Cava el al. • DIET OF URBAN BREEDING COOPER’S HAWKS 
777 
TABLE !. Percent frequency of avian and mammalian 
prey items by three sampling schemes at Cooper's Hawk 
nests in Victoria. British Columbia. 1995-2010. 
% Frequency 
Prev remains 
in = 3,231) 
Direct observations 
(n = 437) 
Video analysis 
(n - 783) 
Avian 
SC 1 
32 
46 
62 
SC 2 
61 
33 
32 
>SC 2 
3 
3 
0 
Unidentified 
1 
11 
9 
Totals 
97 
93 
96 
Mammalian 
3 
7 
1 
Unidentified 
0 
0 
3 
classes. We also estimated biomass for items 
observed in the video with the same methods used 
in direct observations. Data from the two nests 
were pooled because proportions of avian prey 
were similar (96% at both nests). 
RESULTS 
Small (< 27 g) to medium-size (2S-9I g) birds 
provided most of the diet in terms of frequency 
(79-94%) and biomass (67-93%) among all three 
sampling schemes; mammalian items were a 
small component of the diet at typical Victoria 
nests (Tables I, 2). There was a silc bias 
concerning one species of mammalian prey; 
nearly all European rabbits (26 of 27) were 
recorded from nest sites associated with the 
University of Victoria campus (6 of 87 nest sites 
over 16 years). Introduced species were a major 
component of the diet, contributing >50% of the 
TABLE 2. Percent biomass of avian and mammalian 
prey items by direct observations and video at Cooper’s 
Hawk nests in Victoria, British Columbia, 1995-2010. 
Total biomass in grams is provided below sampling 
scheme headings. 
Cc Biomass 
Direct observations* 
Video analysis 
(g = 21.086) 
(g = 30.379) 
Avian 
SC 1 
23 
40 
SC 2 
44 
53 
>SC 2 
14 
9 
Totals 
81 
95 
Mammalian 
19 
5 
Estimates do not 
include data from nests 
on the University of Victoria 
campus due lo site bias. 
items in terms of frequency and a similar 
proportion of the biomass ol items identified as 
introduced or native (Tables 3, 4). The most 
prevalent avian prey species were House Sparrow 
(Passer dome slum), European Starling (Stumus 
vulgaris), and American Robin (Jurdus migratorius) 
(Table 5). These three species combined represented 
85. 88. and 97% of the avian items identified to 
species in direct observations, prey remains, and 
video analysis, respectively. These species contrib¬ 
uted a majority of the avian biomass (69% in direct 
observations and 91% in video analysis). 
Young of Ihe year represented a majority of 
identifiable avian items, contributing 76% of prey 
remains. 63% of direct observations, and 92% in 
video analysis. These young of the year also 
contributed a majority of biomass in direct 
observations (69%) and video (97%). The iden¬ 
tifiable mammalian items in the prey remains and 
direct observations were mostly subadults (100 
and 80%. respectively). Only one mammalian 
item was classified to age in the video analysis; a 
subadult rat. Avian young of the year were taken 
w ith increased frequency in the prey remains and 
direct observation samples as the breeding season 
progressed. All identifiable items recorded before 
late April were adult or subadult mammals, while 
avian young of the year dominated in the diet 
from late May to the end of July (Fig. 1). Nest 
video data documented that high proportions of 
young of the year prey (92%) were captured in 
June and July, corresponding to similarly high 
proportions of young prey recorded in the same 
months by direct observations and prey remains 
(Fig. I). 
DISCUSSION 
Small and medium-size birds 27 g and 
28-91 g. respectively), and especially introduced 
species, were the primary prey taken by Cooper’s 
Hawks breeding in the city of Victoria, British 
Columbia. Mammals were taken at low frequen¬ 
cies and did not contribute substantial biomass at 
typical Victoria nests. 
High variability of urban environments should 
cause variation in the ecology of a raptor species 
that inhabits different cities, which appears to be 
the case for several investigations of the diet of 
urban breeding Cooper’s Hawks. This species 
preyed mainly on small and medium-size birds in 
Victoria, many of which were introduced. Estes 
and Mannan (2003) in Tucson, Arizona reported 
nesting Cooper's Hawks mainly taking native 
