debban. 1980. Habitat pre-
ferences of wintering diurnal raptors in the Sac-
ramento Valley. Western Birds 1 1:25-34.

Department of Biology, Northwest Nazarene College, Nampa,
ID 83651. Present address of first and second authors: Box
1 , Lee Creek Road, Leadore, ID 83464.

Received 10 May 1983; Accepted 10 February 1985

Distribution and Density of Prairie Falcons
Nesting in California During the 1970s

Douglas A. Boyce, Jr., Ronald L. Garrett and Brian J. Walton

The Prairie Falcon ( Falco mexicanus ) is endemic to west-
ern North America, and although extensively studied,
questions still remain regarding distribution of nesting
territories and density throughout its range. Information
on Praire Falcon nest site distribution and
densities in California was limited prior to 1970. Grinnell
and Miller (1944:108) stated that Prairie Falcons were
“extensive throughout the state save in northwest humid
coast belt. Metropolis appears to lie in southeastern desert
and thence northwest along inner coast ranges.” Miller
(1951) used 3 methods to analyze California avifaunal
distribution — life zone, biome, and biotic province — but
did not specifically examine the distribution of Prairie
Falcons statewide based on vegetational schemes. In 1971
the California Department of Fish and Game began a
two-year statewide survey of Prairie Falcon nesting sites
(Garrett and Mitchell 1972) that yielded comprehensive
distribution data and provided impetus for more surveys.
Here we report on Prairie Falcon nesting distribution and
densities in California, and on the unusually low nesting
density in northwestern California that may be the result of
(1) interspecific competition with increasing numbers of
Peregrine Falcons (Falco peregrinus), and/or (2) the pres-
ence of unsuitable foraging habitat.

Garrett and Mitchell (1973) provided us with 256 nest-
ing territory locations that were originally obtained from
ornithologists, egg collectors, museums, falconers, state
and federal biologists and personal records. A territory
was defined as an area 1 km in diameter that may have one
or more alternate nest sites located within it. The existence
of a territory was confirmed when (1) a pair of adults was
found defending a specific location during the nesting
season, (2) adults were observed incubating or brooding
young in or on a ledge-pothole-cavity-or stick nest, (3)

when young were observed at a nesting site or (4) when
recently fledged young in family groups were located. A
territory was “active” when any of the aforementioned
criteria were observed in subsequent years and inactive if
falcons were not found. Visits to territories in following
years revealed that alternate nest locations were some-
times used. When Prairie Falcons shifted nest site loca-
tions within the original 1 km diameter area of the terri-
tory we treated the nest as being within the same territory
if no other Prairie Falcons were present within 1 km of the
nest. Infrequently, two or more pairs of Prairie Falcons
nested simultaneously within 1 km of each other and 1 km
territories overlapped.

We added to Garrett and Mitchell’s data base during the
1970s by conducting surveys, during the nesting season,
of potential nesting areas using helicopters (after 1976),
automobiles, motorcycles and by searching on foot. Addi-
tional records of Prairie Falcon nesting territories were
obtained from the California Department of Fish and
Game and other biologists working for state and federal
agencies. Territories were plotted on U.S.G.S. topo-
graphical maps (1:64,000) and deposited with the West-
ern Foundation of Vertebrate Zoology, Los Angeles,
California.

The entire state was examined during the 1970s
(1970-1979) for nesting Prairie Falcons. Our original
intent was never to locate every single territory in the state.
However, as we worked during the 1970s we were able to
cover the entire state in great depth. We were not able to
examine (in depth) the entire state in any single year and
we did not have the manpower to visit all known territories
in a single year. Furthermore, even with the use of
helicopters, we could not be certain that we located all
territories within every habitat type visited as Prairie Fal-

72

Short Communications

Vol. 20, No. 2

Oregonian

Californian

North Cascade

Artemesian

Californian

Central Valley

Californian

Sierra

Californian

Central Coast

Californian

Southern Coast

Mojavian

Sonoran

Californian

Inyo

Figure 1. Biotic provinces of California (after Dice 1943). Within the Californian Province floristic regions are
designated (after Stebbins and Major 1965).

cons were not always present at each territory every year.

Although our nesting densities were not absolute (i.e.,
conservative estimates because of variable sampling inten-
sities), they do provide for meaningful density compari-
sons by biotic province. We examined new areas each year
and reexamined areas previously covered until the end of
the 1970s. Our search image for nests was biased in that
we examined only geologic formations for nesting pairs.

We did not attempt to locate tree-nesting falcons (MacLa-
ren et al. 1 984) or falcons nesting on man-made structures
(Boyce et al. 1980). We examined all territories reported
on here at least once during the 1970s.

We report our findings by biotic province (Dice 1943,

Fig. 1), which is similar to the U.S. Forest Service’s
ecoregions of the United States (Bailey 1980). Biotic
provinces are delineated for all of North America and
provide a means for researchers to report future com-
parative densities for the remaining biotic provinces. We
subdivided the California Province, because of its recog-
nized floristic heterogeniety, and reported nesting de-
nsities within it by floristic region (Stebbins and Major
1965). The number of territories reported for each area is
the maximum count made during the entire study period.

Prairie Falcon nesting territories were distributed un-
evenly throughout California, but the mechanism(s)

causing the uneven distribution remains unknown (Table
1). More than 1,250 nesting attempts were observed at 520
nesting territories during the 1970s. The greatest number
of active territories observed in a single year was 300 in
1977. During 1977, only 60% of the known territories
were visited. Based on the number of territories found
active in 1977, we estimate between 300-500 breeding
attempts may occur annually within the state. Garrett and
Mitchell (1973) provide data that showed an average of
2.18 fledglings were produced per pair studied statewide
for a 3-year period. Based on 300-500 pairs of falcons
attempting to nest each year, we estimate that between 650
and 1,100 fledgling falcons may be produced annually in
California.

Greatest densities of nesting Prairie Falcons were found
in the Mojavian Province (Mojave Desert) and central
coast of the Californian Province (Table 1). During the
latter part of the 1970s, historical nesting territory occu-
pancy rates and fledgling productivity rates for the
Mojave Desert were down (Boyce 1982) from those ob-
served earlier by Garrett and Mitchell (1973). In contrast,
the central coast population experienced an increase in
number of occupied territories during the 1970s (Garrett
and Walton unpub. data). The major difference between
the two areas was that the Mojave Desert remained in

Summer 1986

Short Communications

73

Table 1 , The distribution and density of Prairie Falcon nesting territories in California by biotic province (shown bold;

Dice 1943). Distribution and density of nesting territories by floristic region (Stebbins and Major 1965) within
the Californian province are given.

Biotic Province

Nesting Territories
(per biotic province;)

Area

(Km 2 )

Km 2 /Territory

Mojavian

91

23,210

255

Artemisian

63

22,600

359

Californian

332

267,080

804

Central Coast

136

47,920

352

Central Valley

78

59,390

761

Inyo

39

34,520

885

Southern Coast

44

47,500

1,080

Cascade/N. Coast

30

34,370

1,146

Sierra

5

43,380

8,676

Sonoran

25

29,450

1,178

Oregonian

9

55,000

6,111

TOTALS

520

397,340

764

public ownership (U.S. Bureau of Land Management)
while the central coast remained in private ownership.
Land use policies differ drastically between the two areas.
Private land owners in the central coast prevented high
levels of human disturbance from occurring. In contrast,
the Bureau of Land Management’s multiple land use
philosophy for the Mojave Desert resulted in nesting
Prairie Falcons being subjected to tremendous human
disturbance, and fledgling productivity rates were sig-
nificantly lower for nests located near roads versus nests
away from roads (Boyce 1982).

Northwestern California (west of longitude 123) had
the lowest nesting density within the state despite an
abundance of cliffs. Data suggest their absence in north-
western California may be due in part to interspecific com-
petition with Peregrine Falcons for nest sites and lack of
suitable foraging habitat. Walton (1978) observed two
adult peregrines attack and apparently kill a Prairie Fal-
con flying near the peregrines’ nest site in northwestern
California. Northwestern California contained the
greatest density of nesting peregrines remaining in the
state and, during our study, their density slowly increased
as the population recovered from presumed DDE
poisoning (Monk 1981). However, interspecific competi-
tion for nest sites may not be important throughout the
state. Dixon and Bond (1937) reported that Prairie and
Peregrine Falcons nested concurrently on the same cliff
near Tule Lake in northeastern California. We have also
observed both species nesting in close proximity in other
areas of the state. Presence of nesting peregrines and
Prairie Falcons on the same cliff may be understood in

terms of habitat (no trees in sagebrush habitat) that ap-
parently was conducive to hunting success of both species.
At Tule Lake, Prairie Falcons primarily hunt ground
squirrels (Haak 1983) while Peregrine Falcons probably
captured exclusively birds.

