(1S£N OB92-1016) 


The 

Journal 

OF 

Raptor Research 


Volume 26 


December 1992 


Number 4 


Contents 


Energy Requirements of Adult Cape Vultures {Gyps coprotheres). 


Joris Komen 213 

Organochlorines and Mercury in Osprey Eggs from the Eastern 
United States. Daniel J. Audet, David S. Scott and Stanley N, Wiemeyer 219 

Kleptoparasitism and Cannibalism in a Colony of Lesser Kestrels 

{FaLCO NAUMANNI). Juan Jose Negro, Jose Antonio Donazar and Fernando Hiraldo 225 

Home Range and Activity of a Pair of Bald Eagles Breeding in 


Northern Saskatchewan. Jon M. Gerrard, Alan R. Harmata and P. Naomi Gerrard 229 


Seasonal and Sexual Variation in the Diet of the Common Buzzard 
IN Northeastern Spain. Santi Manosa and Pedro J. Cordero 235 

Diet Changes in Breeding Tawny Owls {Strix aluco). David a. Kirk 239 

Foraging Ecology of Bald Eagles on a Regulated River, w. Grainger 
Hunt, J. Mark Jenkins, Ronald E. Jackman, Carl G. Thelander and Arnold T. Gerstell 243 

Short Communications 

Increased Parental Care in a Widowed Male Marsh Harrier {Cirus aeruginosus). 

Carmelo Fernandez and Paz Azkona 257 

Bats as Prey of Stygian Owls in Southeastern Brazil. Jose C. Motta Junior and Valdir 

A. Taddei 259 

Food-stressed Great Horned Owl Kills Adult Goshawk: Exceptional Observation or 

Community Process? Christoph Rohner and Frank I. Doyle 261 

Nesting Association Between the Woodpigeon (Columba palumbus) and the Hobby 

(Falco subbuteo). Giuseppe Bogliani, Eugenio Tiso and Francesco Barbieri 263 

Letters 266 

Thesis Abstract 270 

News and Reviews 271 


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Copyright 1992 by The Raptor Research Foundation, Inc. Printed in U.S.A. 


THIS PUBLICATION IS PRINTED ON ACID-FREE PAPER. 


THE JOURNAL OF RAPTOR RESEARCH 

A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC. 
VoL. 26 December 1992 No. 4 


/ Raptor Res. 26(4):213-218 
© 1992 The Raptor Research Foundation, Inc. 

ENERGY REQUIREMENTS OF ADULT GAPE 
VULTURES {Gyps coprotheres) 

JORIS Komen 

The State Museum of Namibia, P.O. Box 1203, Windhoek, Namibia 

Abstract. — Outdoor feeding trials were used to determine gross energy intake, energy assimilation efficiency 
and metabolizable energy intake of captive adult Cape Vultures (Gyps coprotheres). The mean ash-free dry 
energy density of daily pooled samples of feces and urine was 14.0 ± 0.2 kj/g. A consistently high mean 
energy assimilation efficiency of 86.2 ± 2.7% caused daily energy content of excreta to fluctuate according to 
the quantity of energy assimilated. Mean gross energy intake was 2926.8 ± 349.1 kj/day and mean metab- 
olizable energy intake was 2552.9 ± 300.9 kJ/day for birds with changes in body mass of 2% or less between 
start and end of feeding trials. The daily energy expenditure of a free-living adult, weighing 8.3 kg, was 
estimated to be 3006 kJ/day (DEE = 826.7 kJ/day x kg“^’). 


Requerimientos de energia del buitre de la esjjecie Gyps coprotheres 

Extracto. — Experimentos de alimentacion al aire libre, con buitres de la especie Gyps coprotheres en cautividad, 
fueron realizados para determinair: la cantidad total de energia ingerida en el alimento; la eficiencia en la 
asimilacion de esa energia; y la cantidad de energia ingerida y disponible para el metabolismo. La media de 
la energia, libre de carbon de todas las muestras diarias de la combinacion de heces y orina, fue de 14.0 ± 
0.2 kJ/g. La media de la eficiencia de asimilacion de energia, que fue consistentemente edta: 86.2 ± 2.7%, 
causo que el contenido de energia excretada por dia fluctuara de acuerdo con la cantidad de energia asimilada. 
La media de la energia total ingerida fue 2926.8 ± 349.1 kj/dia; y la media de la energia ingerida y disponible 
para el metabolismo fue 2552.9 ± 300.9 kJ/dia, para aves con cambios en la masa corporal de 2% 6 menos, 
entre el comienzo y el fin del experimento. El gasto diario de energia de un adulto que vive en libertad, y que 


pesa 8.3 kg, se estimo en 3006 kJ/dia (GDE = 826.7 

Vultures include some of the heaviest raptorial birds, 
yet constitute a poorly studied group in bioenergetics 
research (Komen 1991). Intrinsically difficult to study 
in the field, bioenergetic research of vultures has only 
been done under captive conditions, and has mostly 
been limited to measurements of gross food intake for 
a few vulture species (Jarvis et al. 1974, Hiraldo 1976, 
1983, Houston 1976, Mendelssohn and Leshem 1983, 
Komen 1991). 

Like their congeners elsewhere, Cape Vultures {Gyps 
coprotheres) are scavengers of ungulate carcasses, and, 
in rural areas which no longer support wild ungulate 
herbs, scavenge domestic livestock (Mundy 1982, Rob- 
ertson and Boshoff 1986). Foraging success of adult 
Cape Vulures is dictated by unpredictable food re- 
sources and climatic conditions (Boshoff et al. 1984, 


kJ/dia X kg° ^'). 

[Traduccion de Eudoxio Paredes-Ruiz] 

Robertson and Boshoff 1986), and, while rearing nest- 
lings, vultures may only forage once every two days 
(Komen 1991). If we assume that the maximum food 
intake of Cape Vultures is dictated by stomach and 
crop capacity (Komen 1986), measurements of daily 
metabolizable energy intake for existence, and esti- 
mates of daily energy expenditure in this study, provide 
an indication of energetic constraints on these large 
raptorial birds while breeding. 

Methods 

Ten adult Cape Vultures were maintained in captivity at 
the De Wildt Raptor Research Centre (25°4TS 27°56'E), 
Transvaal, South Africa. For the purpose of this study an 
“adult” vulture is defined as a full-grown vulture which is 
one year old or older, and falls within the normal range of 
adult body mass and standard wing- length (Mundy 1982, 
Komen 1986). 


213 


214 


JORIS Komen 


VoL. 26, No. 4 


Table 1. Water, lipid, protein, ash and carbohydrate 
content, and energy density in kj/g (Ash-free dry (AFD) 
and wet) of different meat types consumed by adult Cape 
Vultures during three feeding trial periods. 


Trial 
Period . 

AND 

Food- 

Type 


Percent 


Energy 

Density 

kJ/g kJ/g 
AFD Wet 

Wa- 

ter 

Lip- 

id 

Pro- 

tein 

Ash 

Car- . 

BOHY- 

DRATE 

1 (Horse) 

71.2 

1.7 

22.0 

3.4 

1.8 

25.1 

6.4 

2 (Cow 1) 

72.5 

2.4 

21.7 

2.8 

0.6 

27.0 

6.7 

3 (Cow 2) 

69.7 

1.9 

19.2 

8.7 

0.5 

26.0 

5.6 

Mean 

71.1 

2.0 

21.0 

5.0 

1.0 

26.0 

6.2 

±SE 

1.4 

0.4 

1.5 

3.3 

0.7 

1.0 

0.6 

CVi 

2.0 

20.0 

7.1 

66.0 

70.0 

3.9 

9.7 


^ CV = Coefficient of variation. 


Of the ten vultures, three, five, amd seven vultures were 
used in three feeding trial sessions, for a total of 15 feeding 
trials, with some of the vultures used in more than one trial. 
Trial vultures were removed from their flight aviary, weighed 
and placed in trial-cages on the same day at the start of the 
acclimation period of a particular feeding trial session. During 
these outdoor feeding trials, the vultures were maintained 
separately in visually-isolated cages (2 x 1.5 x 1 m), large 
enough to allow the birds to turn around and extend their 
wings fully. Each cage had a wire-mesh floor under which 
a removeable plastic-lined tray was placed to facilitate the 
collection of excreta. During the feeding trials, monthly min- 
imum temperatures ranged between 3-8"C during June 
through September (trial sessions 1 and 2) and \A-\1°C, 
during October and November (trial session 3). Maximum 
monthly temperatures were 21-25°G during July through 
September and 26-33°C during October and November. Am- 
bient temperature did not appear to have an effect on food 
intake (see results). 

Energy requirements for maintenance (gross energy intaike 
and metabolizable energy) were determined by the food con- 
sumption method (Gessaman 1973). Each vulture was al- 
lowed to acclimatize for a few days proceeding the trial. 
During this acclimation period the birds were fed to satiation 
and provided free drinking water. Depending on how quickly 
each individual settled down behaviorally (e.g., cessation of 
restlessness and acceptance of hand-fed meat), a vulture’s 
pre-feeding starvation period would be initiated. This period 
lasted between 2-4 d to ensure a post-absorptive state. Food 
was first offered on the morning when the most recently 
voided excreta no longer had a visible black fecal fraction; 
this suggested that all meat last consumed (from two to four 
days previously) had been assimilated and excreted. At this 
stage of starvation the white urinary fraction had a green 
tinge in most individuals. 

On the first day of feeding birds mostly ate to satiation (in 
excess of 1.0 kg meat). Thereafter food intake decreased to 
almost negligible amounts after 4-5 d of feeding (pers. ob- 
servations). Accordingly, the number of days on which food 


was offered was dictated by individual demand; a feeding 
trial was ended when a bird no longer demanded food. 

To measure existence metabolism which requires the 
maintenance of “constant” body mass (i.e., 2% or less change 
in body mass between the start and end of a trial), feeding 
trials included post-feeding starvation periods lasting as long 
as 5 d, depending on the body mass of a bird on the morning 
after the day of last feeding. The 15 trials (including pre- 
and post-feeding starvation periods) therefore lasted between 
12 and 18 d, with periods of actual feeding ranging from 6- 
11 d. 

Each bird was weighed at least four times during a feeding 
trial: prior to the pre-feeding starvation period, prior to the 
feeding period, after the feeding period, and at the end of the 
post-feeding starvation period. Water was not offered at any 
stage of the feeding trial. The vultures were fed lean cow or 
horse meat obtained from livestock carcasses. Their diet was 
supplemented with a mineral-vitamin mixture (Beefee, Cen- 
taur Co., Johannesburg, South Africa). Three different types 
of meat were used in the trials. Meat was deep frozen m 
labeled plastic bags, and on the evening prior to the next 
morning’s feeding bags were removed and defrosted. Care 
was taken to give all trial birds the same type of meat on a 
given day of feeding. The same three types of meat were 
available for all three feeding trial sessions. On feeding days, 
each bird was fed once by hand to satiation, and food intake 
was determined by weighing meat before feeding and left- 
overs immediately after feeding. Spilled food and regurgita- 
tions were collected whenever these occurred. Regurgitations 
were oven-dried at 60°C to constant mass. The dry mass of 
a regurgitation was subtracted from the calculated dry mass 
of food consumed on the same day the regurgitation occurred. 

Five samples (100 g each) of each meat type (Table 1) 
used for a feeding trial were taken for analyses. Each Scimple 
was oven-dried at 60°C to constant mass. Water loss was 
calculated by subtraction, and mean water content calculated 
for the five samples. The dried samples were pooled, ground 
to a powder and analysed for energy density (Gallenkamp 
ballistic bomb calorimeter) and inorganic content by ashing 
(see Komen 1986, 1991 for details). Metabolizable energy 
and assimilation efficiency of birds in any one trial period 
were calculated using the mean composition data for all meat 
types consumed during that trial. The mean “wet” energy 
density of all meat types consumed was 6.2 kj /g (SE ±06 
kJ/g, range 5.6-6.7 kJ/g; Table 1). 

All feces and urine (hereafter referred to as excreta) were 
collected from each bird twice daily, for each day after the 
initiation of feeding until the end of post-feeding starvation 
period (i.e., when a black fecal fraction was no longer present 
in excreta) and pooled. These daily quantities of excreta from 
each bird were oven-dried at 60®C to constant mass and 
weighed separately. They were then analysed for energy 
density and inorganic content (see Komen 1986 for details). 
To determine assimilation efficiency the daily excreta weights 
were pooled for each trial. Twenty-three day-seunples were 
randomly selected from all individual day-samples (N = 1 86) 
collected during the three feeding trial sessions and analysed 
for energy density (kJ/g ash-free dry mass) and ash content. 

The efficiency with which birds assimilate energy was 
determined following Gessaman (1973): 

ME = GEI - (F ± U) 


December 1992 


Vulture Energy Requirements 


215 


Table 2. Changes in body mass during 15 feeding trials of 10 Cape Vultures (A-J), showing percentage change 
between the pre-feeding and post-trial body masses. 


Body Mass (g) 


Period 

AND 

Bird 

Duration of 
Trial* in 
Days 

Pre-Trial 

Pre- 

Feeding 

(A) 

Post- 

Feeding 

Post-Trial 

(B) 

Mean 

±SE 

% Change 
(B - A)/ 
A. 100 

1 A 

15 (12) 

9150.0 

8800.0 

9220.0 

8840.0 

9002.5 

184.7 

0.5 

B 

17 (14) 

9580.0 

9200.0 

10 080.0 

9445.0 

9576.3 

321.2 

2.7 

C 

14(11) 

9600.0 

9250.0 

9455.0 

9200.0 

9376.3 

160.7 

0.5 

2D 

17 (14) 

7100.0 

6720.0 

7165.0 

6785.0 

6942.5 

192.8 

1.0 

E 

17 (14) 

7620.0 

7220.0 

7615.0 

7225.0 

7420.0 

197.5 

0.1 

F 

12 (9) 

8050.0 

7720.0 

7715.0 

7715.0 

7800.0 

144.4 

-0.1 

G 

13 (10) 

8150.0 

7770.0 

7465.0 

7465.0 

7712.5 

281.6 

-3.9 

B 

18 (15) 

9070.0 

8670.0 

10 665.0 

9630.0 

9508.8 

749.6 

11.1 

3 D 

15 (12) 

7550.0 

7170.0 

7675.0 

7140.0 

7383.8 

233.2 

-0.4 

E 

17 (13) 

7740.0 

7370.0 

8275.0 

7640.0 

7756.3 

328.7 

3.7 

F 

17 (13) 

7445.0 

7070.0 

7975.0 

7350.0 

7460.0 

327.7 

4.0 

G 

17 (13) 

6650.0 

6270.0 

7225.0 

6585.0 

6682.5 

344.6 

5.0 

H 

17 (13) 

7575.0 

7170.0 

7975.0 

7300.0 

7505.0 

308.2 

1.8 

I 

14 (11) 

6860.0 

6470.0 

7175.0 

6550.0 

6763.8 

278.6 

1.2 

J 

15 (12) 

6825.0 

6470.0 

7275.0 

6600.0 

6792.5 

306.2 

2.0 

Mean 


7931.0 

7556.0 

8197.0 

7698.0 

7845.5 


1.9 

±SE 


955.4 

960.1 

1084.9 

1026.7 

985.8 


3.2 


* First numeral represents duration of each trial, numeral in brackets represents number of days between pre-feeding and post-trial 
weighings. 


where ME = metabolizable energy, GEI = gross energy 
content of food consumed, F = energy content of feces and 
U = energy content of urine; and assimilation efficiency (AE): 

AE = 100 X {GEI ± (F + U)}/GEI[%] 

Results 

The mean change in individual adults’ pre-feeding 
and post-trial body masses was 1.9% (SE ± 3.2%, 
range 0.1-11.1%, N = 15 trials) and the majority of 
birds gained body mass during the trials (Table 2). 
During 10 of the 15 trials, the vultures had body mass 
changes of about 3% or less, and of the remaining 
trials, only one bird had a marked decrease (—3.9%) 
in body mass, the rest {N = 4) had body mass increases 
ranging between 3.7% and 11.1%. Mean pre-feeding 
body mass of all birds was 7556.0 g (SE ± 960.1 g, 
range 6270-9250 g) and mean post-trial body mass 
was 7698.0 g (SE ± 1026.7 g, range 6550-9630 g). 

There was eonsiderable variation in individual gross 
daily food intake (Table 3). Mean daily gross food 
intake was 479.2 g meat/day (SE ± 52.9 g meat/day, 
range 372.6-558.7 g meat/day), and represents 6.5% 
of body mass (SE ± 1.3%, range 4.0-8.4%; Table 3). 


Gross energy content of the daily excreta of indi- 
vidual birds changed proportionally to the energy eon- 
tent of food consumed. The mean ash-free dry (AFD) 
energy density of daily excreta was 14.0 kj/g AFD 
(SE ± 0.2 kJ/g AFD, V = 23 individual daily samples 
analysed, range 13.9-14.1 kJ/g AFD). The mean in- 
organic content of these excreta samples was 7.1 7o (SE 
± 0.2%, N = 23, range 6.9-7.2%). 

There was no significant diflference between mean 
gross energy intake of each trial session (ANOVA, F 
= 3.29, df = 2, 12, P > 0.05), and the results for all 
three trial sessions were eombined. Mean daily gross 
energy intake for all birds was 2926.8 kj/day (SE ± 
349.1 kJ/day, A/^= 15 trials, range 2347.5-3743.3 kj/ 
day). Gross energy assimilation efficieney was consis- 
tently high at 86.2% (SE ± 2.7%, range 83.2-88.2%) 
and mean daily metabolizable energy was 2522.9 kJ/ 
day (SE ± 300.9 kJ/day, range 2023.6-3226.7 kJ/ 
day, V = 1 5 trials). 

Existence metabolism, which is equivalent to me- 
tabolisable energy providing the birds undergo body 
mass ehanges of about 2% or less between the start 
and end of a trial {sensu Gessaman 1973), was 2420.3 
kJ/day (SE ± 93.2, V = 9 trials). 


216 


JORIS Komen 


VoL. 26, No. 4 


Table 3. Gross food intake (g meat/day and % of pre- 
feeding body mass (BM)), and daily energy requirements 
(kj/day, gross energy intake (GEI) and metabolizable 
energy (ME)) of ten captive adult Cape Vultures (A-J) 
calculated for the number of days between pre-feeding and 
post-trial weighings (see Table 2) during feeding trials. 


Trial 

Period 

AND 

Bird 

Total 

Meat 

Con- 

sumed 

S 

Meat/ 

Day 

g/day 

% or 
BM % 

Daily 

GEI 

kJ/day 

Daily 

ME^ 

kJ/day 

1 A 

5005.0 

417.1 

4.7 

2669.4 

2301.0 

B 

6735.0 

481.1 

5.2 

3079.0 

2654.1 

G 

4099.0 

372.6 

4.0 

2384.6 

2055.5 

2 D 

6965.0 

497.5 

7.4 

3333.3 

2873.3 

E 

5904.0 

421.7 

5.8 

2825.4 

2435.5 

F 

4145.0 

460.6 

6.0 

3086.0 

2660.1 

G 

4531.0 

453.1 

5.8 

3035.8 

2616.9 

B 

8380.0 

558.7 

6.4 

3743.3 

3226.7 

3 D 

5020.0 

419.2 

5.8 

2347.5 

2023.6 

E 

6615.0 

508.8 

6.9 

2849.3 

2456.1 

F 

7145.0 

549.6 

7,8 

3077.8 

2653.1 

G 

6610.0 

508.5 

8.1 

2847.6 

2454.6 

H 

6220.0 

478.5 

6.7 

2679.6 

2309.8 

I 

5960.0 

541.8 

8.4 

3034.1 

2615.4 

J 

6235.0 

519.6 

8.0 

2909.8 

2508.3 

Mean 

5971.3 

479.2 

6.5 

2926.8 

2522.9 

±SE 

1168.0 

52.9 

1.3 

349.1 

300.9 


' ME calculated using mean energy assimilation efficiency of S6.2%. 


Discussion 

Gape Vultures are relatively inactive raptors; soar- 
ing flight presumably allows them to forage in an 
energetically inexpensive fashion (Pennycuick 1972), 
and they spend a major part of each day roosting 
(Mundy 1982, Boshoff et al. 1984, Robertson and 
Boshoff 1986). Taking advantage of prevailing climatic 
conditions, wind and thermals, Cape Vultures in sum- 
mer rainfall areas of southern Africa generally leave 
their colonial roosts to forage from early to mid-morn- 
ing and return in the afternoon, generally precluding 
more than one foraging trip per day (Brown 1988, J. 
Komen unpubl.). While rearing young, each parent 
may therefore only forage once every 2 d (Komen 
1991). Cape Vultures have been reported to forage as 
little as once every 3 d (Robertson and Boshoff 1986) 
and during the post-fledging dependency, young birds 
may go without food for much longer periods (up to 
16 d; Robertson 1985). 

The results of adult Cape Vulture feeding trials in 


captivity provide data on gross energy intake and me- 
tabolizable energy which probably represent minimum 
requirements for existence, taking into account the rel- 
ative inactivity of both free-living vultures and captive 
birds. Starved Cape Vultures can consume 1.5 kg meat 
in one feeding (Komen 1991). This equivalent to a 
gross energy intake of 9300 kj, which, with a high 
assimilation efficiency (86.2%), represents a maximum 
metabolizable energy intake of 8017 kJ. Kirkwood 
(1980) predicted that the mean maximum daily me- 
tabolizable energy intake by any animal is 1713 kJ/ 
day X kg° ’^ (SE of slope ± 0.008). Using Kirkwood’s 
(1980) equation, the predicted maximum daily me- 
tabolizable energy intake for Gape Vultures ranges 
between 6409 and 8499 kJ/day (using the lowest and 
highest post- starvation body masses in this study; Table 
2). Starved, low weight, adult Cape Vultures therefore 
appear to exceed the theoretical maximum for daily 
metabolizable energy intake, by being able to consume 
a single large quantity of food which provides metab- 
olizable energy for more than one day of existence. 
This suggests a strategy to counter unpredictable food 
resource and extended periods of inclement weather. 

Daily energy expenditure is estimated to be about 
1 .2 times existence metabolism in small, non-breeding 
(and thus relatively inactive), diurnal raptors (Sapsford 
and Mendelsohn 1984). Since metabolism does not 
scale in direct proportion to body mass, birds with 
large bodies would have relatively lower metabolic rates 
than birds with small bodies (Lasiewski and Dawson 
1967, Walsberg 1980). Accordingly, daily energy ex- 
penditure of free-living Cape Vultures is probably not 
greatly elevated above existence metabolism, especially 
since they are inactive for a major part of each day, 
with little seasonal variability in this behavior. 

The captive adults in this study were generally less 
massive than free-living adults, and assuming that en- 
ergy requirements for existence are scaled to body mass 
regardless of differences in body constituent propor- 
tions, the energy requirements of free-living adults will 
be proportionally higher than that of captive adults. If 
existence metabolism and daily energy expenditure are 
scaled on body mass according to an exponent of 0.61 
(Walsberg 1980), I estimate that daily existence me- 
tabolism of free-living adults is 2505 kJ/day (688.9 
kJ/day x kg*’ ^^) for body mass equal to 8300 g (mean 
body mass 8298.3, SE ± 477.8 g, 11 wild adults 
weighed; Komen 1986), and daily energy expenditure 
(1.2 times existence metabolism) is 3006 kJ/day or 
826.7 kJ/day x kg°^^ This relatively low predicted 
value for daily energy expenditure is supported by 


December 1992 


Vulture Energy Requirements 


217 


evidence from field and captive studies on diurnal and 
nocturnal raptorial birds (Koplin et al. 1980, Sapsford 
and Mendelsohn 1984, Wijnandt 1984, see also Wals- 
berg 1980). 

A single maximal feeding (9300 kj) should theo- 
retically provide enough energy to maintain an adult 
Cape Vulture for about 3 d, without expending body 
fat reserves. Fat content of free ranging adult Gape 
Vultures ranges between 9.5-1 5.77o of body mass, ac- 
counting for a fat depot of as much as 1 346 g (Komen 
1991). Assuming that stored fat has an energy density 
of 38 kj/g AFD (Johnston 1970), and that 98% may 
be re-absorbed for metabolism, to the point where body 
condition is still “reversible” in the sense that an in- 
dividual may be re-fed and thus survive the fast (Robin 
et al. 1988), then this fat reserve could theoretically 
maintain an adult bird during periods of food depri- 
vation (assuming daily extence metabolism of 2505 kJ/ 
day) for as long as 20 d, and probably longer if met- 
abolic rate diminishes during fasting. 

While rearing young, each parent should optimally 
be able to provide enough food on 1 d of every 2-d 
foraging cycle to satisfy the gross energy requirements 
of the nestling, for the duration of the nestling period 
of 136 d (Komen 1986, 1991). Daily gross energy 
intake of the growing nestling increases with age, and, 
during the period of maximum growth (about 40 d, 
or 20 parent foraging cycles), may be twice as much 
as the daily adult energy requirement for existence 
(Komen 1991, in preparation). However, at no stage 
does daily nestling gross energy intake exceed the max- 
imum quantity set by adult crop and stomach capacity. 
Except during unusual periods of food deprivation, 
resulting from inclement weather conditions (Komen 
1986, in preparation), both adult and nestling energy 
requirements should be satisfied, without undue de- 
mands on body fat reserves. 

Acknowledgments 

I am grateful to C.S. Sapsford and P.J. Mundy for their 
advice and discussion during this study. This paper forms 
part of a dissertation submitted to the University of the Wit- 
watersrand. South Africa. The work was supported by the 
State Museum of Namibia, Vulture Study Group, Endan- 
gered Wildlife Trust, Witwatersrand Bird Club, Southern 
African Ornithological Society, and C.S.I.R. I thank C.J. 
Brown, S. Chaplin, D. Houston, J.S. Kirkley and M. L. 
Morton for their constructive criticism of an earlier version 
of this manuscript. 

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Received 20 May 1991; accepted 15 April 1992 


J. Raptor Res. 26(4);219-224 
© 1992 The Raptor Research Foundation, Inc. 


ORGANOGHLORINES AND MERCURY IN OSPREY 
EGGS FROM THE EASTERN UNITED STATES 

Daniel J. Audet^ and David S. Scott^ 

U.S. Fish and Wildlife Service, Chesapeake Bay Estuary Program, 180 Admiral Cochrane Drive, 

Annapolis, MD 21401 


Stanley N. Wiemeyer^ 

U.S. Fish and Wildlife Service, Patuxent Wildlife Research Center, Laurel, MD 20708 


Abstract. — Organochlorine and mercury concentrations were determined in Osprey eggs collected from 
Maryland, Virginia, and Massachusetts during 1986-87. DDE concentrations were significantly different 
among locations. Median DDE concentrations did not decline significantly in eggs from Glenn L. Martin 
National Wildlife Refuge, Maryland, between 1973 and 1986. The median DDE residue for eggs from 
Martin Refuge in 1986 surpassed the value associated with 107o eggshell thinning, but was below the 
value associated with production of 1.0 young per active nest, a level assumed to represent a stable 
population. DDD, DDT, dieldrin, PCB, and mercury residues in all eggs appeared insignificant with 
regard to potential effects on shell thickness or reproduction. DDE and PCB residues were lower in eggs 
collected in 1986-87 than in those collected in the 1970s for each area. DDD, DDT, and dieldrin were 
not detected in Martin Refuge eggs in 1986, representing a significant reduction since 1973. DDD, DDT, 
and dieldrin levels in Massachusetts and Virginia eggs in 1986-87 were similar to those in eggs from 
the 1970s for each state. Mercury residues in eggs from Martin Refuge may be increasing and although 
not significant in this study, may warrant future monitoring. 


Mercuric y compuestos organoclorados en huevos de Aguila Pescadora del este de los Estados Unidos 

Extragto. — Concentraciones de mercuric, y compuestos organoclorados normalmente usados en pesti- 
cidas, fueron determinadas en huevos de aguilas de la especie Pandion haliaetus colectados en Maryland, 
Virginia y Massachusetts durante 1986-87. Las concentraciones de DDE fueron significativamente 
diferentes de un lugar a otro. La media de las concentraciones de DDE no declino significativamente en 
huevos colectados en el Refugio Nacional de Vida Silvestre Glenn L. Martin, Maryland, entre 1973 y 
1986. La media de residues de DDE, en huevos del Refugio Martin en 1986, sobrepaso el valor asociado 
con el 10% de disminucion en el espesor de la cascara; pero estuvo por debajo del valor asociado con la 
produccion de una cria por nido active, lo que es un nivel que se asume representa una poblacion estable. 
Los residues de DDD, DDT, dieldrine, bifenil policlorinado (PCB), y mercuric en todos los huevos 
parecian tener insignificantes potenciales efectos en el grosor de la casara o en la reproduccion. Los 
residues de DDE y PCB en huevos colectados en 1986-87, fueron mas bajos que los de aquellos colectados 
en los anos 70 en cada area. DDD, DDT y dieldrine no fueron detectados en huevos del Refugio Martin 
en 1986, lo que representa una significativa reduccion desde 1973. Los niveles de DDD, DDT y dieldrine 
en huevos colectados en Massachusetts y Virginia en 1986-87, fueron similares a aquellos de los colectados 
en los anos 70 en cada estado. Puede que haya un incremento en los residues de mercuric en huevos 
procedentes del Refugio Martin; y aunque no haya sido significative para este estudio, puede que justifique 
futures controles. 

[Traduccion de Eudoxio Paredes-Ruiz] 


^ Present address: U.S. Fish and Wildlife Service, 1201 
Ironwood Drive, Couer d’Alene, ID 83814 
^ Present address: Ohio Division of Wildlife, Olentangy 
Wildlife Experiment Station, 8589 Horseshoe Road, Ash- 
ley, OH 43003 

^ Present address: U.S. Fish and Wildlife Service, 4600 
Kietzke Lane Building C-125, Reno, NV 89502. 


Osprey populations in the eastern United States, 
including Chesapeake Bay, began to decline in the 
late 1950s, continuing through the 1970s (Ames 1966, 
Schmid 1966, Henny 1977, Reese 1977). During 
this period, high concentrations of organochlorines, 
including DDE, dieldrin, and polychlorinated bi- 
phenyls (PGBs), were found in eggs of populations 


219 


220 


Daniel J. Audet et al. 


VoL. 26, No. 4 


with poor reproductive success (Wiemeyer et al. 1975, 
Wiemeyer et al. 1978, Spitzer et al. 1978, Wiemeyer 
et al. 1988). By the early 1970s, productivity began 
to increase and continued through the 1970s in many 
portions of the Chesapeake Bay (Reese 1975, 1977). 
Also, preliminary data showed a decline in organo' 
chlorine levels found in Chesapeake Bay Ospreys in 
1975-82 compared to 1971-73 (Wiemeyer et al. 
1987). However, Osprey nestlings in areas such as 
Poplar Island, Tilghman Island, Glenn L. Martin 
National Wildlife Refuge and the mouth of the 
Choptank River, Maryland, have had decreased sur- 
vival with mortality rates ranging as high as 40- 
75% (P.R. Spitzer unpubl.). 

One possible cause of these isolated declines in 
nestling survival is contaminant accumulation in eggs 
or young. We present data on contaminants in Os- 
prey eggs collected in 1986 from Martin Refuge, 
which supports one of the largest Osprey concen- 
trations on the east coast. This area represents an 
Osprey “colony” where the cause of recent decreased 
nestling survival is unknown. Data on contaminants 
in Osprey eggs from the eastern United States col- 
lected in 1986-87 and Martin Refuge in 1973 are 
also presented. Our objective was to determine if 
concentrations of contaminants found in Osprey eggs 
from the eastern United States were at levels asso- 
ciated with adverse effects to reproduction, including 
nestling survival. 

Materials and Methods 

Martin Refuge is located on the northern end of Smith 
Island, Somerset County, Maryland, and is bordered to 
the west by Chesapeake Bay, to the north by Kedges 
Straits, and to the east by Tangier Sound. Five freshly 
laid Osprey eggs were collected from randomly selected 
active nests in the spring of 1986. The eggs were double 
wrapped in aluminum foil, placed in plastic bags, and 
refrigerated soon after collection. Eggs were prepared for 
analysis in cooperation with staff of the Patuxent Ana- 
lytical Control Facility of the U.S. Fish and Wildlife Ser- 
vice. The contents of each egg were emptied into separate 
chemically cleaned jars. Addled eggs collected from coastal 
Massachusetts (between Narragansett Bay and Buzzards 
Bay), and Virginia (York River area, Mobjack Bay, and 
Rappahannock River) as part of the 1986-87 U.S. Fish 
and Wildlife Service’s “Northeast Bird Egg and Tissue 
Project” were prepared for analyses in a similar manner 
as Martin Refuge eggs collected in 1986. Field collection 
techniques and contaminant analyses for Osprey eggs col- 
lected at Martin Refuge in 1973 were described by Wie- 
meyer et al. (1988). 

