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THE WILSON JOURNAL OF ORNITHOLOGY • Vul 124. No. 4. December 2012 
TABLE 3. Percent frequency of introduced and nativi 
prey items recorded by three sampling schemes at Cooper": 
Hawk nests in Victoria. British Columbia. 1995-2010. 
% Frequency 
Prev remains 
In = 2.893) 
Direct observations 
(n = 217) 
Video analysis 
l« = 155) 
Introduced 
Avian 
50 
52 
54 
Mammalian 
3 
12 
4 
Totals 
53 
64 
58 
Native 
Avian 
47 
35 
42 
Mammalian 
0 
I 
0 
Totals 
47 
36 
42 
dove species and introduced species had little role 
in the diet (7.3% frequency, 4.6% biomass) 
Bielcfeldt el al. (1992) found Eastern chipmunks 
(Tarnias striatus) contributed a majority of the 
biomass at a semi-urban nest near a rural town in 
Wisconsin and the Eastern chipmunk is a common 
prey item of breeding Cooper’s Hawks in and 
around the city of Stevens Point. Wisconsin 
(Nicewander and Rosenfield 2006; RNR, unpuhl. 
data). We found local variation within our study 
population; only Cooper's Hawks nesting on the 
Univeisity of Victoria campus preyed upon 
European rabbits in addition to the avian prey 
commonly taken at nests elsewhere in the study 
area. The variation in diet among populations and 
opportunistic use of locally abundant prey within 
Victoria underscores the plasticity of Cooper’s 
TABLE 4. Percent biomass of introduced and native 
prey items by two sampling schemes at Cooper’s Hawk 
nests in Victoria. British Columbia. 1995-2010 Total 
biomass in grams is provided below sample scheme 
headings. 
% Biomass 
Direct observations' 
<g = 14.397) 
Video analysis 
(g = 9,390) 
Introduced 
Avian 
41 
38 
Mammalian 
24 
14 
52 
Totals 
65 
Native 
Avian 
35 
48 
Mammalian 
0 
0 
48 
Totals 
35 
campus due to sile bias. 
Hawk feeding behavior. This plasticity may in 
part explain this hawk's ability to occupy a wide 
variety of habitats across North America iRosen- 
field et al. 2007b, 2010; Sonsthagen et al. 2012). 
A commonality in age of prey taken by Cooper’s 
Hawks underlies these variations in prey species 
used across cities. Bielefeld! et al. (1992) and this 
study report young of the year birds are major 
components ol the urban breeding season diet; 
Estes and Mannan (2003) did not report the ages 
ot the avian prey taken by urban Cooper's Hawks 
in Arizona. 
T lie type and availability of prey can (or has the 
potential to) dcleteriously affect reproductive 
success of Cooper’s Hawks. Boa! et al. (1998) 
recorded high levels of trichomoniasis, an upper 
digestive iract disease, among urban Cooper's 
Hawk nestlings in Tucson. Arizona, likely due to 
high consumption of abundant columbid (Colum- 
bidae) prey infected with the protozoan Tricho¬ 
mona* gullinae. Boal and Mannan (1999) found 
relatively lower reproductive rates at urban versus 
anal nests, largely the result of nestling mortality 
due to trichomoniasis; this led them to suggest the 
ciiy of I ucson was an 'ecological trap' for 
Cooper’s Hawks (but see Mannan et al. 2008). 
I he diet ol Cooper’s Hawks in Victoria was low 
in columhids and this disease has not been 
documented in hawks there (Rosenfield et al. 
2002). Rosenfield et al. (2009) reported a low 
prevalence ol /. \>allincte( 7%), but no evidence of 
the disease (or deaths) in nestling urban Cooper's 
Hawks in Grand Forks, North Dakota. However, 
sampling there in 201 I found -30% prevalence 
of T. gal/inae (/? = 70 hawks at 13 nests), but no 
documented deaths of nestlings due to trichomo¬ 
niasis (T. G. Driscoll, unpubl. data). Both the 
Victoria and Grand Forks populations exhibit 
some of the highest nesting densities and 
production indices recorded for Cooper's Hawks 
in North America (Rosenfield et ai. 2007a). 
although the Grand Forks population could 
potentially be affected by trichomoniasis in future 
years (- 14% of the diet of Cooper's Hawks in 
Grand Forks is columbid prev: T. G. Driscoll, 
unpubl. data). 
Bielcfeldt et al. (1992) challenged the assump¬ 
tion that birds were the most frequent prey of 
Cooper’s Hawks as reported in pre-1990 studies 
°* '* 1e biology of this species. They indicated 
mammals provided a majority of biomass in some 
studies and argued that previous studies could 
iave overestimated avian prey in the diet due to 