Even where Peregrine Falcons were not occupying suit-
able cliffs (cliffs having ledges or potholes available) in
northwestern California, Prairie Falcons were still absent.
Unsuitable hunting habitat, in addition to interspecific
competition, may have been a factor limiting Prairie Fal-
con densities since northwestern California is primarily
forested with scattered open meadows. In northwestern
California Peregrine Falcons often forage over forested
valleys (Enderson and Kirven 1983) and river systems for
avian prey that is attacked and captured in air (Hoddy
pers. comm.). Prairie Falcons, on the other hand, gener-
ally hunt over unforested habitats for birds, mammals and
reptiles, capturing prey mostly on or near the ground
(15-100 m; Brown and Amadon 1968). In the Mojave
Desert, Prairie Falcons have been observed to hunt, at-
tack, and captaure prey while within 10 m of the ground
(Harmata et al. 1978; Boyce pers. obs.). The greatest
nesting density of Prairie Falcons within California occurs
in areas having few trees. Prairie Falcons, therefore, may
be maladapted for exclusive predation on birds in
forested habitat types.

Nesting densities and productivity rates have been re-
duced by urban sprawl in some areas (Boyce and Garrett
1977). Prairie Falcon distribution statewide, however, did
not appear to be seriously affected during the 1970s,
because (1) Prairie Falcons continued to occupy territories

74

Short Communications

Vol. 20, No. 2

that had a history of use in the last 30 years, (2) most
territories studied by Garrett and Mitchell (1973) re-
mained active until the end of the 1970s and, (3) during
the later part of the 1970s we found additional territories
in the same areas that had previously been examined.

N ewton (1979) stated food or nest site availability set the
upper limit of raptor pairs in an area. Undoubtedly, suita-
ble habitat (unforested) with adequate nesting cliffs and
ample vulnerable prey were fundamental factors involved
in determining distribution and density of Prairie Falcons
in California during the 1970s. Other factors possibly
influencing Prairie Falcon distribution and abundance
between and within biotic provinces were rainfall pattern
and human population levels. We did not qttempt to
quantify cliff availability, prey availability or rainfall pat-
tern and these were undoubtedly ultimate factors affect-
ing distribution. There were definite differences in den-
sity of human populations in different areas. Northwest-
ern California has few nesting Prairie Falcons and rela-
tively low numbers of people. The Mojave Desert had
high numbers of nesting falcons and low numbers of
resident humans. However, an incredible number of
recreationalists poured out of the megalopolis of Los
Angeles and inundated the Mojave Desert during
weekends, holidays and summer vacations. Lowered
fledgling productivity in the Mojave Desert is attributed to
high levels of human disturbance (Boyce 1982).

Prairie Falcons will occupy the same nesting territory
year after year (not necessarily the same pair), but nest
locations within territories may change. In some cases
territories will remain unoccupied for 4-5 yrs in a row and
then be reused. Data on nesting territory use through the
years is important for; (1) determining the potential qual-
ity of individual territories by examining occupancy rates
of territories between and within provinces, (2) examining
use of alternate nests within the territory over time to
determine critical habitat, (3) observing fledgling pro-
duction through a series of years to determine levels of
harvest or accessing population health, (4) observing how
varying levels of human disturbance influence occupancy
and reproduction and (5) assisting land managers in
making informed decisions regarding land use practices.

Acknowledgments

This manuscript benefited from the comments of W. Mader, S. Looman
and A. Harmata.

Literature Cited

Bailey, R.G. 1980. Description of the ecoregions of the
United States. U.S. Department of Agriculture, Misc.
Publ. No. 1391,77 pp.

Boyce, D. A. 1982. Prairie Falcon fledgling productivity
in the Mojave Desert, California. Unpubl. M.S. thesis.
Humboldt State Univ., Areata, CA. 98 pp.

Boyce, D.A., Jr., L. Fisher, W.E. Lehman, R. Hipp and
J, Peterson. 1980. Prairie Falcons nest on an artifi-
cial ledge. Raptor Res. 14:46-50.

Boyce, D.A., and R.L. Garrett. 1977. Status of the
California Desert Prairie Falcon population. Trans.
Cal-Nev. Wildlife Soc. 108-111.

Brown, L. and D. Amadon. 1968. Eagles, Hawks and
Falcons of the World. McGraw-Hill Book Company,
New York. 945 pp.

Dice, L.R. 1943. The Biotic Provinces of North
America. Univ. Mich. Press, Ann Arbor. 76 pp.

Dixon, J.B. and R.M. Bond. 1937. Raptorial birds in the
cliff areas of Lava Beds National Monument, Califor-
nia. Condor 39:97-102.

Enderson, J.H. and M.N. Kirven. 1983. Flights of
nesting Peregrine Falcons recorded by telemetry.
Raptor Res. 17:33-37.

Garrett, R.L. and D.J. Mitchell. 1972. A preliminary
report — the California Prairie Falcon nesting survey,
1971. Trans. Cal-Nev. Wildlife Soc. 26-28.

1973. A study of Prairie Falcon

populations in California. Calif. Dept, of Fish and
Game Wildl. Manag. Branch Adm. Rep. #73-2. 15 pp.

Grinnell, J. and A.H. Miller. 1944. The Distribution
of the Birds of California. Cooper Ornith. Club., Ber-
keley. 607 pp.

Haak, B.A. 1983. Population status of Prairie Falcons in
northern Lake County, Oregon. M.S. Thesis, Oregon
State Univ., Corvallis, Oregon.

Harmata, A.R., J.E. Durr and H. Geduldig. 1978.
Home range, activity patterns and habitat use of
Prairie Falcons nesting in the Mojave Desert. Unpubl.
Rep. U.S.D.I. Bureau of Land Management contract
YA-512-CT8-43. On file Denver Federal Center, De-
nver, Colorado. 89 pp.

MacLaren, P.A., D.E. Runde and S.H. Ander-
son. 1984. A record of tree-nesting Prairie Falcons in
Wyoming. Condor 86:487-488.

Miller, A.H. 1951. An analysis of the distribution of the
birds of California. Univ. Calif. Publ. Zool. 50:531-643.

Monk, J.G. 1981. Distribution of DDE residues in prey
species of California peregrine falcons. M.S. Thesis,
Univ. of Calif., Berkeley. 29 pp.

Newton, I. 1979. Population Ecology of Raptors. Buteo
Books, Vermillion, South Dakota. 399 pp.

Stebbins, G.L. and J. Major. 1965. Endemism and
speciation in California flora. Ecol. Monogr. 35:1-35.

Walton, B.J. 1978. Peregrine-Prairie Falcon interac-
tion. Raptor Res. 12:46-47.

Department of Zoology, Brigham Young University, Provo,
Utah 84602. Current address second author: U.S. Fish and
Wildlife Service, Yukon-Kuskokwim Delta Nat’l Wildlife
Refuge, Bethel, Alaska 99559. Current address third au-
thor: Predatory Bird Research Group, Lower Quarry, Univ.
of Calif., Santa Cruz, Calif. 95064

Received 15 July 1985; Accepted 1 April 1986.

Summer 1986

Short Communications

75

Bald Eagle Pursues and Injures Snow Goose in Flight

Richard S. Bennett, Jr., and Erwin E. Klaas

The Bald Eagle (, Haliaeetus leucocephalus) employs sev-
eral hunting methods in capturing avian prey. They have
been observed capturing waterfowl (Parris et al. 1 980) and
ptarmigan (Sherrod et al. 1977) on land by approaching
close to the ground, and eagles also have been seen flying
low between ocean swells, (White 1974) diving from a high
altitude, (Meinertzhagen 1959; Sherrod et al. 1977) or
returning to the surface from an underwater dive (Bent
1937; Munro 1938) to take sitting birds. All these methods
depend on surprise and require little or no aerial pursuit.
Brewster (1880) reported Bald Eagles capturing water-
fowl, including wild geese in flight, by swooping under the
prey, turning upside down, and grasping the breast of the
prey with its talons.

Griffin et al. ( 1982) during 3 seasons of field work saw
many attempts by Bald Eagles to kill apparently healthy
waterfowl. Only 5 attempts were successful of which 1
capture was a Snow Goose ( Chen caerulescens caerulescens).
The authors give no details except to say the goose was
forced down into a field and fed upon.

On 21 January 1983, we observed an adult Bald Eagle
pursue and injure a Snow Goose near DeSoto National
Wildlife Refuge located in Harrison County, Iowa, and
Washington County, Nebraska. The weather was dear,
with a temperature of about 2° C, a slight north wind, and
the ground partly covered by snow.

