Dietary Responses of Three Raptor Species to Changing Prey Densities in a Natural Environment

Abstract

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.

Diets of Wedge-tailed Eagles (Aquila audax) and Little Eagles (Hieraaetus morphnoides) Breeding Near Canberra, Australia

“…Following Steenhof (1987), we defined an occupied nesting territory as an area containing one or more nest sites within the home range of a pair of eagles. Between July 2002 and January 2008 we searched the area inside a 40-km-radius of the Canberra Central Business District for occupied territories of both species and for nests containing eggs or young.…”

“…Studies of dietary relationships between raptor species are often performed at a regional as opposed to a local scale, and it is assumed that niche overlap indicates potential competition (Jaksić 1983, Jaksić and Delibes 1987, Marti et al 1993, Burton and Olsen 1997, Garcia and Arroyo 2005, Olsen et al 2006a. However, to clearly understand the effect of competition, studies should occur at a local scale, between neighboring pairs (Steenhof and Kochert 1985, 1988, Marti et al 1993. Studies of this sort are less common (e.g., Nilsson 1984), and few have been undertaken for Australian raptors (Olsen et al 2006a(Olsen et al , 2006b, although Aumann (2001c) compared mean prey weight, dietary diversity (Shannon's Index) and dietary overlap (Pianka's Index) for a suite of diurnal raptors including WTE and LE.…”

Food-limitation in a generalist predator

“…Newton 1979; Steenhof & Kochert 1988;Tornberg et al 1999), or disperse to food-rich areas (e.g. Krebs et al 2001), when one of their staple prey becomes scarce.…”

Future of biodiversity in the Barents Region

“…Golden eagles generally avoid human disturbance (López-López et al 2007) and prefer mixed habitat with forests for nesting and open lands for hunting (Moss 2011). They have a rather broad food niche (Nyström et al 2006), which permits them to adapt their diet to prey availability (Steenhof and Kochert 1988) and allows them to stay in the same territory year after year (Steenhof et al 1997). Nonetheless, they still favour some prey species over others (Steenhof and Kochert 1988); in Sweden the main prey species are mountain hare (Lepus timidus) and Tetraonid species, except for Gotland where they mainly feed on rabbits (Oryctolagus cuniculus) and hedgehogs (Erinaceus europaeus) (Tjernberg 1981).…”

“…They have a rather broad food niche (Nyström et al 2006), which permits them to adapt their diet to prey availability (Steenhof and Kochert 1988) and allows them to stay in the same territory year after year (Steenhof et al 1997). Nonetheless, they still favour some prey species over others (Steenhof and Kochert 1988); in Sweden the main prey species are mountain hare (Lepus timidus) and Tetraonid species, except for Gotland where they mainly feed on rabbits (Oryctolagus cuniculus) and hedgehogs (Erinaceus europaeus) (Tjernberg 1981). The breeding success of the golden eagle appears to be closely linked to the availability of these prey species (Nyström et al 2006) and the presence of these species has therefore been accounted for in the modelling.…”