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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society SUMMARY(1) Diets of three nesting raptor species were evaluated from 5939 prey items collected from nests in south-western Idaho during a 10-year period that included a complete jack rabbit population cycle and an unusual ground squirrel population crash.(2) Jack rabbits were the principal prey of golden eagles; Townsend's ground squirrels were the main prey of prairie falcons and red-tailed hawks. Prairie falcons had the most specialized diets, and red-tailed hawks the most diverse. None of the three raptor diets reflected the relative abundance of prey types in the environment.(3) Diet diversity of each of the three raptor species expanded as the abundance of their main prey declined. Ground squirrels and birds were alternate prey for eagles; gopher snakes, kangaroo rats, and r4bbits were alternate prey for red-tailed hawks. Prairie falcons had no single important alternate prey species.(4) Yearly frequencies of main prey in each of the three raptor diets were correlated with the annual abundance of that prey in the environment. Frequencies of alternate prey were correlated not with their own abundance but inversely with the abundance of the principal prey.(5) Eagle preference for jack rabbits was strong and unaffected by changes in prey densities. Red-tailed hawk selectivity for jack rabbits was inversely related to ground squirrel abundance, suggesting 'switching' behaviour. Prairie falcon selectivity for ground squirrels did not vary with ground squirrel densities.(6) Prey choice was generally consistent with predictions of the original optimal diet model, but red-tailed hawk prey selection appeared to depend on relative prey densities. Degree of diet specialization and plasticity are probably related to a raptor's life-history characteristics, and may influence a raptor's effects on its prey populations.

SAS INSTITUTE INC. 1987. SAS/Stat guide for personal computers. Version 6 ed. SAS Inst. Inc., Cary, N.C. 1029pp. STAHLECKER, D. W. 1975. Effect of a new transmission line on wintering prairie raptors. Condor 80:444-446. 1979. Raptor use of nest boxes and platforms on transmission towers. Wildl. Soc. Bull. 7:59-62. STALMASTER, M. V. 1980. Salmon carrion as a winter food source for red-tailed hawks. Murrelet 61:43-44. WILLIAMS, R. D., AND E. W. COLSON. 1989. Raptor associations with linear rights-of-way. Pages 173-192 in B. G. Pendleton, proj. ed. Proc. western raptor management symposium and workshop. Natl. Wildl. Fed.,

Global climate change has affected avian migration patterns and nesting phenology. Changes in one phase of a bird's cycle will most likely affect other stages, but few studies focus simultaneously on multiple life‐history events. We used western North American ringing records and Christmas Bird Counts to examine whether changes in migration patterns were concordant with advancing American kestrel Falco sparverius nesting phenology. Consistent with previous findings, male kestrels migrated shorter distances than female kestrels, and kestrels nesting in southern latitudes migrated shorter distances than kestrels nesting in more northern areas. In addition, kestrel migration distance decreased significantly from 1960 to 2009 and was negatively associated with winter minimum temperatures. Christmas Birds Counts from the same time period showed increasing indices of overwintering kestrel abundance in northern states (Washington, Idaho, and Utah), where winter minimum temperatures have increased significantly, and concomitant decreases in southern states (California and Arizona). Finally, changes in nesting phenology of kestrels in southwestern Idaho were best explained by warmer winters, not springs. Warmer winters may decrease energetic demands on migrants by allowing for shorter migration distances, decreasing thermoregulatory costs, or both. Decreased energy demands during winter may allow birds to gain resources necessary for reproduction earlier in the nesting season. Higher winter temperatures that decrease (former) constraints on early nesting may be a particularly important mechanism leading to advancing nesting phenology for species with strong seasonal declines in fecundity or intense early season competition for high‐quality nesting areas.

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