At 1621 H, a flock of about 500 Snow Geese was flushed
from the ice by a Bald Eagle near an open-water area on
DeSoto Lake. The flock circled briefly and flew eastward
from the refuge for several kilometers, evidently to feed
in surrounding croplands. The geese flew in a typical
searching pattern characterized by wide wavy lines, low
altitude, and circling over prospective fields. About 1650
H, a Bald Eagle was observed following several hundred
meters behind the flock at. the same altitude. The flock
then gained altitude and flew northwest along a nearly
straight course at ground speeds in excess of 50 kph, as
measured on our vehicle’s speedometer.

The eagle continued to pursue and slowly gained on the
flock for about 5 km. At about 1700 H, the eagle flew into
the middle of the flock causing it to suddenly split into 2
groups. The lead group continued flying north while the
trailing group briefly circled, as if confused, and then
continued northward. At this point, the eagle pursued a
single goose from the flock. On 3 occasions, the goose
tried to dive below the eagle, but each time the eagle would
also dive until it was below the goose and they would
briefly fly horizontally. Finally the goose dove toward a
wooded fencerow; however, the eagle dove and caught
the goose during the vertical descent. Although it could
not be determined how the eagle grasped the goose, both

birds fell and separated just before hitting the ground and
glided into a corn stubble field about 1 km from the road.

As we approached the area on foot, we saw an adult
Bald Eagle and an adult Snow Goose standing in alert
postures about 10 m apart. The eagle flushed about 0.5
km from us, while the goose stood motionless until we
were 10 m away. It then flew along the ground for about
0.5 km. As it flew, we saw blood over much of the rump
area, especially posterior to the left wing. After relocating
the goose, we again approached. It flushed atabout 10-15
m and flew strongly toward the refuge lake about 4 km
away. We believe the eagle would have eventually killed
the goose on the ground, if we had not intervened.

Although we have been conducting field studies of
Snow Geese in the fall since 1976, in the middle Missouri
River valley, this was our first observation of such an
extended pursuit of geese by an eagle. While Snow Goose
flocks on water or land will invariably flush when an eagle
flies overhead, the Canada Goose (. Branta canadensis ) and
ducks rarely do. We have occasionally observed -eagles
pursuing individual Snow Geese for short distances after
flushing feeding flocks from crop fields. In each instance,
the goose escaped by diving into tall vegetation or out-
maneuvering the eagle in flight. On the basis of the in-
tense response of Snow Geese to flying eagles, predation
may occur more often than our observations indicate.

Journal paper No. J-l 1016 of the Iowa Agriculture and
Home Economics Experiment Station, Ames, Iowa,
Project No. 2427.

Literature Cited

Bent, A. C. 1937. Life histories of North American birds
of prey. Part I. U.S. Natl. Mus. Bull. 167:1-409.
Brewster, W. 1880. Prowess of the Bald Eagle
(. Haliaeetus leucocephalus). Bull. Nuttall Ornithol. Club
5(1) : 57-58 .

Griffin, C.R., T.S. Baskett and R.W. Sparrowe.
1983. Ecology of bald eagles wintering near a water-
fowl concentration. U.S. Dept, of Interior, Fish and
Wildlife Service, Spec. Sci. Report-Wildlife No.
247:1-12.

Meinertzhagen, R. 1959. Pirates and predators. Oliver
and Boyd, Edinburgh.

Munro, J.A. 1938. The northern Bald Eagle in British
Columbia. Wison Bull. 50(1): 28-35.

Parris, S.D., E.E. Klaas and R.A. Wilson. 1980. Coyote
steals snow goose from bald eagles. Raptor Res.
14(3) : 88-89.

76

Short Communications

Vol. 20, No. 2

Sherrod, S.K., C.M. White and F.S.C. Williamson.
1977. Biology of the bald eagle on Amchitka Island,
Alaska. Living Bird 15:143-182.

White, C.M. 1974. Studies on peregrine falcons in the
Aleutian Islands. Raptor Res. Rep. 3:33-49.

Department of Animal Ecology, Iowa State University, Ames,
Iowa 50011. Present address of first author: Corvallis En-
vironmental Research Lab, USEPA, 200 S.W. 35th St., Cor-
vallis, Oregon 97330. Address of second author: Iowa
Cooperative Wildlife Research Unit, Iowa State University,
Ames, Iowa 50011.

Received 20 May 1985; Accepted 1 May 1986

Osprey Captures Gray Squirrel

Peter Taylor

At approximately 0800 H on 6 February 1982, 1 saw an
Osprey (Pandion haliaetus) capture an Eastern Gray Squir-
rel ( Sciurus carolinensis) at Blue Springs State Recreation
Area, near Orange City, Florida. Several squirrels were
foraging on the ground, in and around a picnic site in a
partly cleared area of live oak ( Quercus virginiana) and
unidentified palms adjoining the St. Johns River. Using
7x binoculars, I watched the Osprey glide in a shallow dive
for about 100 m along a curving flight-path. It flew from
the river over the clearing, snatched the squirrel from the
ground without pausing, then flew back over the river,
where it was lost to view behind trees.

The capture occurred about 100 m away; unfortu-
nately, my view of the moment of capture was obscured by
shrubbery, so I was unable to observe the reactions of the
victim, or other squirrels, to the attack. The squirrel was
grasped firmly in the Osprey’s talons, and was immobile; I
did not record whether it was carried in one or both feet.

Wiley and Lohrer (1973) reviewed records of non-fish
prey taken by Ospreys. These included mammals as large
as ground squirrels and rabbits, as well as a variety of
birds, reptiles, amphibians and invertebrates. More recent
instances of atypical prey have been reported by Castrale
and McCall (1983), Kern (1976), Layher (1984) and
Proctor (1977). This is apparently the first record of an
Osprey taking a tree squirrel (albeit from the ground),
and is one of rather few observations of the actual attack.
It is unfortunate that the ultimate use of the squirrel could
not be observed.

Wiley and Lohrer (1973) recorded at least 9 non-fish
prey, mainly cotton rats ( Sigmodon hispidus), taken by a pair
of Ospreys near Tampa, Florida, in 1972. They suggested
that some inland-nesting Ospreys might exploit an abun-
dant alternative food source, since they are presented with
more opportunities for non-fish prey while travelling
between the nest and scattered bodies of water than are
coastal-nesting Ospreys.

The incident described here took place alongside a
large, clear river in mild, calm weather under a light
overcast sky. The long, stealthy approach and adept cap-
ture by the Osprey suggests that this incident may have

been a regular feature of its hunting routine. These cir-
cumstances support the suggestion that some Ospreys
take prey other than fish in an opportunistic manner,
without the pressure of poor fishing conditions, inexperi-
ence, or other constraints on more typical hunting be-
havior.

Literature Cited

Castrale, J.S. and J. McCall. 1983. Osprey captures
mammal at edge of soybean field. Raptor Res. 17 :92.
Kern, P.R. 1976. Osprey with muskrat. Blue Jay 34:55.
Layher, W.H. 1984. Osprey preys on Canada Goose
gosling .Wilson Bull. 93:469-470.

Proctor, N.S. 1977. Osprey catches vole. Wilson Bull
89:625.

Wiley, J.W. and F.E. Lohrer. 1973. Additional records
of non-fish prey taken by Ospreys. Wilson Bull.
85:468-470.

P.O. Box 597, Pinawa, Manitoba, ROE 1L0, Canada.

Received 8 October 1985; Accepted 31 March 1986

Summer 1986

Short Communications

77

Adoption of a Juvenile by Breeding Spanish Imperial Eagles
During the Postfledging Period

Jose L. Gonzalez, Borja Heredia, Luis M. Gonzalez and Juan C. Alonso

Fostering and crossfostering of nestlings are manage-
ment techniques that have been widely used in the field
with different endangered raptor species (Meyburg 1977;
Temple 1978; Olendorff et al. 1981). However, we know
of only 2 published reports on adult breeding raptors
feeding fledglings that were not their own (Ellis and Groat
1982; Poole 1982). Since this kind of adoption does not
take place naturally in territorial species, these have prob-
ably not developed an intolerance of intruder fledglings
(Wilson 1975). In this paper we describe the introduction
of a fledged Spanish Imperial Eagle (. Aquila heliaca adal-
berti) into a family group consisting of 2 adults and 2
young.

