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Timing of reproduction by red-tailed hawks, northern goshawks and great horned owls in the Kluane Boreal Forest of Southwestern Yukon

University of British Columbia

In this thesis I examine the timing of breeding in 3 raptorial birds, red-tailed hawks (Buteo jamaicensis), goshawks (Accipiter gentilis) and great horned owls (Bubo virginianus). Specifically, I test Lack's 1954 theory that birds typically begin to breed such that the young bird's greatest demand for food will coincide later with the greatest abundance of available prey. Lack's theory predicts that birds which successfully match the timing of breeding to the peak in prey fledge more young than pairs which do not. This study was part of the larger Kluane Boreal Forest Ecosystem Project (Krebs et al. In press). Detailed information on weather, prey density and timing of the peaks in prey availability was gathered annually. I examined the timing of breeding over 8 years (1989 - 1996) in an environment with both harsh winter weather and cyclic prey populations. The 3 species differ in both life history and morphology and yet share the same prey base for much of the year. These shared prey enabled me to explore if and how the three species adjusted their breeding time to exploit the peaks in prey availability. The migrant red-tailed hawk bred every year and adjusted breeding to match the peak in prey availability. It bred early when its main prey, the Arctic ground squirrel, bred early thus ensuring that prey consumption needs of young red-tailed hawks corresponded with the synchronous emergence of young ground squirrels. In contrast, the resident great horned owl only bred when its main prey was abundant. Great horned owls have a long breeding period, and therefore bred before a peak in prey was available to match the peak in prey consumption demands of its young. The other resident, the goshawk, has a shorter breeding period than the owl, and bred such that a broad peak in available prey matched the peak consumption needs of its young. However, it is unclear if the goshawk adjusted breeding to match the predicted peak in prey, or if it had an average breeding time that corresponded to the average breeding time of its preferred prey. Slight annual variation in the timing of breeding in the goshawk, like that of the other resident, the owl, may have been in response to fluctuations in winter prey density. These fluctuations in prey availability could through its influence on the raptors body condition, reduced the bird's ability to breed at the average time to which they have become adapted. Alternatively, goshawks may have adjusted their timing of breeding in response to a predicted later broad peak in prey, initiated by the increased availability of those prey during mating. Within a year, seasonal declines in the number of young that fledged were absent in the two resident species, but present for the migrant red-tailed hawk. Reduced fledging success in red-tailed hawks was attributed to attacks on the young by blackflies. Attacks from blackflies were so intense in 2 of the years that all young hawks from 5 of the 12 intensively monitored nests died. These nests were all late and the young were <3 weeks of age at the time of blackfly emergence. Attacks by blackflies on young red-tailed hawks, largely explained why the birds failed to attain maximum reproductive success every year, even though they matched peak needs by their young to the peak in prey availability. The resident goshawks and great horned owls only bred when they could do so successfully, while all red-tailed hawks bred every year. I propose that migration by the redtailed hawk avoided the loss of body condition caused by low prey abundance in winter. However, the red-tailed hawk had a lower overall success rate, with only 25% of pairs fledging young in the least successful year. Over all years, when birds did breed, red-tailed hawks typically fledged 1-2 young and both goshawks and horned owls 2 -3 young.

Thesis/Dissertation

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2009-07-27

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http://hdl.handle.net/2429/11269

Zoology

University of British Columbia

UBCV

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