Eggs collected in 1986-87 were analyzed by laboratories 
under contracts administered by the Patuxent Analytical 
Control Facility, Laurel, Maryland, which monitored per- 


formance and assured quality. Organochlorines were an- 
alyzed by Weyerhaeuser Analytical and Testing Services, 
Tacoma, Washington. Briefly, portions of homogenized 
samples were mixed with sodium sulfate and extracted for 
20 hr with petroleum ether. Lipid cleanup of extracts was 
by gel permeation chromatography. Analysis was con- 
ducted with a Hewlett Packard 5880A gas chromatograph 
with dual columns (DBl and DB 1701) and dual electron 
capture detectors. Lower limits of detection, before cor- 
rections for dehydration, that varied among samples were 
<0.1 ppm for pesticides and PCBs, except for PCBs in 
Maryland eggs where the limit was <0.6 ppm. In addition 
to the contaminants reported here, the samples were also 
analyzed for chlordane isomers and metabolites, hepta- 
chlor epoxide, endrin, hexachlorobenzene, mirex, and sev- 
eral other compounds, none of which were detected. The 
Osprey eggs were analyzed in a batch process with other 
lots. The batch size for soxhlet extraction was 12 (11 
samples and 1 blank). No analytes were detected in the 
blank at concentrations greater than 0.5 ppb. Duplicate 
analysis of one of the Martin Refuge eggs collected in 1986 
resulted in standard deviations of 0.21 and 0.13 for DDE 
and PCBs, respectively. Duplicate analysis of one of the 
Northeast Egg and Tissue Project eggs resulted in stan- 
dard deviations of 0.19, 0.12, 0.12, 0.05, and 0.50 for 
DDE, DDD, DDT, dieldrin and PCBs, respectively. 
Mercury was analyzed by Environmental Trace Sub- 
stances Research Center, Columbia, Missouri, using cold 
vapor atomic absorption with a Perkin Elmer Model 403 
AA. The limit of detection for mercury was 0.02 ppm. 
Duplicate analysis of a Martin Refuge egg collected in 
1986 and a Northeast Egg and Tissue Project egg resulted 
in standard deviations of 0.07 and 0.04, respectively. Spike 
recoveries of individual eggs were 97% for eggs collected 
at Martin Refuge in 1986 and 107% for eggs collected for 
the Northeast Egg and Tissue Project. Eggshell thickness 
was not measured in eggs collected in 1986-87. 

The volume of all eggs was measured by water dis- 
placement or estimated as described by Stickel et al. (1973). 
Contaminant concentrations were adjusted by egg wet 
weight to volume ratios (ppm) assuming a specific gravity 
of 1.0 (Stickel et al. 1966). To aid in quantitative data 
analyses, 0,05 ppm was used for eggs where a particular 
contaminant was not detected. However, when a contam- 
inant was not found in any eggs for a particular location, 
residue levels were simply listed as “not detected.” 

Due to small sample sizes and uncertainty regarding 
the sampling distribution associated with our egg contam- 
inant data, nonparametric statistical tests were used to 
differentiate between and among median contaminant con- 
centrations. While median values and geometric means 
reported elsewhere in the literature are not directly com- 
parable, both are valid measures of central tendency for 
a data set. Kruskal- Wallis tests (Chi-square approxima- 
tion) were used to determine if differences existed among 
all locations with data on a particular contaminant (Sokal 
and Rohlf 1981). If significant differences in median values 
were found among locations, all pairwise multiple com- 
parisons were made using Wilcoxon’s signed-rank test for 
unpaired data (normal approximation, Sokal and Rohlf 
1981). Statistical significance was assumed at P < 0.05, 
and Bonferroni’s multiple comparison technique was used 


December 1992 


Contaminants in Osprey Eggs 


221 


Table 1. Median (and range) contaminant concentrations (ppm fresh wet weight) in Osprey eggs from several 
locations in the eastern U.S., 1973-87. 


^ ^ Contaminant 

Location and Collection 


Year 

N 

DDE 

DDD 

DDT 

Dieldrin 

PCB 

Mercury® 

Maryland 


Glenn L. Martin NWR, 1973 

11 

3.4 

0.44 

0.14 

0.05 

2.8 

0.05 


(1.3-5. 9) 

(0.27-1.3) 

(n.d.'^-1.2) 

(n.d.-0.20) 

(1. 8-4.3) 

(0.03-0.11) 

Glenn L. Martin NWR, 1986 

5 

2.3 

n.d. 

n.d. 

n.d. 

1.0 

0.11 


(0.82-3.0) 


(0.59-2.3) 

(0.70-0.24) 

Virginia 


York River area, Mobjack 

5 

0.65 

0.05 

0.13 

0.01 

3.7 

0.11 

Bay and Rappahannock 
River, 1987 


(0.38-0.83) 

(0.04-0.11) 

(0.11-0.80) 

(0.01-0.02) 

(2.2-5.7) 

(0.05-0.21) 

Massachusetts 


Between Narragansett 

4 

0.56 

0.13 

0.23 

0.03 

2.4 

0.06 

Bay and Buzzards Bay, 1986 


(0.45-0.68) 

(0.10-0.18) 

(0.12-0.29) 

(0.0‘^-0.04) 

(2.16-2.50) 

(0.05-0.23) 


^ Sample size for mercury analysis was five for Glenn L. Martin National Wildlife Refuge in 1973 and three for Massachusetts in 1986 
^ n d. = not detected. 

^ Actual value calculated as 0.001 but reported as 0,00 when rounded for consistency. 


to control the Type I error rate at 0.05 (Miller 1981). All 
data analyses were performed using the PC version of SAS 
(SAS Institute, Inc. 1985). Statistical differences found 
between 1973 and 1986-87 data should be viewed with 
caution based on variations in chemical analytical tech- 
niques and laboratories used for these two separate data 
sets. 

Results and Discussion 

Median contaminant concentrations and range 
(ppm) in Osprey eggs from each area and year are 
given in Table 1. Residues of DDE were detected 
in all eggs from all locations and collection periods. 
Median concentrations of DDE did not decline sig- 
nificantly in eggs from Martin Refuge between 1973 
and 1986 (Z = 1.47, P = 0.14). However, DDE 
concentrations were significantly different among lo- 
cations sampled in 1986-87 (x^ = 8.52, df = 2, T = 
0.014). Multiple comparisons did not reveal statis- 
tically significant differences between location pairs, 
although eggs from Martin Refuge appeared to con- 
tain higher DDE residues than those from either 
Virginia (P = 0.06) or Massachusetts (P = 0.06). 
Eggs from similar areas of Virginia collected in 1976- 
77 contained geometric mean DDE concentrations 
of 1 .8 to 2.6 ppm with the lowest concentration being 
0.92 ppm (Wiemeyer et al. 1988). Eggs from the 
Westport River, Massachusetts in 1972-73, an area 
within the region sampled in 1986, had a geometric 


mean of 4.2 ppm DDE, with the lowest concentra- 
tion being 2.0 ppm (Wiemeyer et al. 1988). 

DDE residues have been clearly associated with 
adverse effects on Ospreys including decreased re- 
productive success and associated population de- 
clines, whereas other organochlorine pesticides have 
not been associated with such effects (Wiemeyer et 
al. 1988). Median values for DDE reported from 
Virginia and Massachusetts in 1986-87 were well 
below reported values associated with biologically 
significant effects on eggshell thickness and repro- 
ductive success (Wiemeyer et al. 1975, 1988). The 
median residue for DDE from Martin Refuge in 
1986 surpasses the 2.0 ppm DDE concentration as- 
sociated with 1 0% eggshell thinning but is well below 
the 4.2 ppm DDE associated with 15% eggshell 
thinning (Wiemeyer et al. 1988). Also, the median 
residue value for Martin Refuge in 1986 is less than 
the 2.6 ppm DDE value associated with a production 
rate of 1.0 young per active nest and assumed to 
represent a healthy and stable population (Wie- 
meyer et al. 1988). A production rate of 0.8 young 
per active nest is considered necessary to maintain 
a stable population (Spitzer et al. 1983), Eggs col- 
lected at Martin Refuge in 1973 contained higher 
median DDE residues (3.4 ppm); eggshell thinning 
was 17% (Wiemeyer et al. 1988) and young pro- 
duced was about 1.5 per active nest (S.N. Wiemeyer 


222 


Daniel J. Audet et al. 


VoL. 26, No. 4 


unpubl.) which was considered excellent. Wiemeyer 
et al. (1988) had predicted these egg residues to be 
associated with about 14% thinning and a production 
rate of about 0.9 young per active nest. The equation 
estimating the relationship between DDE concen- 
trations and brood size for eggs collected after failure 
to hatch, gave production estimates that were below 
actual levels of production in nearly all sampled 
populations (Wiemeyer et al. 1988) and should be 
used with caution. 

Residues of DDD, DDT, and dieldrin were not 
detected in any eggs from Martin Refuge in 1986; 
therefore, this location and colletion period was as- 
sumed to have the lowest concentration of these con- 
taminants and data analyses include only the other 
locations. The median concentration of DDD plus 
DDT was significantly higher in eggs from Mas- 
sachusetts than in those from Virginia (Z = 2.08, P 
= 0.037). DDD and DDT residues were combined 
in the statistical analysis because DDT is metabo- 
lized to DDD during embryonic development (Abou- 
Donia and Menzel 1968) and reductive dechlori- 
nation occurs in embryonated eggs following death 
(Walker and Jefferies 1978). DDD plus DDT res- 
idues in Virginia and Massachusetts eggs collected 
in similar areas in the 1970s (Wiemeyer et al. 1988) 
were similar to those found in 1986-87. The DDD 
and DDT residues appear insignificant with regard 
to potential effects on shell thickness or reproduction. 

The median concentration of dieldrin did not dif- 
fer between eggs collected in Virginia and Massa- 
chusetts (Z = 1.11, P = 0.27). Dieldrin was seldom 
detected in Virginia eggs collected from similar areas 
in 1976-77, whereas eggs from the Westport River, 
Massachusetts, collected in 1972-73 contained a 
mean of 0.14 ppm. The median dieldrin values in 
the present study are similar to mean values reported 
to have no significant impact on Osprey productivity 
(Wiemeyer et al. 1988). 

PCBs were detected in all eggs from all locations 
and collection periods. Significantly lower PCB res- 
idues were found in eggs collected at Martin Refuge 
in 1986 than in 1973 (Z = 2.606, P = 0.01), sug- 
gesting a decline in the loading of PCBs. Overall, 
median concentrations of PCBs were significantly 
different among locations sampled in 1986-87 (x^ 
= 8.63, df = 2, P = 0.01). Multiple comparisons did 
not reveal statistically significant differences between 
location pairs, although eggs from Martin Refuge 
may have contained lower residues than eggs from 
Virginia {P = 0.06) or Massachusetts {P = 0.11). 


The median PCB residue concentration for eggs from 
Virginia was the highest among all locations and 
collection periods reported in this study. Although 
this value is within the range of reported values for 
Osprey eggs collected from similar areas of Virginia 
in 1976-77, eggs from these areas contained mean 
concentrations of 5.0 to 9.2 ppm. Eggs collected from 
Westport River, Massachusetts, in 1972-73 con- 
tained a geometric mean of 8.3 ppm PCBs (range 
2.2-23.0 ppm). PCB concentrations of the magni- 
tude reported here have not been associated with 
adverse effects on Osprey reproduction (Wiemeyer 
et al. 1988). However, concentrations of highly toxic 
coplanar dioxin-like PCB cogeners and related com- 
pounds were not measured. These compounds have 
been implicated in reproductive impairment of fish- 
eating birds in other areas (Kubiak et al. 1989). 

Mercury was detected in eggs from all locations 
and collection periods. No significant difference in 
mercury concentrations between collection periods 
was noted for eggs from Martin Refuge (Z = 1.67, 
P = 0.09). Further, no significant differences were 
detected among locations for eggs collected in 1986- 
87 (x^ = 0.96, df = 2, P = 0.62). The slightly higher 
mercury levels found at Martin Refuge in 1986 com- 
pared to 1973 suggest that an increase in mercury 
contamination may have occurred. Mercury is being 
increasingly used in gold mining in Brazil in the 
Amazon Basin, much of which pollutes the aquatic 
environment (Martinelli et al. 1988, Pfeiffer et al. 
1989). This is an important wintering area for Os- 
preys that breed in the Mid- Atlantic and Northeast 
areas of the United States (Poole and Agler 1987). 
Mercury concentrations in Osprey eggs were below 
those associated with adverse effects on reproduction 
(Wiemeyer et al. 1988). 

DDE and PCB residues were lower in Osprey 
eggs collected in 1986 than in 1973 at Martin Ref- 
uge. Further, residues of DDD, DDT, and dieldrin 
were not detected in 1986 leading us to assume that 
a significant reduction in these contaminants has 
occurred as well. Concentrations of DDE and PCBs 
also appear to have declined in eggs from Virginia 
and Massachusetts. Although not significant, mer- 
cury residues in Osprey eggs from Martin Refuge 
may be increasing and warrant future monitoring. 
The concentrations of contaminants found appear 
far too low to impact nestling survival. 

Geometric mean DDE concentrations in Osprey 
eggs from the Atlantic Coast and Delaware Bay of 
New Jersey that were collected in 1985-89 (Steidl 


December 1992 


Contaminants in Osprey Eggs 


223 


et al. 1991) bracketed the median concentration in 
eggs from Martin Refuge in 1986, whereas the New 
Jersey eggs contained somewhat higher DDE con- 
centrations than eggs from Massachusetts and Vir- 
ginia in 1986 and 1987. Dieldrin concentrations in 
the New Jersey eggs were similar to those we found 
in Virginia and Massachusetts eggs, whereas the 
New Jersey eggs, especially those from Delaware 
Bay, contained higher PCB concentrations than the 
eggs we analyzed. The differences in residue con- 
centrations in Osprey eggs among these areas are an 
indication of exposure of the adults on their breeding 
areas, for they share common wintering grounds 
(Henny and Van Velzen 1972, Poole and Agler 
1987). 

Osprey eggs from Eagle Lake, California, col- 
lected after failure to hatch in 1983-84 (Littrell 
1986), contained DDE concentrations similar to those 
in eggs from Martin Refuge in 1986. The California 
eggs contained much lower PCB concentrations than 
our samples from Virginia and Massachusetts, pos- 
sibly due to the remote location of the California site 
from industrial contamination. 

The ratios of DDE to DDD + DDT in the recent 
eggs from Virginia and Massachusetts compared to 
that in eggs from a variety of areas in earlier years 
(Wiemeyer et al. 1988), and the presence of DDT 
in all eggs from these two states suggests that these 
Ospreys were recently exposed to low levels of un- 
metabolized DDT; however, the source is unknown. 

Bald Eagle {Haliaeetus leucocephalus) eggs from 
Maryland and Virginia that were collected from 1 5 
territories after failure to hatch in 1980-84, con- 
tained geometric means of 4.4 ppm DDE, 0.42 ppm 
DDD + DDT, 0.31 ppm dieldrin, 14 ppm PCBs, 
0.07 ppm mercury, and low concentrations of a va- 
riety of other organochlorines (S.N. Wiemeyer un- 
publ.). The higher concentrations of organochlorines 
in these eggs reflects the higher position of Bald 
Eagles in the food chain than that of Ospreys. Also, 
Chesapeake Bay Ospreys are exposed to contami- 
nants on their breeding grounds for only about one- 
half of the year due to their migration, whereas 
breeding pairs of Bald Eagles are resident on the 
Chesapeake Bay. 

One Peregrine Falcon {Falco peregrinus) egg col- 
lected in 1984 from South Marsh Island, Maryland, 
just to the north of Smith Island and Martin Refuge, 
contained 14 ppm DDE, 0.36 ppm heptachlor epox- 
ide, 0.75 ppm oxychlordane, and 8.2 ppm PCBs 
(Gilroy and Barclay 1988). These elevated concen- 


trations are also an indication of the high position 
of this species in the food chain and its presumed 
resident status in the region. 

Acknowledgments 

We thank the Blackwater and Martin National Wildlife 
Refuge staffs for collection of the Martin Refuge Osprey 
eggs and background information on the study site. Steve 
Goodbred of the U.S. Fish and Wildlife Service Annapolis 
Field Office was involved in processing the eggs and pre- 
liminary review of the manuscript. Gary H. Heinz, Charles 
J. Henny and Glen A. Fox provided helpful reviews of 
drafts of the manuscript. Mickey Hayden and Deborah 
Senior typed the manuscript. 

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Received 22 August 1991; accepted 22 April 1992 


J. Raptor Res. 26(4):225-228 
© 1992 The Raptor Research Foundation, Inc. 


KLEPTOPARASITISM AND CANNIBALISM IN A 
COLONY OF LESSER KESTRELS 

{Falco naumanni) 

Juan Jose Negro, Josfe Antonio Donazar and Fernando Hiraldo 

Estacion Biologica de Donana, CSIC, Apdo, 1056, 41080 Seville, Spain 


Abstract. — ^We describe kleptoparasitism and cannibalism for the first time in a colony of Lesser Kestrels 
{Falco naumanni). Kleptoparasitism was practiced almost exclusively by females, the larger sex, while 
males received most of the attacks. Kleptoparasitic Lesser Kestrels had a relatively high success (43.1%, 
N = S2 attempts) compared to other species in which kleptoparasitism occurs frequently. Two cases of 
chick cannibalism by adults were also recorded. 


Cleptoparasitismo y canibalismo en una colonia de Cernicalos Primillas {Falco naumanni). 

Extracto.— El cleptoparasitismo y el canibalismo han sido observados por vez primera en una colonia 
de Cernicalos Primillas {Falco naumanni). El cleptoparasitismo fue practicado casi exclusivamente por 
hembras, que son de mayor tamano que los machos, mientras que estos recibieron la mayoria de los 
ataques. El exito de los ataques fue relativamente alto (43.1%, N ^ 52 intentos) en comparacion con el 
observado en otras especies donde el cleptoparasitismo es frecuente. Tambien registramos dos casos de 
canibalismo practicado contra polios por Cernicalos Primillas adultos. 


Despite the fact that kleptoparasitism has been 
reported in several birds of prey (for a review see 
Brockman and Barnard 1979), it seems to be rare 
among species of the genus Falco. The Lesser Kestrel 
{Falco naumanni) is a colonial small falcon. Although 
both sexes do not show significant differences for 
most body traits, females are up to 24% heavier than 
their mates (Cade 1982). Males feed their females 
from before to a few days after egg laying, share 
incubation and deliver most of the nourishment for 
their offspring (Donazar et al. 1992). None of the 
authors who have studied the species described klep- 
toparasitic behavior (see Glutz et al. 1971, Cramp 
and Simmons 1980). In this study we describe the 
occurrence of kleptoparasitism in a colony of Lesser 
Kestrels and discuss the role of the reversed size 
dimorphism (RSD) exhibited by the species in the 
directionality of the kleptoparasitic attacks. Addi- 
tionally, we comment on two cases of chick canni- 
balism by adult Lesser Kestrels. 

Study Area and Methods 

The observations were carried out during 1989 and 
1990 in a colony of Lesser Kestrels nesting in Mairena 
del Alcor (37"22'N 5°45'W), Seville, southern Spain. We 
counted 42 breeding pairs in 1989 and 40 in 1990. A 
sample of nests in two adjacent walls of a tower was 
selected for systematic recording of behavior (see Negro 
et al. 1992 for details). The portion of the colony observed 
consisted of 7 nests in 1989 and 6 nests in 1990 (26 focal 


individuals). The observations of behavioral interactions, 
feedings and the type of prey delivered were carried out 
from a distance of 70 m with a telescope (20-40 x). The 
observations lasted from dawn to dusk, 2-3 d a week, from 
the beginning of the period of pair formation (February) 
until the independence of the fledglings (end of July). 
Observations amounted to 475 hr in 1989 and 567 hr in 
1 990. Simultaneously, one or two additional observers ra- 
diotracked seven males and six females which were breed- 
ing in the portion of the colony under observation. Ra- 
diotracking amounted to 305 hr in 1989 and 647 hr in 
1990. 

All the nests in the colony were visited 1-3 times during 
the breeding season. Adults were trapped on the nest and 
were banded with laminated plastic bands (wearing two 
characters) which allowed them to be identified by tele- 
scope. In 1989 a quarter of the adult Lesser Kestrels wore 
these bands and in 1990 the proportion was two-thirds of 
the adults in the colony. All the young in the colony were 
also marked with plastic bands in the two years of the 
study. 

Results 

Food Supply. The availability of food in the en- 
vironment was determined indirectly using feeding 
of nestlings as an approximate measure. The fre- 
quency of chick feedings in the colony was 1.8 feed- 
ings/hr in 1989 and 1.9 feedings/hr in 1990. Both 
values are below those observed in southern France 
(2 feedings/hr during 5.5 hr of observation; Blondel 
1964 or 3.1 feedings/hr during 22.7 hr; Hovette 
1971) and in northeastern Spain (5.4 feedings/hr 


225 


226 


Juan Jose Negro et al. 


VoL. 26, No. 4 


during 6.7 hr; M. Pomarol pers. comm.). Our values 
are higher, however, than those given by Bijlsma et 
al. (1988) for colonies in Extremadura in south- 
western Spain (1.3 feedings/hr during 26 hr) where, 
according to the authors, the availability of food was 
very high. Nonetheless, these authors collected their 
data at the beginning of the post-hedging period, 
when the rate of feeding of the young is reduced 
(Bustamante 1990). 

The prey consumed in our area might have been 
smaller than prey in the other studies. Most prey 
were insects. The percentage of vertebrates was low 
(0.9% of 1113 items) compared to 6.3% vertebrates 
observed by Franco and Andrada (1977) in the same 
general area several years ago, 2.6% observed in 
Provence (Hovette 1971), and 5.7% observed in Ex- 
tremadura (Bijlsma et al. 1988). 

Kleptoparasitism and Cannibalism. At least 4 
individuals from the colony, but only 1 of the 26 
focal individuals acted as kleptoparasites. Klepto- 
parasitic attacks were directed at 14 (53.8%) of these 
26 focal birds. Kleptoparasitism occurred during the 
chick rearing period (June- July), when the parents 
delivered prey directly to their young in the nest. As 
they perched in the entrance of the nest, the attacker 
flew in and tried to snatch the food. Kleptoparasitic 
attacks were never observed while radiotracking the 
birds in the hunting areas, nor during the period of 
mate-feeding (April-May; see Donazar et al. 1992). 
Of the 51 attempts at stealing food, 29 (577o) failed. 
Of these failed attempts, 14 (487o) were because of 
the aggressive response by the victim, 1 3 (447o) were 
because the adult managed to transfer the food to 
the chicks and two (6.9%) were because the victim 
appeared to have anticipated the attack and escaped 
without feeding the chicks (although they returned 
later on). 

Males fed chicks more than females in the period 
during which kleptoparasitism occurred (61.57o vs. 
38.57o, N = 894 feedings); they were also the victims 
of a disproportionate number of the kleptoparasitic 
attacks (82.47o vs. 17.67o, N = 51; ~ 9.004, P = 

0.002). Females were responsible for the majority 
of attacks (947o). Of the others, two attacks (3.97o) 
were made by males and one by a bird of unknown 
sex (goodness of fit test assuming sex ratio of 1:1, x^ 
= 40.50, P < 0.001). When the attacker was a 
female, success tended to be higher when attacking 
males (45.27o, TV = 41) than when attacking females 
(14.27o, TV = 7), although the difTerence was not 
significant (Fisher’s exact test, P > 0.05). 


One banded female (GK) carried out 627o of the 
attacks. She attacked at least nine different males 
and four females. Another two identified females 
carried out one and two of the attacks, respectively. 
Banded but not identified females (possibly GK) car- 
ried out 13 attaeks (25.47>). Another two attaeks 
were carried out by a male and a third by an in- 
dividual of unknown sex. 

The importance of kleptoparasitism for the feed- 
ing of the female GK may be greater than that ob- 
served. Her nest, although near, was in a different 
portion of the colony so she may have made attacks 
which we did not observe. This female was the most 
successful breeder in the colony in the two years of 
the study. In 1989, she successfully reared three 
chicks when the average number of chicks fledged 
per breeding pair in the colony was 1.8 ± 0.17 (TV 
= 42 pairs). In 1990, she successfully reared four 
chicks, when the average per pair in the colony was 
1.4 ± 0.88 (AT = 39 pairs). 

The parasitic activity of GK was not limited to 
stealing food from adults carrying prey to the nest. 
On one occasion the fresh carcass of a 7-day-old 
chick, which did not correspond to any of her young, 
was found in her nest. On another occasion, GK was 
seen trying to steal a chick 10 d old from a nest. 
This attempt was prevented by the parent female 
when GK had already managed to take the chick 
out of the nest entrance. CK’s objective was evidently 
the chick as she fought violently with the resident 
female for its possession. In 1991, we observed one 
adult male stealing and eating a chick in a neigh- 
boring colony. 

Discussion 

Our data suggest that the food supply to the young 
by parent Lesser Kestrels was lower in the studied 
colony than that encountered by other investigators 
several decades ago in the same area or in other 
regions in the Western Palearctic. Additionally, we 
observed a high nestling mortality (about 50%) in 
the two years of study due to starvation (Negro 1991). 
Such high mortality rates have not been observed by 
other investigators cited here and they seem to be 
uncommon among raptors of similar size (e.g., New- 
ton 1979). Both lines of evidence, the low provi- 
sioning rate and the high nestling mortality, suggest 
that the period of study was a time of food shortage 
for the Lesser Kestrels. Kleptoparasitism and can- 
nibalism have been said to be favored in stressful 


December 1992 


Kleptoparsitism in Lesser Kestrels 


227 


food conditions (Brockmann and Barnard 1979, Jorde 
and Lingle 1988, Tones and Manez 1990, Bortolotti 
et al. 1991). 

Kleptoparasitism was practiced mainly by female 
Lesser Kestrels, the larger sex, with most attacks 
made on males. In other species, size is apparently 
a determining factor for success in kleptoparasitic 
attempts (Knight and Knight 1988, Tershy and 
Breese 1990). If the frequency of kleptoparasitic 
attempts is influenced by the RSD (i.e., the larger 
size of the females) in the Lesser Kestrel, it would 
also be expected that males were the subject of suc- 
cessful attacks more frequently than females. Our 
results do not show a clear tendency in this respect, 
although this could be due to the low frequency of 
attacks by females on other females. It might also 
be that, since females spent more time in the colony 
throughout the breeding cycle (Donazar et al. 1992), 
they would have more opportunities to carry out 
kleptoparasitic attempts. However, the attacks were, 
in the main, produced at the end of the nestling 
period when males and females spent a similar 
amount of time in the colony, and not in other periods 
of the breeding cycle when the females’ colony at- 
tendance was higher than that of males. Kleptopar- 
asitism was practiced by a few individuals, such as 
the female GK, who apparently had specialized in 
this behavior. The systematic kleptoparasitism by 
the female CK may have been highly profitable, 
given her high reproductive success in both years of 
the study. We cannot discount, however, that other 
factors were involved, such as a high provisioning 
rate of her mate. 

Lesser Kestrels had a relatively high success rate 
in their kleptoparasitic attempts (43.1%). Other spe- 
cies practicing intraspecific kleptoparasitism showed 
lower figures; Common Tern {Sterna hirundo) 6.2% 
(Hopkins and Willey 1972), Bald Eagle {Haliaeetus 
leucocephalus) 8.1% (Fischer 1985), Black Kite {Mil- 
vusmigrans) 3.4-16.6% (Sunyer 1988). In other rap- 
tor species where intraspecific kleptoparasitism is 
widespread, individuals use display behaviors to hide 
the prey and deceive potential pirates (Brown 1976, 
Fischer 1985, Sunyer 1988). Such patterns of be- 
havior were not evident in the case of the Lesser 
Kestrels (only 4% of the victims apparently detected 
the attacker). Nevertheless, in 1990 we twice ob- 
served atypical behavior by two individuals that had 
been recently kleptoparasitized. These males circled 
over the colony before feeding the chicks, and then 
dived to the nest. 


Acknowledgments 

M. de la Riva assisted in the field work. S. Flemming 

and W.M. Iko helped us revise the manuscript. A. Krupa 

helped to translate an early Spanish draft. The CSIC- 

CIGYT provided financial support (project PB87-0405). 

Literature Cited 

Bijlsma, S., E.J.M. Hagemeijer, G.J.M. Verkley and 
R. Zollinger. 1988. Ecological aspects of the Lesser 
Kestrel Falco naumanni in Extremadura (Spain). Rap- 
port 285, Katholieke Universiteit Nijmegen, The 
Netherlands. 

Blondel, J. 1964. Notes sur la biologie et le regime 
alimentaire du Faucon Crecerellette Falco naumanni 
Nos Oiseaux 28:295-296. 

Bortolotti, G.R., K.L. Wiebe and W.M. Iko. 1991. 
Cannibalism of nestling American Kestrels by their 
parents and siblings. Can. J. Zool. 69:1447-1453. 

Brockmann, H.J. and C.J. Barnard. 1979. Klepto- 
parasitism in birds. Anim. Behav. 27:487-514. 

Brown, L.H. 1976. Birds of prey, their biology and 
ecology. Hamlyn Publication Co., London, U.K. 

Bustamante, J. 1990. Condi cionantes ecologicos del 
periodo de emancipacion en Falconiformes. Diserta- 
cion doctoral. Universidad Autonoma de Madrid, Ma- 
drid, Spain. 

Cade, T.J. 1982. The falcons of the World. Cornell 
University Press, Ithaca, NY. 

Cramp, S. and K.E.L. Simmons. 1980. The birds of 
the Western Palearctic. Vol. 2. Oxford University Press, 
Oxford, U.K. 

DonAzar, J.A., J.J. Negro and F. Hiraldo. 1992. 
Functional analysis of mate-feeding in the Lesser Kes- 
trel Falco naumanni. Ornis Scandinavica 23:190-194. 

Fischer, D.L. 1985. Piracy behavior of wintering bald 
eagles. Condor 87:246-251. 

Franco, A. and J. Andrada, 1977. Alimentacion y 
seleccion de presa en el Falco naumanni. Ardeola 23: 
137-187. 

Glutz, U.N., K.M. Bauer and E. Bezzel. 1971 
Handbuch der Vogel Mitteleuropas. Vol. 4. Akadem- 
ische Verlagsgesellschaft, Frankfurt, Germany. 

Hopkins, C.D. and R.H. Wiley. 1972. Food parasitism 
and competition in two terns. Auk 89:583-594. 

HovETTE, C. 1971. Notes sur la reproduction du Faucon 
Crecerellette Falco naumanni en Provence. Nos Oiseaux 
31:82-90. 

Jones, A.M. and M. MaSez. 1990. Cannibalism by 
Black Kite {Milvus migrans). J. Raptor Res. 24:28-29 

Jorde, D.G. AND G.R. Lingle. 1988. Kleptoparasitism 
by Bald Eagles wintering in south-central Nebraska. 
J. Field Ornithol. 59:183-188. 

Knight, R.L. and S. Knight. 1988. Agonistic asym- 
metries and the foraging ecology of Bald Eagles. Ecol- 
ogy 69:1188-1194. 

Negro, J.J. 1991. Ecologia de poblaciones del cernicalo 


228 


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VoL. 26, No. 4 


primilla Falco naumanni. Disertacion doctoral. Univer- 
sidad de Sevilla, Sevilla, Spain. 

, J.A. DonAzar and F. Hiraldo. 1992. Cop- 

ulatory behavior in a colony of Lesser Kestrels: sperm 
competition and mixed reproductive strategies. Anim. 
Behav. 43:921-930. 

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

SuNYER, C. 1988. El cleptoparasitismo en el Milano 


Negro. M.Sc. thesis. Universidad Complutense de Ma- 
drid, Madrid, Spain. 

Tershy, B.R. and D. Breese. 1990. The influence of 
sexual dimorphism on kleptoparasitism of Blue-footed 
Boobies by Brown Boobies. Can. J. Zool. 68:197-199. 


Received 6 February 1992; accepted 1 May 1992 


J. Raptor Res. 26(4);229-234 
© 1992 The Raptor Research Foundation, Inc. 


HOME RANGE AND ACTIVITY OF A PAIR OF 
BALD EAGLES BREEDING IN 
NORTHERN SASKATCHEWAN 

Jon M. Gerrard 

Manitoba Institute of Cell Biology, WO Olivia Street, Winnipeg, MB, Canada R3E 0V9 


Alan R. Harmata 

Department of Biology, Montana State University, Bozeman, MO 59717 


P. Naomi Gerrard 

Box 113, R.R. 1, Headingly, MB, Canada ROH OfO 


Abstract. — A male and female adult Bald Eagle (Haliaeetus leucocephalus) were radiotracked for 12 d 
during the summer of 1982. Size of the range and territory was 7 km^ and 4 km^, respectively. The 
female spent significantly more time within 200 m of the nest than the male, from 0400-1100 H. During 
the same period the male spent significantly more time flying than did the female. The greater proportion 
of time spent flying in early morning hours by the male may be a function of lower wing loading, 
facilitating energetically less expensive flight in the absence of updrafts and thermals. The results suggest 
that 400 locations of both members of a pair at 1 5 min intervals evenly distributed throughout the day, 
which is equivalent to 100 hr of observations, are adequate to describe 90% of the home range. The 
radio-tagged eagles usually responded only to intruders of the same sex. 


Espacio habitado, y actividades de una pareja de Aguila Cabeciblanca (Haliaeetus leucocephalus), en el 
norte de Saskatchewan 

Extracto. — Una pareja adulta de Aguila Cabeciblanca {Haliaeetus leucocephalus) fue radiocontrolada 
por 12 dias durante el verano de 1982. La extension de area habitada y el territorio a defender fueron 
de 7 km^ y 4 km^ respectivamente. La hembra paso significativamente mas tiempo que el macho dentro 
de 200 m cerca del nido, entre las 0400 y las 1100 horas. Durante el mismo periodo el macho void 
significativamente mas tiempo que la hembra. La mayor proporcion de tiempo gastado por el macho, en 
sus vuelos de las tempranas horas de la manana, puede ser una funcion de las alas que son proporcio- 
nalmente mas grandes en relacion con el peso del cuerpo; lo que facilita energeticamente menos costosos 
vuelos en ausencia de termales y de vientos ascendentes. 

Se observaron 400 ubicaciones consecutivas con 15 minutos de intervalo de ambos miembros de esta 
pareja de aguilas, ello es equivalente a 100 h (igualmente distribuidas durante las horas del dia) de 
observacion con ambas aguilas a la vista. Estos resultados son adecuados para describir el 90% de la 
extension habitada por una pareja de Haliaeetus leucocephalus en su ciclo reproductive. Estas radiocon- 
troladas %uilas generalmente respondieron solo a intrusos del mismo sexo. 