The experiment was carried out during a study of the
post-fledging behavior of Spanish Imperial Eagles in the
National Park of Donana, SW Spain, during the summer
of 1 984. A young female fledgling that had lost its parents
from electrocution was found starving on 31 July at 91 d
of age and weighing 2300 g (the age of young was calcu-
lated from hatching dates). She was housed and fed for 9
days and reached a weight of 3550 g. Once recovered, the
young female was released at an age of 100 d into the
home-range of a family group with 2 young approxi-

mately 99 d of age and already out of the nest. Both the
normal young and the introduced one were provided with
radio-transmitters and wing-tags for visual identification.
At the moment of release 2 dead rabbits were left near the
young. Approximately 1 1 min after the fostered eaglet
was released, she began feeding on one of the rabbits. She
was soon joined by the 2 original young and all 3 fed
together without obvious aggressive interactions. For the
first 3 d after release, the fostered young remained
perched near its stepbrothers. From the fourth day, it
started to use the same perches as they did, being fed by
the adults, without any evidence of rejection or aggression
(Table 1). It seemed to be totally adopted, and followed
the normal process of transition to independence. Un-
fortunately, on 19 August (age 109 d) one of the true
young died in collision with an electric powerline. The
data for this bird have been included in Table 1. Prior to
dispersal, both the original and the adopted fledgling
were fed and simultaneously chased by the adults (Table
1). Both made long distance flights on 30 August (age 120
d) and 16 September (age 138 d) respectively, and finally
left on 1 September (age 122 d) and 20 September (age
142 d). Chasing consisted of stoops towards the young,

Table 1. Summary of feeding and chasing flights of adult Spanish Imperial Eagles towards young. The dates of final
dispersal were 1 September for the original young and 20 September for the adopted young.

No. of Feedings

No. of Chasing
Flights by Adults

Date

Original

Original

Adopted

Original

Original

Adopted

Young A

Young B

Young

Young A

Young B

Young

1 1 August

2

0

1

0

0

0

18 August

0

1

0

12

0

2

19 August

0

0

0

0

0

0

22 August

1

*

1

4

*

4

24 August

1

2

0

0

26 August

1

1

4

2

30 August

0

0

0

1

8 September

-

0

-

0

10 September

-

1

-

1

16 September

-

0

-

0

20 September

"

0

0

Died on 19 September.

78

Short Communications

Vol. 20, No. 2

sometimes when they were perched, but mostly flying;
often the adults hit the young, with their talons inter-
locked, making them lose their balance and to lose height.

On 2 September the introduced young (age 124 d) was
seen trying to kleptoparasitize a Booted Eagle ( Hieraaetus
pennatus) which had caught a rabbit. On 8 September, one
adult was seen eating in an oak tree 200 m from the
fosteredjuvenile, which begged but was not fed. Two days
later it was flushed from the ground while it was feeding
on a rabbit, but we cannot be sure whether the rabbit had
been killed by the eaglet or brought to it by the adults.
These observations are in accordance with the decreasing
feeding rate of the adults to their offspring which oc-
curred during the postfledging period in other family
groups studied (Gonzalez et al. in prep.).

In conclusion, these observations proved that the intro-
duction was successful, and the adopted young reached
independence and dispersed normally. This kind of fos-
tering during the postfledging period could be a valuable
management technique useful in reintroducing salvage or
confiscated young raptors or for the release of captive
bred birds.

We thank L. Garcia, M. Ferrer, M. de la Riva and the
Conservation Staff of the National Park for their help
during the field work. Financial support was provided by
ICONA and CAYCIT.

Literature Cited

Ellis, D.H. and D.L. Groat. 1982. A Prairie Falcon
fledgling intrudes at a Peregrine Falcon eyrie and pi-
rates prey. Raptor Res. 16:89-90.

Meyburg, B.U. 1977. Protective management of
eagles by reduction of nestling mortality. Proc.
I.C.B.P. World Conf. on Birds of Prey. Vienna, 1975,
pp. 387-392.

Olendorff, R.R., R.S. Motoni and M.W. Call.
1981. Raptor Management. The state of the art in
1980. Technical note, 385 pp. U.S. Dep. of Interior,
Bureau of Land Management, Washington.

Poole, A. 1982. Breeding Ospreys feed fledgling that
are not their own. Auk 99:781-784.

Temple, S. (Ed.). 1978. Endangered Birds: manage-
ment techniques for preserving threatened species
University of Wisconsin Press, Madison.

Wilson, E.O. 1975. Sociobiology. Cambridge, Mass-
achusetts, Belknap Press.

Address of first and third authors: Unidad de Zoologia
Applicada, Consejeria de Agricultura y Ganaderla, El Encin,
Alcala de Henares, Madrid, Spain. Address of second and
fourth authors: Museo Nacional de Ciencias Naturales, Jose
Gutierrez Abas cal 2. 28006 Madrid, Spain. Present address
of first author: Museo Nacional de Ciencias Naturales, Jose
Gutierrez Abascal 2. 28006 Madrid, Spain. Present address
of second and third authors: ICONA. Subdireccion General
de Recursos Naturales Renovables, Gran Via de S. Fran-
cisco, 35, 28005 Madrid, Spain.

Received 15 August 1985; Accepted 1 January 1986

Harrir Kills Mobbing Willet

Marilyn E. England

On 2 July 1984, while studying the breeding biology of
the Northern Harrier ( Circus cyaneus hudsonius) , on a Long
Island, New York barrier beach, I saw a female harrier kill
one of two mobbing Eastern Willets ( Catoptrophorus
semipalmatus). The incident took place on a tidal salt marsh
where both species breed. Vegetation is predominantly
Spartina grasses interspersed with marsh elder (Iva frutes-
cens) in the slightly elevated, drier areas where willets nest.
Dense stands of common reed ( Phragmites communis) flank
the marsh and provide nesting habitat for the harriers. I
witnessed frequent encounters between harriers and wil-
lets, especially as the breeding season progressed. Female
harriers hunted over the marsh areas adjacent to nests,
which usually brought them near to, or over, willet nests.
When this occurred, willet pairs mobbed the intruder.
Prior to this observation, however, such incidents had
failed to elicit more than a mild response (e.g., head-
turning to look at the mobbers and/or leg-dropping) from
a harrier.

Generally, harriers respond to mobbing by increasing
their flight speed while moving away from the disputed
area (Bildstein 1982; M. England pers. obs.). Sometimes a
particularly persistent or bold assault is acknowledged
with short bursts of chattering, a vocalization similar to
“kekking”, which I have heard used during nest defense.
Avoidance maneuvers, such as those described by Bild-
stein (1982) for harriers being mobbed by passerines were
also observed. These include “rolling”, a change in flight
elevation, and a shift from the “to and fro” pattern of
“quartering” flight to straight-line flight, again while
moving away from the area.

On 2 July, a female harrier, hunting low over the marsh,
apparently passed close to a willet nest, as both members
of the pair flew up and began mobbing the harrier. At
first, the harrier continued moving in her original direc-
tion. Then, as one of the willets flew under the harrier
from behind, the harrier slowed, then suddenly, at high
speed, seized the willet in mid-flight in one foot. With the

Summer 1986

News and Reviews

79

prey dangling from the talons of her outstretched foot,
the harrier glided approximately 50 m before landing.
The harrier remained on the ground plucking her kill for
8 min while the surviving willet continued circling and
calling overhead. When the harrier rose carrying the dead
willet in two feet pressed close to her body, and flew
approximately 0.5 km to the northeast, the remaining
willet ceased calling.

An observation by Watson (1976) for the Hen Harrier
(' Circus cyaneus cyaneus) in Europe, concerns the taking of a
mobbing Lapwing ( Vanellus vanellus) in a similar fashion.
In that instance, the Lapwing was released alive several
minutes later. Harriers are dietary generalists capable of
taking relatively large prey (Errington 8c Breckenridge
1936; Bent 1937; Craighead & Craighead 1956;
Hamerstrom 1969). However, in the course of my three-
year study, this is the first time predatory behavior was
observed during mobbing.

Acknowledgments

I thank C. Safina, J. Greenlaw, K. Blumer and W. Kolodnicki for providing
helpful comments on an earlier draft of this paper. K. Bildstein, W. Scharf
and J. Schmutz greatly improved the manuscript by their suggestions. Special
thanks to the Theodore Roosevelt Sanctuary for cooperation during field
season and the Huntington Audubon Society for a grant in partial support of
1984 research.

Literature Cited

Bent, A. C. 1937. Life histories of North American birds
of prey. U.S. Natl. Mus. Bull. 167.

Bildstein, K.L. 1982. Responses of Northern Harriers
to mobbing passerines./. Field Ornith. 53:7-14.

Craighead, J.J. and F.C. Craighead. 1956. Hawks,
owls and wildlife. Stackpole, Harrisburg, Pennsyl-
vania.

