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British Columbia Male Cooper’s Hawks Were Smaller and Took Smaller Prey Than Wisconsin Males

Cooper's hawk with a common starling as prey in Quebec City, Canada
Research: Ecology, Reproduction & development

A study combining long-term data from southern Vancouver Island in Canada and Wisconsin in the United States examined whether regional differences in male Cooper’s hawk (Astur cooperii) body size were associated with the size and age of their prey. The authors also tested a separate idea: whether successful urban nests on Vancouver Island produced more young than rural nests.

The prey comparison was especially focused on the period before incubation, when males provision females during courtship and egg laying. This matters for the authors’ prey-agility hypothesis because an earlier objection was that Cooper’s hawks often catch inexperienced nestlings and fledglings later in the breeding season, when high aerial agility may be less important.

Adult prey dominated before incubation

All 56 prey deliveries recorded from individually marked Wisconsin males before incubation involved adult prey. In the Victoria study area on southern Vancouver Island, 93% of age-determined individuals among the three dominant bird prey species were also adults. The authors therefore argue that breeding males regularly have to catch experienced prey during this important early part of the nesting cycle.

The diet also differed between regions. British Columbia males relied more heavily on birds, whereas mammals made up a larger share of prey in Wisconsin. Eastern chipmunk (Tamias striatus) was among the important Wisconsin prey, alongside American robin (Turdus migratorius) and mourning dove (Zenaida macroura); in British Columbia, the dominant prey included American robin, common starling (Sturnus vulgaris) and house sparrow (Passer domesticus).

Smaller males took smaller predominant prey

Breeding males on Vancouver Island averaged 296 g, about 10% lighter than Wisconsin males at 327 g. The three predominant prey types averaged about 57 g in British Columbia and about 100 g in Wisconsin.

The authors interpret this geographic match — smaller males taking smaller prey and larger males taking larger prey — as support for the idea that prey size and agility may help shape male body size. Agility itself was not measured directly. Instead, prey age, size and type were used as indicators: experienced adult birds and smaller birds were treated as more difficult aerial prey than young or larger birds. The study therefore supports the hypothesis without providing a direct measurement of prey manoeuvrability.

Urban and rural nests produced similar numbers of young

Successful urban nests on Vancouver Island averaged 3.64 older nestlings, compared with 3.69 at successful rural nests. The authors found no clear difference between the two environments.

These British Columbia data therefore did not support the frequently suggested idea that Cooper’s hawks necessarily produce more young in cities because suitable bird prey is more abundant there. Taken together, the study argues that regional variation in male size, prey size, prey age and diet should be considered together when describing Cooper’s hawk foraging ecology across North America.

Sources and image credit

  1. Rosenfield, R. N., Stewart, A. C., Frater, P. N., & Holmgren, E. L. (2026). Does Seasonal Variation in Agility of Avian Prey Influence Body Size of Breeding Male Cooper's Hawks? And Comments on the Putative Influence of Avivory on Reproductive Output in Cities. Ecology and Evolution, 16(1), e72837. https://doi.org/10.1002/ece3.72837Primary open-access article, published 9 January 2026; CC BY 4.0. Full text was checked. The paper combines long-term prey and reproductive data from Wisconsin and southern Vancouver Island and explicitly treats prey agility as a qualitative/ordinal inference rather than a directly measured variable.

Image: Cephas — Cooper's hawk with a common starling as prey, photographed at Domaine Maizerets, Quebec City, Quebec, Canada, 5 May 2013; Wikimedia Commons; representative image, not a bird from the British Columbia or Wisconsin study populations · CC BY-SA 3.0 · Source

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