[Traduccion de Eudoxio Paredes- Ruiz] 


Previous studies have estimated territory size and 
home range of breeding Bald Eagles {Haliaeetus leu- 
cocephalus), but in most studies estimates were based 
on visual locations of unmarked eagles or linear dis- 
tance between nests (e.g., Broley 1947, Hensel and 
Troyer 1964, Retfalvi 1965, Mattson 1974, Gerrard 
et al. 1980, Mahaffey 1981). Radiotelemetry permits 
identification of individual eagles, allows locating 
target eagles at will, and permits more precise def- 
inition of ranges and movements. 

Understanding the relative roles of male and fe- 


male Bald Eagles also has been difficult due to prob- 
lems identifying unmarked individuals. Similarity in 
plumage has made definite and continuing identi- 
fication of genders difficult. Size is the best criteria 
for distinguishing gender of eagles in the field, and 
although reasonably reliable when two eagles are 
together, gender assignment of solitary eagles is dif- 
ficult. Radiotracking eagles of known gender allows 
identification and improves determination of the rel- 
ative roles of male and female during the breeding 
cycle. 


229 


230 


Jon M. Gerrard et al. 


VoL. 26, No. 4 


Table 1. Measurements of two mated adult Bald Eagles 
breeding at Besnard Lake, Saskatchewan in 1982. 


Measurement 

Male 

Female 

Ratio 

Fe- 

male/ 

Male 

Weight (g) 

3920 

4540 

1.15 

Wing span (cm) 

207 

211 

1.02 

Wing area (cm^) 

5601 

6014 

1.07 

Wing loading (g/cm^) 

0.70 

0.75 

1.07 

Wing 


Chord (cm) 

56.8 

60.3 

1.06 

Flattened 

59.7 

61.6 

1.03 

Culmen length (mm) 

49.4 

55.0 

1.11 

Bill depth 

33.2 

35.6 

1.07 

Tarsus (mm) 


Largest width 

14.4 

17.0 

1.18 

Smallest width 

13.3 

16.5 

1.24 

Footpad (mm) 

131.3 

136.9 

1.04 


Methods 

A pair of Bald Eagles breeding on Besnard Lake were 
identified through previous study and chosen for radiotag- 
ging based on logistics and previous knowledge of habits 
(Gerrard et al. 1983, Gerrard and Bortolotti 1988). Eagles 
were captured by padded leg-hold traps placed in shallow 
water (0.1 -0.3 m deep; Harmata 1985). Four or six cap- 
ture devices were set around a Northern Pike (Esox lucius) 
or Walleye {Stizostedion vitreum) bait carcass staked in 
place. One capture site was in a shallow area with a mud 
and rock bottom in a wide shallow bay. Emergent vege- 
tation surrounded the capture site to prevent the eagle 
from attempting to take the bait by air. Another capture 
site was on submerged rocks near the edge of a small rocky 
island which had several White Spruce (Picea glauca) used 
regularly by eagles for perching. Both capture sites were 
within 2 m of shore where adult eagles had caught fish 
previously. 

Each capture adult was weighed and measured; a 54 
gram radiotransmitter was attached to the two central tail 
feathers on each eagle. Wing area was measured after 
tracing the outline of the right wing onto a sheet. Wing 
outline was later transferred to graph paper with 1 mm 
squares. Area was determined by counting the number of 
inclusive and partially inclusive squares. Wing loading is 
the bird’s weight divided by area of the two wings (Brown 
and Amadon 1968) and was here expressed as grams/cm^. 

Eagles were located using receivers and hand held yagi 
antennae, and observed with binoculars and spotting scope. 
Observations were nearly continuous during daylight hours 
during the first 2 d. After a one day hiatus, we located 
both eagles at 15 min intervals from 0415-2200 H for the 
next 9 d. At each sampling interval we determined the 
location and activity of both eagles and scanned with 10 x 
binoculars and 20-45 x spotting scope to locate other ea- 


gles. Most monitoring was from an elevated rock located 
1.3 km southeast of the nest. This point permitted good 
visibility of many of the eagles’ perch sites as well as the 
nest. During midday, when eagles were often soaring, a 
mobile tracker moved throughout the eagles’ range to tri- 
angulate eagles when out of visible range. Visual contact 
with the radio-tagged eagles was made to verify the ac- 
curacy of the telemetry locations whenever possible. Range 
was determined using the minimum convex polygon meth- 
od (Mohr 1947, Jennrich and Turner 1969). Territory 
was defined as the part of the range that was defended 
(i.e., from whieh other adult eagles were excluded, Pet- 
tingill 1970). Defended area was that enclosed by locations 
where we saw chase flights with one or other of the ter- 
ritorial pair chasing other eagles away. Activity and spatial 
relationships were calculated by dividing the number of 
15 min observation records during which the eagle was 
flying and more than 200 m from the nest, respectively, 
by total records engaged in that activity for that hourly 
period. Total records per period was approximately 20 in 
each case (range 15-22). 

Results 

A target pair of mated adult Bald Eagles breeding 
on Besnard Lake was captured in July 1982. The 
male was captured on 17 July at the Shallow Bay 
site and the female on 19 July at the Rocky Island 
site. Both eagles were caught the same day as re- 
spective capture sites were set. 

Mensural data showed considerable difference in 
size between the two eagles. Greatest differences 
were in weight, culmen length, bill depth, and tarsal 
width (Table 1). Measurements of the larger eagle 
were well within those of known females and those 
of the smaller eagle were well within those of known 
males (Bortolotti 1984). 

Radiotracking of the male began after release and 
continued through 0900 H 28 July 1982. Male and 
female eagles were monitored for 126 and 105 hr 
over 12 and 10 d, respectively. During this time, 
their nestlings were between 48 and 59 d old. Both 
eaglets fledged normally in early August 1982. 

Range and Territory. Visual locations of eagles 
were obtained for 48% of 964 telemetry locations. 
Location of the eagle monitored were equivocal as 
to whether it was < or >200 m from the nest for 
236 of 964 telemetry locations. Range for both eagles 
was 7 km^, with no appreciable differences between 
the range of the male and that of the female (Fig. 
1). Size of range in relation to cumulative number 
of observations is shown in Figure 2. There was 
little expansion in range size during the latter half 
of the observation period. 

The defended area was a minimum of 4 km^ but 
might have been larger, particularly as few inter- 


December 1992 


Bald Eagle Ranges and Activities 


231 


SIZE 

OF 

HOME RANGE 
(Km®) 


NUMBER OF OBSERVATIONS 


Figure 2. Size of home range of a mated pair of Bald 
Eagles radiotagged on Besnard Lake, Saskatchewan, in 
relation to cumulative number of locations. 


1 km 

Figure 1. Range (7 km^) of a mated pair of radio-tagged 
Bald Eagles on Besnard Lake, Saskatchewan between 17- 
28 July 1982. Sightings of the male and female eagles 
were considered individually. The size of the range in- 
cluded the area encompassed by the outer extent of these 
flights. Symbols denote: # nest of eagles equipped with 
radios, ■ nests of adjacent bald eagle pairs, -» territorial 
defense flights by marked eagles; A, B small lakes visited 
by mated pair, * capture sites; OP observation hill. 

actions were seen to the north of the nest. On three 
occasions when both male and female were near the 
nest, other adults entered the territory and perched 
on a small island 300 m from the nest. Size of the 
intruders indicated that all were females. On all 
three occasions, the male did not pursue the intruder 
but the female did. On four occasions, the male was 
involved in chasing and pursuing other eagles which 
entered the territory. Gender of intruding eagles was 
not determined during the latter encounters, al- 
though at least two were thought to be males. 

Perching Behavior. One adult eagle was within 
200 m of the nest most of the time. The female spent 
significantly more time within 200 m of the nest 
than the male from 0400-1100 H (x^ = 25.7, P < 
0.01; Table 2). There was no difference between 
male and female regarding distance from the nest 
from 1100-1800 H (Table 2) or from 1800-2200 
H, except that the male tended to roost more than 
200 m from the nest while the female roosted near 
the nest (Fig. 3). Both male and female used perches 
when near the nest. The male tended to perch on 


the topmost branch of the tallest spruce within 30 
m of the nest (11.0% of time perched), or topmost 
branch of the tallest spruce on the nest island (5.2% 
of time perched). The female tended to perch on top 
of spruce trees which were slightly lower but did, 
on occasion, use the same perches as the male (4.7% 
and 0.5% of time perched, respectively). Both eagles 
spent a small proportion of their time away from 
the nest at the small lakes A and B away from 
Besnard Lake (4.9% for male, 6.1% for female; Fig. 
1 ). 

Activity Patterns. Activity of the eagle monitored 
(perched or flying) could not be determined for 228 
(24%) of 964 telemetry locations. The data showed 
that female and male spent nearly equal time in 
flight, 18% and 17% respectively, but they distrib- 
uted their activity differently between morning and 
midday. The male spent significantly more time fly- 


Table 2. Spatial and temporal relationships of male and 
female Bald Eagles relative to their nest at Besnard Lake. 


Number of Locations 

Hours 

Male 

Female 

0400-1100 

Within 200 m of nest 
More than 200 m from nest 
Total 

60 (31%) 
132 (69%) 
192 

93 (58%) 
67 (42%) 
160 

1100-1800 

Within 200 m of nest 
More than 200 m from nest 
Total 

91 (43%) 
121 (57%) 
212 

64 (41%) 
94 (59%) 
158 


232 


Jon M. Gerrard et al. 


VoL. 26, No. 4 


TIME OF DAY (HOUR) 

Figure 3. Location of male ( ) and female ( ) Bald 

Eagles in relation to the location of their nest at Besnard 
Lake, Saskatchewan by hour of the day. Total records 
were approximately 20/hr. 

ing in early morning than did the female (x^ = 8.1, 
P < 0.01; Table 3). There was no signficant dif- 
ference in amount of time spent flying by male or 
female during midday, although the female tended 
to fly longer and/or more often (Table 3). In the 
evening (1800-2200 H), the flying activity of the 
male and female was similar (Fig. 4). 

Discussion 

Movements and activities of radio-tagged eagles 
suggested little efifect of capture, handling, and mon- 
itoring on normal behavior. Upon release, the male 
immediately flew to perch on a tall tree on the nest 
island. Within 2.5 hr of release, the male had chased 
both an intruding immature and an intruding adult 
Bald Eagle and then caught a fish which it brought 
back to the nest. Capture and handling may have 
affected the female briefly, however. When released 
at 0820 H she did not return to the nest immediately, 
but flew to a perch near the small lake (B; Fig. 1) 
over 2 km from the nest and remained there until 


Table 3. Relative activity of male and female Bald Eagles 
in early morning and during midday at Besnard Lake. 


Number of Locations 

Hours 

Male 

Female 

0400-1100 

Flying 

29 (15%) 

9 (6%) 

Perched 

164 (857o) 

152 (94%) 

Total 

193 

161 

1100-1800 

Flying 

46 (21%) 

46 (28%) 

Perched 

172 (79%) 

118 (72%) 

Total 

218 

164 


o 


Figure 4. Percent time the radio- tagged male ( ) and 

female ( ) Bald Eagle engaged in flight by hour of day. 

Total records were approximately 20/hr. 


1125 H. This may not have been normal behavior. 
Subsequent monitoring indicated that she was more 
often within 200 m of the nest during the morning 
hours. However, within 4 hr both radio-tagged ea- 
gles were using their usual perches near the nest 
and both the young fledged normally. Research ac- 
tivities apparently had no effect on habitat use or 
productivity. 

Mensural data illustrated the normal size dimor- 
phism in a mated pair of Bald Eagles, Proportion- 
ally, weight was greater for the female than wing- 
span and wing area, resulting in a higher wing 
loading. Wing loading values suggest active flight 
may be more energetically expensive in calm air for 
females than for males. Indeed, the male spent more 
time flying early in the day when the thermals were 
weaker (or non-existent). Both male and female spent 
considerable time flying during mid-day when ther- 
mals were strongest. Results were similar to Bald 
Eagle activity patterns on wintering grounds, where 
males tended to be active earlier and over a greater 
part of the day than females (Harmata 1984). 

Consistent with the observation that the female 
was less active in the morning, she also spent more 
time within 200 m of the nest between 0400-1100 
H. Although some bias may exist due to the pro- 
portion of locations where we did not unequivocally 
establish whether an eagle was more or less than 
200 m from the nest or flying versus perched, we 
have no reason to suspect this would have differ- 
entially affected the data for males versus females. 
Indeed, our visual confirmation of 48% of all loca- 
tions provided a reasonable corroboration of the re- 
liability of the data. Additionally, signal character- 
istics differed noticeably between flying and perched 
eagles. Therefore, assessment of activity for eagles 


December 1992 


Bald Eagle Ranges and Activities 


233 


out of visual range could be determined relative to 
signal type and receiving antenna position during 
strongest signal (horizontal = flying, vertical = 
perched). 

Responses of radio-tagged eagles to intruders in- 
dicated gender-specific defense of territory. The fe- 
male clearly reacted to other females but ignored 
eagles we thought to be males. The opposite ap- 
peared to be true for the male. Gender specific de- 
fense of territory has been noted in Golden Eagles 
(Aquila chrysaetos) and would facilitate rapid re- 
placement of lost mates (Harmata 1982). 

Home ranges of Bald Eagles vary from an esti- 
mated 10-15 km^ for other adult eagles on Besnard 
Lake (Gerrard et al. 1980) to about 30 km^ used by 
a pair of eagles on the San Juan Islands in Wash- 
ington (Retfalvi 1965) to a mean range of 47.5 km^ 
in the Greater Yellowstone Ecosystem (Harmata 
and Oakleaf 1991). Home ranges in the Greater 
Yellowstone Ecosystem were annual ranges, and this 
may explain the relatively large differences in re- 
lation to other estimates made during the breeding 
season. Size of the defended area (about 4 km^) did 
not differ appreciably from an estimated 6 km^ for 
a pair farther southwest on Besnard Lake, but did 
differ from estimates of 1. 5-2.0 km^ in Florida and 
Michigan (Broley 1947, Mattson 1974). 

Four other pairs of eagles nested successfully in 
close proximity to the monitored pair (Fig. 1). These 
four nests were previously recorded in this region of 
the lake and may have induced the slightly smaller 
territory size relative to estimates from elsewhere on 
the lake in 1978 (Gerrard et al. 1980). Our findings 
that perches may be preferentially used by one eagle 
of a pair is similar to that of Retfalvi (1965) and 
illustrate the importance of adequately describing 
Bald Eagle ranges for management purposes. 

Precise size of the home range of Bald Eagles may 
depend on available food supply and proximity of 
neighboring eagles. Range utilized may also vary 
with season, time of the breeding cycle and nesting 
habitat (river, lake or marine). Range size also is a 
function of monitoring time. Figure 2 shows little 
increase in size of the range after 400 observation 
points were accrued, suggesting that by this point 
we were close to determining the maximum extent 
of the home range. 

Several recovery and management plans for the 
Bald Eagle in the United States suggest the devel- 
opment of site- or pair-specific management plans 
for each nesting pair before “delisting” from endan- 


gered status should occur (e.g.. Pacific Bald Eagle 
Recovery Plan, USFWS 1986, and Montana Bald 
Eagle Management Plan, MBEWG 1986). The 
utility of site plans for efifective management has 
been slow because the management strategy was 
based on an inadequate description of range and 
habitat use. Data deficiencies were mostly a conse- 
quence of insufficient monitoring effort, spawned by 
a lack of guidelines. 

In this study, 400 consecutive 15 min telemetry 
locations determined 93% of the range of a pair of 
Bald Eagles. Doubling the effort added only 7% to 
range size (Fig. 1). Therefore, a minimum of 400 
telemetry locations, accrued consecutively at 15 min 
intervals over daylight hours, or 100 hr of obser- 
vation with both eagles in view, distributed evenly 
throughout daylight hours may be used as a guide- 
line for observational effort. This effort should de- 
lineate over 90% of a range of breeding Bald Eagles 
and provide adequate data for site- or pair-specific 
management purposes, at least on lakes. 

Literature Cited 

Bortolotti, G.R. 1984. Plumage polymorphism and 
sex dimorphism in size of Bald Eagles. J. Wildl. Man- 
age. 48:72-81. 

Broley, C.L. 1947. Migration and nesting of Florida 
Bald Eagles. Wilson Bull. 59:3-20. 

Brown, L. and D. Amadon. 1968. Eagles, hawks, and 
falcons of the World. McGraw-Hill Book Co., New 
York. 

Gerrard, J.M. and G.R. Bortolotti. 1988. The Bald 
Eagle: haunts and habitats of a wilderness monarch. 
Smithsonian Press, Washington, DC. 

, P.N. Gerrard, G.R. Bortolotti and D.W.A. 

Whitfield. 1983. A 14-year study of Bald Eagle 
reproduction on Besnard Lake, Saskatchewan. Pages 
47-57 in D.M, Bird [Ed.], The biology and manage- 
ment of Bald Eagles and Osprey. Harpell Press, Mon- 
treal, PQ, Canada. 

, P.N. Gerrard and D.W.A. Whitfield. 1980 

Behavior of a non-breeding Bald Eagle. Can. Field- 
Nat. 94:391-397. 

Harmata, A.R. 1982. What is the function of undu- 
lating flight display in Golden Eagles? /. Raptor Res 
16:103-109. 

. 1984. Bald Eagles of the San Luis Valley, Col- 
orado: their winter ecology and spring migration. Ph.D. 
thesis, Montana State University, Bozeman, MT. 

. 1985. Capture of wintering and nesting Bald 

Eagles. Pages 139-159 in J.M. Gerrard and T.M. 
Ingram [Eds.], The Bald Eagle in Canada. Proceedings 
of Bald Eagle Days 1983. White Horse Plains Pub- 
lishing, Headingly, MB, Canada. 


234 


Jon M. Gerrard et al. 


VoL. 26, No. 4 


AND R. Oakleaf. 1991. A management oriented 

study of the ecology of Bald Eagles in the Greater 
Yellowstone Ecosystem. Draft I. Wyoming Game and 
Fish Department, Lander, WY. 

Hensel, R.J. and W.A. Trover. 1964. Nesting studies 
of the Bald Eagle in Alaska. Condor 66:282-286. 

Jennrich, R.I. AND F.B. Turner. 1969. Measurements 
of noncircular home range. /. Theo. Biol. 22:227-237. 

Mahaffey, M.S. 1981. Territorial behavior of the Bald 
Eagle on the Chippewa National Forest. M.Sc. thesis, 
University of Minnesota, Minneapolis, MN. 

Mattson, M.P. 1974. Interaction of a breeding pair of 
Bald Eagles with subadults at Sucker Lake, Michigan. 
M.A. thesis, St. Cloud State University, St. Cloud, 
MN. 

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


MBEWG (Montana Bald Eagle Working Group). 
1986. Montana Bald Eagle management plan. USDI, 
Bureau of Land Management, U.S. Department of the 
Interior, Billings, MT. 

Pettingill, O.W., Jr. 1970. Ornithology in laboratory 
and field. Burgess Publishing Company, Minneapolis, 
MN. 

Retfalvi, L.I. 1965. Breeding behavior and feeding 
habits of the Bald Eagle {Haliaeetus leucocephalus) on 
San Juan Island, Washington. M.S. thesis, University 
of British Columbia, Vancouver, BC, Canada. 

USFWS (U.S. Fish and Wildlife Service). 1986. Pa- 
cific Bald Eagle recovery plan. Fish and Wildlife Ser- 
vice, U.S. Department of the Interior, Portland, OR. 


Received 22 June 1990; accepted 20 August 1992 


J. Raptor Res. 26(4):235-238 
© 1992 The Raptor Research Foundation, Inc. 


SEASONAL AND SEXUAL VARIATION IN THE 
DIET OF THE COMMON BUZZARD IN 
NORTHEASTERN SPAIN 

Santi Map^osa 

Departament de Biologia Animal, Universitat de Barcelona, 08028 Barcelona, Spain 


Pedro J. Cordero 

Consejo Superior de Investigaciones Cientificas, Instituto Pirenaico de Ecologia, 
Apartado 202, 50080 Zaragoza, Spain 


Abstract. — We examined the diet of Common Buzzards {Buteo buteo) from a Mediterranean area 
(Catalonia, NE Spain), by analyzing prey remains and pellets found in the nest, and stomach contents. 
The diet was seasonal. Relatively large items, such as young rabbits and Ocellated Lizards (Lacerta 
lepida), predominated in the breeding season, orthopterans and mantodeans in autumn and insects, rodents 
and soricidans in winter. Males presented an empty stomach more often than females, but only small 
differences were found in the diet of males and females. 

Resumen. — Se analiza la variacion estacional en la dieta del ratonero comiin {Buteo buteo) en una zona 
mediterranea (Cataluna, NE Espaha) a base de restos de presas y egagropilas encontrados en los nidos 
y al analisis de contenidos estomacales. La dieta vario estacionalmente. Durante el periodo reproductive, 
el ratonero consumio presas relativamente grandes, tales como gazapos y lagartos, mientras que en otono 
consumio preferentemente ortopteros y mantidos. En invierno los insectos, roedores y musaranas constitu- 
yeron la base de la dieta. Los machos presentaron el estomago vacio con mayor frecuencia que las hembras 
pero solo se detectaron pequehas diferencias en la dieta de ambos sexos. 


The Common Buzzard {Buteo buteo) feeds on a 
wide range of prey, mainly rodents, but also on other 
vertebrates and invertebrates of appropriate size 
(Cramp and Simmons 1980). Its diet has been stud- 
ied in most parts of its geographical range, and it 
reflects underlying differences in prey availability 
(Bustamante 1985). However, seasonal variation has 
received much less attention, probably because of 
difficulty in studying the diet outside the breeding 
season. In this paper we investigate the seasonal 
variation of the diet of the Common Buzzard in a 
Mediterranean area, where the species is present 
throughout the year. 

Study Area and Methods 

Diet outside the breeding season was studied by ana- 
lyzing the stomach contents of 69 Common Buzzards con- 
fiscated from hunters in the Mediterranean area of Cat- 
alonia (NE Spain) between October and February of 1982- 
87 The sex of 39 individuals was identified. Diet during 
the breeding season was studied in La Segarra county of 
Catalonia by collecting prey remains and pellets from 20 
nests, during and after reproduction in 1985-89. Pellets 
were especially useful to identify small prey, which are 
rarely found as items in the nest (Manosa 1991, Real 
1991). The importance of each prey was expressed as the 
percentage of appearance of that prey among all prey items 
in nests, pellets or stomachs. 


Results and Discussion 

Diet During the Breeding Season (Spring- 
Summer). European Rabbit {Oryctolagus cuniculus) 
was the most frequent prey species (Appendix 1). 
This has been found in other Mediterranean areas 
(Veiga 1982, Real 1987) but is unlike the deciduous 
forest region of Northern Spain where invertebrates 
form the bulk of the diet (Bustamante 1985). The 
Common Buzzard captured mainly young rabbits, 
very abundant in spring and summer (Soriguer 1981). 
The mean tarsus length of the rabbits taken was 
37.5 mm (SD = 5.7, range = 26-64, N = 122), 
which corresponds to a mean weight of less than 550 
g (Manosa 1991). The second most consumed prey 
were reptiles, especially Ocellated Lizard {Lacerta 
lepida), also very common in spring and summer 
(Castilla 1989). Several species of birds formed an 
important percentage of the diet. Invertebrates, am- 
phibians, rodents and shrews were taken only oc- 
casionally (Table 1). 

Diet Outside the Breeding Season. Only 45 
(65%) of the 69 stomachs analyzed contained at least 
one prey. A total of 240 prey items were found 
(Appendix 1). Insects were the most frequent prey 
both in autumn and winter (Table 1). Rabbits were 


235 


236 


Santi Mai^Josa and Pedro J. Cordero 


VoL. 26, No. 4 


Table 1. Diet of Common Buzzard in Catalonia (NE Spain) expressed in percentages. Autumn includes October- 
November, winter December- February and spring and summer the breeding season. 


Spring and Summer 
Remains Pellets 

Autumn 

Winter 

Autumn and 
Winter 

Mammals 

69.90 

49.75 

7.28 

38.2 

18.74 

Shrews 

0.33 

0.00 

0.00 

15.73 

5.83 

Rabbits 

66.55 

21.89 

0.66 

2.25 

1.25 

Voles 

0.00 

6.47 

3.31 

6.74 

4.58 

Mice 

1.34 

5.47 

1.32 

10.11 

4.58 

Other mammals 

1.67 

15.92 

1.99 

3.37 

2.50 

Birds 

16.50 

12.44 

0.00 

3.37 

1.25 

Reptiles 

13.88 

35.82 

3.31 

3.37 

3.33 

Amphibians 

0.17 

0.00 

0.66 

7.87 

3.33 

Insects 

0.00 

1.99 

87.42 

43.82 

71.26 

Mantodeans 

0.00 

0.00 

31.79 

17.98 

26.67 

Orthopterans 

0.00 

1.49 

54.97 

19.10 

41.67 

Coleopterans 

0.00 

0.50 

0.66 

6.74 

2.92 

Other invertebrates 

0.00 

0.00 

1.32 

3.37 

2.08 

Total 

598 

201 

152 

88 

240 


taken only occasionally, and rodents and shrews were 
the most common mammalian prey. Because of the 
low temperatures, reptiles are not available during 
the autumn and winter periods (Castilla 1989), and 
their presence in the diet was restricted. In winter, 
most insect populations decrease, and small mam- 
mals (rodents and shrews) increase in their impor- 
tance in the diet. Then they are especially abundant 
in open fields, where they lack cover as they feed. 
Amphibians increased their presence in the winter 
diet, when they concentrate around their breeding 
pools (Valverde 1967). 

Seasonal and Sexual Comparison. Compared 
with pellet analysis, collections of prey remains in 
nests underestimates small preys (invertebrates, small 
mammals and reptiles). On grouping prey into in- 
vertebrates, poikilotherm vertebrates, birds, rabbits, 
and small mammals, differences were significant (x^ 
= 199.038, df = 4, P < 0.01; Table 1). We analyzed 
seasonal variation by comparing pellet data with 


stomach contents, eliminating by this way the bias- 
associated with prey remain collections. On grouping 
prey into invertebrates, amphibians, reptiles, birds, 
rabbits and small mammals, we found differences 
between breeding-season, autumn and winter diets 
(x^ = 341.436, df = 10, P < 0.01; Table 1). Buzzards 
consumed bigger prey during the breeding season 
than outside it, but it is possible that adults carried 
only large prey to the nests and consumed small prey 
themselves, as was observed in other raptors (Veiga 
1982, Donazar 1988). This possibility should be 
taken into account when interpreting our results. 
The sex ratio of buzzards killed did not differ sig- 
nificantly from unity (20 males and 19 females; x^ 
= 0.026, P > 0.05) with no variation between au- 
tumn and winter (x^ = 0.779, P > 0.05). Data from 


Table 3. Number of Common Buzzard stomachs in which 
different prey were found in relation to sex. 


Table 2. Frequency of full compared to empty stomachs 
according to sex of Common Buzzards. 


Males 

Females 

Total 

Full 

9 

17 

26 

Empty 

10 

3 

13 

Total 

19 

20 

39 


Males 

(V = 9) 

Females 

(V = 17) 

Invertebrates 

5 

8 

Amphibians 

3 

0 

Reptiles 

1 

2 

Birds 

1 

1 

Rabbits 

0 

2 

Small mammals 

8 

10 


December 1992 


Seasonal Diets of Common Buzzards 


237 


both seasons could therefore be pooled to analyze 
differences in diet between the sexes. Empty stom- 
achs were found more often in males than females 
(x^ = 6.278, P < 0.025; Table 2). Although sample 
sizes were small, we found amphibians more often 
in stomachs of males than females (Fisher exact test 
P = 0.03) and Rabbits were taken only by females 
(Table 3). These differences may be related to the 
sexual dimorphism of the species (male weight = 
828 g, female weight = 1052 g; Cramp and Simmons 
1980) and can be explained either by prey selection 
or by habitat partitioning, as shown in Hen Harriers 
(Circus cyaneus; Newton 1979, Marquiss 1980), Eu- 
ropean Sparrowhawks (Accipiter nisus; Marquiss and 
Newton 1982, Newton 1986), or American Kestrels 
(Falco sparverius; Smallwood 1987, 1988). 

Acknowledgments 

We are grateful to Sheila Hardie for improving the 
English of the manuscript. We are also very grateful to 
J.E. Jimenez and R. Kenward for their useful comments 
of an early version of our manuscript. 

Literature Cited 

Bustamante, J.M. 1985. Alimentacidn del ratonero 
comun (Buteo buteo) en el norte de Espana. Donana 
Acta Vertebrata 12:51-62. 

Castilla, A.M. 1989. Autoecologia del lagarto ocelado 
(Lacerta lepida). Tesis doctoral, Universidad Autonoma 
de Madrid, Madrid, Spain. 

Cramp, S.C. and K.E.C. Simmons [Eds.]. 1980. The 
birds of the western Palearctic. Vol. 2. Oxford University 
Press, Oxford, U.K. 

Donazar, J.A. 1988. Variaciones en la alimentacion 
entre adultos reproductores y polios del Buho Real 
(Bubo bubo). Ardeola 35:278-284. 

Ma51osa, S. 1991. Biologia trofica, us de I’habitat i 
biologia de la reproduccio de I’astor (Accipiter gentilis, 


Linnaeus, 1758) a la Segarra. Tesis doctoral, Univer- 
sitat de Barcelona, Barcelona, Spain. 

Marquiss, M. 1980. Habitat and diet of male and fe- 
male Hen Harriers in Scotland in winter. British Birds 
73:555-560. 

and I. Newton. 1982. Habitat preference in 

male and female Sparrowhawks Accipiter nisus. Ibis 
124:324-328. 

Newton,!. 1979. Population ecology of raptors. T. and 
A.D. Poyser, Berkhamsted, U.K. 

. 1986. The sparrowhawk. T. and A.D. Poyser, 

Calton, U.K. 

Real, J. 1987. L’organitzacid d’una comunitat de rapin- 
yaires a la Catalunya mediterranea. I: Accipitriformes 
i Falconiformes. Unpubl. report. Caixa de Barcelona, 
Barcelona, Spain. 

. 1991. h’ kliga perdigucra. Hieraaetus fasciatus 2 l 

Catalunya: status, ecologia trofica, biologia reproduc- 
tora i demografia. Tesis doctoral, Universitat de Bar- 
celona, Barcelona, Spain. 

Smallwood, J.A. 1987. Sexual segregation by habitat 
in American Kestrels wintering in south-central Flor- 
ida: vegetative structure and responses to differential 
prey availability. Condor 89:842-849. 

. 1988. A mechanism of sexual segregation by 

habitat in American Kestrels (Falco sparverius) win- 
tering in south-central Florida. Auk 105:36-46. 

Soriguer, R.C. 1981. Biologia y dinamica poblacional 
de una poblacidn de conejos (Oryctolagus cuniculus, L) 
en Andalucia occidental. Donana Acta Vertebrata 8(3) 

Valverde, J.A. 1967. Estructura de una comunidad de 
vertebrados terrestres. Consejo Superior de Investiga- 
ciones Cientificas, Madrid, Spain. 

Veiga, J.P. 1982. Ecologia de las rapaces de un ecosiste- 
ma de montaha. Aproximacion a su estructura co- 
munitaria. Tesis doctoral, Universidad Complutense 
de Madrid, Madrid, Spain. 

Received 5 December 1991; accepted 20 August 1992 


238 


Santi Manosa and Pedro J. Cordero 


VoL. 26, No. 4 


Appendix 1. Prey items of the Common Buzzard during the breeding and non-breeding season in Catalonia (NE 
Spain). Species with less than five representatives are grouped in the “other” category. 


Breeding Season 

Remains 

Pellets 

Fall and Winter 


N 

% 

N 

% 

N 

% 

Mammals 

416 

69.60 

100 

49.75 

445 

18.75 

Crocidura russula 

2 

0.33 


11 

4.58 

Oryctolagus cuniculus 

398 

66.55 

44 

21.89 

3 

1.25 

Sciurus vulgaris 

4 

0.67 

15 

7.46 


Microtus duodecimeostatus 


13 

6.47 

11 

4.58 

Apodemus sylvaticus 

6 

1.00 

11 

5.47 

7 

2.92 

Other^ mammals 

6 

1.00 

2 

1.99 

7 

2.92 

Unidentified small mammals 


15 

7.46 

6 

2.50 

Birds 

96 

16.05 

25 

12.44 

3 

1.25 

Alectoris rufa 

30 

5.02 


1 

0.42 

Columba palumbus 

15 

2.51 


Garrulus glandarius 

11 

1.84 


Unidentified Passeriformes 

12 

2.01 

17 

8.46 


Other^ birds 

12 

2.01 


2 

0.83 

Unidentified birds 

16 

2.67 

8 

3.98 


Reptiles 

83 

13.88 

72 

35.82 

8 

3.33 

Psammodromus algirus 

7 

1.17 

24 

11.94 


Lacerta lepida 

44 

7.36 

25 

12.44 


Ophidians 

31 

5.18 

8 

3.98 

1 

0.42 

Other^ reptiles 

1 

0.17 


7 

2.92 

Unidentified reptiles 


15 

7.46 


Amphibians 

1 

0.17 


8 

3.33 

Bufo sp. 