Errington, P.L. and W.J. Breckenridge. 1936. Food
habits of marsh hawks in the glaciated prairie region of
north-central United States. Am. Midi. Nat. 7:831-848.

Hammerstrom, F. 1969. A harrier population study.
Pp. 367-383, In J.J. Hickey (Ed.), Peregrine Falcon
populations; their biology and decline. Univ. Wiscon-
sin Press, Madison, Wisconson.

Watson, D. 1977. The Hen Harrier. Poyser, Berkham-
sted, U.K.

Theodore Roosevelt Sanctuary, 134 Cove Road, Oyster Bay,
New York 11771.

Received 10 January 1986; Accepted 1 June 1986

News and Reviews

Working Bibliography of the Golden Eagle and the Genus Aquila by Maurice N. LeFranc, Jr. and William S. Clark. National Wildlife
Federation. Scientific Technical Series No. 7, 1983. xxx + 234 pp. Price $14.95 U.S. + $1.55 U.S. postage and handling. Available from
Institute for Wildlife Research, National Wildlife Federation, 1412 16th St. N.W., Washington, D.C. 20036.

The Raptor Information Center of the National Wildlife Feder-
ation has earned a debt of gratitude from raptor biologists
worldwide for its production of a series of working bibliographies
on major taxa of birds of prey. This is the third of these
keyworded, computer-generated reference works, having been
preceded by volumes dealing with owls (Clark, R.J., D.G, Smith
and L.H. Kelso, 1978. Working Bibliography of Owls of the
World, NWF Scient./Tech. Ser. No. 1) and the Bald Eagle (Lincer,
J.L., W.S. Clark and M.N. LeFranc, Jr. 1979. Working Bibliog-
raphy of the Bald Eagle, NWF Scient./Tech. Ser. No. 2). The three
books are the concrete results of a project devoted to the compila-
tion of literature citations that was begun under Jeff Lincer’s
administration as first director of the Raptor Information Center
and continued to successful completion under his successor, Bill
Clark.

Because the present volume includes references to the genus
Aquila worldwide, the organization of the specific format is more
like that of the owl volume than the Bald Eagle bibliography. This
format, by now familiar to raptor students here and abroad, con-
sists of introductory material followed by a master list of citations,
arranged alphabetically by author, with each entry assigned a
reference number. This is succeeded by a permuted list of
keywords, which provides cross-references to each author entry
on the basis of up to a dozen or more selected keywords, which

collectively describe the major content of that particular entry;
hence specific keywords deal with such subjects as nesting, dis-
tribution, research techniques, management and conservation.

The highlight of the introductory material in this volume is a
foreword written by the late Leslie Brown (in his inimitable salty
style) wherein he reviews some general features of the genus and
describes the tedious“hack-work” (his word) required of raptor
students who attempted to conduct literature searches before
working bibliographies of this type were available. Chapter 1
contains the authors’ introduction and acknowledgments, with
information on the production and organization of the bibliog-
raphy. Chapter 2 is an introduction to the members of the genus
Aquila, with a paragraph or two devoted to the description, range
and general characteristics of each of ten species (instead of 9 as
recognized by Brown and some other authorities; the authors state
their good reasons for treating the Steppe Eagle, Aquila nipalensis,
as a full species instead of as a race of A. rapax). This completes the
general and introductory material, and the working bibliography
itself comprises the remainder of the chapters, which are arranged
as follows: Chapter 3 - Master List of Citations (3459 entries),
Chapter 4 - Permuted List of Keywords (“Abandonment” through
“Wintering Population”), Chapter 5 - Species Index to Citations
(ranging from 18 for Gurney’s Eagle to 2305 for the Golden
Eagle), Chapter 6 - Geographic Index of Citations (listed by sub-

80

News and Reviews

Vol. 20, No. 2

region, nation, state, province, etc., for each continent), and
Chapter 7 - List of Citations of Occurrences. There is also an
appendix, with a useful glossary or dictionary of all keywords used
in the bibliography.

Many good things could be said about the finished product, but
its ultimate value will lie in the utility it provides in the context of
an otherwise tedious literature search. I, for one, am grateful that
the authors have not felt it necessary to restrict entries to only
those appearing in referred scientific journals. Golden Eagle af-
ficianados are aware that much useful information may be
gleaned from the semi-popular or anecdotal accounts of the
species that are included (e.g., Gordon, S.P. 1955. The Golden
Eagle, king of birds, Citadel Press, New York, Olendorff, R.R.
1975. Golden Eagle Country, A.A. Knopf, Inc., N.Y.), and even
from fictional works (e.g., Murphy, R. 1965. The Golden Eagle,
E.P. Dutton & Co., N.Y.)

As is to be expected, the work is not without flaws. Any product
that is largely the result of computer programming is bound to
suffer from a few “glitches” and inconsistencies. My main concern
is with the ambiguity which inevitably results from the use of
keywords, which are the utilitarian “heart” of the method. The
number of keywords assigned to a given entry ranges from a
minimum of 3, generally for short notes with limited information,
to 20 or more for more extensive monographs. What constitutes a
valid or relevant keyword, and what criteria are used to determine
the number of them selected to cross-reference a given bibli-
ographic entry? Obviously, this is largely a matter of arbitrary
choice by the compilers, and it is bound to produce some confu-
sion and unsatisfactory allocations of information. To cite a few
examples: there is only one entry under the keyword “Nest Con-
struction” (!); the 13 entries under “Nest Adornment” do not
include several references which should be listed, including the
important monograph by D.H. Ellis (1979). Development of be-
havior in the Golden Eagle, Wildlife Monographs, No. 70. The
Wildlife Society, Inc.); the book by Olendorff (1975) is cited under
the keyword “Popular Monograph,” but the equally anecdotal
work by Gordon (1955) is not; Walter Spofford’s report on the
Golden Eagle aerial shootoff in west Texas (1964. The Golden
Eagle in the Trans Pecos and Edwards Plateau of Texas, Audubon
Cons. Rept. No. 1. National Audubon Society), is not listed under

“Conservation” despite its bearing the label of a conservation
report; nor is it listed under the keyword “Aircraft.”

There are numerous other examples, but these are enough to
emphasize the cautionary message; use of a given keyword does
not guarantee that all literature containing that specific informa-
tion will be cited thereunder. People using the bibliography will
soon discover this problem and adjust their search methods ac-
cordingly, and no doubt all readers will recognize the difficulty
facing the editors in determining how many different keyword
entries to extract from a given publication.

Among other items that would benefit from clarification, the
heading of Chapter 7, “List of Citations of Occurrences,” is some-
what misleading, and needs to include the further information
given on page xx of the introduction so that readers will under-
stand that this is a list of additional locality references for the
Golden Eagle not included in the master list of citations (Chapter
3). I found another inconsistency in the fact that some foreign
titles were given English translations in parentheses, while others
were not. I wondered why some entries were listed as “title only,”
with no keyword cross-referencing. A case in point is the manu-
script report by Glover and Heugly (1970). Golden Eagle Ecology
in West Texas. Report to Natl. Audubon Soc., Colorado Coop
Wildl. Res. Unit. Fort Collins), which contains important informa-
tion on economic relations and management of eagles vis a vis
livestock; perhaps the compilers didn’t have access to a copy
Among other minor errors, I noticed that the two-part paper on
Black Eagles in Tanzania by E.G. Rowe (1947). The breeding
biology of Aquila verreauxi Lesson. Ibis 89:387-410 (Part I); 576-
606 (Part II) was cited in reverse order, i.e., the second part is cited
(and numbered) first.

These comments are not intended to detract from the basic
value and genuine contribution of the book as a labor-saving
repository of a great deal of useful information. Given the rapid
rate of expansion of published material on the genus, it is to be
hoped that the compilers of the parent organization will plan to
update the listings from time to time (this should be another of the
virtues of the computer-generated approach). The community of
raptor enthusiasts are well served by these bibliographers; let’s
hope that the momentum will continue, and that similar works on
other taxa will be forthcoming. — Joseph R. Murphy.

The 1986 Annual Meeting of The Raptor Research Foundation, Inc. — the Raptor Research Foundation will hold its
annual meeting at the University of Florida, Gainsville, FL. 20-23 November 1986. MICHAEL W. COLLOPY, Dept
opf Wildlife and Range Sciences, 118 Newins-Ziegler Hall, Univ. of Florida, Gainesville, FL 32611, will chair the Local
Committee. KEITH L. BILDSTEIN, Dept, of Biology, Winthrop College, Rock Hill, SC 29733, will chair the Programs
Committee.

European Haliaeetus East-West Symposium 1986. The European Haliaeetus Symposium is organized as a forum for the
exchange of scientific findings in the field of raptor biology, as well as for an exchange of experience between experts
from East and West on the subject of nature conservancy and conservation of species. The symposium is scheduled to
take place 18-28 September 1986 in Heidelberg, Federal Republic of Germany. Plenary meetings will take place from
19-21 September, and excursions and tours are scheduled to take place 22 - 28 September, plus another series of
meetings dealing with specialized topics. Please address inquiries regarding the symposium to: Deutsche Greifenwarte
(German Raptor Center), Burg Guttenberg or Claus Fentzloff, D-6954 Burg Guttenberg/Neckar, Federal Republic of
Germany.

Summer 1986

News and Reviews

81

RESOLUTIONS

The following resolutions were adopted, by the Board of Directors and the General Membership of The Raptor Research Foundation,
Inc., at The 1 985 Raptor Research Foundation International Meeting and Symposium on the Management of Birds of Prey held 2 -
10 November 1985 in Sacramento, California.

Resolution #85-1. RE: The California Condor

WHEREAS, the wild population of California condors, currently only six birds, has been rapidly
approaching extinction, in spite of all efforts to counteract the man-induced mortality factors that are the
principal causes of the decline; and

WHEREAS, no effective means to reduce these mortality factors are yet in sight, and these mortality
factors appear to be associated with existing foraging traditions of the population; and

WHEREAS, there appears to be no hope for recovery of the species in the absence of a fully viable
captive population; and

WHEREAS, the fraction of the 2 1 currently-held captives that may become effective breeders is unknown;
and

WHEREAS, the genetic viability of the existing captive population has been seriously questioned by all
population geneticists that have been consulted by the recovery team; and

WHEREAS, the capture of the remaining wild birds might significantly increase the chances that a
viable captive population can be achieved; and

WHEREAS, effective methods have been developed for reestablishment of wild vulture populations
from captivity;

THEREFORE BE IT RESOLVED that (1) the remaining wild California condors be taken captive as
soon as possible; and (2) until a healthy self-sustaining captive population has been achieved, no release of
captives to the wild should be attempted, and (3) that releases of California condors to the wild be made as
soon as advisable, and that they be directed to historic regions that offer effective protection from
detrimental human influences.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-2. RE: Old Growth Management on Federal, State, and Private Lands

WHEREAS, many of the raptor species inhabiting the forests and woodlands of the United States,
including such sensitive species as the Goshawk, Great Grey Owl and Spotted Owl, are wholly or partially
dependent on mature and old growth trees and/or snags for nesting, roosting, cover and/or foraging; and

82

News and Reviews

Vol. 20, No. 2

WHEREAS, present silvicultural systems do not adequately address the maintenance and recruitment
of old growth tree groves in densities to sustain adequate raptor populations;

THEREFORE BE IT RESOLVED that The Raptor Research Foundation, Inc., does hereby take the
position that comprehensive old growth policies, in the spirit of multiple use-sustained yield principles, be
implemented which allow for natural fluctuation and movement of raptor populations by local, state and
federal agencies responsible for management of wildlife populations and their habitats.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-3. RE: Appreciation to RRF Founders

WHEREAS, the need for an organization focusing on communication, raptor management and conser-
vation was apparent in the mid-1960s because of widespread population declines and habitat loss; and

WHEREAS, in an effort to accomplish the above goals, Donald V. Hunter, Jr., Byron E. Harrell and
Paul F. Springer became the founding members, and George M. Jonkel, the first secretary of the Raptor
Research Foundation; and

WHEREAS, the Raptor Research Foundation (RRF) has since grown in stature, membership, and
influence as a result of those original efforts; and

WHEREAS, the efforts of RRF and its membership is believed to have significantly contributed to the
increased awareness, conservation, and proper management of birds of prey;

NOW, THEREFORE BE IT RESOLVED that The Raptor Research Foundation, Inc., formally, and
with great pleasure, recognizes the insight, commitment and leadership of our founding members,
Donald V. Hunter, Jr., Byron E. Harrell and Paul F. Springer and its first secretary, George M. Jonkel;

FURTHER, that a copy of this resolution, appropriately prepared, be presented to these key individu-
als.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Summer 1986

News and Reviews

83

Resolution #85-4. RE: Societal Priorities

WHEREAS, birds of prey, being at the top of the food chain, are extremely sensitive to environmental
degradation such as habitat loss and chemical pollutants; and

WHEREAS, these negative impacts are all directly or indirectly the result of human population
pressures; and

WHEREAS, conservation issues cannot be fully addressed until socioeconomic justice and political
stability are achieved;

THEREFORE BE IT RESOLVED that The Raptor Research Foundation, Inc., recognizes that human
population and increasing per capita resource use in the developed world are one of the greatest threats to
the survival of raptor populations and that The Raptor Research Foundation, Inc., encourages approp-
riate agencies to effect the transition from a growth and consumer-oriented society to a stable conserver
society.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-5. RE: Government Game and Nongame Programs

WHEREAS, non-game wildlife provides important recreational, aesthetic, ecological, existence,
economic, and genetic benefit to the people of the United States; and

WHEREAS, the U.S. Fish and Wildlife Service and other federal agencies are charged with the
responsibility of managing and protecting migratory birds; and

WHEREAS, recent surveys and studies clearly demonstrate that the public supports management of
non-game wildlife and prevention of extinction; and

WHEREAS, fish and wildlife service programs for non-endangered wildlife species currently are
predominantly game-oriented; and

WHEREAS, early detection of species declines is an important part of species protection and prevention
of extinction;

WHEREAS, current bird monitoring programs do not adequately represent trends for many bird
species, including raptorial birds;

NOW, THEREFORE BE IT RESOLVED that The Raptor Research Foundation, Inc., urges the U.S.
Fish and Wildlife Service to redirect its research and management efforts to create a reasonable balance
between game and non-game programs; and

84

News and Reviews

Vol. 20, No. 2

BE IT FURTHER RESOLVED that The Raptor Research Foundation, Inc., urges the U.S. Fish and
Wildlife Service and other appropriate agencies to adopt, as a principal objective of their non-game
programs, the prevention of non-game wildlife from becoming threatened or endangered and that the
non-game program include, but not be limited to, monitoring non-game bird populations that are not now
being effectively monitored.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-6. RE: Welfare of Raptors in Mexico

WHEREAS, Mexico is comprised of high ecological diversity and varied natural habitats; and

WHEREAS, there exists very little knowledge on natural resources in terms of inventories, manage-
ment, conservation, and possible impact of industries, for example, petroleum; and

WHEREAS, the high species diversity of raptors is under constant threat by habitat modification,
pollution, hunting pressure, and trade;

THEREFORE BE IT RESOLVED that The Raptor Research Foundation, Inc., respectfully suggests
that the government of Mexico pay special attention to the following points:

(1) urgently ratify and implement the convention on trade in endangered species;

(2) habitat conservation and management;

(3) stopping the illegal trade of wildlife, with particular emphasis on Peregrine Falcons and other
raptors, at the national and international levels;

(4) develop educational programs for wildlife conservation integrating the concept of planned de-
velopment;

(5) monitor the use, abuse and impact of pesticides on wildlife with particular emphasis on raptors;

(6) support the training and education of specialists in wildlife management either in the country or
abroad; and

(7) support long-term research projects oriented toward the integration of conservation strategies into
the developmental process of the country.

Summer 1986

News and Reviews

85

BE IT FURTHER RESOLVED that The Raptor Research Foundation, Inc,, willingly offers its assis-
tance to the Mexican government and expertise that would facilitate the accomplishment of these
objectives.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer,

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-7. RE: Illegal Trade of Peregrine Falcons in Mexico

WHEREAS, Falco peregrinus is a bird species in danger of extinction, included in Appendix I of the cites
convention, and protected by the convention on migratory birds signed by Canada, USA, and Mexico; and

WHEREAS, information from international organizations indicates that an illegal international trade
exists between Mexico and other countries of the world involving live and dead individuals of the species;
and

WHEREAS, some of those individuals have been identified as migratory birds from the northern
breeding grounds in transit through Mexico to their wintering areas to the south;

THEREFORE BE IT RESOLVED that The Raptor Research Foundation, Inc., supports the ongoing
trade investigations aimed at ending violations of relevant Mexican and international laws protecting Falco
peregrinus.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-8. RE: International Year for Birds of Prey

WHEREAS, birds of prey are found in virtually every country of the world, often migrating freely across
international borders; and

WHEREAS, birds of prey, being at the top of the food chain, constitute important barometers of
environmental change; and

86

News and Reviews

Vol. 20, No. 2

WHEREAS, there is an international recognition and appreciation for birds of prey; and

WHEREAS, there is international concern for the welfare of those birds of prey due to declines in some
populations because of habitat loss, widespread pollution, illegal trade, persecution and other human-re-
lated problems; and

WHEREAS, the ICBP World Working Group on Birds of Prey is hosting the next world conference on
birds of prey in Eilat, Israel in 1987; and

WHEREAS, The Raptor Research Foundation, Inc., consists of over 700 specialists on the biology,
conservation, and public awareness of birds of prey, from all over the world;

THEREFORE BE IT RESOLVED that The Raptor Research Foundation, Inc., urges the United
Nation’s UNEP-IUCN to proclaim 1988 as “The International Year for Birds of Prey.”