5 

2.08 

Other^ amphibians 

1 

0.17 


3 

1.25 

Arthropods 


4 

1.99 

175 

72.92 

Mantis religiosa 


64 

26.67 

Grillotalpa grillotalpa 


13 

5.42 

Unidentified Acrididae 


5 

2.08 

Other orthopterans 


3 

1.49 

6 

2.50 

Coleopterans 


1 

0.50 

7 

2.92 

Other^ arthropods 


4 

1.67 

Anelids 


1 

0.42 

Oligochets 


1 

0.42 

Total prey 

598 


201 


240 


® Other prey items include; Mammals: Suncus etruscus, Eliomys quercinus, Rattus rattus, Mus spretus, Mus sp., Unidentified Muridae, 
Mustela nivalis. Birds: Columba oenas, Otus scops, Athene noctua, Alaudidae, Saxicola torquata, Turdus merula, Turdus viscivorus, Onolus 
oriolus, Sturnus vulgaris, Fringilla coelebs, Emberiza cirlus. Miliaria calandra. Nestling Passeriforme. Ophidians: Malpolon monspessulanus, 
Elaphe scalaris, Unidentified Ophidians. Reptiles; Podarcis hispanica, Blanus cinereus. Matrix natrix, Matrix sp., Anguis fragilis, Vipera 
latastii. Amphibians: Bufo calamita, Hyla meridionalis, Rana perezi. Goleopterans: Tenebrionidae, Carabidae, Timarcha tenebricosa, Cetonia 
aurata, Ceotrupes stercorarius. Unidentified Goleopterans. Orthopterans: Gryllus campestris, Oedipoda sp., Anacridium sp., Unidentified 
Orthopterans. Arthropods: Camponotus cruentatus, Lepidoptera Larvae, Disdera sp.. Unidentified Isopoda. 


J Raptor Res. 26(4):239-242 
© 1992 The Raptor Research Foundation, Inc. 


DIET CHANGES IN BREEDING TAWNY OWLS 

{Strix aluco) 


David A. Kirk^ 

Department of Zoology, 2 Tillydrone Avenue, University of Aberdeen, 
Aberdeen, Scotland, U.K. ABO 2TN 


Abstract. — I examined the contents of Tawny Owl (Strix aluco sylvatica) pellets, between April 1977 
and February 1978, in mixed woodland and gardens in northeast Suffolk, England. Six mammal, 14 
bird and 5 invertebrate species were recorded in a sample of 105 pellets. Overall, the Wood Mouse 
(Apodemus sylvaticus) was the most frequently taken mammal prey and the House Sparrow (Passer 
domesticus) was the most frequently identified bird prey. Two types of seasonal diet change were found; 
first, a shift from mammal prey in winter to bird prey in the breeding season, and second, a shift from 
small prey in the winter to medium-sized (>30 g) prey in the breeding season. Contrary to some findings 
elsewhere in England, birds, rather than mammals, contributed significantly to Tawny Owl diet during 
the breeding season. 


Cambios en la dieta de buhos de la especie Strix aluco durante el periodo de reproduccion 

Extracto. — He examinado el contenido de egagropilas del buho de la especie Strix aluco sylvatica, entre 
abril de 1977 y febrero de 1978, en florestas y huertos del noreste de Suffolk, Inglaterra. Seis mamiferos, 
catorce aves y cinco especies invertebradas fueron registrados en una muestra de 105 egagropilas. En el 
total, entre los mamiferos, el roedor Apodemus sylvaticus fue el que con mas frecuencia fue presa de estos 
buhos; y entre las aves, la presa identificada con mas frecuencia fue el gorrion Passer domesticus. Dos 
tipos de cambio en la dieta estacional fueron observados; primero, un cambio de clase de presa: de 
mamiferos en invierno a la de aves en la estacion reproductora; y segundo, un cambio en el tamano de 
las presas: de pequenas en el invierno a medianas (>30 g) en la estacion reproductora. En contraste con 
hallazgos realizados en otras partes de Inglaterra, las aves, en vez de los mamiferos, contribuyeron 
significativamente a la dieta del Strix aluco sylvatica durante las estacion reproductora. 

[Traduccion de Eudoxio Paredes-Ruiz] 


The diet of many owl species is influenced by 
habitat and season (e.g., Marti 1974, Yalden 1985, 
Mikkola 1983). Among sedentary “generalist” spe- 
cies, Tawny Owls (Strix aluco sylvatica) inhabiting 
deciduous woodland in England preyed on Bank 
Voles (Clethrionomys glareolus) and Wood Mice 
(Apodemus sylvaticus) in winter, but switched to 
Moles (Talpidae), young Rabbits (Oryctolagus cun- 
iculus), Cockchafers (Melolontha melolontha) and 
earthworms (Lumbricina) in summer (Southern 1954, 
1969). In urban or other open habitats, birds may 
form important components of Tawny Owl diet (e.g., 
Harrison 1960, Beven 1965, Yalden and Jones 1971, 
Glue 1972), but these have generally been aggre- 
gated in analyses so that the seasonal importance of 
different species or size classes cannot be investi- 
gated. 


' Present address; Canadian Wildlife Service, Ontario Re- 
gion, 49 Camelot Drive, Nepean, Ontario, Canada KIA 
0H3. 


Few data exist with regard to Tawny Owl diet 
in discontinuous woodland habitats, where prey spe- 
cies and hunting techniques may differ from that of 
owls inhabiting larger forest tracts (Nilsson 1978). 
In this paper, I report on seasonal variation in the 
diet of Tawny Owls from a site in south-eastern 
England in relation to breeding and possible changes 
in prey selection or availability. Because Tawny Owls 
disgorge pellets before roosting (Guerin 1932), pel- 
lets are scattered throughout territories, making them 
difficult to find. However, in this study sufficient 
numbers of pellets were found by intensive searching 
and knowledge of roost sites of individual owls. 

Study Area and Methods 

This study was carried out between April 1977 and 
February 1978 at Herringfleet, north-east Suffolk, in a 
0.06 km^ woodland dominated by Scots Pine (Pinus syl- 
vestris), with mixed woods of birch (Betula pendula), oak 
(Quercus robur), rowan (Sorbus aucuparia), maple (Acer 
platanoides) and ash (Fraxinus excelsior), interspersed with 
large gardens. Marshes used for grazing and reedbeds 
(Phragmites australis) occur along a river to the west and 


239 


240 


David A. Kirk 


VoL. 26, No. 4 


Table 1. Total numbers and percentage contribution by weight of prey species recovered in Tawny Owl pellets, 
during and outside the breeding season (April- August; non-breeding season September-February). 


Prey Species 

Breeding Season 

Winter Season 

No. 

Weight 

(g) 

% 

Weight 

No. 

Weight 

(g) 

% 

Weight 

Common Shrew Sorex araneus 

0 

0 

0 

1 

8 

0.3 

Wood Mouse Apodemus syluaticus 

4 

72 

4.4 

34 

612 

20.7 

Field Vole Microtus agrestis 

0 

0 

0 

14 

294 

10.0 

Bank Vole Clethrionomys glareolus 

1 

16 

1.0 

29 

464 

15.7 

Rabbit Oryctolagus cuniculus 

1 

100 

6.1 

0 

0 

0 

Norway Rat Rattus norvegicus 

3 

180 

10.9 

4 

240 

8.1 

Kestrel Falco tinnunculus 

1 

220 

13.3 

0 

0 

0 

Wren Troglodytes troglodytes 

0 

0 

0 

1 

8 

0.3 

Dunnock/Robin Prunella modularis / Erithacus rubecula 

1 

20 

1.2 

1 

20 

0.7 

Blackbird/Song Thrush Turdus merula/T. philomelos 

7 

599 

36.3 

3 

257 

8.7 

Redwing/Starling Turdus iliacus / Sturnus vulgaris 

0 

0 

0 

5 

368 

12.5 

Coal Tit/Blue Tit Parus ater/P. caeruleus 

0 

0 

0 

4 

48 

1.6 

Jay Garrulus glandarius 

1 

161 

9.8 

0 

0 

0 

Starling Sturnus vulgaris 

2 

164 

9.9 

0 

0 

0 

House Sparrow Passer domesticus 

0 

0 

0 

12 

294 

10.0 

Chaffinch Fringilla coelobs 

0 

0 

0 

2 

49 

1.7 

Greenfinch Carduelis chloris 

0 

0 

0 

2 

52 

1.8 

Small bird (unidentified) 

5 

100 

6.1 

10 

200 

6.8 

Dor beetle Geotrupes stercocarius 

0 

0 

0 

30 

30 

1.0 

Dung Beetle Typhaeus typhoeus 

13 

13 

0.8 

6 

6 

0.2 

Cockchafer Melolontha melolontha 

4 

4 

0.2 

0 

0 

0 

Beetles Carabidae 

0 

0 

0 

4 

0.4 

0.01 

Earthworms Lumbricidae 

1 

0 

0 

34 

0 

0 

Total 

43^ 

1649 


162^ 

2950 


® Excluding earthworms. 


farmland to the east. Exotic shrubs such as rhododenron 
{Rhododendron spp.) and laurel {Prunus laurocerasus) pro- 
vide roosts for small birds during winter. 

I collected pellets at weekly intervals at roosts in 2-3 
ha of mature Scots Pine in two large wooded gardens. Of 
the 105 pellets, 77% were collected during the first 5 mo, 
the remaining 23% were collected between September and 
February. One pair of Tawny Owls nested in a nestbox, 
approximately 300 m from the roost sites used for pellet 
collection. However, few pellets were found beneath the 
nestbox. The principal source of pellets was from this pair 
of owls but due to territorial infringements some pellets 
might have been from other individuals (territories in dis- 
continuous woodland in Wytham averaged 22 ha; Hirons 
1985). I collected up to 16 pellets per week from October- 
February (21% of pellets cast by owls, assuming 1.27 
pellets/day are produced in winter; Lowe 1980), but be- 
tween April and September relatively few pellets (1-6 per 
week) were found (6% of pellets cast, assuming 1.03 pel- 
lets/day are produced in summer) for the reasons described 
by Southern and Lowe (1968). 

Mammal remains were identified to species by dental 
and cranial features (Yalden 1977), while birds were iden- 
tified by comparing skulls or bills with reference skeletons 
collected locally. Other remains used to identify birds were 


feet, pelvises, gizzard size and feathers in the pellet matrix. 
The number of individuals represented was determined 
by counts of skulls, jaws or pelvises for mammals, and 
skulls, mandibles and long bones for birds as suggested by 
experiments with Tawny and other owl species (Short and 
Drew 1962, Raczyhski and Ruprecht 1974). 

Coleoptera were identified by elytra striations and chi- 
tinous remains. Earthworms were identified by chaetae 
and I estimated earthworm numbers by the proportion of 
fibrous material and sand in pellets (Southern 1954), Es- 
timates of earthworms were excluded from table totals 
because they were not comparable with counts of other 
prey. Average weights of bird species were calculated by 
the length of humeri recovered in pellets using the re- 
gression equation; log weight = (2.706 x log humerus 
length) — 2.062 (Yalden 1977) or by using average weights 
in Hickling (1983, Appendix 12). I used data in Yalden 
(1977, 1985) for weights of small mammals and Cole- 
optera. 

Results and Discussion 

I recorded 6 mammal, 14 bird and 5 invertebrate 
species in the 105 pellets examined (Table 1). Of a 
total of 204 prey items recovered from pellets (ex- 


December 1992 


Tawny Owl Diet 


241 


eluding earthworms), 45% by number were mam- 
mals. Wood Mice predominated (19%), followed by 
Bank Voles (15%) and Field Voles (7%). Birds com- 
prised 28% of total prey; birds smaller than 30 g 
estimated body weight contributed 19%. Of the spe- 
cies identified, House Sparrows (6%) and thrushes 
{Turdus spp. 5%) were most important. Numerically, 
Coleoptera represented 30% of total diet. By weight, 
mammals formed 43% and birds 56%, respectively. 
European Blackbirds and Song Thrushes were most 
important by weight (19%), followed by Wood Mice 
(15%), Bank Voles (10%) and Norway Rats (9%). 
The contribution of Coleoptera by weight was neg- 
ligible. 

Significantly more birds than mammals were taken 
between April-August than between September- 
February (G = 8.08, P < 0.005), suggesting a switch 
from small rodents to birds during the breeding pe- 
riod. Also, significantly more medium-sized than 
small vertebrate prey were taken by owls during the 
breeding season than in autumn and winter when 
the converse was true (G = 24.29, P < 0.001; Table 
2). Similar results were found when bird prey were 
considered separately (G = 8.57, P < 0.005), al- 
though the total biomass intake of small and me- 
dium-sized birds was similar between the breeding 
and non-breeding season (Table 2). The weight of 
vertebrate prey was significantly higher in the breed- 
ing than the non-breeding season (breeding season 
X = 62.8 g, SE = 9.7, N = 26; non-breeding x - 
23.9 g, SE = 1.5, V = 122; Mann Whitney U test, 
Z = 4.473, P < 0.001), but no difference was found 
when invertebrates were included. 

During the breeding season, thrushes, European 
Starlings, Jay, and Eurasian Kestrel together con- 
tributed 69% of the diet by weight. Wood Mice and 
Bank Voles comprised only 5%. In winter. Wood 
Mice, Bank Voles, Field Voles and Common Shrews 
accounted for 47% of the diet by weight. Small birds 
(e.g.. House Sparrows) comprised 23% of the winter 
diet by weight (Tables 1 and 2). Earthworms were 
taken mostly in the non-breeding season, particu- 
larly in October and December, probably due to the 
wet conditions in these months resulting in increased 
availability of earthworms. The importance of earth- 
worms was probably greatly underestimated since I 
did not weigh the granular content of pellets as rec- 
ommended by Yalden (1985). Two species of dung 
beetles {Geotrupes stercocarius and Typhaeus ty- 
phoeus) were also taken in large numbers. Geotrupes 
occurred in pellets most frequently in autumn and 
winter, showing that Tawny Owls foraged over 


Table 2. The contribution of different-sized prey in 
Tawny Owl diet during and outside the breeding season 
(percentage figures refer to weight of prey items in grams). 


Breeding 
Season 
(24 Pellets) 

Winter Season 
(81 Pellets) 


No. 

% Weight 
( g) 

No. 

% Weight 
( g) 

Small mammals 

5 

5.3 

78 

46.7 

Medium- sized 

4 

17.0 

4 

8.1 

mammals 
Small birds 

6 

7.3 

32 

22.7 

Medium-sized birds 

11 

69.4 

8 

21.2 

Invertebrates^ 

18 

1.0 

44 

1.2 

Total number 
Total weight 

43 

1649 

166 

2950 


® Excluding earthworms. 

marshes where there were cattle. Typhaeus was most 
commonly taken in the summer (Table 1). Cock- 
chafer beetles were found in pellets from June and 
July, the period of emergence for this species. 

A shift from small to larger (mammal) prey in 
Tawny Owl diet during the breeding season was 
also found in Derbyshire, England, but in contrast 
to my study bird prey was most important in the 
‘winter’ season (Yalden 1985, Table 2). Increased 
weight of prey taken by Tawny Owls during the 
breeding season was suggested by Nilsson (1984) in 
Sweden, and by Southern (1954) in Oxfordshire, 
England. However, in the latter study owls appar- 
ently did not prey on abundant fledgling passerines. 

The increase in medium-sized (often fledgling) 
bird prey in this study during the breeding season 
suggested that Tawny Owls might selectively take 
larger prey when they have chicks, as noted by Mik- 
kola (1983, Table 26) and as documented in some 
Common Barn Owl {Tyto alba) pairs (Buckley and 
Goldsmith 1975). Southern (1969) also recorded diet 
changes in Wytham owls when their young were 
half-grown and no longer brooded. Tawny Owls are 
sexually dimorphic, and the larger size of females 
(26% heavier than males; Hardy et al. 1981) might 
allow them to select larger prey than the male (for 
other owl species see Earhart and Johnson 1970, 
Mikkola 1983), especially when they have limited 
hunting time due to demands from their chicks. Also, 
individual Tawny Owls can specialize on particular 
prey types, so perhaps the female owl in this study 
selectively took large bird prey at this time. Con- 
versely, more small birds were found in winter pel- 


242 


David A. Kirk 


VoL. 26, No. 4 


lets probably as a result of owls feeding on com- 
munally roosting birds as occurs in Long-eared Owls 
{Asia otus; Glue and Hammond 1974). 

Prey availability for Tawny Owls is determined 
by ground cover (Southern and Lowe 1968) which 
may account for seasonal change in the diet of Taw- 
ny Owls at Herringfleet. Dense vegetation cover (es- 
pecially Bracken Pteridium aquilinum) in summer 
could prevent owls from locating small mammal prey. 
Conversely, dieback of vegetation in autumn may 
mean that small rodents are more vulnerable to owl 
predation. Rodents might make more noise moving 
through leaf litter on the ground in autumn and 
winter and thus be more easily located by foraging 
owls. The fact that a major prey species, the Wood 
Mouse, spends less time foraging outside the nest in 
winter on moonlit nights (Wolton 1983) also indi- 
cates that small mammals are more vulnerable to 
Tawny Owl predation in the winter season. Thus, 
changes in vegetation cover could account for the 
abundance of small rodents in Tawny Owl diet at 
Herringfleet during the winter season. 

My results suggest one, or a combination of factors 
in the apparent diet shift; 1) small mammals were 
more vulnerable to owl predation outside the breed- 
ing season; 2) owls switched, opportunistically, to 
fledgling birds during the breeding season because 
they were easier to catch or 3) medium-sized birds 
were taken selectively by owls because they were 
more ‘profitable’ prey (with a higher nutrient intake 
per handling time) than rodents, during the period 
when owls had dependent young. 

Acknowledgments 

I thank A. Johnson, I. Kirk, T.C. Kirk, D, Knott, and 
K. Robinson for their encouragement with this work and 
F Bulsara for inspiration. Earlier versions of the manu- 
script were improved by comments from P.J. Ewins, C.D. 
Marti, M. Mbnkkbnen and D.G. Smith. 

Literature Cited 

Beven, G. 1965. The food of Tawny Owls in London. 
London Bird Kept. 29:56-72. 

Buckley, J. and J.G. Goldsmith. 1975. The prey of 
the Barn Owl {Tyto alba alba) in east Norfolk. Mammal 
Review 5:13-16. 

Earhart, C.M. and N.K. Johnson. 1970. Size di- 
morphism and food habits of North American owls. 
Condor 72:251-264. 


Glue, D.E. 1972. Bird prey taken by British owls. 

Study 19:91-95. 

AND G.J. Hammond. 1974. Feeding ecology of 

the Long-eared Owl in Britain and Ireland. Br. Birds 
67:361-369. 

Guerin, G. 1932. La Hulotte et son regime. Encyclo- 
pedie Ornithologique. P. Lechevalier, Paris, France. 

Hardy, A.R., G.J.M. Hirons, P.I. Stanley and L.W. 
Huson. 1981. Sexual dimorphism in size of Tawny 
Owls (Strix aluco): a method for sexing in field studies. 
Ardea 69:181-184. 

Harrison, C.J.O. 1960. The food of some urban Tawny 
Owls. Bird Study 7:236-240. 

Hickling, R. (Ed.). 1983. Enjoying ornithology. T. and 
A.D. Poyser, Caltoh, U.K. 

Hirons, G.J.M. 1985. The effects of territorial behavior 
on the stability and dispersion of Tawny Owl (Stnx 
aluco) populations. /. ZooL, Lond. (B) 1:21-48. 

Lowe, V.P.W. 1980. Variation in digestion of prey by 
the Tawny Owl (Strix aluco). J. ZooL, Lond. 192:283- 
293. 

Marti, C.D. 1974. Feeding ecology of four sympatric 
owls. Condor 76:45-61. 

Mikkola, H. 1983. Owls of Europe. T. and A.D. Poy- 
ser, Gallon, U.K. 

Nilsson, I.N. 1978. Hunting in flight by Tawny Owls 
Strix aluco. Ibis 120:528-531. 

. 1984. Prey weight, food overlap, and repro- 
ductive output of potentially competing Long-eared 
Owls and Tawny Owls. Ornis Scand. 15:176-182. 

Raczynski, j. and A.L. Ruprecht. 1974. The effect 
of digestion on the osteological composition of owl pel- 
lets. Acta Ornithologica 14:25-38. 

Short, L.L. and L.C. Drew. 1962. Observations con- 
cerning behavior, feeding and pellets of Short-eared 
Owls. Amer. Midi. Nat. 67:424-433. 

Southern, H.N. 1954. Tawny Owls and their prey. 
Ibis 96:384-410. 

. 1969. Prey taken by Tawny Owls during the 

breeding season. Ibis 111:293-299. 

AND V.P.W. Lowe. 1968. The pattern of dis- 
tribution of prey and predation in Tawny Owl terri- 
tories. J. Anim. Ecol. 37:75-97. 

Wolton, R.J. 1983. The activity of free-ranging Wood 
Mice, Apodemus sylvaticus. J. Anim. Ecol. 52:781-794. 

Yalden, D.W. 1977. The identification of remains in 
owl pellets. Occasional Publication. Mammal Society 
Reading, U.K. 

. 1985. Dietary separation of owls in the Peak 

District. Bird Study 32:122-131. 

AND R. Jones. 1971. The food of suburban 

Tawny Owls. The Naturalist 914:87-89. 

Received 18 March 1992; accepted 21 August 1992 


J Raptor Res. 26(4):243-256 
© 1992 The Raptor Research Foundation, Inc. 


FORAGING ECOLOGY OF BALD EAGLES ON A 

REGULATED RIVER 


W. Grainger Hunt 

BioSystems Analysis, Inc., 303 Potrero, No. 203, Santa Cruz, CA 95060 


J. Mark Jenkins 

Technical and Ecological Services, Pacific Gas and Electric Company, 
3400 Crow Canyon Road, San Ramon, CA 94583 


Ronald E. Jackman, Carl G. Thelander and Arnold T. Gerstell 

BioSystems Analysis, Inc., 303 Potrero, No. 203, Santa Cruz, CA 95060 


Abstract. — We studied the habitat, foraging behavior, and prey of eight pairs of Bald Eagles (Haliaeetus 
leucocephalus) nesting along northern California’s Pit River where flows and reservoir elevations were 
regulated by five hydroelectric facilities. Prey remains (N = 1166) and photographic data {N = 117) 
indicated that eagles fed on a variety of fishes (88%), birds (9%), and mammals (4%), but one species, 
Sacramento Sucker (Catostomus occidentalis) dominated the diets of all pairs. Bald Eagle prey utilization 
at Britton Reservoir was directly related to the abundance of fish species inventoried by surface gill nets. 
Bald Eagles ate Sacramento Sucker and Tule Perch (Hysterocarpus traski) as carrion in late May, June, 
and July when these species became numerous on the surface of the reservoir. Eagles nesting near 
relatively small run-of-river reservoirs downstream of Britton Reservoir foraged in both lacustrine and 
riverine habitats. On the river sections, eagles selected hunting perches near pools rather than runs or 
riffles. In pools, live suckers were taken mainly in shallow areas where there was no surface turbulence. 
Inventories indicated that fish were less common in pools than in runs or riffles, suggesting that physical 
conditions promoting prey vulnerability were more important to eagles than those influencing prey density. 
However, eagles did not use a large section of river where suckers of appropriate sizes for eagles were 
uncommon. 


Habitos de alimentacion de Aguila Cabeciblanca en un rio de corriente regulada 

Extracto. — Hemos estudiado el habitat, la conducta en la alimentacion, y las presas de ocho parejas 
de Aguila Cabeciblanca {Haliaeetus leucocephalus) que anidaban a lo largo de rio Pit en California del 
norte. En este rio el volumen del flujo del agua y la cantidad de ella en las represas estaban regulados 
por medio de cinco plantas hidroelectricas. Los residuos de presas {N = 1166) asi como datos fotograficos 
{N = 117) indicaron que las aguilas se alimentaron de una variedad de peces (88%), de aves (9%), y de 
mamiferos (4%); pero una especie de pez perteneciente a la especie Catostomus occidentalis domino la dieta 
de todas las parejas. La utilizacidn de las presas de Aguila Cabeciblanca en la represa Britton, estuvo 
directamente relacionada con la abundancia de especies de peces cogidos por redes tendidas en la superficie 
del agua. Hacia fines de mayo, en junio y julio, las aguilas comieron carroiia de peces C. occidentalis y 
Hysterocarpus traski, cuando estas especies se hacen numerosas en la superficie del estanque. Aguilas que 
anidaban cerca de relativamente pequenos estanques, los que se Henan con agua de la represa Britton, se 
alimentaron tanto en habitats lacustres como fluviales. En las secciones riverenas, las aguilas seleccionaron 
las perchas de observacion para cazar prefiriendo la cercania a albercas que a corrientes rapidas o 
turbulentas. En las albercas, peces vivos fueron cogidos principalmente en areas de poca profundidad 
donde no habia turbulencia superficial. Los conteos indicaron que los peces fueron menos numerosos en 
albercas que en secciones de rapidos y turbulencias; lo que sugiere que las condiciones fisicas que 
promueven la vulnerabilidad de las posibles presas, fueron mas importantes para las aguilas que las 
condiciones que influencian la abundancia de las presas. Sin embargo, las aguilas no usaron una gran 
seccion del rio donde los peces del tamano apropiado para ellas no fueron comunes. 

[Traduccidn de Eudoxio Paredes-Ruiz] 


Foraging success of raptors depends on the com- 
position, densities, life histories, and behaviors of 
prey species, and the physical and biotic elements of 


habitat that contribute to prey vulnerability. Raptor 
foraging patterns may coincide with prey abundance 
(Hunt et al. 1992a) or depend on the distribution 


243 


244 


W. Grainger Hunt et al. 


VoL. 26, No. 4 


of specific habitats where prey are vulnerable but 
not necessarily abundant (Hunt and Ward 1988). 
Life history, behavioral, and ecological factors af- 
fecting vulnerability may differ among prey species 
and habitats. 

In this paper, we present the results of a two-year 
study on the foraging ecology of Bald Eagles {Hal- 
laeetus leucocephalus) on northern California’s Pit 
River, where flows are controlled by five hydroelec- 
tric facilities occurring along 70 river km (study 
area). Eight Bald Eagle nesting territories are known 
in the area, and in winter and spring eagle numbers 
are augmented by migrants. 

To explore the interrelationships between eagle 
diet, foraging habitat selection, and factors affecting 
prey availability, we investigated: 1) the distribution 
of nesting and wintering eagles in the study area 
using visual surveys and telemetry, 2) diets of the 
eagles, 3) habitat use in both lacustrine and riverine 
habitats, 4) river habitat distribution, 5) distribution, 
relative abundance, and size classes of prey species, 
and 6) how each major prey fish species became 
vulnerable to Bald Eagles. 


Study Area 

The Pit River originates in the Warner Mountains of 
northeastern California and flows through several broad, 
irrigated valleys to Fall River Mills where it enters a 
narrow, steep-sided canyon that extends for 90 km to 
Shasta Reservoir. Our study area included 70.3 km of this 
canyon, from the river section upstream from Britton Res- 
ervoir downstream to Reservoir 6 (Fig. 1). Within this 
zone are 24.5 km of reservoirs (Britton Reservoir and 
Reservoirs 4, 5, and 6) and 45.8 km of flowing, regulated 
river (Reaches 3, 4, and 5). Rather than producing electric 
power at Dams 3, 4, and 5, water is transported from 
them in underground conduits to powerhouses (turbines) 
located 10-16 river km downstream near the inflow of the 
next reservoir. 

Because of habitat differences, we distinguish between 
Britton Reservoir (13 km long, 520 ha) and the mainly 
riverine environment downstream from it (Lower Study 
Area) where two relatively small run-of-river (currented) 
reservoirs (Reservoirs 4 and 5, 42 ha and 13 ha respec- 
tively) lie between river sections 9.6 to 15.9 km in length. 
In discussions of the river reaches, we sometimes differ- 
entiate between the upper (upstream) and lower (down- 
stream) halves of each reach. 

The area around Britton Reservoir is primarily Pon- 
derosa Pine {Pinus ponderosa) forest (elevation ca. 860 m 
MSL); Sierran mixed-conifer forest is the dominant hab- 
itat type in the lower study area (elevation at Pit 6 Dam 
ca. 430 m MSL). Rainfall averages about 1 m/yr, Rec- 


December 1992 


Foraging Ecology of Bald Eagles 


245 


reational use peaks during May-October and includes 
fishing and camping throughout the study area, and boat- 
ing on Britton Reservoir. 

In warmer months, the level of Britton Reservoir fluc- 
tuates with power demand, resulting in a highly variable 
pattern of drawdown (1-2 m/wk) during weekdays and 
refilling during weekends. Flashboards raise the height of 
the dam almost 2 m and increase generating capacity; they 
are removed in winter when increased flows result in 
spillage over the dam. The reservoir is often turbid with 
algae, particularly in the warmer months. 

The three river reaches (3, 4, and 5) are confined to 
narrow canyons and have coarse-textured substrates, most- 
ly cobbles and boulders covered with algae. During spring 
runoff, flow rates in the reaches are about 100 m^/sec and, 
rarely, up to 565 m^/sec. In the summer, Dams 4 and 5 
provide minimum flow releases into the river sections of 
1.4-4. 2 mVsec on behalf of fisheries (D. Bowers pers. 
comm.). During our study, no water was released from 
the dam at Lake Britton (Dam 3), but about 1.4 m^/sec 
seeped from the dam and underground springs. Because 
none of the warm turbid water from Britton Reservoir 
was released into Reach 3 in summer, water was cooler 
and clearer than in reaches 4 and 5. The fish community 
in Reach 3 reflected these differences. 

Methods 

Bald Eagle Distribution and Habitat Selection. We 

determined the distribution of Bald Eagles in the study 
area by censuses conducted from helicopters, boats, and 
vehicles. We made 82 helicopter censuses from a Bell Jet 
Ranger helicopter, flying at 95-125 km/hr downriver or 
along reservoir shores above the tree tops. Weather per- 
mitting, these censuses were done weekly from March 
1983 to December 1984, usually in the early morning. On 
Britton Reservoir, we censused Bald Eagles and waterfowl 
on 36 surveys (approximately 2/mo) from a boat moving 
slowly along the shore. At Reservoir 4, we censused Bald 
Eagles and waterfowl 104 times from a vehicle slowly 
moving along a road adjacent to the reservoir. 

We recorded the age class of each eagle observed. For 
this analysis, juvenile/immature birds (dark head), sub- 
adults (mottled head), and near-adults (“dirty” white head) 
were all grouped as subadults; only birds with completely 
white heads were considered adults. For each eagle ob- 
served, we noted its location, distance to water, perch type, 
and habitat. We also collected information on waterfowl 
distribution, noting the location, number and species of 
waterfowl observed. Location data were based on a 0.1 
km scale following the river centerline. 

We affixed radio transmitters to seven nesting adults (5 
females, 2 males) and nine subadults of unknown natal 
origin. The radio-tagged adults included four nesting fe- 
males at Britton Reservoir (nests 1, 3, 4, and 5). In the 
lower study area, we radiotagged the adult male at Nest 
7 and both members of the pair at Nest 6. We mounted 
nine of the transmitters on retrices (Young 1983); the other 
seven were backpack-mounted, using teflon ribbons se- 
cured with cotton string over the carina. We captured 
eagles with either floating, noosed fish (Frenzel and An- 
thony 1982, Gain and Hodges 1989) or with padded leg- 
hold traps (Harmata 1985). 


We used telemetry to locate and identify individual ea- 
gles during surveys. Telemetry monitoring sessions of ra- 
dio-tagged adults were conducted by ground vehicle or 
boat throughout the morning hours in both breeding and 
non-breeding months. For analysis of relocation data with- 
in the study area, we considered only the first detection 
per day per location and excluded instances of soaring 
flight; we defined a relocation as a movement of at least 
100 m. Outside the study area, we recorded the movements 
of radio tagged eagles on periodic aerial telemetry surveys 
around the northern California region. 

From a boat and from the shoreline we observed eagles 
foraging in the reservoirs. A dirt road paralleling the river 
allowed access during tracking, although the forest canopy 
often obscured our view. We therefore constructed eight 
blinds along the forested river banks to allow observations 
of foraging in riverine habitat. We chose blind locations 
based on concurrent telemetry data and occupied several 
of them each morning. When a foraging attempt was ob- 
served, and after the eagle departed, we measured; 1) water 
depth, 2) substrate characteristics (e.g., cobble, sand, sed- 
iment), 3) surface turbulence (visually estimated), 4) water 
velocity (time for a floating object to travel 1 m), 5) stream 
habitat type (e.g., pool, run, riffle, see below), and 6) 
vegetation at the strike point. Even if the exact strike point 
could not be observed, certain data could be obtained if 
conditions such as depth and surface turbulence were ho- 
mogeneous over wide areas. If possible, we visually iden- 
tified the prey at the time it was taken, and also searched 
the foraging site later for prey remains. 

Bald Eagle Diets. We determined diet by: 1) collecting 
prey items in and below nests and under perches, 2) ob- 
serving foraging eagles, and 3) time-lapse photography. 
We identified prey remains by comparison with a reference 
collection of study area fishes, using scale keys (Casteel 
1972, 1973), and by comparison with museum bird and 
mammal collections. Using bone length to fish length and 
fish length to weight equations empirically derived from 
fish captured during electrofishing (see below), we com- 
puted estimated total weights for non-duplicate prey items. 
By subtracting bone and scale weights (plus 5% total weight 
to account for inedible biomass) from fish weights in the 
prey reference collection, we obtained values of edible 
biomass. To calculate size and minimum number of fish 
in scale samples we determined scale age (Bagenal and 
Tesch 1978). We used standard weights for estimating 
non-fish prey biomass (Steenhof 1983, Dunning 1984). 

We placed time-lapse movie cameras (Minolta Super-8 
with intervolometers and light-activated switches) at three 
nests in 1983. These cameras, installed in boxes 3-5 m 
above nests, exposed one frame per 90 seconds during 
daylight. 