FURTHER, that copies of this resolution be sent to the executive director of the Office of Environmen-
tal Programs, UNEP, and all other appropriate international bodies.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this I9th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-9. RE: Falconer’s Contribution

WHEREAS, important raptor conservation techniques, including captive breeding and reintroduction
of raptors, have been pioneered and developed largely by falconers;

THEREFORE BE IT RESOLVED that the conservation community owes falconers a debt which seldom
has been recognized, and encourages the continued participation of falconers in raptor conservation
projects.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Summer 1986

News and Reviews

87

Resolution #85-10. RE: Grassland Raptors

WHEREAS, many birds of prey are vitally dependent upon grasslands for their food base, i.e., small
rodents; and

WHEREAS, the absence of this food base will reduce breeding effort or cause these species to not breed
at all; and

WHEREAS, some of these species of raptors are considered as endangered or threatened in many
states; and

WHEREAS, grasslands also are important to many species of ground nesting birds; and

WHEREAS, the conversion of grasslands to cropland has resulted in serious soil erosion problems; and

WHEREAS, the “sodbuster” and conservation reserve provisions will protect and restore grassland
habitat;

THEREFORE, BE IT RESOLVED that The Raptor Research Foundation, Inc., strongly urges Con-
gress to include the “sodbuster” and conservation reserve provisions in the 1985 farm bill.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-11. RE: Gray’s Harbor Bay

WHEREAS, the 500 acre section of Gray’s Harbor Bay known as Bowerman Basin has been documented
to be the largest concentration of migrant shorebirds on the Pacific coast south of the Copper River Delta,
Alaska; and

WHEREAS, Bowerman Basin is an intensively used feeding and resting area for 3 subspecies of
wintering and migrant Peregrine Falcons; and

WHEREAS, the port of Gray’s Harbor proposes to fill this estuarine wetland/mudflat for speculative
industrial development; and

WHEREAS, federal and state environmental protection acts/regulations and the respective federal and
state resource agencies have been unable to secure permanent protection of Bowerman Basin;

88

News and Reviews

Vol. 20, No. 2

THEREFORE, LET IT BE RESOLVED that The Raptor Research Foundation endorses and supports
proposals to establish the Bowerman Basin National Wildlife Refuge.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Resolution #85-12. RE: Appreciation to Local Committee

WHEREAS, the 1985 annual RRF meeting in Sacramento, California, was an unprecedented success,
involving approximately 900 individuals, representing 40 countries; and

WHEREAS, the success of this meeting was due primarily to the efforts of the local committee
co-chairmen, Butch Olendorff, Nancy Venizelos, Dave Harlow, and their committee members; and

THEREFORE, BE IT RESOLVED that the RRF gratefully expresses their appreciation for a job well,
done to those individuals; and

FURTHER, the RRF similarly expresses their appreciation to the BLM, San Francisco Zoological
Society, USFWS, and other agencies and organizations for allowing their employees the time to make this
annual professional meeting the success that it was.

THE RAPTOR RESEARCH FOUNDATION, INC.

Jeffrey L. Lincer

President, The Raptor Research Foundation, Inc.

WITNESS my hand and official seal, this 19th day of December, A.D. 1985

Joan Ann Carter

Notary Public, State of Florida

Summer 1986

News and Reviews

89

Hawk Mountain Sanctuary Association 10th Annual Award for Raptor Research - The Hawk Mountain Sanctuary
Association is accepting applications for its 10th annual award for raptor research. To apply for the $750 award,
students should submit a brief description of their research program (5 pages maximum), a curriculum vitae, and 2
letters of recommendation by 15 October 1986 to Stanley E. Senner, Executive Director, Hawk Mountain Sanctuary
Association, Route 2, Kempton, Pennsylvania 19529. The Association’s Board of Directors will make a final decision
early in 1987. Only students enrolled in a degree-granting institution are eligible. The award will be granted on the basis
of a project’s potention to improve understanding of raptor biology and its ultimate relevance to the conservation of
North American raptor populations.

Leslie Brown Memorial Grant (Deadline Extended) — The Raptor Research Foundation, Inc., announces the
availability of a $500 grant to provide financial assitance to promote the research, or the dissemination of information,
on birds of prey, applicants must send a resume (vitae), specific study objectives, an account of how funds will be spent,
and a statement indicating how the proposed work would relate to other work by the applicnat and to other sources of
funds. Proposals concerning African raptors will receive highest priority between proposals of otherwise equal merit.

Application material must be received by 7 November 1986. Proposals, donations and inquiries about tax-exempt
contributions to the fund should be sent to: Dr. Jeff Lincer, Chairman, RRF Leslie Brown Memorial Fund, 4718 Dunn
Drive, Sarasota, FL 33583, U.S.A.

THESIS ABSTRACTS

The Biogeochemistry of Peregrine Falcon Feathers

Secondary remiges were collected from 92 Peregrine Falcon (Falco peregrines) nestlings in the summer of 1979 and
1 980 at 1 West Greenland and 4 North American breeding localities. This study’s purpose has been to determine if natal
origin of peregrines could be identified using feather trace element quantities obtained by instrumental neutron
activation analysis (INAA). Gamma-ray spectra analysis of irradiated feather samples identified 14 trace element
quantities present in the distal most feather portion. Multivariate discriminant function analysis of spectral data
provided a means of separating Peregrine Falcons from 5 breeding localities into groups of similar natal origin.

Twenty-two additional feather samples were collected from migrant peregrines at South Padre Island, Texas and
Mobile Point, Alabama during autuminal migration periods. Trace element analysis of migrant peregrine feather
samples and comparison with nestling peregrine feather sample analyses provided a method potentially useful in
predicting natal origin of North American migrant peregrines with substantial accuracy. Variation in prediction
capability results when groupings of selected traced element quantities are utilized in formulating predictive mul-
tivariate discriminant functions.

Trace element analysis of feathers using INAA provided an effective means of identifying levels of environmentally
harmful elements such as Mercury which could adversely affect population densities in remote regions of the
peregrine’s range. As peregrine wintering locality studies by others expand in Latin America, similar “ground truth”
data from feathers grown by peregrines in these regions will enhance isolating discrete wintering localities of remote
northern breeding populations, information vital to overall management of this endangered species. Parrish, J.R., M.S.
Thesis, Univ. of Alabama, Tuscaloosa. Thesis directed by D.T. Rogers, Jr., 1983.

Some Aspects of the Reproductive Biology of Tundra Peregrine Falcons

A dense, productive population of tundra peregrine falcons (Falco peregrinus tundrius ) was studied over five years on a
450 km 2 study area located along the northwest coast of Hudson Bay. The number of territorial pairs varied over the
years from 17 to 26. A mean internest distance of 3.3 km represents a nesting density for the population that is among
the highest on record for the species. Morphometric and plumage characteristics of adults in the study population

90

Thesis Abstracts

Vol. 20, No. 2

suggest clinal variation between populations of F.p.tundrius from north and F.p. anatum to the south. Pairs of falcons
nested on cliffs of less than 30 m in height. Most nest cliffs had a southern exposure, but pairs nesting at north-facing
sites did not suffer lower reproductive success. Falcons arrived on the breeding grounds before most migratory prey
species (mid-May) and left the area before October. Mean date of clutch initiation over three years was June 7, but this
date was significantly delayed in one year of poor spring weather (relatively lower temperatures and high precipitation).

Nineteen species of birds and three species of mammals were used as prey by the study population; however, in most
years the bulk of the diet consisted of six species of birds and one mammal. A dramatic increase in the number of
territorial pairs of falcons, coupled with changes in various measures of reproductive performance, in a year of high
microtine rodent abundance suggests that the use of mammals by falcons can be siginficant in some years.

In most years, successful pairs produced about 35 young, or about two young per occupied territory per year. The sex
ratio of nestlings was not significantly different from equality. Despite changes in reproductive performance noted in a
year of high prey abundance, there were no significant differences among years in clutch size, brood size, or number of
young fledged per territorial pair.