Habitat Mapping. River habitat downstream from 
Britton Reservoir was mapped in 1984. The distribution 
of riverine habitats did not change with the flow releases 
under study (2.8-8. 5 mYsec), but might change with spring 
run-ofif flows (>50 m^/sec). Aerial photos and ground 
checking were used to classify river sections into the fol- 
lowing categories: “Pools” are depressions in the stream- 
bed, with a major hydraulic control at the downstream 
end. Throughout most of the length and width of the pool 
habitat, current velocities are low relative to prevailing 


246 


W. Grainger Hunt et al. 


VoL. 26, No. 4 


Mammals 
Birds 
Other Fish 
Bullhead (spp.) 

Tui Chub 
Sacramento Squawfish 
Hardhead 
Sacramento Sucker 

0 1 0 20 30 40 50 60 70 

Percent 

Figure 2. Diet of Bald Eagles in the Pit River study area 
as determined from a sample of 1166 prey items identified 
in remains (representing an estimated 938.1 kg of edible 
biomass). 

streamflow. “Runs” are relatively deep, usually narrow 
channels. There is little or no white water in this habitat 
type and the hydraulic control is less distinct than in a 
pool; current velocity is relatively fast. “Riffles” are char- 
acterized by relatively shallow, fast-moving water flowing 
down gradients less steep than cascades and over substrates 
usually no larger than small boulders. “Cascades” are 
steep gradient white water with less than 10% quiet water. 
“Pocket water” usually contains boulders, with fast water 
liberally interspersed across the width of the stream. Pock- 
ets of quiet water (1-3 m in diameter) are frequent. 

The principal river pools where eagles foraged were 
mapped on aerial photos and ground-checked in summer 
1 984. A digital planimeter was used to determine the area 
of various pool characteristics under normal summer flows 
and three experimental release flows. Assessment at each 
flow level included: 1) the presence and surface area of 
three water depth categories (less than 0.6 m (classified 
as “shallow”), 0.7-1. 2 m, and over 1.3 m), 2) the presence 
or absence of surface turbulence (a rippling of the water 
surface that obscures visibility into the pool), 3) the esti- 
mated percentage of green algae or macrophyte coverage, 
4) the total pool area, and 5) the length of the pool tail 
(the shallow area at the downstream end of the pool). 

Prey Fish Distribution and Abundance in Reser- 
voirs. Data on fish abundance and distribution in reser- 
voirs were collected by gill netting, electroshocking, and 
carrion surveys. Vondracek et al. (1989) detailed the gill 
netting and electro fishing procedures. To summarize, gill 
nets were set monthly at either of two coves on Britton 
Reservoir using variable mesh gill nets set at surface (0- 
4 m), midwater (4-8 m), and bottom locations (8-16 m). 
We selected coves known to be eagle foraging areas. Nets 
were 36-38 m long and 1.5-1. 8 m deep. Five nets at each 
depth were set for a minimum of 4 hr during all sampling 
periods. Variable mesh in equal- sized panels (3 m) ranged 
from 20-152 mm. The electrofishing surveys were con- 
ducted monthly at 27 stations on Britton Reservoir, al- 


Biomass 

^53 No. of Individuals 


-V= 


=r 


though activities were suspended during the second Bald 
Eagle breeding season (March-July 1984) to avoid biasing 
Bald Eagle food habit data (electrofishing can kill fish and 
create a carrion food source for eagles). A Cofelt boat- 
mounted electrofisher was generally set at 350 volts DC 
and 60 pulses per second (Vondracek et al. 1989). Elec- 
trofishing stations were about 50 m in length and concen- 
trated in shoreline locations. A diversity of shoreline hab- 
itats were electrofished, including shallow and deep water 
with various bottom substrates. Each captured fish was 
measured and weighed (to develop a length-weight re- 
gression relationship) and then released. 

We surveyed carrion semi-monthly by boat 34 times on 
Britton Reservoir and 32 times on the three reservoirs in 
the lower study area. We used hoopnets to sample dead 
and injured fish emerging from powerhouse turbines at 
the inflow tailrace of Reservoir 4. 

We compared the biomass and frequency of fish species 
in eagle prey remains at Britton Reservoir to the biomass 
and relative abundance of fish species in electrofishing 
samples and in surface (0-4 m) gill nets. We excluded 
fish from the comparison if they were less than the min- 
imum size found as prey (250 mm for most species). 

Prey Fish Distribution and Abundance in the River. 
Snorkeling surveys conducted in early summer and fall 
were used to determine fish abundance and distribution 
in the stream sections (see Baltz et al. 1987 for methods). 
Surveys were stratified by reach, river segment, and hab- 
itat, and were selected to cover various habitats within 
each stream section. Two to four snorkelers worked in an 
upstream direction starting below a selected habitat (see 
Baltz et al. 1987). Survey lengths were determined by 
habitat length and ranged from 25-150 m; 5-20 minutes 
were required to complete each survey. Seventy-three 
stream locations were surveyed four times each; data from 
31 surveys were eliminated from the analysis because of 
poor visibility. Sampling area sizes were calculated from 
measurements of river lengths and widths. Each fish es- 
timated to be over 50 mm SL (standard length: snout to 
base of tail) was recorded. 

We obtained information on fish behavior from blinds 
above two pools in Reach 4 (July, August, and October 
of 1984) and from incidental observations. At half-hour 
intervals from 0600-1130 H, we identified, counted, es- 
timated the size, and noted the location and activity of all 
fish visible in the pool. When visibility was low, we es- 
timated overall fish activity by noting the number of rises 
during the observation period. 

Results 

Eagle Occurrence in the Study Area. During 
the study period, paired eagles occupied eight nesting 
territories: five at Britton Reservoir and one each at 
reservoirs 4, 5, and 6 (Fig. 1). All nest sites were 
within 1 km of reservoirs. Only one was within 100 
m of shore, and this nest was in the area least dis- 
turbed by humans. All nests but one were in mature 
Ponderosa Pines; Nest 8 was in a Douglas-fir {Pseu- 
dotsuga menziesii). 

During our study, mated adults remained near 


December 1992 


Foraging Ecology of Bald Eagles 


247 


their nesting territories throughout the year. They 
generally laid eggs in late February and early March 
with young fledging in mid- to late June. The eight 
pairs fledged 17 young in 1 5 nesting attempts during 
the two years of study. Fledglings departed from the 
study area in late July or early August. We radio- 
tracked five individuals on northward migrations 
apparently directed toward salmon runs in Canada 
or Alaska (Hunt et al. 1992b). 

We observed the greatest number of eagles during 
January and February (13.5 birds per helicopter 
survey); subadults represented 37% of the total. Dur- 
ing this time, subadult eagles were attracted to the 
powerhouse tailrace at Reservoir 4 where small fish 
from Britton Reservoir passed through the turbines 
and became available as carrion. Fewer eagles were 
observed along reservoirs and river sections in March 
and April when adults were incubating or perched 
near nests. In May, June, and July, 33% of all eagles 
were subadults. However, by early August, virtually 
all subadults had vacated the study area; they com- 
prised only 5% of total sightings in September and 
October and 2% in November and December. All 
seven of the subadults radiotagged in winter later 
frequented the Klamath Basin 120 km to the north, 
and three subsequently returned to the study area. 

Diet. Bald Eagles in the Pit River study area fed 
on a variety of prey species taken either alive or as 
carrion. Fish comprised 87%, birds 9%, and mam- 
mals 4% of the 1166 items in our samples (Fig. 2). 
Sacramento Sucker (Catostomus occidentalis) was the 
most important fish species (numbers of individuals 
and biomass) taken by eagles in all parts of the study 
area, followed by Hardhead (Mylopharodon cono- 
cephalus) and Sacramento Squawfish {Ptychocheilus 
grandis) (Table 1). Lower Britton Reservoir eagles 
utilized less suckers and more cyprinids — namely 
Hardhead, Tui Chub {Gila bicolor), and Sacramento 
Squawfish — than in the other regions. 

Chi-square comparisons of prey remains data with 
those collected by time-lapse cameras at individual 
nests and in total (Table 2) did not suggest that 
larger species were over-represented in remains be- 
cause of larger and more persistent bones (see Todd 
et al. 1982). The results also did not indicate that 
Tui Chub, a relatively delicate species, was under- 
represented. Sample sizes for the time-lapse data 
were larger than for prey remains; some remains 
were likely dropped or taken from the nest by the 
eagles while other prey items may have been entirely 
consumed. 


Of 17 species of birds identified in prey remains 
collected throughout the study area, all but 2 were 
waterbirds (Table 1). Birds were most numerous in 
prey samples collected in winter and spring and were 
absent in those obtained in July through October. 
Waterbird numbers in the study area were highest 
in winter and lowest in summer, but the number of 
species was highest (20) in spring. Canada Geese 
{Branta canadensis) were the most abundant (786 of 
2608 bird records) and were present throughout the 
year. American Coots were the second most common 
{N = 576) but were observed only during fall and 
winter. Other common waterbirds were gull {Larus 
spp., N = 196), Common Merganser (Mergus mer- 
ganser, N = 155), Double-crested Cormorant (Phal- 
acrocorax auritus, N = 147), American Widgeon (Anas 
americana, N = 100), and Mallard {Anas platyrhyn- 
chos, N = 99). There was no significant association 
between numbers of the five most common waterbird 
species found in Bald Eagle prey remains and rel- 
ative abundance of these five species recorded in 
waterbird surveys throughout the study area (Spear- 
man rho = 0.20, P > 0.05). 

Foraging on Britton Reservoir. The five pairs 
of bald eagles nesting on Britton Reservoir foraged 
in all portions of the reservoir, but rarely visited the 
river sections upstream or downstream. The linear 
ranges along the reservoir of three radio-tagged adult 
females were 0.7, 2.4, and 2.7 km. Although we 
radiotagged no breeding male eagles on Britton 
Reservoir, visual observations suggested that their 
foraging ranges were similar to those of the radio- 
tagged females. 

The eagles foraged on carrion and moribund fish, 
as well as live prey. We observed 42 forage attempts 
(52% successful) including 22 (52%) in open water 
or flowing reservoir habitat, 17 (41%) in cove, back- 
water, shallow gravel bar, or marsh habitat; 3 (7%) 
were piracies from Osprey {Pandion haliaetus) and 
Great Blue Heron {Ardea herodias). Prey taken were 
fish {N = 22), namely Sacramento Sucker, Carp 
{Cyprinus carpio), Hardhead, and small fish, prob- 
ably Tule Perch {Hysterocarpus traski). At least 8 
fish (36%) were taken as carrion. 

Eagles flying out from shore over deep water took 
carrion fish or attacked fish swimming at or near 
the surface. Alternatively, eagles foraged in reservoir 
shallows, partieularly in coves where they launched 
their attacks at live fish from perches. Fish spawned 
in and around the mouths of tributaries in coves 
where the clear inflow of springs and creeks in- 


248 


W. Grainger Hunt et al. 


VoL. 26, No. 4 


Table 1. Number and edible biomass of fishes, birds and mammals found in Bald Eagle prey remains in five sub- 
units of the Pit River study area. Remains were collected in and below nests and from below perches, during all 
seasons. 


Britton Reservoir Lower Study Area 


Upper 

Lower 

Reservoir 4 

Reservoir 5 

Reservoir 6 

Nests 1, 2 

Nests 3, 4, 5 

Nest 6 

Nest 7 

Nest 8 

% 

% 

% 

% 

% 

Bio- 

Bio- 

Bio- 

Bio- 

Bio- 


No . (%) 

MASS 

No . (%) 

mass 

No . {%) 

MASS 

No . (%) 

MASS 

No . (%) 

MASS 

Fish 

Sacramento Sucker 

284 ( 52 . 3 ) 

72.8 

84 ( 24 . 9 ) 

44.9 

80 ( 51 . 0 ) 

60.2 

32 ( 38 . 1 ) 

32.2 

24 ( 54 . 6 ) 

68.5 

Bullhead sp. 

84 ( 15 . 5 ) 

3.1 

39 ( 11 . 5 ) 

2.0 

4 ( 2 . 5 ) 

2.0 

6 ( 7 . 1 ) 

0.2 

3 ( 6 . 8 ) 

0.4 

Hardhead 

63 ( 11 . 6 ) 

7.2 

66 ( 19 . 5 ) 

14.3 

24 ( 15 . 3 ) 

7.6 

9 ( 10 . 7 ) 

6.2 

6 ( 13 . 6 ) 

8.8 

Tui Chub 

21 ( 3 . 9 ) 

2.6 

30 ( 8 . 9 ) 

8.2 

5 ( 3 . 2 ) 

2.9 

0 ( 0 . 0 ) 

0.0 

0 ( 0 . 0 ) 

0.0 

Sacramento Squawfish 

19 ( 3 . 5 ) 

3.6 

17 ( 5 . 0 ) 

6.6 

6 ( 3 . 8 ) 

5.9 

1 ( 1 . 2 ) 

0.7 

1 ( 2 . 3 ) 

1.6 

Other® 

25 ( 4 . 6 ) 

3.3 

61 ( 18 . 1 ) 

11.6 

17 ( 10 . 8 ) 

6.2 

6 ( 7 . 2 ) 

4.5 

1 ( 2 . 3 ) 

0.4 

Total (% of total) 

496 ( 91 . 4 ) 

92.6 

297 ( 87 . 9 ) 

87.6 

136 ( 86 . 6 ) 

84.8 

54 ( 64 . 3 ) 

43.8 

35 ( 79 . 6 ) 

79.7 

Birds^ (% of total) 

30 ( 5 . 5 ) 

4.9 

29 ( 8 . 6 ) 

8.5 

14 ( 8 . 9 ) 

11.2 

26 ( 30 . 9 ) 

46.3 

3 ( 6 . 8 ) 

7.2 

Mammals^ (% of total) 

17 ( 3 . 1 ) 

2.5 

12 ( 3 . 5 ) 

3.9 

7 ( 4 . 5 ) 

4.0 

4 ( 4 . 8 ) 

9.9 

6 ( 13 . 6 ) 

13.1 


^ Other fish species (and total number of occurrences) included: 8 Channel Catfish {Ictalurus punctatus), 8 Carp, 38 minnows (Cyprinidae 
sp.), 24 crappie (Pomoxis sp.), 14 Tule Perch, 3 Rainbow Trout, 7 trout {Salmo sp.), 6 Largemouth Bass, and 2 sunfish (Centrarchidae 
sp.) 

Birds included: 29 American Coot (Fulica americana), 18 Dabbling ducks {Anas spp.), 11 Mallard, 10 geese (Anserinae), 8 grebes 
(Podicipedidae), 6 Tundra Swan, 5 Common Merganser (Mergus merganser), 4 unidentified birds, 3 Ruddy Duck {Oxyura jamaicensis), 
2 Great Blue Heron, 2 gull {Larus sp.), 1 Double-crested Cormorant, 1 Common Goldeneye {Bucephala aclangula), 1 Ring-necked 
Pheasant {Phasianus colchicus), and 1 Screech Owl {Otis kennicottii). 

Mammals included: 12 Muskrat {Ondatra zibethica), 8 California Ground Squirrel {Citellus beecheyi), 7 Western Gray Squirrel {Sciurus 
griseus), 5 rabbits (Leporidae), 5 unidentified squirrels (Sciuridae), 3 Black-tailed Deer {Odocoileus hemionus), 1 Mountain Beaver 
{Aplodontia rufa), 2 Domestic Cow {Bonus domesticus), 1 Yellow-bellied Marmot {Marmota flaviventris), and 1 Striped Skunk {Mephitis 
mephitis). 


creased fish visibility to eagles in the otherwise turbid 
reservoir. 

We occasionally observed bottom feeders such as 
sucker and catfish swimming slowly near the surface 
of Britton Reservoir. Surface gill netting and hy- 
droacoustic surveys indicated that frequency and di- 
versity of fish swimming near the surface were great- 
est at dusk, intermediate at dawn, and lowest at 
midday, and that fish were most abundant near the 
surface during the warmer months (Vondracek et 
al. 1989). During the nesting season, Bald Eagle 
foraging occurred mostly in the morning. Of 236 
prey deliveries recorded by time-lapse cameras at 
three nests (Nests 5, 6, and 8) totaling 98 camera- 
days, 49.6% occurred between 0600-1100 H, 29.2% 
between 1100-1600 H, and 21.2% occurred between 
1600-2100 H. 

Fish carrion was available on Britton Reservoir 
in late spring and early summer. In June and July, 


we found 12.9 items per survey (range = 3-30) 
compared with 1.7 items per survey from August 
through May (range = 1-6). Sacramento Sucker and 
Tule Perch represented 57% and 35%, respectively, 
of 99 carrion fish found in the June-July surveys. 
Many of these fish had apparently died from spawn- 
ing stress, and some of the Tule Perch counted in 
the surveys were still alive, floating moribund at the 
surface on their sides. Some Sacramento Squawfish 
and Hardhead were killed by anglers. We were un- 
able to determine whether significant numbers of 
dead fish were stranded during reservoir level fluc- 
tuations, but we occasionally found dead suckers 
along flat, grassy shorelines of Britton Reservoir and 
other backwaters in the study area. 

Fish Abundance Versus Eagle Diet at Britton 
Reservoir. Sacramento Sucker comprised only 1 1 % 
of the number of fish in electrofishing samples on 
Britton Reservoir, but because of their large size 


December 1992 


Foraging Ecology of Bald Eagles 


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249 


250 


W. Grainger Hunt et al. 


VoL. 26, No. 4 


Table 3. Number and percent of the seven most abundant fish species collected in Lake Britton by electrofishing and 
surface gill netting. 


Electrofishing 


Surface Gill 
Netting 


No. 

% 

Biomass (g) 

% 

No. 

% 

Tule Perch {Hysterocarpus traskiY 

2130 

38.8 

25 705 

3.6 

27 

12.0 

Hardhead {Mylopharodon conocephalusY 

1120 

20.4 

80 921 

11.2 

125 

55.6 

Sacramento Sucker {Catostomus occidentalisY 

605 

11.0 

452 884 

62.6 

33 

14.7 

Sacramento Squawfish (Ptychocheilus grandisY 

588 

10.7 

68 796 

9.5 

19 

8.4 

Black Grapple {Pomoxis nigromaculatusY 

457 

8.3 

24 398 

3.4 

20 

8.9 

Bluegill (Lepomis macrochirusY 

322 

5.9 

3 614 

0.5 

0 

0 

Largemouth Bass (Micropterus salmoidesY 

269 

4.9 

66 561 

9.2 

1 

0.4 

Total 

5491 

100.0 

722 879 

100.0 

225 

100.0 


® Native species. 

^ Introduced species. 


represented over 60% of the total biomass (Table 3). 
Sacramento Sucker, Hardhead, and Sacramento 
Squawfish together accounted for over 80% of the 
total fish biomass. Tule Perch, the most numerically 
abundant fish, comprised only 3.6% of the biomass 
samples. In the gill netting sample, suckers repre- 
sented over 807o of the biomass. 

Eagle prey selection at Britton Reservoir was sig- 
nificantly associated with fish abundance as indi- 
cated by gill netting data (Spearman rho = 0.626; 
P < 0.05) but not electrofishing data (Spearman rho 
= 0.191; P < 0.10). Ictalurids and Tui Chub were 
well represented in the eagles’ diet but were rare in 
the electrofishing surveys (Vondracek et al. 1989). 
Conversely, Tule Perch, Largemouth Bass {Microp- 
terus salmoides), and other centrarchids were abun- 
dant in the electrofishing surveys, but relatively un- 
important to eagles. The relative percent of biomass 
of Hardhead and sucker in the diet was very similar 
to that in electrofishing; sucker and Hardhead com- 
prised 73.8% of the 723 kg of fish sampled by elec- 
trofishing (Table 3) and averaged 73.5% of the ea- 
gles’ diet. 


Eagles nesting on the downstream portion of Brit- 
ton Reservoir took fewer Sacramento Suckers than 
eagles nesting on the upstream section (Table 1). To 
evaluate the difference, we compared the relative 
abundance of Sacramento Sucker >200 mm, col- 
lected by electrofishing in upper and lower Britton 
Reservoir. We found that the upstream section con- 
tained more Sacramento Sucker >200 mm (19.1 per 
station; Vondracek et al. 1989) than the lower part 
of the reservoir (8.5 per station) where eagles relied 
more heavily on other species. 

Foraging on the Small Reservoirs. Eagles nest- 
ing near the small downstream reservoirs took live 
fish and carrion fish (and waterfowl) in the reservoir 
bodies and inflow areas and dead and moribund fish 
emanating from the turbines of powerhouses situated 
on the reservoirs. Sacramento Sucker were the most 
abundant fish species both in terms of numbers and 
biomass identified in the downstream reservoirs (Res- 
ervoirs 4, 5, and 6) during electroshocking surveys. 
Hardhead, Sacramento Squawfish, and Tule Perch 
were also common. We saw eagles attempt to catch 
Hardhead and sucker near shore and in the main 


Table 4. Habitat use by radio-tagged adult Bald Eagles nesting near riverine habitat as determined by radio-telemetry 
locations. Data include only the first detection of the day per location and exclude instances of soaring flight. 


Terri- 

tory 

Sex 

Reach 3 

Reservoir 4 

Reach 4 

Reservoir 5 

Reach 5 

Tunnel 

Reservoir 

Total 

Detec- 

tions 

Nest 7 

Male 

— 

— 

114 (17.2%) 

271 (40.9%) 

236 (35.7%) 

41 (6.2%) 

662 

Nest 6 

Male 

1 (0.4%) 

142 (64.3%) 

78 (35.3%) 

— 

— 

— 

221 

Nest 6 

Female 

0 (0.07o) 

20 (38.5%) 

32 (61.5%) 

— 

— 

— 

52 


December 1992 


Foraging Ecology of Bald Eagles 


251 


channels of reservoirs 4 and 5. At the upstream end 
of Reservoir 4, we frequently observed adults (Nest 
6) taking suckers in the spring and summer in the 
currented shallows of an island gravel bar at the 
reservoir inflow where suckers were spawning. A 
backwater inlet at Reservoir 5 stranded several suck- 
ers on at least one occasion when water levels dropped. 

We quantified habitat characteristics for 50 for- 
aging attempts (60% successful) on the downstream 
reservoirs. Of these, 27 (54%) occurred in open water 
or flowing reservoir habitat, 13 (26%) in backwaters 
or marshes, 9 (18%) in powerhouse tailrace waters, 
and 1 (2%) unknown. Fourteen of 30 prey items 
appeared to be carrion. Tule Perch and crappie {Po- 
moxis sp.) carrion emerged from the powerhouse 
tailrace at Reservoir 4 primarily during winter and 
spring. Peaks in small carrion fish emerging from 
the tailrace into the reservoir corresponded with in- 
creased eagle attendance near the tailrace. 

Bald Eagle Use of the River Sections. Bald Ea- 
gles that nested near the small reservoirs in the lower 
study area frequently perched and hunted along the 
river sections. In over half of 662 telemetry reloca- 
tions of the radio-tagged adult male from Nest 7 
(August 1983-February 1984 and May-December 
1984) the eagle was in riverine habitats upstream 
and downstream of Reservoir 5 (Table 4). The total 
range was 22 river km. 

Similarly, 35.7% of recorded relocations by the 
radio-tagged male at Nest 6 (6 June to 10 December 


N«st 


Figure 3. Home ranges of the adult pair of Bald Eagles 
at Nest 6 in the Pit River study as revealed by radiote- 
lemetry. 

1984) were on the river rather than the reservoir. 
During 9 August to 30 September 1983, his mate 
perched mainly in riverine habitat (61.5% of relo- 
cations). Figure 3 shows that the ranges of the pair 
of radio-tagged adults at Nest 6 were very similar, 
both in extent (ca. 1 1 km) and distribution. Although 
the upstream river section (Reach 3) was just as 
accessible to the pair as the downstream reach (Reach 
4), we observed the male in Reach 3 only once and 


Table 5. Mean number and biomass per hectare of the two major Bald Eagle prey species recorded in three riverine 
habitats on snorkel surveys of the Pit River in 1983 and 1984. 


Hardhead 


Sacramento Sucker 


No. OF 
Surveys 

No. 

Biomass 

(kg) 

Mean 
Size (g) 

No. 

Biomass 

(kg) 

Mean 
Size (g) 

Reach 3 

Pool 

45 

135.8 

47.2 

(347.6) 

102.5 

29.2 

(284.9) 

Run 

40 

7.1 

2.1 

(295.7) 

345.5 

23.0 

(66.6) 

Riffle 

26 

0.9 

0.3 

(333.3) 

142.8 

13.5 

(94.5) 

Reach 4 

Pool 

12 

57.8 

23.2 

(401.3) 

138.0 

100.9 

(731.2) 

Run 

32 

33.0 

13.0 

(393.9) 

352.2 

227.8 

(646.8) 

Riffle 

25 

12.9 

2.0 

(155.0) 

443.2 

279.5 

(630.6) 

Reach 5 

Pool 

18 

26.5 

2.8 

(105.7) 

180.7 

62.7 

(346.9) 

Run 

34 

55.6 

12.4 

(223.0) 

447.0 

133.6 

(298.9) 

Riffle 

29 

21.8 

5.1 

(233.9) 

475.5 

134.2 

(282.2) 


252 


W. Grainger Hunt et al. 


VoL. 26, No. 4 


3 

z 

X 

s 


A. NEST 7 MALE 

(n - 1 1 S TELEMETRY OBSERVATIONS) 


85.2 


POOLS 


100 B. NEST 6 MALE 

90 ■ 

(n - 20 TELEMETRY OBSERVATIONS) 


95.0 


C. HELICOPTER SURVEYS 


POCKET 

WATER 


RUNS 


Expected Q Observed 


Figure 4. Observed and expected utilization of riverine habitats by Bald Eagles in Reaches 4 and 5. Graphs A and 
B show the expected percentages of telemetry observations of perchings in riverine habitats based on the proportional 
occurrence of each habitat within the home range of the eagle. For observations during helicopter surveys (C), percentages 
are based on habitat availability throughout Reaches 4 and 5. 


the female never. In Reach 4 the ranges of the Nest 

6 pair overlapped only slightly with that of the Nest 

7 male. 

Fish Occurrence in the River Reaches. Snor- 
keling surveys on the three river reaches (3, 4, and 
5) provided data on the occurrence of Sacramento 
Sucker, Hardhead, Sacramento Squawfish, and 
Rainbow Trout {Oncorhynchus mykiss). Table 5 pre- 
sents mean number and biomass per ha for sucker 
and Hardhead identified in the snorkeling surveys. 
Suckers were large and numerous in the riffles and 
runs of Reach 4, equally plentiful but smaller in 
Reach 5, and very small and least frequent in Reach 
3. Conversely, Hardhead numbers were highest in 
Reach 3 pools, although they appeared larger in 
Reach 4. Trout numbers followed a similar pattern 
to Hardhead; they were more numerous in Reach 3 
(especially riffles), intermediate in Reach 4, and few- 
est in Reach 5. Squawfish were most abundant in 
Reach 5 and intermediate in the other reaches; how- 
ever, their numbers and biomass were comparatively 
low. We will later show an apparent connection 
between the distribution of these fishes, namely suck- 
ers, and the occurrence of foraging eagles. 

Fish Behavior in the River Reaches. Our ob- 
servations of fish behavior in Reach 4 showed that 
both Sacramento Sucker and Hardhead exhibited 
activity peaks during the morning. Suckers spent 
most of their active period slowly grazing on algae- 
covered cobble substrate. It was apparent that as 
they moved into the shallow areas (tails) of pools 
they came close enough to the surface to be caught 


by eagles. Their movement into pool tails may have 
also been related to spawning. Sacramento Suckers 
typically spawn in riffles (Moyle 1976), and in our 
study area riffles are usually preceded by pools. 
Therefore, suckers may pass though pool tails on 
their way to and from spawning areas. 

Hardhead activity was variable. These sight feed- 
ers hovered in the middle of the water column and 
cruised along the river bank. We observed Hardhead 
feeding at the surface and in aquatic vegetation, 
browsing on the bottom, and apparently feeding on 
invertebrate drift in the water column. On two oc- 
casions, we observed eagles capture Hardhead swim- 
ming around beds of rooted aquatic vegetation. 
Hardhead feeding in this manner appeared to have 
their heads obscured by the plant material and ap- 
peared unaware of the eagle attack. 

Riverine Habitat Selection. The riverine habi- 
tats used by the two radio-tracked male eagles (Nests 
7 and 6) differed significantly from the proportional 
occurrence of aquatic habitats within their home 
ranges (Fig. 4). From 115 telemetry observations of 
the Nest 7 male, we recorded significantly more oc- 
currences {N = 98, 85.2%) on river pool habitat than 
expected by chance (x^ = 297, df = 3, T < 0.001). 
Similarly, in 20 river habitat observations of the Nest 
6 male, he chose pools {N =19, 95%) more often 
than expected by chance (x^ = 18.5, P < 0.001). 
Helicopter surveys also showed eagles selecting pools 
disproportionately to pool occurrence (x^ = 94.9, P 
< 0.001, 83% use in 34 of 41 observations compared 
to 31% availability; Fig. 4). 


December 1992 


Foraging Ecology of Bald Eagles 


253 


Foraging Behavior at River Pools. From blinds 
situated at riverine pools we noted that eagle attacks 
typically began high above the water from tree 
perches (20 of 25 observations); only one eagle struck 
without perching first. Attack distances ranged from 
10 to 75 m. The success rate for all riverine forages 
observed from blinds was 16 in 25 attempts (64%), 
with 1 outcome undetermined. Exposed boulders 
were used as sites to drag and eat large fish. We 
identified the prey taken in ten instances: eight were 
Sacramento Sucker and two were Hardhead. 

Water depth ranged from 0.1-1.26 m at 15 for- 
aging strike points. Of 17 assessments of surface 
conditions in strike areas, only 2 showed a distur- 
bance greater than swirls, whereas 11 had a glassy 
surface. The bottom was visible in 17 of 18 mea- 
surements of strike point turbidity; the exception 
showed visibility to a depth of 0.4 m. Water velocity 
at strike points was usually low; 7 of 16 observations 
showed no measurable current. 

An analysis of river pool habitat characteristics 
and prey distribution at 1 1 pools indicated that eagle 
occurrence (total number of visits by telemetered 
eagles and eagles observed in helicopter surveys) was 
positively associated with the number of prey-sized 
fish per 100 m^ (as determined by the snorkeling 
surveys, Pearson correlation coefficient, r = 0.77, P 
< 0.01). We also found a significant positive cor- 
relation of eagle occurrence with percent of pool area 
classified as “smooth/shallow” (no surface turbu- 
lence and <0.6 m deep, Pearson r = 0.67, P < 0.03). 
Comparisons of eagle occurrence with pool area, 
maximum depth of pool, percent algae coverage, 
length of pool tail (as a percent of pool length), and 
the total estimated number of prey fish per pool were 
not significant. 

An experimental increase in flow above summer- 
time conditions (4.2 m^/sec) reduced the amount of 
shallow areas of no surface turbulence. The loss of 
smooth/shallow habitat for Reach 4 was quite high, 
with decreases for all seven pools averaging over 50% 
(minimum 27.6%, maximum 100%) at 8.5 mVsec 
flow. Water velocity in the studied pools generally 
increased with greater flows, but changes within 
smooth/shallow areas were inconsistent, with ve- 
locities both increasing and decreasing. Decreases in 
areas of no surface turbulence also resulted at each 
of the three pools measured in Reach 5 (minimum 
7.2%, maximum 54.3%) when flows were increased 
from 2. 8-4.2 m^/sec. 

Increased flows did not widen the river at most 


pools because of the relatively steep-sided canyon; 
therefore, availability of tree perches was not af- 
fected. Because pool length was dictated by hydraulic 
factors and did not change with flows, perch posi- 
tions relative to pool boundaries did not change. 
However, increased flow reduced the number of ex- 
posed boulders at water level, which are often im- 
portant to Bald Eagles as perches for manipulating 
heavy prey. 

We caution the reader that specific management 
implications suggested by these results (i.e., man- 
aging for smooth/shallow habitat) may not apply to 
other river systems where Bald Eagles forage. Dif- 
fering hydrologic and biotic factors may diversely 
influence the occurrence of catostomids and other 
prey fishes in pools, their activities within them, and 
their vulnerability to eagle attack. In Arizona, Hunt 
et al. (1992c) found pools least favored among the 
riverine habitats where Bald Eagles foraged on Des- 
ert Sucker (Catostomus clarki), Sonora Sucker (C. 
insignis), and Carp. 

Eagle Distribution Versus Prey Occurrence. 
Based on the number of Bald Eagle sightings during 
helicopter surveys and the number and biomass of 
sucker per ha for six river segments (each river reach 
divided into upper and lower segments), eagle pref- 
erence for the different river reaches was likely as- 
sociated with the abundance of large suckers. The 
total number of eagle sightings in helicopter surveys 
were 1 and 5 for lower and upper Reach 3, respec- 
tively, and 24 for upper Reach 4. We saw eagles 
15-18 times in each of the remaining segments. 

The segments with the lowest eagle sightings also 
were estimated to have the lowest sucker populations 
(32.2 and 1.4 sucker per ha for lower and upper 
Reach 3, respectively). Upper Reach 4 had the larg- 
est sucker population (388.7/ha), the highest bio- 
mass (286.9 kg/ha) and the highest number of eagle 
sightings. The value for Spearman’s rank correlation 
representing the correspondence between number of 
eagle sightings and biomass of suckers was 0.89 (P 
< 0.05). There was no significant correlation with 
numbers of suckers (rho = 0.77). 

We reported above that the telemetered nesting 
pair of eagles at Reservoir 4 often foraged in the 
river section downstream of the reservoir (Reach 4), 
but rarely ventured upstream to the equally acces- 
sible Reach 3. Although suckers in size categories 
taken by these eagles (200-450 mm) were found in 
all habitats during snorkeling surveys of Reach 4 
and electrofishing surveys of Reservoir 4, there were 


254 


W. Grainger Hunt et al 


VoL. 26, No. 4 


Ul 


lU 

Scv 

BS 

B2 

(Cff 

uioe 

^lU 

BS: 

C 03 


o> 

s. 


XauGnBeij OAjteieu 


IS 

■ 


S i 

d o o 

AauenBsjj aAoeiey 


€> 

I 


Figure 5. Relative length-frequenq^ distributions of Sacramento Sucker in prey remains from Nest 6 compared with sucker length-frequencies observed in fish 
population surveys. 


December 1992 


Foraging Ecology of Bald Eagles 


255 


relatively few such fish observed in Reach 3 where 
smaller suckers were numerous (Fig. 5). Pools in 
upper Reach 4 contained four times the biomass of 
sucker per unit area (176.5 kg/ha) as lower Reach 
3 pools (44.4 kg/ha). Sucker growth was limited in 
Reach 3 probably because of reduced algae growth 
and colder water than is optimum for suckers. Be- 
cause of the diversion tunnel to the downstream pow- 
erhouse, Reach 3 was the only reach that did not 
receive relatively warm, nutrient-rich reservoir wa- 
ter in summertime. 

Discussion 

Several points suggest a simple relationship be- 
tween relative prey species abundance and prey se- 
lection. These points include the dominance of Sac- 
ramento Sucker in eagle diets in all parts of the study 
area, the increased use of other species in Lower 
Britton Reservoir where suckers were less common, 
the disproportionate use of pools with the highest 
densities of prey fish, and the rarity of eagle visits 
to Reach 3 where Sacramento Sucker of appropriate 
body size were in relatively low density. 

Our data also show how prey behavior and life 
history can influence vulnerability to predation. Sac- 
ramento Sucker were not only numerous, they were 
also vulnerable to eagles in more ways than other 
species. They became available when they: 1) for- 
aged in shallow water, 2) spawned in shallows, and 
3) appeared as carrion. The first two components of 
vulnerability, characteristic of many catostomids, no 
doubt account for their occurrence in the diets of 
Bald Eagles over much of their inland range (Dun- 
stan and Harper 1975, Todd et al. 1982, Swenson 
et al. 1986, Haywood and Ohmart 1986, Gerrard 
and Bortolotti 1988, Hunt et al. 1992c). Not only 
do suckers typically enter shallow water to spawn 
and graze (photosynthesis is highest in shallow hab- 
itats), but their downward visual orientation must 
leave them more vulnerable to eagle attack than sight- 
feeding fish (see Swenson 1979, Todd et al. 1982 for 
discussion). This point helps to explain the apparent 
contradiction of a bottom- feeding fish being the ma- 
jor prey of a surface-feeding predator (Haywood and 
Ohmart 1986). Accordingly, sight-feeding cyprinids 
(Hardhead and Sacramento Squawfish) appeared in 
significantly less frequency in prey remains than 
predicted by their occurrence relative to Sacramento 
Sucker in the gill netting and electroshocking sam- 
ples on Britton Reservoir (Tables 1 and 3). Trout, 
also sight-feeders, were common in the river reaches. 


but were rarely taken by eagles. Both Hardhead and 
trout often wait near the surface for insects, but these 
fish tend to be oriented upward and are more aware 
than suckers of any movement above them. 

We believe the timing and occurrence of sucker 
mortalities on Britton Reservoir may contribute to 
the unusually high nesting density of Bald Eagles 
there. This carrion “bloom” coincides with the sec- 
ond half of the nestling cycle, including the post- 
fledging period. Large post-spawning dieofTs are 
atypical among catostomids, and P.B. Moyle (pers. 
comm.) believes that the proportion of the total suck- 
er spawners dying each year as a result of spawning 
stress is small. However, dead and dying fish, drift- 
ing down from the relatively long river reach up- 
stream, tend to accumulate in the reservoir inflow 
area where they are highly visible to the eagles. 
Carrion fish may also be produced by stranding as 
a result of flow variation from the powerhouse up- 
stream of the reservoir. 

Another point of difference between eagle diet and 
species occurrence in the fisheries inventories in- 
volved ictalurids. Bullheads {Ictalurus melas or I. 
nebulosus) were absent in the extensive electrofish- 
ing, gill netting, and carrion survey samples on Brit- 
ton Reservoir, and yet we identified 1 23 individuals 
in eagle prey remains (Table 1). Dunstan and Harp- 
er (1975) and Van Daele and Van Daele (1982) 
mention that bullheads often swim or “bask” near 
the surface, and indeed, we witnessed unidentified 
ictalurids doing so on Britton Reservoir. How they 
avoided the gill nets is unknown to us. 

Finally, although relative eagle use of the three 
river reaches was directly related to the abundance 
of large suckers (>200 mm), our data show that 
physical conditions promoting prey vulnerability were 
more important in attracting foraging eagles to spe- 
cific habitats than were factors influencing prey den- 
sity. Eagles chose river pool habitat despite the fact 
that sucker densities in Reaches 4 and 5 were in- 
variably lower in pools than in runs or riffles, and 
there were no consistent size-class differences of 
suckers between the three habitats (Table 5). 

Acknowledgments 

Fisheries field work and data analyses thereof were 
conducted by University of California (Davis) personnel 
including P. Moyle, D. Baltz, B. Vondracek, L. Brown, 
D. Longanecker, and B. Spence. We are especially grateful 
to Drs. Moyle and Vondracek for enlightening conver- 
sations on fish ecology. Bald Eagle researchers included 
R. Lehman, J. Linthicum, L. Young, C. Himmelwright, 


256 


W. Grainger Hunt et al. 


VoL. 26, No. 4 


G. Sanders, L. Speigel, and K. Austin. We are grateful 
to the following people for their support and advice: R. 
Olendorff, P. Detrich, D. Smith, D. Harlow, T. Brumley, 

C. Eckert, J. Adams, R. Jurek, and C. Simpkins. Tech- 
nical assistance was provided by H. Stubbs, P. Bloom, D. 
Garcelon, G. Bienz, L. Kiff, I. Kanehiro, K. Hart, and 
BioSonics, Inc. We thank S. Rohwer (T. Burke Memorial 
Museum, University of Washington), R. Laybourne 
(Smithsonian Institution), and R. Paynter (Museum of 
Comparative Zoology, Harvard University) for allowing 
us to examine museum specimens and for help with prey 
identification. S. Cressey and S. Foreman of Western Eco- 
logical Services Company mapped the river habitat. We 
thank the U.S. Fish and Wildlife Service, California De- 
partment of Fish and Game, U.S. Forest Service (Lassen 
and Shasta-Trinity forests), and the California Bald Eagle 
Working Team for providing permits and/or advice. 
Graphics and word processing were provided by N. Far- 
well, J. Burnette, K. Williams, P. Woodside, D. Kane, J. 
Gilardi and the BioSystems Graphics Department. We 
thank D. Arora, N. Smith, J. Barclay, G. Bortolotti, and 

D. Stahlecker for comments on the manuscript. We are 
particularly grateful to K. Steenhof who helped and en- 
couraged us during several revisions. The study was fund- 
ed entirely by Pacific Gas and Electric Company (PG&E) 
which owns and operates the Pit River hydroelectric fa- 
cilities under license of the Federal Energy Regulatory 
Commission. We also thank PG&E, particularly its Shasta 
Division, for logistical support during the study. 

Literature Cited 

Bagenal, T.B. AND F.W. Tesch. 1978. Age and growth. 
Pages 98-130 in T.B. Bagenal [Ed.], Methods for as- 
sessment of fish production in fresh waters. IBP 
Handbook No. 3. Blackwell Scientific Publications, 
Oxford, U.K. 

Baltz, D.M., B. VoNDRACEK, L.R. Brown and P.B. 
Moyle. 1987. Influence of temperature on micro- 
habitat choice by fishes in a California stream. Trans. 
Am. Fisheries Soc. 116:12-20. 

Gain, S.L. and J.I. Hodges. 1989. A floating-fish snare 
for capturing Bald Eagles. J. Raptor Res. 23:10-13. 
Casteel, R.W. 1972. A key based on scales to the fam- 
ilies of native California freshwater fishes. Proc. Calif. 
Acad. Sci. 39:75-86. 

. 1973. The scales of native freshwater fish fam- 
ilies of Washington. Northwest Sci. 47:230-238. 
Dunning, J.B. 1984. Body weights of 686 species of 
North American birds. Monograph No. 1. Western 
Bird Banding Association, Cave Creek, AZ. 
Dunstan, T.C. and J.F. Harper. 1975. Food habits 
of Bald Eagles in north-central Minnesota. /. Wildl. 
Manage. 39:140-143. 

Frenzel, R.W. AND R.G. Anthony. 1982. Method for 
live-capturing Bald Eagles and Osprey over open wa- 
ter. Bull. No. 82-13. Fish and Wildlife Service Re- 
search Information, U.S. Dept, of the Interior. 


Gerrard, J.M. AND G.R. Bortolotti. 1988. The Bald 
Eagle: haunts and habits of a wilderness monarch. 
Smithsonian Institution Press, Washington, DC. 

Harmata, A.R. 1985. Capture of wintering and nesting 
Bald Eagles. Pages 139-159 in J.M. Gerrard and T.N. 
Ingram [Eds.], The Bald Eagle in Canada: proceedings 
of Bald Eagle Days, 1983. Whitehorse Plains Publ., 
Headingly, Manitoba. 

Haywood, D.D. and R.D. Ohmart. 1986. Utilization 
of benthic-feeding fish by inland breeding Bald Eagles. 
Condor 88:35-42. 

Hunt, W.G., B.S. Johnson and R.E. Jackman. 1992a. 
Carrying capacity for Bald Eagles wintering along a 
northwestern river. J. Raptor Res. 26:49-60. 

, R.E. Jackman, J.M. Jenkins, C.G. Thelander 

and R.N. Lehman. 1992b. Northward post-fledging 
migration of California Bald Eagles. /. Raptor Res. 26 
19-23. 

, D.E. Driscoll, E.W. Bianchi and R.E. Jack- 

man. 1992c. Ecology of breeding Bald Eagles in Ar- 
izona. Report to U.S. Bureau of Reclamation, Contract 
No. 6-CS-30-04470. BioSystems Analysis, Inc., Santa 
Cruz, CA. 

and F.P. Ward. 1988. Habitat selection by 

spring migrant peregrines at Padre Island, Texas. Pages 
527-535 in T.J. Cade, J.H. Enderson, C.G. Thelander 
and C.M. White [Eds.], Peregrine Falcon populations' 
their management and recovery. The Peregrine Fund, 
Inc., Boise, ID. 

Moyle, P.B. 1976. Inland fishes of California. Univer- 
sity of California Press, Berkeley, CA. 

Steenhof, K. 1983. Prey weights for computing percent 
biomass in raptor diets. J. Raptor Res. 17:15-27. 

Swenson, J.E. 1979. Factors affecting status and re- 
production of Ospreys in Yellowstone National Park 
Condor 43:595-601. 

, K.L. Alt and R.L. Eng. 1986. Ecology of Bald 

Eagles in the Greater Yellowstone Ecosystem. Wildl 
Monogr. 95:1-46. 

Todd, C.S., L.S. Young, R.B. Owen, Jr. and F.J. 
Gramlich. 1982. Food habits of Bald Eagles in 
Maine. J. Wildl. Manage. 46:636-645. 

Van Daele, L.J. and H.A. Van Daele. 1982. Factors 
affecting the productivity of Ospreys nesting in west- 
central Idaho. Condor 84:292-299. 

Vondracek, B., D.M. Baltz, L.R. Brown and P.B. 
Moyle. 1989. Spatial, seasonal, and diel distribution 
of fishes in a California reservoir dominated by native 
fishes. Fisheries Res. 7:31-53. 

Young, L.S. 1983. Movements of Bald Eagles associated 
with autumn concentrations in Glacier National Park. 
M.S. thesis, University of Montana, Missoula, MT. 

Received 2 June 1992; accepted 15 September 1992 


Short Communications 


J Raptor Res. 26(4):257-259 
© 1992 The Raptor Research Foundation, Inc. 


Increased Parental Care in a Widowed Male 
Marsh Harrier {Circus aeruginosus) 

Garmelo FernAndez^ 

Estacion Biologica de Donana (CSIC) , Avda. M“ Luisa s/n, Pabellon de Peru, 

41013 Sevilla, Spain 

Paz Azkona 

Soc. Estudios Biologicos Ugarra, Tafalla 34, 4”, 31003 Pamplona, Spain 


Pair bonding and distribution of functions is essential 
to the successful breeding of birds (Trivers 1972). This is 
especially important in raptors, where food is generally 
not abundant and its capture requires a considerable hunt- 
ing effort (Newton 1979). Besides food supply, parental 
care involves more activity including; protection of chicks 
from inclement weather and predators, nest maintenance 
and sanitation, and territorial defense (Johannesson 1975, 
Newton 1986, Fernandez 1992), which require much time 
and energy and can hardly be carried out by a single 
parent. 

In the spring of 1990, during the study of several nests 
of Marsh Harriers (Circus aeruginosus), we noted the dis- 
appearance of one of the breeding females. Hence, we had 
the opportunity to see to what extent the loss of his mate 
affected the parental behavior of the widowed male and 
compare his behavior with that of other neighboring breed- 
ing pairs. 

Methods 

The study was carried out at the Dos Reinos wetland 
(Ebro Valley, Spain), where ten monogamous pairs of 
Marsh Harriers bred in 1990 (Fernandez 1990). Two of 
the nests failed during incubation. The other eight breed- 
ing pairs were observed from a hide about 300-500 m 
from the nests. We recorded separately for each sex the 
time spent by adults in the nest area (limits of wetland 
area), food items supplied to nestlings, territorial chases 
and other aspects of parental care (Fernandez 1992). To 
estimate the breeding stages, chicks were aged according 
to body development (body mass, tarsus length and 6th 
primary length; Altenburg et al. 1987, Gonzalez 1991). 

During our study we observed the disappearance, for 
unknown reasons, of the hen in one pair. The disappear- 
ance occurred on 20 June 1990 when the chicks were 37- 


’ Present address; Soc. Est. Biologicos Ugarra, c/Tafalla 
34, 4°, 31003 Pamplona, Spain. 


38 d old. A week before, two chicks had been observed in 
the nest but whether the loss of one young was caused by 
the female’s disappearance or happened previously is not 
certain. The chick that survived flew when 43 d old, within 
the usual range of first flight times in the Marsh Harrier 
(Cramp and Simmons 1980). 

The behavior of the male before female loss had been 
studied over 4 d for 28 hr. Following the disappearance 
of the female, the behavior of its mate was monitored for 
3 d (a total of 22 hr), until the only surviving chick flew. 
Three aspects of the behavior of the widowed male (percent 
time spent in nest area, number of food items delivered per 
hour and number of territorial chases per hour) were 
compared to those of neighboring pairs over the same 
breeding period. Observations of neighboring pairs oc- 
curred on 20 d (120 hr) before loss and on 16 d (108 hr) 
during the last days before flying. Statistical comparisons 
between hours with and without ehases and prey delivered 
were made by means of x^-tests in 2 x 2 contingency tables 
(Sokal and Rohlf 1969). 

Results 

As shown in Table 1, the input of parental care by the 
widowed male before the loss of his mate was similar to 
input of other males. He spent only slightly more time in 
nest area than neighboring males (x^ = 1.07, P > 0.05), 
delivered a few less food items to nestlings (x^ = 1.39, P 
> 0.05) and defended his nesting area as much as other 
males (x^ = 0.03, P > 0.05). 

After female loss (Table 1) the widowed male: a) spent 
significantly more time in the nesting area than neigh- 
boring males in the same nestling period (x^ = 103.02, P 
< 0.001) and about the same proportion of the time as 
females, b) supplied slightly more food items per hour 
than mated males (x^ = 1.75, P > 0.05) though signifi- 
cantly fewer than males and females together (x^ = 3.91, 
P < 0.05), and c) defended his nesting area more fre- 
quently than other males (x^ — 6.20, P < 0.05) though 
not more frequently than mated birds taken together. The 
number of chases in relation to time spent in nesting area 
was, however, similar for the single male (1.34 chases/ 
hr) and other male birds (1.58 chases/hr). 


257 


258 


Short Communications 


VOL. 26, No. 4 


Table 1 . Comparison of parental care of a widowed male Marsh Harrier with neighboring paired males, before and 
after female loss in the Ebro Valley, Spain. 


After Female Loss 


Before Loss 


Male and 
Female 


Widowed Male Paired Males Widowed Male Paired Males Combined 


Observation time (hr) 

28 

120 

22 

108 

108 

% time in nest area 

22.5 

14.5 

33.9 

12.6 

36.4 

Food items/hr 

0.39 

0.51 

0.64 

0.48 

0.82 

Defense chases/hr 

0.25 

0.27 

0.46 

0.20 

0.51 


Discussion 

Disappearance of one of the parents during breeding is 
not uncommon among birds of prey (Newton 1979). In 
most cases, if the lost mate is not rapidly replaced, aban- 
donment of the nest results (Newton 1976). Occasionally, 
however, especially if the death of a parent occurs toward 
the end of the nestling period (Newton 1986), the re- 
maining adult can successfully rear young. Such has been 
reported previously for several raptors, including Marsh 
Harrier (Cramp and Simmons 1980). Successful single- 
parent broods must likely involve a greater parental effort 
(Trivers 1972) by the remaining bird or a reduction in 
number or quality of fledglings. Our observations indicate 
that the male Marsh Harrier widowed at the late nestling 
stage of the breeding season made a greater parental effort 
than mated males, at least in several facets of parental 
care. He spent more time in the nesting area, delivered a 
few more (although not significantly more) food items to 
nest, and carried out more territorial defenses. The in- 
crease in number of chases was probably related to the 
greater amount of time spent in the nesting area. Sasvari 
(1990) has found experimentally that in the Great Tit 
{Farm major) widowed birds fed nestlings more frequently 
than either of the mated parents but less than both together, 
as was also the case with the Marsh Harrier we studied. 

Our results suggest that male harriers are not neces- 
sarily at the limit of their parental care capabilities during 
the later stages of nesting. In this sense, Altenburg et al. 
(1982) found that monogamous birds reduced their pa- 
rental input, in terms of number of prey items delivered 
per hour, toward the end of the nestling stage, perhaps 
indicating that mated males’ parental abilities are also 
“underused” at this time. 

A division of sexual roles is usual among birds of prey 
(Newton 1979) and the presence of both sexes seems es- 
sential to successful breeding. However, this division in 
function becomes blurred in the later stages of reproduc- 
tion (Newton 1986). As breeding progresses the females 
contribute gradually more to prey capture and less to other 
aspects of brood care; sexual roles become similar and can 
perhaps be undertaken equally by either member of the 
pair. Role division itself does not necessarily preclude the 
rearing of nestlings by a single parent, at least in circum- 
stances where the remaining parent is able to increase its 
investment, as may occasionally occur in some potentially 
polygynous birds of prey (such as Circus; Newton 1976, 


Cramp and Simmons 1980) when they are breeding mo- 
nogamously (Altenburg et al. 1982). 

Resumen. — El macho de un pareja de Aguiluchos lagu- 
neros quedo viudo cuando los polios contaban con 37-38 
dias de edad. El macho consiguio sacar adelante un polio 
intensificando el esfuerzo reproductor y supliendo en parte 
las funciones realizadas por la hembra: aumento el mimero 
de cebas, el tiempo invertido en el area de cria y las defensas 
del territorio. Elio parece indicar que al final de la cria 
los aguiluchos monogamos con polladas escasas no se en- 
cuentran al limite de sus posibilidades y que la contri- 
bucion relativa de cada sexo a los cuidados parentales es 
susceptible de variar en funcion de las necesidades fami- 
liares de cada momento. 

Acknowledgments 

This research was supported by an FPI grant from the 
Departamento de Educacion and the Direccion de Medio 
Ambiente del Gobierno de Navarra in collaboration with 
the Estacion Biologica de Donana (CSIG). We are most 
grateful to J. Bielefeldt, J.A. Donazar, F. Hiraldo, M. 
Marquiss, D.T. Parkin and W.C. Scharf for their con- 
structive comments on an earlier draft. The English trans- 
lation was made by N.C.B. Bowles. 

Literature Cited 

Altenburg, W., J. Bruinenberg, P. Wildsghut and 
M. Zijlstra. 1987. Colonization of a new area by 
the Marsh Harrier. Ardea 75:213-220. 

, S. Daan, j. Starkenburg and M. Zijlstra. 

1982. Polygamy in the Marsh Harrier Circus aerugi- 
nosus: individual variation in hunting performance and 
number of mates. Behaviour 79:272-312. 

Cramp, S. and E. Simmons. 1980. Handbook of the 
birds of Europe, the Middle East and North Africa. 
Vol. II. Oxford University Press, Oxford, U.K. 
Fernandez, C. 1990. Genso, fenologia y exito reproduc- 
tor del Aguilucho lagunero. Circus aeruginosus , en Na- 
varra. Munibe 41:89-93. 

. 1992. Nest material supplies in the Marsh Har- 
rier Circus aeruginosus-. sexual roles, daily and seasonal 
activity patterns and rainfall influence. Ardea 80. 
281-284. 


December 1992 


Short Communications 


259 


GonzAlez, J.L. 1991, El Aguilucho lagunero Circus 
aeruginosus en Espana: situacion, biologia de la re- 
produccion, alimentacion y conservacion. Ed. ICONA, 
Madrid, Spain. 

JOHANNESSON, H. 1975. Activities of breeding Marsh 
Harriers Circus aeruginosus. Vdr Fdgelvdrld 34:197- 
206. 

Newton, I. 1976. Population limitation in diurnal rap- 
tors. Can. Field-Nat. 90:274-300. 

. 1979. Population ecology of raptors. T. and 

A.D, Poyser, Berkhamsted, U.K. 

. 1986. The Sparrowhawk. T. and A.D. Poyser, 

Berkhamsted, U.K. 


Sasvari, L. 1990. Feeding response of mated and wid- 
owed bird parents to fledglings: an experimental study 
Ornis Scand. 21:287-292. 

SoKAL, R.R. AND F.J. Rohlf. 1969. Biometry. Freeman 
and Co., San Francisco, CA. 

Trivers, R.L. 1972. Parental investment and sexual 
selection. Pages 136-179 in B. Campbell [Ed.], Sexual 
selection and the descent of man. Aldine Press, Chi- 
cago, IL. 


Received 26 March 1992; accepted 20 August 1992 


J. Raptor Res. 26(4):259-260 
© 1992 The Raptor Research Foundation, Inc. 


Bats as Prey of Stygian Owls in Southeastern Brazil 

Jose C. Motta Junior 

Departamento de Ecologia e Biologia Evolutiva, Vniversidade Federal de Sdo Carlos, 

13560 Sdo Carlos, SP, Brazil 

Valdir a. Taddei 

Departamento de Zoologia, Vniversidade Estadual Paulista, 

Institute de Biociencias, Letras e Ciencias Exatas, 15055 Sdo Jose do Rio Preto, SP, Brazil 


Few studies have examined quantitatively large num- 
bers of pellets or stomachs for assessing the relative fre- 
quency of bats as prey of owls (cf., Uttenddrfer 1943, 
Ruprecht 1979, Mikkola 1983). Pellets cast by three or 
four Stygian Owls (Asio stygius) were collected during 25 
mo, mostly between June 1985 and February 1987 and 
sporadically in August- September 1989, December 1990 
and February 1991. We collected pellets under trees in a 
Pmus sp. plantation located in Sao Carlos, Sao Paulo State, 
southeastern Brazil (21°58'S, 47°52'W) at an altitude of 
840 m. The climate of the study area is a transition between 
Koppen’s Cwai and Awi, or rainy tropical with dry (April 
to September) and wet (October to March) seasons (To- 


lentino 1967). The nocturnal foraging activities of the owls 
took place in savannah (“campo cerrado”) and grassland 
(“campo”) habitats near the Pinus plantation, which was 
used for diurnal roosting. All data were gathered through 
direct observation in the study area. 

A total of 422 pellets were analyzed after treatment 
with a 3% boiling solution of NaOH (Schueler 1972). 
Prey remains were identified by comparison with reference 
collections. The bulk of the prey items consisted of small 
birds (Table 1), mostly finches (e.g., Volatinia jacarina 
which alone comprised 62.57o of all birds or 56.3% of all 
prey), weighing 10-15 g (J.C. Motta Junior unpubl.). 
Bats were the second most frequent prey whereas insects 


Table 1. Numbers of prey items found in pellets of Stygian Owls in two climatic seasons in southeastern Brazil. 


Prey 

Dry Season 

Wet Season 

Total 

N 

(%) 

N 

(%) 

N 

(%) 

Bats 

49 

(5.7) 

26 

(6.8) 

75 

(6.1) 

Birds 

793 

(93.1) 

318 

(83.7) 

nil 

(90.2) 

Frogs 

0 


1 

(0.3) 

1 

(0.1) 

Insects 

10 

(1.2) 

35 

(9.2) 

45 

(3.6) 

Total Prey 

852 

(100.0) 

380 

(100.0) 

1232 

(100.0) 

Total Pellets 

265 


157 


422 


260 


Short Communications 


VoL. 26, No. 4 


Table 2. Bats {N = 75) found as prey of Stygian Owls 
in southeastern Brazil. Body weights were obtained from 
museum specimens, collected in Sao Paulo state. 


Species 

Weight (g) 
(Range) 

(N) 

No. (%) of 
Bats in 
Pellets 

Molossidae 

Eumops glaucinus 

28.6-38.6 

(12) 

47 (62.7%) 

Nyctinomops laticaudatus 

8.5-13.8 

(6) 

10 (13.3%) 

Nyctinomops macrotis 

26.0 

(1) 

1 (1.3%) 

V espertilionidae 

Eptesicus furinalis 

5.0-7.2 

(8) 

3 (4.0%) 

Histiotus velatus 

7.0-10.8 

(9) 

1 (1.3%) 

Lasiurus blossevillii 

8.7-11.4 

(3) 

1 (1.3%) 

Lasiurus cinereus 

14.3-23.5 

(3) 

3 (4.0%) 

Lasiurus ega 

14.3-15.0 

(3) 

6 (8.0%) 

Phyllostomidae 

Glossophaga soricina 

o 

ci 

1 

00 

(86) 

1 (1.3%) 

Chiroderma doriae 

26.9-33.0 

(18) 

1 (1.3%) 

Pygoderma bilabiatum 

15.4-15.9 

(2) 

1 (1.3%) 


(Scarabaeidae and Orthoptera) and a frog seemed to be 
of minor importance. The absence of rodents in the diet 
of the Stygian Owls studied was surprising. Rodents were 
abundant in the study area as evident from their frequent 
occurrence in pellets of Barn Owls (Tyto alba) living in 
the same area (Motta Junior 1988). 

G-tests (Sokal and Rohlf 1969) applied to seasonal fre- 
quencies of prey items (Table 1) demonstrated that insects 
were more frequently preyed upon during the wet season 
(G = 39.92, P < 0.001), whereas consumption of birds 
and bats did not show seasonal trends (G = 2.58, P > 
0 10 and G = 0.35, P > 0.50, respectively). 

Eleven species of bats in eight different genera and three 
families were recorded. The largest bat {Eumops glaucinus) 
was also the most frequently preyed upon by Stygian Owls 
(Table 2). 

Data from Colombia (Borrero 1967), Belize (Franz 
1991) and from Colima, Mexico (based on three pellets) 
were similar to ours. Except for the Black-and- White Owl 
{Ciccaba nigrolineata) that forages heavily on bats (Ibanez 
et al. 1983) and the Stygian Owl (this study), apparently 
no other owls include bats so frequently (6.1%) in their 
diet (cf., Uttendorfer 1943, Earhart and Johnson 1970, 
Burton 1973, Mikkola 1983). 

Resumen. — Durante 25 meses, entre junio de 1985 y 
febrero de 1991, estudiamos la ocurrencia de murcielagos 
en la dieta del Tecolote Fusco {Asia stygius) en el sudeste 
de Brasil. El analisis de 422 egagropilas rindio 1 232 presas 


entre las cuales las aves representaron 90,2%, los murcie- 
lagos 6,1%, los insectos 3,67o, y los anuros 0,1%. Identi- 
ficamos 11 especies de murcielagos entre 75 individuos. 
Eumops glaucinus fue la especie mas frecuente (47 indi- 
viduos). La predacion de murcielagos fue regular a lo largo 
de las estaciones. Asio stygius es apuntada como una de las 
especies de Strigiformes que mas preda murcielagos en 
todo el mundo. 

Acknowledgments 

We thank Storrs L. Olson for allowing us to examine 
three pellets of Stygian Owls from Mexico, and David De 
Jong for reviewing the English manuscript. We greatly 
appreciated the comments of F.M. Jaksic, A.R. Craig and 
S.I. Tiranti on an earlier manuscript. This research was 
partly funded by grants from FAPESP to J.C. Motta 
Junior and from CNPq to V.A. Taddei. 

Literature Cited 

Borrero, J.I. 1967. Notas sobre habitos alimentarios 
de Asio stygius robustus. Hornero 10:445-447. 

Burton, J.A. [ed.]. 1973. Owls of the world. E.P. Dut- 
ton and Co., New York. 

Earhart, C.M. and N.K. Johnson. 1970. Size di- 
morphism and food habits of North American owls. 
Condor 72:251-264. 

Franz, M. 1991. Field observations on the Stygian Owl 
Asio stygius in Belize, Central America. (Abstract). J. 
Raptor Res. 25:163. 

IbaNez, C., C. Ramo and B. Busto. 1983. La lechuza 
blanquinegra {Ciccaba nigrolineata) como depredador 
de murcielagos. Page 123 in F.G. Stiles and P.G. Agui- 
lar [Eds.], Primer Simposio de Ornitologia Neotrop- 
ical, Lima, Peru. 

Mikkola, H. 1983. Owls of Europe. Buteo Books, Ver- 
million, SD. 

Motta Junior, J.C. 1988. Alimentagao diferencial da 
suindara (Tyto alba) (Aves, Strigiformes) em duas es- 
tagoes do ano em Sao Carlos, estado de Sao Paulo 
Anais do Seminario Regional de Ecologia 5:357-364. 
Ruprecht, A.L. 1979. Bats (Chiroptera) as constituents 
of the food of Barn Owls Tyto alba in Poland. Ibis 121: 
489-494. 

Schueler, F.W. 1972. A new method of preparing owl 
pellets; boiling in NaOH. Bird-Banding 43:142. 
Sokal, R.R. and F.J. Rohlf. 1969. Biometry. W.H 
Freeman and Co., San Francisco, CA. 

Tolentino, M. 1967. Estudo critico sobre o clima da 
regiao de Sao Carlos. Concurso de Monografias Muni- 
cipals. Sao Carlos, Sao Paulo, Brazil. 

Uttendorfer, O. 1943. Fledermause als Raubvogel — 
und Eulenbeute. Z. Sdugetierk. 15:317-319. 

Received 12 February 1992; accepted 21 August 1992 


/. Raptor Res. 26(4):261-263 
© 1992 The Raptor Research Foundation, Inc. 


Food-Stressed Great Horned Owl Kills Adult Goshawk: 
Exceptional Observation or Community Process? 

Christoph Rohner 

Department oj Zoology, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 

Frank I. Doyle 

Kluane Lake Research Base, Mile 1054 Alaska Highway, Yukon Territory, Canada Y1A 3V4 


Great Horned Owls (Bubo virginianus) can prey on 
other owl and diurnal raptor species up to the size of 
Northern Goshawks (Accipiter gentilis), and the question 
has been raised why this behavior occurs and whether it 
affects the structure of raptor communities (Craighead and 
Craighead 1956, Mikkola 1983, Voous 1988). Most re- 
ports originate from analyses of pellets of prey remains 
collected at owl nests and roost sites. There is little specific 
information on how the owls kill potential harmful prey, 
nor about the ecological conditions that facilitate such pre- 
dation. During our study of avian predation in the boreal 
forest ecosystem near Kluane Lake in the southwestern 
Yukon (Krebs et al. 1992), we encountered circumstantial 
evidence for an owl predation of an adult female goshawk, 
which led us to a revised assessment of such interspecific 
killings among raptors. 

On 18 June 1991, we found a goshawk nest on the flat 
top of a dead White Spruce (Picea glauca) about 10 m 
high. The nest was unusually exposed above canopy height 
of the surrounding trees (all other 28 goshawk nests found 
in our study areas were 4-8 m below canopy height). Fresh 
streaks of whitewash and two plucking sites with fresh 
prey remains indicated that the nest was active, and we 
were attacked by both parents. Because we heard loud 
begging calls, but the chicks were not yet visible at the 
nest edge, we estimated their ages to be 2-3 wk. 

On 25 June 1991, the nest area was quiet and there 
were no fresh whitewash or new prey remains. We found 
numerous breast feathers and the left wing of an adult 
goshawk 2 m from the base of the nest tree, together with 
four Great Horned Owl feathers. More goshawk feathers, 
including a goshawk’s right wing, were found under a 1 
m high log perch about 12 m from the nest tree. The wings 
measured 356 mm, indicating they were from a female 
goshawk (Mueller and Berger 1968). Because the goshawk 
remains were several days old on 25 June, we estimated 
that the predation occurred between 18 and 22 June. 