Mean clutch size (3.62± 0.59) was comparable to that for peregrine populations from both arctic and more temperate
latitudes. Mean brood size (2.79± 0.78) was most similar to that reported for populations in the central Canadian arctic
and Greenland, but was also similar to, and even greater than, that from other populations at more temperate latitudes.
Earlier suggestions by some authors that peregrines lay fewer eggs in the arctic part of their range was not supported by
data from the study population.

Twenty-nine separate nesting territories were used in the five years of the study. Territories were held by adult
peregrines in all cases. The oldest female on territory was at least five years old, the youngest was three. The oldest male
was at least five years old, the youngest was two. Annual turnover of adults was estimated to be 25% (males 16%, females
30%). Maximum annual mortality was estimated to be 23% (males 16%, females 27%). Peregrines showed a high degree
of fidelity to previously used nest sites; movement between territories was recorded only once.

Peregrines competed intensively for nesting territories early in the spring, but competition dropped off as breeding
commenced. For the most part, territorial interactions were sex specific. Males were more territorial than females in
terms of interaction rate and in the distance to which they would pursue intruders. Observation of territorial
interaction, and evidence of the existence of non-breeding, non-territorial adults, suggested that the breeding density
of the falcon population was limited by a shortage of nesting habitat created through territoriality. Apparent change in
territory size in a year of high prey abundance supports the hypothesis that expressions of territoriality and territory size
in birds are often mediated by food supply.

Total and partial nesting failures were recorded in the study population. Ten percent of all territorial pairs failed to
lay. Failure of territorial pairs to lay accounted for 41% of all failures. Losses of entire clutches accounted for 48%
failures, while losses of complete broods made up only 1 1%'of failures. Seventy-four percent of all territorial pairs were
successful in fledging young. Peregrines managed a full hatch with 44% of all clutches. At successful nests, hatching
success (% of all eggs to hatch) was 87% and nestling mortality (% of all young hatched that do not fledge) was 14%.

Female peregrines on nesting territories for the first time appeared to lay later, had significantly smaller clutches,
but did not show a difference in the number of young fledged when compared to females with greater experience on
nesting territories. Falcon pairs nesting in close proximity or in areas of higher intraspecific nesting density did not show
lower clutch size, hatch success, or number of young fledged, suggesting that the effect of intraspecific strife on
reproductive success was negligible.

Asynchronous hatching was commonly observed in the study population. An investigation of the effects of hatch
asynchrony on reproductive success, conducted over two years, showed that 7% of all chicks produced died as a result of
brood reduction associated with asynchronous hatching. Most chicks that died were the last-hatched in asynchronous
broods of four; about half of the last-hatched members of these broods died within their first five days of life. Mortality
occurred through starvation. Surviving last-hatched members of asynchronous broods of four did not show signifi-
cantly different growth rates compared to other chicks from different hatch positions. Mortality did not appear related
to the food available to the parents, but to the inability of last-hatched young to compete with older siblings during
feedings. Differential survival of last-hatched chicks in asynchronous broods was linked to differences in the attentive-
ness of adult females (feeding rate and time spent brooding), prey size, and possibly, nest ledge size. Asynchrony of
hatching and brood reduction observed in the study population did not suggest an adaptation in keeping with the brood
reduction hypothesis.

Court, Gordon S. 1986. Thesis. Department of Zoology, University of Alberta, Alberta T8A, IV7, CANADA.
Present address: 913 Alder Avenue, Sherwood Park, Alberta, CANADA T8A 1 V7.

RAPTOR RESEARCH

A Quarterly Publication of The Raptor Research Foundation, Inc.

EDITOR: Clayton M. White, Department of Zoology, 161 Widtsoe Building, Brigham Young University, Provo,
Utah 84602

ASSISTANT EDITOR: Jimmie R. Parrish, Department of Zoology, 159 Widtsoe Building, Brigham Young Univer-
sity, Provo, Utah 84602
ASSOCIATE EDITORS

Jeffrey L. Linger - Environmental Chemistry and Toxicology
Richard Clark - Order Strigiformes
Ed Henckel - Family Cathartidae
Gary E. Duke - Anatomy and Physiology

Patrick T. Redig - Pathology, Rehabilitation and Reintroduction
Jim Mosher - General Ecology and Habitat Analysis

INTERNATIONAL CORRESPONDENT: Richard Clark, York College of Pennsylvania, Country Club Road,
York, Pennsylvania 17405

Raptor Research (ISSN 0099-9059) welcomes original manuscripts dealing with all aspects of general ecology, natural
history, management and conservation of diurnal and nocturnal predatory birds. Send all manuscripts for considera-
tion and books for review to the Editor. Contributions are welcomed from throughout the world, but must be written in
English.

INSTRUCTIONS FOR CONTRIBUTORS: Submit a typewritten original and two copies of text, tables, figures and
other pertinent material to the Editor. Two original copies of photographic illustrations are required. Raptor Research is
published in a double-column format and authors should design tables and figures accordingly. All submissions must
be typewritten double-spaced on one side of 8 x 1 1-inch (21 Vi x 28 cm) good quality, bond paper. Number pages
through the Literature Cited section. The cover page should contain the full title and a shortened version of the title (not
to exceed 30 characters in length) to be used as a running head. Author addresses are listed at the end of the Literature
Cited section. Authors should indicate if present addresses arc different from addresses at the time the research was
conducted. When more than one author is listed, please indicate who should be contacted for necessary corrections and
proof review. Provide an abstract for each manuscript more than 4 double-spaced typewritten pages in length. Abstracts
are submitted as a separate section from the main body of the manuscript and should not exceed 5% of the length of the
manuscript. Acknowledgments, when appropriate, should immediately follow the text and precede the Literature
Cited. Both scientific and common names of all organisms are always given where first appearing in the text and should
conform to the current checklists, or equivalent references, such as the A.O.U. Checklist of North American Birds (6th
ed., 1983). Authors should ensure that all text citations are listed and checked for accuracy. If five or fewer citations
appear in the text, place the complete citation in the text, following these examples: (Brown and Amadon, Eagles,
Hawks and Falcons of the World. McGraw-Hill, New York. 1968), or Nelson (Raptor Res. 16(4):99, 1982). If more than
five citations are referenced, each should include author and year (e.g., Galushin 1981), or in a citation with three or
more authors, the first author and year (e.g., (Bruce et al. 1982)). Citations of two or more works on the same topic
should appear in the text in chronological order (e.g., (Jones 1977, Johnson 1979 and Wilson 1980)). Unpublished
material cited in the text as “pers. comm.,” etc., should give the full name of the authority, but must not be listed in the
Literature Cited section. If in doubt as to the correct form for a particular citation, it should be spelled out for the Editor
to abbreviate.

Metric units should be used in all measurements. Abbreviations should conform with the Council of Biology Editors
(CBE) Style Manual, 4th ed. Use the 24-hour clock (e.g., 0830 and 2030) and “continental” dating (e.g., 1 January 1984).

Tables should not duplicate material in either the text or illustrations. Tables are typewritten, double-spaced
throughout, including title and column headings, should be separate from the text and be assigned consecutive Arabic
numerals. Each table must contain a short, complete heading. Footnotes to tables should be concise and typed in
lower-case letters. Illustrations (including coordinate labels) should be on 8 ] /4 x 1 1 -inch (21 V 2 x 28 cm) paper and must
be submitted flat. Copies accompanying the original should be good quality reproductions. The name of the author(s)
and figure number should be penciled on the back of each illustration. All illustrations are numbered consecutively
using Arabic numerals. Include all illustration legends together, typewritten double-spaced, on a single page whenever
possible. Line illustrations (i.e., maps, graphs, drawings) should be accomplished using undiluted india ink and
designed for reduction by 1/3 to 1/2. Drawings should be accomplished using heavy weight, smooth finish, drafting
paper whenever possible. Use mechanical lettering devices, pressure transfer letters, or calligraphy. Typewritten or
computer (dot matrix) lettering is not acceptable for illustrations. Use of photographic illustrations is possible but
requires that prior arrangements be made with the Editor and the Treasurer.

A more detailed set of instructions for contributors appeared in Raptor Research, Vol. 18, No. 1, Spring 1984, and is
available from the Editor.

NON-PROFIT ORG.

THE RAPTOR RESEARCH FOUNDATION, INC.
TREASURER’S OFFICE
12805 St. Croix Trail
Hastings, MN 55033

U.S. POSTAGE

PAID

PERMIT #66
PROVO, UTAH

Return Postage Guaranteed