During the same period, we monitored a Great Horned 
Owl family with a nest 1.0 km from the goshawk nest. 
The two owl fledglings were tethered to an elevated ar- 
tificial platform for diet study (Petersen and Keir 1987). 
We moved two additional young Great Horned Owls to 
the platform for a brood size manipulation experiment 
from 10-20 June. The adult female owl was equipped 
with a backpack radiotransmitter, and we recorded her 


locations once every second night. Food stress during the 
brood addition experiment was suggested by a decrease in 
the amount of food brought to the platform, declining owlet 
weights, and increased hunting distances from its nest by 
the female owl. The goshawk nest was within the territory 
of the owl pair, but the telemetry locations did not reveal 
any relation to the goshawk nest. On 27 June, we found 
the remains of the right leg of an adult female goshawk 
beside the owl tethering platform. The remains were sev- 
eral days old, and presumably were dropped by the owls 

Discussion 

Why Publish a Single Observation? Because of the 
nature of rare events, a sufficient sample size for testing 
hypotheses can only be achieved as a collaborative effort 
of different researchers who publish few or even single 
observations on this topic (Schmutz 1992). The fact that 
Great Horned Owls kill other birds of prey has been well 
documented (reviews in Craighead and Craighead 1956, 
Mikkola 1983, Voous 1988), and no further publications 
are needed to simply report this behavior. We agree with 
Bortolotti (1992) that the publication of single observations 
should allow links to the analysis or interpretation in a 
higher-level context. As a consequence, we suggest not 
publishing short notes that simply report the interspecific 
killing among raptors— instead we should ask the question 
when and why it occurs, and focus on the context of these 
observations. In our case, we present a single observation 
with additional information that shows potential links to 
causes and implications of this behavior: we will 1) try to 
estimate how rare such events were during our study, and 
2) discuss how the documented details of the ecological 
context of both predator and prey relate to hypotheses on 
the evolution of interspecific killing among raptors. 

How Frequently do Great Horned Owls Kill Other 
Birds of Prey? We monitored 17 goshawk nests during 
1989-91 and found a maximum of two possible cases of 
Great Horned Owl predation on goshawks. The second 
case was a brood that disappeared for unknown reasons. 
The described goshawk nest was exposed above canopy 
height, which is an unusual situation in our study area 
and elsewhere (Shuster 1980, Hall 1984). Owl predation 
may rather affect the nest site selection than the population 
dynamics and density of other raptors. Predation by Great 
Horned Owls, however, has been reported to account for 
higher mortalities in other species: up to 30% of juvenile 
Spotted Owls (Strix occidentalis; Forsman et al. 1984, Gu- 
tierrez et al. 1985, Miller and Meslow 1986), 65% of 
juvenile Great Gray Owls (S. nebulosa; Duncan 1987), 0- 


261 


262 


Short Communications 


VoL. 26, No. 4 


44% of young Red-tailed Hawks {Buteo jamaicensis; 
Mclnvaille and Keith 1974, Houston 1975), up to 27% 
(Walton and Thelander 1988) or locally even more (Steidl 
et al. 1991a) of fledged or released Peregrine Falcons 
{Falco per egrinus), up to 21% of hatched Ospreys (Pandion 
haliaetus; Steidl et al. 1991b), 25 predations on young 
Harris’ Hawks from 64 nests (Parabuteo unicinctus; Daw- 
son and Mannan 1991). It is possible that the literature 
is biased toward high predation, because surprising re- 
sults may be more likely to be published. We encourage 
also the reporting of low predation rates in areas where 
the populations of several raptor species are known. 

Killing Other Birds of Prey: a Response to Food 
Stress? We found it interesting that the goshawk was killed 
by an owl under food stress, which we had induced ex- 
perimentally. During our study, the overall prey base was 
high because Snowshoe Hares {Lepus americanus) were at 
the peak of their population cycle (Krebs et al. 1992), and 
the overall predation by owls on goshawks was low. It is 
intriguing to hypothesize that top-predators kill lower- 
level predators more often when other prey is scarce. In 
support of this hypothesis, Mclnvaille and Keith (1974) 
found a lower predation rate by Great Horned Owls on 
Red-tailed Hawks when Snowshoe Hares were at peak 
densities. More predation rates on raptors should be re- 
ported in conjunction with estimates of other prey species. 

Raptors Killing Raptors: Predation or Competition? 
Observations of raptors killing raptors have been consid- 
ered anomalies. As a consequence many short notes and 
specific lists in handbooks have been published (review in 
Voous 1988). This perspective is based on the assumption 
that raptors are a costly prey because of the high risk of 
injury to an attacking predator. Why raptors kill other 
raptors despite the high costs involved, has been explained 
by the additional benefits of removing a potential com- 
petitor (review in Mikkola 1983). Benefits other than re- 
duced competition for food may be reduced competition 
for nest sites, increased protection of young from predation, 
and increased protection from harassment (Klem et al. 
1985). 

When raptors kill other raptors, do they really suffer a 
higher risk of injury? We are not aware of analyses of 
risks involved in capturing different prey. Our case of an 
owl possibly attacking a brooding or sleeping goshawk 
suggests that there may be no more risk involved than 
when attacking any other prey. The most parsimonious 
explanation is that raptors kill raptors simply to obtain 
food, or in other words, to obtain direct and immediate 
benefits. At the present state of our knowledge, we should 
take this simple explanation as a null-hypothesis, and our 
scientific effort should be directed toward testing it. We 
can reject this null-hypothesis only if field data do not meet 
the predictions derived from it. For example, the null- 
hypothesis predicts that killed raptors are as likely to be 
consumed as any other prey, or that the proportion of 
raptors in the diet should reflect their availability as much 
as any other prey. 

Conclusions 

A Great Horned Owl killing an adult goshawk was a 
rare event with little impact on the goshawk population 
during our study. The frequency of such predation may 


vary with prey abundance, however, and may be more 
pronounced when other prey is scarce. Based on the de- 
tailed knowledge of the ecological situation of our case, 
we question the current perspective that raptors killing 
raptors are anomalies that involve a high risk and require 
competition as an explanation. More observations in a 
known context are needed to test hypotheses on why this 
phenomenon occurs. 

Resumen. — Hemos estudiado los nidos del Gavilan Azor 
{Accipiter gentilis) y del Tecolote Cornudo {Bubo virginia- 
nus) que estuvieron ubicados a 1 km de distancia el uno 
del otro. Los residues encontrados en ambos nidos son 
evidencia de que uno de los buhos de la especie B. virgi- 
nianus mato a un A. gentilis hembra cerca de su nido. Los 
buhos estuvieron sometidos a escasez de comida, la que 
fue inducida por nosotros al aumentar el numero de polios 
en el nido. El nido del A. gentilis estuvo extremadamente 
expuesto. Durante nuestro estudio, esta depredacidn fue 
un evento raro, con poco impacto en la poblacion de A. 
gentilis. La frecuencia de tales depredaciones puede ser 
mas numerosa cuando la presa es escasa. Basados en el 
conocimiento detallado de la situacion ecoldgica de nuestro 
estudio, nosotros dudamos de la creencia de que la muerte 
de una ave raptora causada por otra, es una anomalia que 
lleva un gran riesgo, y que s6lo se explica por la compe- 
tencia entre raptoras. 

[Traduccion de Eudoxio Paredes-Ruiz] 

Acknowledgments 

We thank G. Kullberg and B. Zimmermann for their 
help with field work on owls. We received helpful sug- 
gestions from U. Breitenmoser, R.J. Cannings, J.W. 
Dawson, J.R. and P. A. Duncan, C.L. Esser, C.J. Krebs, 
R.N. Rosenfield, A.R.E. Sinclair, and J.N.M. Smith. We 
were supported by the Natural Sciences and Engineering 
Research Council of Canada (grant to C.J. Krebs), and 
an R.J. Thompson Wildlife Fellowship to C. Rohner. 
This is contribution 15 of the Kluane Boreal Forest Eco- 
system Project. 

Literature Cited 

Bortolotti, G.R. 1992. Evaluating the merit of single 
observations — response to Schmutz. /. Raptor Res. 26. 
100 . 

Craighead, JJ- and F.C. Craighead. 1956. Hawks, 
owls and wildlife. Stackpole Co., Harrisburg, PA. 
Duncan, J.R. 1987. Movement strategies, mortality, and 
behavior of radio-marked Great Gray Owls in south- 
eastern Manitoba and northern Minnesota. Pages 101- 
108 in R.W. Nero, R.J. Clark, R.J. Knapton, R.H 
Hamre [Eds.], Biology and conservation of northern 
forest owls: symposium proceedings. General Tech- 
nical Report RM-142, U.S. Department of Agricul- 
ture, Forest Service, Ft. Collins, CO. 

Forsman, E.D., E.C. Meslow and H.M. Wight. 1984. 
Distribution and biology of the Spotted Owl in Oregon 
Wildl. Monogr. 87:1-64. 

Gutierrez, R.J., A.B. Franklin, W. Lahaye, V.J. 
Meretsky and J.P, Ward. 1985. Juvenile Spotted 


December 1992 


Short Communications 


263 


Owl dispersal in northwestern California: preliminary 
results. Pages 60-65 in R.J. Gutierrez and A.B. Carey 
[Eds.], Ecology and management of the Spotted Owl 
in the Pacific Northwest. General Technical Report 
PNW-185, U.S. Department of Agriculture, Forest 
Service, Portland, OR. 

Hall, P.A. 1984. Characterization of nesting habitat of 
goshawks (Accipiter gentilis) in northwestern Califor- 
nia. M.Sc. thesis, Humboldt State University. 

Houston, C.S. 1975. Close proximity of Red-tailed 
Hawk and Great Horned Owl nests. Auk 92:612-614. 

Klem, D., B.S. Hillegass and D.A. Peters. 1985. 
Raptors killing raptors. Wilson Bull. 97:230-231. 

Krebs, C.J., R. Boonstra, S. Boutin, M. Dale, S. 
Hannon, K. Martin, A.R.E. Sinclair, R. Tur- 
KiNGTON, AND J.N.M. Smith. 1992. What drives the 
Snowshoe Hare cycle in Canada’s Yukon? Pages 886- 
896 in D. McCullough and R. Barrett [Eds.], Wildlife 
2001: populations, Elsevier, London, U.K. 

McInvaille, W.B. and L.B. Keith 1974. Predator- 
prey relations and breeding biology of the Great Horned 
Owl and Red-tailed Hawk in Central Alberta. Can. 
Field-Nat. 88:1-20. 

Mikkola, H. 1983. Owls of Europe. Buteo Books, Ver- 
million, SD. 

Miller, G.S. and E.C. Meslow. 1986. Dispersal of 
juvenile Northern Spotted Owls in the Pacific North- 
west Douglas-fir Region, Progress Report, Prelimi- 
nary Analysis 1982-1986. General Technical Report 
PNW-82-322, U.S. Department of Agriculture, Forest 
Service, Portland, OR. 


Mueller, H.C. and D.D. Berger. 1968. Sex ratios 
and measurements of migrant goshawks. Auk 85:431- 
436. 

Petersen, L.R. and J.R. Keir. 1976. Tether plat- 
forms — an improved technique for raptor food habits 
study. Raptor Research 10:21-28. 

ScHMUTZ, J.K. 1992. Should single observations be pub- 
lished? J. Raptor Res. 26:99. 

Shuster, W.C. 1980. Northern Goshawk nest site re- 
quirements in the Colorado Rockies. Western Birds 11: 
89-96. 

Steidl, R.J., C.R. Griffin, L.J. Niles and K.E. Clark. 
1991a. Reproductive success and eggshell thinning of 
a re-established Peregrine Falcon population. J. Wildl 
Manage. 55:294-299. 

, , AND . 1991b. Differential re- 
productive success of Ospreys in New Jersey. J. Wildl 
Manage. 55:266-272. 

Voous, K.H. 1988. Owls of the northern hemisphere 
Collins Sons and Co., London, U.K. 

Walton, B.J. and C.G. Thelander. 1988. Peregrine 
Falcon management efforts in California, Oregon, 
Washington, and Nevada. Pages 587-599 in T.J. Cade, 
J.H. Enderson, C.G. Thelander, and C.M. White, 
[Eds.], Peregrine Falcon populations: their manage- 
ment and recovery. The Peregrine Fund Inc., Boise, 
ID. 


Received 11 May 1992; accepted 10 September 1992 


J Raptor Res. 26(4):263-265 
© 1992 The Raptor Research Foundation, Inc. 


Nesting Association Between the Woodpigeon 
{Columba palumbus) AND THE Hobby {Falco subbuteo) 

Giuseppe Bogliani 

Dipartimento di Biologia Animate, University of Pavia, Piazza Botta 9, 27100 Pavia, Italy 

Eugenio Tiso 

Via Carena 106, 27050 Casei Gerola (PV) , Italy 
Francesco Barbieri 

Dipartimento di Biologia Animate, University of Pavia, Piazza Botta 9, 27100 Pavia, Italy 


Nest predation is the main cause of breeding failure in 
birds (Ricklefs 1969). Various mechanisms for defending 
nests against predators have evolved. In their classification 
of nest defenses, Collias and Collias (1984) recognized, 


among others, species which use “protective nesting as- 
sociation with formidable species”; the formidable species 
can be large birds of prey, wasps, bees or termites and 
their nests, or humans and their habitations. It is presumed 


264 


Short Communications 


VoL. 26, No. 4 


that potential predators risk death or serious injury from 
the dangerous species, if they try to prey on nests of the 
protection-seeking species. It is usually said that in this 
kind of symbiosis the species seeking protection do not 
incur intrinsic costs such as predation or egg and chick 
loss due to attacks from the associated dangerous species 
(Gollias and Collias 1984). 

We describe an association between nesting Woodpi- 
geons and Hobbies. Apart from Bijlsma (1984) there have 
only been anecdotal descriptions of this phenomenon by 
various authors, reviewed by Collar (1978) and by Bijlsma 
(1984). Cain et al. (1982) and Kazakov (1976, in Cramp 
1985) have reported the association between Woodpigeons 
and other raptors. 

Study Area and Methods 

The study was carried out in a 62 km^ plot bordering 
the course of the Po River, in northern Italy, where poplar 
{Populus sp. cultivars) plantations predominated. These 
host a dense population of nesting Hobbies with 25.8- 
29.0 pairs/100 km^ and a mean nearest neighbor distance 
of 1.5 km (SD = 0.7, N = 35). This is one of the highest 
densities so far recorded (Bogliani 1992). The poplars are 
mainly from a single clone (1-214) and are therefore ge- 
netically identical. They are planted at regular intervals, 
with 5-6 m between trees. 

The Hobby used only Hooded Crow {Corvus corone 
cornix) nests, selecting those in plots with larger trees 
(circumference at 130 cm greater than 100 cm). Mean 
hatching date was 18 July, based on 29 nests closely ob- 
served from 1985 to 1988. 

Observations on breeding biology were carried out from 
1985-88. Nests were searched for during July and August 
by inspecting all abandoned Hooded Crow and Wood- 
pigeon nests. In 1987 the occurrence of both the Hobby 
and the Woodpigeon was carefully checked in 25 plots 
(surface of each plot 0. 2-1.0 km^) with poplars of the 
suitable size. A detailed map, showing each tree. Hooded 
Crow and Woodpigeon nest within a radius of 100 m from 
1 1 of the 1 8 Hobby nests found in 1 8 plots was drawn. 

Results and Discussion 

Woodpigeon and Hobby nests were significantly asso- 
ciated in the same plots. Of 17 plots with nesting Wood- 
pigeons, only 1 did not also simultaneously host the Hobby. 
On the other hand, the Woodpigeon was absent in only 2 
of the 18 plots with Hobby nests (Fisher exact test, P = 
0 . 001 ). 

Woodpigeon nests tended to be clumped around a Hob- 
by nest, and in most cases were less than 40 m from the 
falcon nest (Fig. 1). We found a significant difi'erence 
between the observed frequency of occurrence in five 20 
m distance classes from the Hobby nest, and the expected 
frequency if the trees in the plot were used at random (x^ 
= 103, df = 4, = 45, P < 0.001; Fig. 1). The ratio of 

used vs. available trees was highest in the nearest distance 
class. The minimum distance was 5 m, the nearest tree to 
the Hobby nest; there were no cases of simultaneous nest- 
ing on the same tree. The 20 to 40 m distance class was 
also preferred, while over 40 m Woodpigeon nests were 


>> 

o 

g 

g- 

<D 


0-20 20-40 40-60 60-80 80-100 


distance from the Hobby nest (m) 

Figure 1. Woodpigeon use of poplars for nesting {N = 
45) at varying distances from Hobby nests (N =11) north- 
ern Italy and availability of poplars within the same dis- 
tance classes (N = 6866). 


less frequent than expected. The average number of Wood- 
pigeon nests present within 100 m of the 11 Hobby nest 
was 4.1 (SD = 1.8). 

The Woodpigeon started nest building after Hobbies 
were settled in their nest and therefore apparently sought 
the association with the raptor. It appears unlikely that 
the association was simply due to their sharing the same 
macro- and micro-habitat requirements. Poplar planta- 
tions with large trees suitable for both species were widely 
available in the study area. 

We were unable to identify a preference by the Hobby 
for certain micro-habitat features such as tree and plan- 
tation structure, using univariate and multivariate statis- 
tical techniques with a set of 1 1 variables. The Hobby 
seemed to choose crow nests only on the basis of the macro- 
habitat and the need to space out the nests, but was un- 
selective as regards micro-habitat (Bogliani et al. 1992). 
Trees close to the raptor nests were highly preferred by 
nest building Woodpigeons. Collar (1978) has suggested 
that the strong preference by the Woodpigeon for nesting 
very close to the Hobby may serve to protect the former 
from nest predators, especially the crows. The Hooded 
Crow is very common in poplar plantations of our study 
area, where it reaches very high densities. Crow nest den- 
sity varies between 14-46 nest per km^ (Quadrelli 1985). 
The Woodpigeon is likely to suffer from crow predation, 
as stated by Tomialojc (1978). The Hobby vigorously 
attacks all large birds, such as Grey Herons (Ardea cinerea) 
Herring Gulls (Larus cachinnans), Black Kites (Milvus 
migrans)^ Common Buzzards {Buteo buteo) and kestrels 
(Falco tinnunculus) which fly within a distance of ca. 50 
m of its nest. Hobbies are especially persistent in attacking 
and driving away Hooded Crows (pers. observation). It 
may be presumed that Woodpigeons gain the advantage 
of reduced nest predation if they nest very close to the 
Hobby, whose attacks keep away crows. The pattern of 
breeding success of Woodpigeons in the Netherlands was 
consistent with this prediction (Bijlsma 1984), Although 


December 1992 


Short Communications 


265 


Woodpigeons expose themselves to the risk of being preyed 
on by the falcon, this risk is apparently low. As a matter 
of fact, only two pluckings containing adult Woodpigeon 
feathers were found on two of the 46 Hobby nests inspected 
in the study area. It is worth noting that no association 
seemed to exist between the Woodpigeon and the four 
Common Buzzard nests found in poplar plantations dur- 
ing this study. Indeed the buzzard commonly preys on 
Woodpigeons (Cramp 1980, G.B. pers. observation) and 
therefore the association would be fatal to this latter spe- 
cies. 

Resumen. — En plantaciones de alamo en el norte de Italia, 
donde halcones de la especie Falco subbuteo anidan con 
relativ^mente alta densidad, palomas de la especie Columba 
paiumbus construyen sus nidos agrupan doles muy cerca a 
los de los halcones. Se presume que esta conducta de las 
palomas reduce la incidencia en la depredacion que en sus 
nidos hacen los cuervos de la especie Corvus corone cornix. 
Estos son fieramente atacados y echados fuera por el F. 
subbuteo. Sin embargo, las palomas adultas corren el riesgo 
de ser presas de los halcones; residuos de C. paiumbus han 
sido hallados entre las plumas de las aves que han sido 
presas de ellos. Este riesgo, sin embargo, podria ser mas 
alto, en la misma area de estudio, si las palomas anidaran 
cerca de los nidos de otra especie de raptora, tal como la 
Buteo buteo, que normalmente hace presa de ellas. Esto 
puede explicar por que las palomas no se asocian con esa 
especie. 

[Traduccion de Eudoxio Paredes-Ruiz] 
Acknowledgments 

We would like to thank R.G. Bijlsma who made helpful 
comments and suggestions on the manuscript. 

Literature Cited 

Bijlsma, R.G. 1984. Over de broedassociatie tussen 
Houtduiven {Columba paiumbus) en Boomvalken {Fal- 
co subbuteo). Limosa 57:133-139. 


Bogliani, G. 1992. Lodolaio {Falco subbuteo). Pages 
651-658 in P. Brichetti, P. De Franceschi and N. 
Baccetti [Eds.], Fauna d’ltalia — Aves. Vol. I. Edizioni 
Calderini, Bologna, Italy. 

, E. Tiso and F. Barbieri. 1992. Breeding bi- 
ology of the Hobby {Falco subbuteo) nesting in poplar 
plantations in Northern Italy. Atti Mus. Reg. Sc. Nat. 
Torino, in press. 

Cain, A.P.E., N. Hillgarth and J.A. Valverde. 1982. 
Woodpigeons and Black Kites nesting in close prox- 
imity. Br. Birds 75: 61-65. 

Collar, N.J. 1978. Association of nesting Woodpigeons 
and Hobbies. Br. Birds 71:545-546. 

COLLIAS, N.E. AND E.C. COLLIAS. 1984. Nest building 
and bird behavior. Princeton University Press, Prince- 
ton, NJ. 

Cramp, S. [Ed.]. 1980. The birds of the Western Pa- 
learctic. Vol. II. Oxford University Press, Oxford, U.K 

. 1985. The birds of the Western Palearctic. Vol. 

IV. Oxford University Press, Oxford, U.K. 

Quadrelli, G. 1985. Elevata densita di nidi di Cor- 
nacchia Grigia {Corvus corone cornix) in una zona go- 
lenale del Po. Proceedings III., Conv. Ital. Orn.:\6S- 
166. 

Ricklefs, R.E. 1969. An analysis of nesting mortality 
in birds. Smithsonian Contrib. Zool. 9:1-48. 

Tomialojc, L. 1978. The influence of predators on 
breeding Woodpigeons in London parks. Bird Study 
25:2-10. 

Received 5 May 1992; accepted 15 September 1992 


J. Raptor Res. 26(4):266 

© 1992 The Raptor Research Foundation, Inc. 


Letters 


Bald Eagles Use Artificial Nest Platform in Florida 


Reports of Bald Eagles (Haliaeetus leucocephalus) nesting in human-made nest structures or on artificial platforms 
are rare. Occasionally, artificial nest structures have been used to replace destroyed natural nests, remove a breeding 
pair from existing or future human activity, or to promote the expansion of a population (M.V. Stalmaster 1987, The 
Bald Eagle, Universe Books, NY). Successful acceptance and use of artificial nests by Bald Eagles have been reported 
from Michigan (S. Postupalsky 1978, Artificial nesting platforms for Ospreys and Bald Eagles. Pages 35-45 in S.A 
Temple [Ed.], Endangered birds; management techniques for preserving threatened species. University of Wisconsin 
Press, Madison, WI) and Arizona (T.G. Grubb 1980, An artificial Bald Eagle nest structure. Research Note RM- 
383, Forest Service, U.S. Department of Agriculture, Tempe, AZ). 

Monitoring of Bald Eagle nesting activities near facilities of Florida Power and Light Company to minimize potential 
impacts on this endangered species has occurred for more than a decade in Florida (N. Williams-Walls et al. 1986, 
Fla. Field Nat. 14:29-37). In the winter and spring of 1987, a pair of Bald Eagles built a small nest at the intersection 
of the cross member supports near the top of a 18.3 m-tall power line structure (H-frame) on a 240 kV line southwest 
of Titusville, Brevard County, Florida. The resident landowner indicated that winds destroyed the nest in 1987. In 
December 1987 and January 1988, a similar nest was built at the same location; incubation by adult Bald Eagles 
began in the latter part of January 1988. Much of the nesting material fell to the ground between mid-April and mid- 
May due to either wind or destruction by the two growing eaglets. Nevertheless, two eagles fledged from this nest. 

Late in 1988, a nest was again rebuilt by Bald Eagles at the same location and incubation began in January 1989. 
Two Bald Eagle chicks hatched in late February 1989 and on 19 April 1989 a wind storm dislodged much of the nest 
material and both young birds fell to the ground where they died. 

In June 1989, we constructed an artificial nesting platform (1.5 m x 1.5 m of 1.9 cm marine plywood attached 
with five wooden supports to a cross member of the structure) and erected it within 1.5 m of the former nest site. Only 
minor adjustment in location of the platform was an improvement as the new nest was not directly over a power line 
and reduced the potential for harm to the birds or interrupted service in the event that wet branches or feces contacted 
this high voltage line. The platform was painted in a camouflage pattern of green and black and had a series of long 
slots to facilitate drainage. Also, the upper surface of the platform had about 20 9-cm pegs projecting up and inward 
to help retain nest material. Vertical structures, sometimes used along sides of the platform to provide protection and/ 
or shade for young, were not placed on this platform. 

We built a 1.3-m diameter nest on the platform, with a 50-cm “cup” in the middle, using Loblolly Pine {Pinus 
taeda) branches from nearby trees. Green pine boughs were added to the top to simulate a natural nest. All nesting 
materials placed on the platform in June 1989 were secured using ropes tied through the slots in the platform. This 
artificial platform was in place several months in advance of the normal October-April Bald Eagle nesting season in 
Florida (Stalmaster 1987). 

In December 1989, adult Bald Eagles were repeatedly seen perched on or near the artificial platform. Because the 
nearest pine trees were >100 m away. Loblolly Pine branches and green boughs were placed in two piles on the 
ground near the power lines. In mid-January 1990, additional sticks were added by the adult Bald Eagles to the nest 
on the platform. A follow-up investigation indicated that only a small quantity of the nest material provided was used 
by the adult eagles in completing the nest. Two Bald Eagles chicks hatched and occupied the platform nest until mid- 
May, when they successfully fledged. This was the first time that Florida Bald Eagles successfully fledged from an 
artificial nest on an artificial platform. Adult Bald Eagles again used the platform and were incubating in January 
1991. Unfortunately, repeated traffic under the nest appeared to disrupt incubation and the platform was abandoned 
in February 1991. 

We thank J.D. Fraser, J.L. Lincer, B.A. Millsap, and an anonymous reviewer for their useful comments on this 
paper. This project was supported by Florida Power and Light Company as part of a long-term monitoring effort 
involving Bald Eagles in Florida. This is contribution R-00902 of the Florida Agricultural Experiment Station, 
Gainesville. — W.R. Marion, Department of Wildlife and Range Sciences, University of Florida, 118 Newins- 
Ziegler Hall, Gainesville, FL 32611-0304 (present address: Hancock Timber Resource Group, 2401 Bristol 
Court S.W., Olympia, WA 98502). P.A. Quincy, Florida Power and Light Company, P.O. Box 078768, West 
Palm Beach, FL 33407-0768; C.G. Cutlip, Jr., Bio-Scan, Inc., 100 Ninth Street East, Lehigh, FL 33936; J.R. 
Wilcox, Florida Power and Light Company, P.O. Box 14000, Juno Beach, FL 33408. 


266 


December 1992 


Letters 


267 


J. Raptor Res. 26(4):267 

© 1992 The Raptor Research Foundation, Inc. 


Great Horned Owl Nesting in Monk Parakeet Colony in 

Suburban Connecticut 

The Great Horned Owl {Bubo virginianus) is one of the most widespread and successful of North American birds 
of prey. Throughout its range, it nests in an extremely wide variety of habitats, ranging from desert cacti in the 
Southwest to forests of the Northeast. Excepting only the Eastern Screech Owl {Otus asio), the Great Horned Owl 
may also be the raptor most tolerant and adaptable to human modified habitats; it has been recorded nesting in a 
variety of urban and suburban open space habitats, where it usually appropriates nests of crows or squirrels or, less 
frequently, large cavities in trees (K.H. Voous 1988, Owls of the Northern Hemisphere, The MIT Press, Cambridge, 
MA). 

We report on the nesting of a Great Horned Owl in a Monk Parakeet (Myiopsitta monachus) colony in a residential 
suburb of Bridgeport, Connecticut. This Monk Parakeet colony has existed since the mid-1970s and was, until recently, 
unique to this site in the state. The colony typically included 90 or more birds and 35-40 active nests at any one time, 
all constructed in a single ornamental fir {Abies sp.) about 19 m tall, in a suburban yard. 

The Great Horned Owls used the top of the largest Monk Parakeet nest located along a branch 15.2-15.4 m high. 
The nest was about 1.8 m in length, 0. 6-0.9 m in width and 0.9 m deep. It housed seven pairs of nesting Monk 
Parakeets, which entered from the bottom or sides. A single Great Horned Owl nestling was first observed in mid- 
April. It fledged on or about 17 May. Backdating (Anderson and Hickey 1970, Wilson Bull. 82:14-28), suggests that 
the egg deposition was in mid-February and hatching occurred in late March. One adult owl typically roosted in the 
tree, usually close against the bole and overlooking the nest. The second adult sometimes roosted in a small line of 
White Pines {Pinus alba) about 90 m away. After fledging, both young and adult continued to roost in the nest site 
tree, always close to the trunk and well within the canopy at heights of 15.2-18.3 m. 

Pellets and prey fragments collected from beneath the nest site tree yielded the remains of 22 prey individuals 
belonging to four species. Of these, 17 (77.3%) were the Norway Rat {Rattus norvegicus), 1 (4.5%) an Eastern Cottontail 
{Sylvilagus floridanus) and 2 (9.1%) each were of Eastern Chipmunk {Tamias striatus) and Gray Squirrel {Sciurus 
carolinensis) . Observations indicated that the Great Horned Owls were taking Norway rats from a small estuary located 
about 0.5 kilometers from the nest site. The other prey species suggest that the adult owls also sometimes foraged along 
the lawns. 

Although several piles of Monk Parakeet feathers were found beneath the nest site none were found in pellets. 
However, we did observe the nestling scurrying along a branch toward a parakeet that had landed about 1.5 m away 
while the adult female watched. Neither it nor the nearby adult were able to capture the parakeet, which flew off as 
the juvenile owl approached. — Arnold Devine, Connecticut Department of Environmental Protection, Hartford, 
CT 06510; Dwight G. Smith, Biology Department, Southern Connecticut State University, New Haven, CT 
06515. 


/ Raptor Res. 26(4): 267-268 
© 1992 The Raptor Research Foundation, Inc. 


Renesting of Mexican Spotted Owl in Southern New Mexico 

Renesting in the wild by Mexican Spotted Owls {Strix occidentalis lucida) has not been documented previously. E.D. 
Forsman et al. (1984, Wildl. Monogr. 87:33) stated that a captive Spotted Owl laid two sets of eggs in two different 
years, but they made no mention of this occurring in the wild. 

We report renesting of a pair of Mexiean Spotted Owls in the Lincoln National Forest in southern New Mexico. 
This pair was included in a study of four mated pairs and one female of a mated pair that were fitted with back-pack 
radiotransmitters in October 1990. Monitored pairs began roosting together in February 1991 and began nesting in 
March. 

We visited the nest sites at least twice per week to check for young after females were thought to be with eggs. On 
3 and 4 May, a single, approximately 10 cm tall owlet was dead at the base of the nest tree of one of the pairs. The 


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VoL. 26, No. 4 


owlet was about 1 wk old. Backdating from the date when this owlet was found, the first clutch of this female was 
laid during the last week of March or the first week of April. After the owlet died, the female remained on the nest 
until 20 May. She was also observed roosting within 1-200 m of the male in the vicinity of the original nest on 10, 
12, and 19 May. 

On 26 May, the female which had lost her young was sitting on another nest in a tree about 100 m farther up the 
canyon than the first nest tree. During daylight hours on 6 July, she was partially erect on the edge of the nest in a 
position that was clearly visible from the ground. Prior to this date, it had been difficult to see the incubating female 
from ground level. Based on her more vertical position on the nest we think that the second clutch of egg(s) hatched 
immediately before 6 July. On 13 July, an owlet was on the edge of the nest. On 16 July, the partially eaten remains 
of the female and an owlet were on the ground near the nest tree. Entrails had been removed from the female. A foot 
was all that remained of the owlet on the ground. Within the nest, we found a portion of the owlet’s beak and the 
female’s wing. — K.W. Kroel and P.J. Zwank, U.S. Fish and Wildlife Service, New Mexico Cooperative Fish and 
Wildlife Research Unit, New Mexico State University, Las Cruces, NM 88003. 


J. Raptor Res. 26(4);268 

© 1992 The Raptor Research Foundation, Inc. 


American Kestrel Completes Clutch Following Movement of Its Nest Box 

Compared with many other raptors, American Kestrels (Falco sparverius) are quite tolerant of disturbance during 
incubation (P.H. Bloom and S.J. Hawks 1983, Raptor Res. 17:9-14; T.J. Wilmers et al. 1985, N. Am. Bird Bander 
10:6-8). Even so, daily disturbance usually causes nest desertion (J.A. Gessaman and P.R. Findell 1979, Comp 
Biochem. Physiol. 63A:57-62). The contents of kestrel nests have been removed and placed in metabolism chambers 
during incubation (Gessaman and Findell 1979), but I am not aware of any published reports where kestrels continued 
to incubate eggs that had been moved to a different location. Here I report such an event. 

The nest box was initially located in a dead maple {Acer sp.) 3 m above the ground in southwest Wood County, 
Ohio. On 3 May 1992, the nest box, containing two eggs (R. Wensick pers. comm.), was removed from the tree and 
placed upright on the ground 3 m away. The tree was then felled and cut into pieces. I learned about the situation 
on 5 May and checked the box at 1205 H, discovering the male incubating four eggs. At 1435 H that same afternoon, 
the female was incubating the eggs. Fearing that the nest would succumb to mammalian predation if left on the ground, 
at 1515 H I secured the box to a steel fence post 1 m above the ground and 10 m from where the nest tree had been. 
At the time, the female was perched on a utility wire 100 m away. At 1620 H, I observed the nest box from a distance 
of 0.5 km with a spotting scope. The female was then perched on a utility wire 10 m away. At 1628 H, she flew to 
the box, hovered at the entrance for several seconds, then flew to the cut up nest tree and circled around the pieces of 
the tree for 2 min. At 1630 H, she flew to the box and entered it. She was still in the box when I left 15 min later. 
During the evening of 5 May the remains of the nest tree were removed and the stump burned. I observed the nest 
from 1122-1142 H at a distance of 0.5 km on 6 May. I noticed no activity in the vicinity of the box, but observed a 
pair of kestrels copulating near a nest box located 1 km to the east, suggesting that the pair had deserted and moved 
to a new nest site. The translocated nest box contained five cold eggs when checked at 1050 H on 10 May. The pair 
was again observed 1 km to the east. 

Thus, after the nest tree was felled, three eggs were laid: two while the box was on the ground and one after it was 
placed on the fence post. Incubation was observed, and probably initiated, while the box lay on the ground. Incubation 
normally commences when the fourth egg is laid in a five egg clutch (R.D. Porter and S.I. Wiemeyer 1972, Condor 
74:46-53). 

These observations show that this pair of kestrels continued to use their nest after it was relocated a short distance 
away at a lower height. This suggests that kestrel nests may be successfully relocated, if disturbance can be kept to a 
minimum following relocation. 

I thank Richard Wensick for providing the fence post and other materials needed to elevate the nest box, for alerting 
me that the nest tree had been removed, and for providing information on the date and nest contents when the tree 
was felled. This, and other nest boxes, were constructed using funds provided by a Paul A. Stewart Award from the 
Inland Bird Banding Association. — Thomas W. Carpenter, Department of Biological Sciences, Bowling Green 
State University, Bowling Green, OH 43403-0212. 


December 1992 


Letters 


269 


/. Raptor Res. 26(4):269 

© 1992 The Raptor Research Foundation, Inc. 


Aerial Mobbing of a Gyrfalgon by Glaucous Gulls 

On 1-2 June 1990, while camped at a small island in Alexandra Fiord on Ellesmere Island (78”54'N 75°45'W) 
near a lowland oasis (B. Freedman and J. Svoboda 1982, Can. Field. -Nat. 96:56-60), I observed a male Gyrfalcon 
(Falco rusticolus) quartering back and forth in front of a cliff where the female was nesting. The orographic lift of the 
Gyrfalcon was interrupted by a group of three Glaucous Gulls (Larus hyperboreus) on two separate occasions, each 
lasting several minutes. During the first encounter, the gulls, which initiated the engagement, acted in unison, each 
gull taking turns diving at the Gyrfalcon, while the others circled close by calling loudly. The Gyrfalcon, in an attempt 
to fight the gulls, swiveled about, talons extended, occasionally even doing so while completely inverted. After returning 
to the nest, the Gyrfalcon ignored the gulls until after 10 min it tried to make a kill of a single passing bird. The 
panicked avoidance by the Glaucous Gull consisted of dropping vertically to several meters above the pack ice and 
calling loudly. Within a few moments, two gulls arrived and the three dissuaded the Gyrfalcon from hunting. The 
falcon then returned to the nest site where it stayed for the next hour of observation, disregarding nearby gulls. 

Gyrfalcons commonly take prey in the air (e.g., G.M. White and R.B. Weiden 1966, Condor 68:517-519; S.A. 
Bengtson 1971, Ibis 113:468-476) and customarily prey upon seabirds (e.g., G.P. Dementiev and N.N. Gortchakovskaya 
1945, Ibis 87:559-565; K.G. Poole and G.A. Boag 1988, Can. J. Zool. 66:334-344) including Glaucous Gulls (G.M. 
White and T.J. Gade 1971, Living Bird 10:107-150). Similar mobbing of Gyrfalcons by Gommon Raven (Corvus corax) 
has been noted previously (M.A. Jenkins 1978, Auk 95:122-127). N. Wooden (1980, Raptor Res. 14:97-124) also 
observed perched Gyrfalcons struck by passing Arctic Terns {Sterna paradisaea) which, however, never grouped to 
drive the raptor away as in the present encounter with the larger Glaucous Gulls. This seemingly paradoxical behavior 
of self-endangerment by mobbers may be necessary to “convince” the predator that their threat is real (S.A. Sordahl 
1990, Wilson Bull. 102:349-353). The result is that both ravens (Jenkins 1978) and Glaucous Gulls (this study) can 
be ignored even when they fly directly beneath or over an occupied Gyrfalcon eyrie. — R.L. France, Department of 
Biology, McGill University, 1205 Ave. Dr. Penfield., Montreal, PQ,, Canada H3A IBl. 


J Raptor Res. 26(4):269-270 
© 1992 The Raptor Research Foundation, Inc. 


An Aggressive Interaction Between a Northern Goshawk and a Red-tailed Hawk 

During September 1987, D. Grannell observed an aggressive encounter between a Northern Goshawk {Accipiter 
gentilis) and a Red-tailed Hawk {Buteo jamaicensis) on the Alsea Ranger District (Township 13 South, Range 09 
West) of the Siuslaw National Forest in the Goast Range of western Oregon. An adult Red-tailed Hawk was observed 
flying erratically, apparently grappling with another bird. The birds tumbled to the ground a short distance away and 
when this location was approached, the Red-tailed Hawk was seen hanging upside down in the talons of a mature 
goshawk about 3 m up in a small tree. The pair of birds were about 5 m from the observer. The goshawk dropped 
the Red-tailed Hawk, possibly because of the close proximity of a human, and after about 60 sec the Red-tailed Hawk 
hopped to an adjacent bush. The two hawks then watched each other for a few seconds, and the Red-tailed Hawk 
flew south across a pasture and landed in a tree. Within seconds, the goshawk pursued the redtail, struck it, and both 
birds went to the ground. The outcome of this last encounter was not observed. 

Vegetation in the area was dominated by second-growth Douglas-fir {Pseudotsuga menziesii), vine maple {Acer 
circinatum) and red alder {Alnus rubra) growing where recent timber harvest had taken place. This habitat was not 
typical of most nesting sites for Northern Goshawks in Oregon, and goshawks are not known to nest in the Goast 
Range of Oregon (R.T. Reynolds et al. 1982, J. Wildlife Manage. 46:124-138), although sightings of goshawks are 
often recorded there. Red-tailed Hawks do nest in the Goast Range. No vocalizations were heard, so it was not known 
if either bird emitted alarm or defensive calls. It is possible that this encounter was an act of predation by the goshawk 
on the redtail rather than aggressive territoriality. We do not know if the goshawk consumed any of the redtail. 
Encounters between Northern Goshawks and Red-tailed Hawks are of interest because of the possibility that the two 
species are being drawn into closer proximity during nesting because of wide-spread alteration of forested habitat due 
to timber harvest (D.G. Grocker-Bedford 1990, Wildl. Soc. Bull. 18:262-269). Physically aggressive encounters between 


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two species of similar size are seldom observed, and may be rare in nature, but such direct sources of mortality or 
competition undoubtedly occur in the wild. 

We thank E.C. Meslow, D.K. Rosenberg, and B. Woodbridge for reviewing this note. — Dan Crannell, Bureau of 
Land. Management, P.O. Box 10226, Eugene, OR 97440-2226; Stephen DeStefano, Oregon Cooperative Wildlife 
Research Unit, Nash 104, Oregon State University, Corvallis, OR 97331-3803. 


J. Raptor Res. 26(4):270 

© 1992 The Raptor Research Foundation, Inc. 


Thesis Abstract 

Habitat Use, Movements, Migration Patterns, and Survival Rates of 
Subadult Bald Eagles in Northern Florida 

The state of Florida supports over half of the breeding population of Bald Eagles {Haliaeetus leucocephalus leucocepha- 
lus) in the southeastern United States; this represents a significant resource for the Southeast and for Florida. Currently, 
primary management emphasis and protection is focused on bald eagle nest sites. No habitat protection or management 
activities are aimed at foraging, roosting or loafing areas for subadult eagles. In fact, habitats and habitat characteristics 
important to subadults have not been quantified. In this study, I examined various aspects of eagle biology that might 
be pertinent to survival or management of the Florida subadult eagle population. Specifically, using radiotelemetry, 
I examined post-fledging habitat needs, factors affecting timing of migration, local movements, habitat use, and survival 
in north-central Florida from spring 1987 through spring 1991. 

Fledgling eagles (birds prior to their initial migration) remained dependent on adults and remained within 4 km of 
the natal nest until they initiated migration (approximately 7 wk post-fledging). It was determined that habitat protection 
within the 229 m primary protection zone used in Florida was not sufficient to meet the habitat needs of fledgling 
eagles and that the protection period should extend until fledglings initiate migration in the summer. Timing of 
migration for fledgling and 1- to 4-year-old eagles appeared to be correlated more with prey availability than with 
temperature, although both factors appear to affect migration. 

Locations of radio-tagged eagles outside of Florida ranged from South Carolina to Prince Edward Island, Canada. 
Data suggest that eagles are philopatric to summering areas, which emphasizes the need for protection of significant 
summering areas. Known and assumed mortality occurred primarily during migration in northern states. The 1 year 
age class had the lowest survival. Survival was significantly lower for eagles fledged from 1 -chick nests and for the 
younger chick in 2-chick nests. The minimum survival rate through AVz years of age was 50% and did not vary by 
sex. 

After subadults returned to the north-central Florida study area in the fall, individuals continued to use the same 
general areas each year. Temporally and locally abundant food sources resulted in temporary small concentrations of 
eagles. Certain portions of the study area were used consistently each year by large numbers of eagles. Subadult eagles 
were not distributed randomly over the study area. Logistic regression analyses revealed that eagles tended to be located 
close to large water bodies, and eagle nests were frequently in cypress and marsh habitats, and avoided main roads 
and developed areas. Immature eagles (1-year olds) tended to be located closer to eagle nests than 2- to 4-year-olds 
Thus, management for subadult populations must include these heavily used concentration areas that supply the habitat 
features preferred by subadults. Survival of subadults may be affected if a highly used area becomes unsuitable. — 
Petra Bohall Wood. 1992. Ph.D. thesis. Department of Wildlife and Range Sciences, University of Florida, 
Gainesville, FL 32611. Present address: West Virginia Coop. Fish and Wildlife Research Unit, West Virginia 
University, P.O. Box 6125, Morgantown, WV 26506-6125. 


J. Raptor Res. 26(4):271-273 
© 1992 The Raptor Research Foundation, Inc. 

News and Reviews 


Trends in European Goshawks {Accipiter gentilis): an overview by R.G. Bijlsma. 1991. Bird Census News. Vol. 
4:3-47. 


The goshawk {Accipiter gentilis) is a large, forest-nesting raptor found across the northern hemisphere. This species 
occurs year-round in a wide variety of habitats in Europe, including forests, woodlands, agricultural and rural-residential 
areas. These habitats are subject to a diversity of land uses and environmental fluctuations. As the author indicates, 
the study of goshawk population trends can provide information on local environmental conditions, such as loss of 
habitat, environmental pollution, human persecution, or declines in other species (goshawk prey). The objective of this 
article was to indicate trends of breeding goshawks in Europe. 

Goshawk population data from 25 countries was examined (Germany is divided into 8 areas, for a total of 32 areas). 
A discussion and supporting figures are presented for each country, including study area size, goshawk density estimates 
and environmental factors influencing trends when available from each literature source. Current goshawk population 
numbers for each country is estimated. The author’s summary of population trends by country is displayed in tabular 
format, for the time period between 1950-90. 

This article is an impressive review of literature on the European Goshawk covering population information and 
environmental threats. Overall, the conclusions of this article appear well supported by references spanning 40 yr. The 
reader must have faith in the author’s interpretive abilities as not all the materials necessary for critical analysis are 
provided. 

The sources referenced are published articles covering a range of study areas, habitats, methodologies and study 
objectives. Thus, a comparison of nesting densities among studies is not possible. The author’s approach is to examine 
trends in individual populations, and to bring together all available information to make conclusions regarding trends 
over larger areas. The author admits that this task has its challenges. For example, information on changes in habitat 
or land use over the length of the study was not always provided by each source. In addition, systematic survey and 
experienced observers may not have been used in all cases. 

The reader might find it difficult to assess the reliability of each source from this article. The original study objectives 
and methodologies are not presented in all cases, and these original references are not likely to be available to most 
non- European readers. The author periodically lends his professional judgment as to whether the cited findings were 
realistic with respect to population trends. 

The discussion for each country is supported by a graph of goshawk population trends. The graphs are not always 
well labeled with country names. Units of measure are used inconsistently in graphs and text to describe goshawk 
densities and study area size, which makes reading challenging. I would have appreciated the inclusion of a map of 
the continent depicting countries and study locations. 

The author summarizes environmental factors which were cited as affecting European Goshawk populations for 
three periods over the total 40-year span. Factors mentioned include pesticides, human persecution of goshawks, and 
changes in habitat and prey availability. 

Data available from the period prior to 1955 suggest a slight to strong increase in goshawk populations, possibly a 
response to prohibition of persecution. In northern Europe, goshawk trends during this period appear to have been to 
a large extent a response to tetraonid population cycles. The use of persistent pesticides is cited as the cause of dramatic 
declines in goshawk populations throughout Europe, during the period between 1956-70. This trend was also seen in 
other bird-eating raptors. The impacts of hunting or persecution on goshawks are difficult to assess but are believed 
by some to have caused local declines and extinctions during this period. In most European countries the use of persistent 
pesticides was discontinued by the early 1970s, and goshawk population trends throughout the continent during this 
period are strongly positive. Densities in western Europe often appeared to peak in the 1980s followed by a decline 
and stabilization at a lower population level. 

Changes in habitat are cited in relatively few sources as causing goshawk declines and cited in fewer cases as resulting 
in increases. A reduction in forested habitats was due to various causes, which included acid rain, clear-cutting, the 
use of “modern forestry” practices, forest fires, and conversion of forests to non-native plantations. The maturing of 
forests and habitat diversification was cited at the potential cause of recent goshawk population increases for one study. 

Fluctuation in prey populations was cited as a factor in goshawk population trends. In the boreal forest of northern 
Europe, cyclic trends in food availability were considered to cause similarly cyclic trends in goshawk numbers. In one 
reference, adverse forestry practices were believed to be the cause of prey population declines, especially forest tetraonids. 


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News and Reviews 


VoL. 26, No. 4 


which in turn were believed to cause declines in goshawk numbers. Agricultural and other rural land practices were 
also suspected causes of depleted prey resources. 

The conclusion of the review is that without further large changes in habitat, food supply, or persecution, European 
Goshawk populations should remain stable in the future. A summary of research over the past 10 yr suggests that 
19% of the countries or areas under consideration have goshawk populations that exhibit at least a probable decreasing 
trend. Forty percent of the areas have goshawk populations that are suspected to be stable or increasing. The remaining 
areas have goshawk populations which appear to exhibit variable trends (16%) or have populations for which no 
reliable information exists (25%). The primary contribution of this article may be to focus attention on European 
Goshawk populations that appear to be declining or for which little is known. — Karen K. Austin 


The International Osprey Foundation (TIOF) is 
seeking applications for its fourth grant to support research 
activities of a graduate student primarily focusing on os- 
preys. Work with other raptor species may be considered, 
however. The award recipient will be expected to provide 
a report on his or her research and use of the funds within 
a year of receiving the grant. 

Applicants should submit a proposal outlining their 


project and the intended use of the funds by December 
31, 1992. The grant will be awarded on March 31, 1993. 
Please send a project description of no more than two 
pages. Also provide an itemized estimate of expenses and 
the name, address and phone number of the graduate 
supervisor. Send applications to; TIOF, Endowment 
Fund, P.O. Box 250, Sanibel, FL 33957-0250. 


Manuscript Referees 

The following persons kindly volunteered their insight and spent valuable time in reviewing manuscripts for the 
Journal: Dean Amadon, James G. Auburn, Thomas G. Balgooyen, Samuel J. Barry, Marc J. Bechard, James C. 
Bednarz, Daniel D. Berger, John Bielefeldt, Robert G. Bijlsma, Keith L. Bildstein, Peter H. Bloom, David A. Boag, 
Gary R. Bortolotti, Thomas Bosakowski, William W. Bowerman, Richard D. Brown, Mitchell A. Byrd, Tom J. Cade, 
Thomas W. Carpenter, Paul M. Cavanagh, Richard J. Clark, William S. Clark, Jack Clinton- Eitnear, Patrick Colgan, 
Charles T. Collins, Michael W. Collopy, A. R. Craig, John L. Curnutt, Dale W. Stahlecker, Martha Desmond, 
Stephen DeStefano, Jose A. Donazar, Gary E. Duke, Jame R. Duncan, David H. Ellis, John T. Emlen Jr., James 
H. Enderson, Dave L. Evans, Roger M. Evans, Peter Ewins, Fran Hamerstrom, John R. Faaborg, Jim Fitzpatrick, 
Stephen P. Flemming, Dale E. Gawlik, Laurie Goodrich, Daniel N. Gossett, Jon S. Greenlaw, Curtice Griffin, Lucinda 
Haggas, Alan H. Harmata, Judy Henckel, Charles J. Henny, Fernando Hiraldo, Anne Hoag Wheeler, Stephen W. 
Hoffman, Denver W. Holt, C. Stuart Houston, Richard Howard, Grainger W, Hunt, William M. Iko, Eduardo E 
Inigo-Elias, Chris Iverson, Fabian M. Jaksic, Paul C. James, Jamie E. Jimenez, Enrique R. Justo, Richard H. 
Kerbes, Paul Kerlinger, Mark Kopeny, Jeff Lincer, Douglas MacCoy, Santi Mahosa, Mark S. Martell, Carl D 
Marti, Kathy Martin, John M. Marzluff, Katherine McKeever, Brian A. Millsap, Douglas W. Mock, Martin L. 
Morton, Charles A. Munn, Robert K. Murphy, Robert W. Nero, R. R. (Butch) Olendorff, Jim W. Parker, Jimmie 
Parrish, James R. Philips, Howard R. Postovit, Patrick T. Redig, Marco Restani, Richard T. Reynolds, Robert J 
Ritchie, Ricardo Rodriguez- Estrella, Christoph Rohner, Robert N. Rosenfield, David A. Ross, J. Stan Rowe, William 
C. Scharf, Wolfgang Scherzinger, Peter E. Scott, Steve Sherrod, Williston Shor, Dwight G. Smith, Noel Snyder, Mark 
Stalmaster, Paul F. Steblein, Karen Steenhof, Ernst Sutter, Ted Swem, Ethan J. Temeles, Jean-Marc Thiollay, Paddy 
Thompson, Sergio Tiranti, Philip L. Trefry, Dan Varland, Ian G. Warkentin, James W. Watson, Clayton M. White, 
Karen L. Wiebe, Edwin O. Willis, Neil D. Woflinden, Petra Bohall Wood, Fridjof Ziesemer, Dale Zimmerman, Fred 
C. Zwickel. 


December 1992 


New and Reviews 


273 


News 

1991 Stephen R. Tully Memorial Grant Recipients 


Keith J. Merkel 

Keith J. Merkel is a native and resident of Marshfield, Wisconsin, where 
he is employed as a Quality Control Supervisor in a manufactured housing 
production facility. He is an active outdoorsman who enjoys backpacking, 
canoeing/kayaking, travel, and photography. He also is an avid amateur or- 
nithologist and bander, with a special interest in raptors. Currently he is 
researching the diet, breeding habitat, nesting success, range, and distribution 
of several raptor species in central and northern Wisconsin. As part of these 
long term studies he has installed over 100 nest boxes and platforms for 
American Kestrel and Eastern Screech, Northern Saw-whet, Barred, and Great 
Gray owls. Annual visits to these nesting structures yield data on clutch size, 
brood size, prey species, fledgling success rates, fledgling dispersal, and nest 
site fidelity. In 1988 Wisconsin’s first documented nesting Great Gray Owls 
successfully fledged four young from one of these platforms, establishing that 
this species does, at least occasionally, breed in the state. 


Neal D. Niemuth 

Neal D. Niemuth was born and raised in Stetsonville, Wisconsin, where 
his early raptor experience involved erecting kestrel nest boxes and climbing 
for local banders. After earning a Bachelor of Science degree in English from 
the University of Wisconsin at Stevens Point, Neal taught high school for five 
years before beginning work on a Master of Science degree in zoology at the 
University of Wyoming. For his thesis Neal is testing the role of nest predation 
in Sage Grouse lek formation, as well as the effect of nest density on predation 
of Sage Grouse nests. 

In addition to his Sage Grouse study, Neal continues to work with raptors. 
He is currently studying natal dispersal of Osprey, philopatry and productivity 
of kestrels, and population ecology of Saw-whet Owls in northern Wisconsin. 


CONTENTS FOR VOLUME 26, 1992 


Number 1 

Letter 1 

Reproductive Parameters for Free Ranging American Kestrels 
{Falco sparverius) Using Nest Boxes in Montana and Wyoming. Anne 

Hoag Wheeler 6 

Observations on the Behavior of Surplus Adults in a Red-shouldered 

Hawk Population. Michael D. McCrary, Peter H. Bloom and Marjorie J. Gibson 10 

Home Range, Habitat Use and Behavior of Prairie Falcons 

W^INTERING IN EaST-CENTRAL COLORADO. Gary Beauvais, James H. Enderson and 
Anthony J. Magro 13 

Northward Post-fledging Migration of California Bald Eagles, w. 

Grainger Hunt, Ronald E. Jackman, J. Mark Jenkins, Carl G. Thelander and Robert N. 

Lehman 19 

Determining Sex of Eastern Screech-Owls Using Discriminant 
Function Analysis. Dwight G. Smith and Stanley N. Wiemeyer 24 

Diet Shifts of Black-chested Eagles {Geranoaetus melanoleucus) 
from Native Prey to European Rabbits in Chile. Eduardo f. Pavez, 

Christian A. Gonzalez and Jaimie E. Jimenez 27 

Short Communications 

Methods of Locating Great Horned Owl Nests in the Boreal Forest, Christoph 

Rohner and Frank I. Doyle 33 

Food Habits of the Short-eared Owl {Asio flammeus) in Southern South America. 

Jaime R. Rau, Marcelo C. Villagra, Marta L. Mora, David R. Martinez and Maria S. 

Tilleria 35 

Golden Eagles Feeding on Fish. Bryan T. Brown 36 

Greater Yellow-headed Vulture {Cathartes melambrotvs) Locates Food by 

Olfaction. Gary R. Graves 38 

Letters 40 

Thesis Abstracts 44 

News and Reviews 47 


Number 2 

Carrying Capacity for Bald Eagles Wintering Along a 

Northwestern River. W. Grainger Hunt, Brenda S. Johnson and Ronald E. Jackman 49 


Demography of Wintering Rough-legged Hawks in New Jersey. Thomas 

Bosakowski and Dwight G. Smith 61 

Prey Use by Eastern Screech-Owls: Seasonal Variation in Central 
Kentucky and a Review of Previous Studies. Gary Ritchison and Paul m. 

Cavanagh 66 

Social Hunting in Broods of Two and Five American Kestrels After 

Fledging. Daniel E. Varland and Thomas M. Loughin 74 

Distribution and Color Variation of Gyrfalgons in Russia. David h. 

Ellis, Catherine H. Ellis, Grey W. Pendleton, Andrei V. Panteleyev, Irena V. Rebrova and Yuri 
M. Markin 81 

Short Communications 

Barn Owl Prey in Southern La Pampa, Argentina. Sergio I. Tiranti 89 

Spring Migration of Honey Buzzards (Perms apivorus) at the Straits of Messina in 

Relation to Atmospheric Conditions. Nicolantonio Agostini 93 

Long-eared Owls Usurp Newly Constructed American Crow Nests. Brian D. Sullivan . . 97 

Letters 99 

News 103 


Number 3 


Preface 106 

Fidelity to Nesting Territory Among European Sparrowhawks in 
Three Areas. I. Newton and I. WylUe 108 

Causes and Consequences of Reversed Sexual Size Dimorphism in 
Raptors: The Head Start Hypothesis. Keith l. Biidstein 115 

Molt of Flight Feathers in Ferruginous and Swainson’s Hawks. Josef 

K. Schmutz 124 

Behavior of Migrating Raptors: Differences Between Spring and 

Fall. Helmut C. Mueller and Daniel D. Berger 136 

Raptor Predation on Rock Ptarmigan (Lagopus mutus) in the 
Central Canadian Arctic. Richard C. Cotter, David a. Boag and Christopher c. 

Shank 146 

The Effect of Man-made Platforms on Osprey Reproduction at Loon 
Lake, Saskatchewan, c. Stuart Houston and Frank Scott 152 

A 24-year Study of Bald Eagles on Besnard Lake, Saskatchewan, jon 

M. Gerrard, Pauline N. Gerrard, P. Naomi Gerrard, Gary R. Bortolotti and Elston H. Dzus . . 159 


The Dho-gaza With Great Horned Owl Lure: An Analysis of Its 
Effectiveness in Capturing Raptors. Peter H. Bloom, Judith L. Henckel, 

Edmund H. Henckel, Josef K. Schmutz, Brian Woodbridge, James R. Bryan, Richard L. 

Anderson, Phillip J. Detrich, Thomas L. Maechtle, James O. McKinley, Michael D. McCrary, 
Kimberly Titus and Philip F. Schempf 167 

Conservation Biology and the Evolution of a Land Ethic. Dale E. 

Gawlik 179 

Raptor Conservation in Veracruz, Mexico. Juan Esteban Martinez-G6mez 184 

Short Communications 

Eye Color of Cooper’s Hawks Breeding in Wisconsin. Robert N. Rosenfield, John 

Bielefeldt and Kenneth R. Nolte 1 89 

The Influence of Gender and Hatching Order on Growth in Hen Harriers (Circus 

CYANEUS CYANEUS). William C. Scharf 192 

Letters 195 

Thesis Abstracts 211 

’ This issue was jointly edited by Josef K. Schmutz and Keith L. Bildstein. Artwork was kindly provided 
by Deann L. De La Ronde, Monica Herzig and Jonathan Wilde. 


Number 4 

Energy Requirements of Adult Cape Vultures {Gyps coprotheres) . 

Joris Komen 213 

Organochlorines and Mercury in Osprey Eggs from the Eastern 

United States. Daniel J. Audet, David S. Scott and Stanley N. Wiemeyer 219 

Kleptoparasitism and Cannibalism in a Colony of Lesser Kestrels 

{FaLCO NAUMANNI) . Juan Jose Negro, Jose Antonio Donazar and Fernando Hiraldo 225 

Home Range and Activity of a Pair of Bald Eagles Breeding in 

Northern Saskatchewan. Jon M. Gerrard, Alan R. Harmata and P. Naomi Gerrard 229 

Seasonal and Sexual Variation in the Diet of the Common Buzzard 
in Northeastern Spain. Santi Mahosa and Pedro J. Cordero 235 

Diet Changes in Breeding Tawny Owls {Strix aluco). David A. Kirk 239 

Foraging Ecology of Bald Eagles on a Regulated River, w. Grainger 

Hunt, J. Mark Jenkins, Ronald E. Jackman, Carl G. Thelander and Arnold T. Gerstell 243 

Short Communications 

Increased Parental Care in a Widowed Male Marsh Harrier (Cirus aeruginosus). 

Carmelo Fernandez and Paz Azkona 257 

Bats as Prey of Stygian Owls in Southeastern Brazil. Jose C. Motta Junior and Valdir 

A. Taddei 259 


Food-stressed Great Horned Owl Kills Adult Goshawk: Exceptional Observation or 


Community Process? Christoph Rohner and Frank I. Doyle 261 

Nesting Association Between the Woodpigeon {Columba palumbus) and the Hobby 

{Falco subbuteo). Giuseppe Bogliani, Eugenio Tiso and Francesco Barbieri 263 

Letters 266 

Thesis Abstract 270 

News AND Reviews 271 


THE RAPTOR RESEARCH FOUNDATION, INC. 

(Founded 1966) 


OFFICERS 

PRESIDENT: Richard J. Clark SECRETARY: Betsy Hancock 

VICE-PRESIDENT: Michael W. Collopy TREASURER: Jim Fitzpatrick 


BOARD OF DIRECTORS 


EASTERN DIRECTOR: Keith L. Bildstein 
CENTRAL DIRECTOR; Thomas Nicholls 
MOUNTAIN & PACIFIC DIRECTOR; 

Stephen W. Hoffman 
CANADIAN DIRECTOR; Paul C. James 
INTERNATIONAL DIRECTOR #1; 

Fabian M. Jaksi6 


INTERNATIONAL DIRECTOR #2; 

Eduardo E. I5Jigo-Elias 

DIRECTOR AT LARGE #1: Michael W. Collopy 
DIRECTOR AT LARGE #2; Robert E. Kenward 
DIRECTOR AT LARGE #3: Jeffrey L. Linger 
DIRECTOR AT LARGE #4; David M. Bird 
DIRECTOR AT LARGE #5; Paul F. Steblein 
DIRECTOR AT LARGE #6; Gary E. Duke 


EDITORIAL STAFF 

JOURNAL EDITOR: Josef K. Schmutz, Department of Biology, University of Saskatchewan, Sas- 
katoon, SK., Canada, S7N OWO 

ASSOCIATE EDITORS 


Keith L. Bildstein Robert E. Kenward 

Susan B. Chaplin Eudoxio Paredes-Ruiz 

Charles J. Henny Patricia P. Rabenold 

C. Stuart Houston Patrick T. Redig 

EDITOR OF RRF KETTLE: PAUL F. Steblein 

The Journal of Raptor Research is distributed quarterly to all current members. Original manuscripts 
dealing with the biology and conservation of diurnal and nocturnal birds of prey are welcomed from 
throughout the world, but must be written in English. Submissions can be in the form of research articles, 
letters to the editor, thesis abstracts and book reviews. Contributors should submit a typewritten original 
and three copies to the Editor. All submissions must be typewritten and double-spaced on one side of 
215 by 280 mm (8V2 x 11 in.) or standard international, white, bond paper, with 25 mm (1 in.) margins. 
The cover page should contain a title, the author’s full name(s) and address(es). Name and address should 
be centered on the cover page. If the current address is different, indicate this via a footnote. Submit the 
current address on a separate page placed after the literature cited section. A short version of the title, 
not exceeding 35 characters, should be provided for a running head. An abstract of about 250 words 
should accompany all research articles on a separate page. 

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

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

Refer to a recent issue of the journal for details in format. Explicit instructions and publication policy 
are outlined in “Information for contributors,” J. Raptor Res., Vol. 24(1-2), which is available from the 
editor. 


d 


1993 ANNUAL MEETING 

The Raptor Research Foundation, Inc. 1993 annual meeting will be held on 3-7 November at the 
Marriott City Center Hotel in Charlotte, North Carolina. Details about the meeting and a call for 
papers will be mailed to Foundation members in the summer, and can be obtained from Keith Bildstein 
or Laurie Goodrich, Scientific Program Chairpersons, Hawk Mountain Sanctuary, Rural Route 2, 
Box 191, Kempton, PA 19529-9449 U.S.A., Voice (215) 756-6961, FAX (215) 756-4468. For further 
information about the meeting or the associated symposium “Raptors Adapting to Human Environ- 
ment” and art show, contact Robert Gefaell, Local Committee Chairperson, P.O. Box 16443, Charlotte, 
NC 28297 U.S.A., Tel. (704) 334-8078 or (704) 875-6521 (Carolina Raptor Center). 


RAPTOR RESEARCH REPORTS 

#1, R.R. OlendorfF. 1971. Falconiform Reproduction: A Review Part 1. The Pre-nestling Period. $10.00 
members, $12.50 non-members. 

#2, F.N. Hamerstrom, B.E. Harrell and R.R. Olendorff [Editors]. 1974. Management of Raptors. Pro- 
ceedings of the Conference on Raptor Conservation Techniques, Fort Collins, CO, 22-24 March 1973. $10.00 
members, $12.50 non-members. 

#3, J.R. Murphy, C.M. White and B.E. Harrell [Editors]. 1975. Population Status of Raptors. Proceedings 
of the Conference on Raptor Conservation Techniques, Fort Collins, CO, 22-24 March 1973. (Part 6). $10.00 
members, $12.50 non-members. 

#4, R.R. Olendorff, A. Miller and R. Lehman [Editors]. 1981. Suggested Practices for Raptor Protection 
on Powerlines: State of the Art in 1981 . $5.00 members, $20.00 non-members. 

#5, S.E. Senner, C.M. White and J.R. Parrish [Editors]. 1986. Raptor Research Conservation in the Next 
Fifty Years. Proceedings of a Conference held at Hawk Mountain Sanctuary, Kempton, PA, 14 October 1984. 
$3.50 members, $4.50 non-members. 

#6, D.M. Bird, and R. Bowman [Editors]. 1987. The Ancestral Kestrel. Proceedings of a Symposium on 
Kestrel Species, St. Louis, MO, 1 December 1983. $10.00 members, $12.50 non-members. 

#7, R.R. Olendorff [Editor]. 1989, The Raptor Research Foundation, Inc. Bibliographic Index (1967-1986) . 
$2.50 members, $5.00 non-members. 

#8, R.R. Olendorff, D.D. Bibles, M.T. Dean, J.R. Haugh and M.N. Kochert. 1989. Raptor Habitat 
Management under the U.S. Bureau of Land Management Multiple-Use Mandate. $5.00 members, $6.50 
non-members. 

Add $2.50 for postage and handling, and $1.00 each for additional reports. 


BOOKS 

Biology and Management of Bald Eagles and Ospreys. Proceedings of the First International Symposium, Montreal, 
Canada. 

D.M. Bird [Editor]. 1983. 

$15.00 members, $18.00 non-members plus $5.00 shipping. 


JOURNAL BACK ISSUES 

Journal Back Issues are available. For details write: Jim Fitzpatrick, Treasurer, Raptor Research Foundation, 
Inc., Carpenter St. Croix Valley Nature Center, 12805 St. Croix Trail, Hastings, MN 55033. 


The Journal of Raptor Research has been selected for abstracting/indexing by several organizations. Articles 
appearing in the fournal are covered in Current Contents/ Agriculture, Biology and Environmental Sciences, 
Ecological Review, Science Citation Index, Wildlife Review and Zoological Record.