Effect of Annual and Habitat Variations in Prey on the Growth and Productivity of Red-Tailed Hawks (Buteo jamaicensis)

Abstract

The effects of different habitats, years and brood sizes on the growth rates, asymptotic body sizes, biomass fledged per nest and nestling mortality in red-tailed hawks are compared in this investigation. Habitats were categorized as crop, pasture or mixed. During three years of the study, drought and wet years occurred. Both male and female hawks showed similar growth rates and patterns of growth. Differences in habitat and brood size had no effect on the four parameters measured. Among years, there was a difference in biomass fledged per nest: more were fledged during 1982 than in previous years. This difference was taken to indicate larger clutches that year, given that rate of mortality was essentially similar among years. Prey availability affected the reproductive biology of red-tailed hawks before the eggs were laid rather than afterwards. Prey availability affects the reproductive biology of raptors in several ways. Limited prey availability can (1) decrease growth rates (Moss, 1979), (2) decrease weight at fledging (Cave, 1968; Moss, 1979), (3) increase nestling mortaltiy (Bechard, 1983; Mebs, 1964; Southern and Lowe, 1968) and (4) decrease size of clutch (Cave, 1968; Linkola and Myllymaki, 1969; McInvaille and Keith, 1974; Mebs, 1964; Smith et al., 1981; Southern and Lowe, 1968). The extent that prey availability affects each of these aspects of reproductive biology may vary among species of raptors. This study determined which aspects of the red-tailed hawk's (Buteo jamaicensis) reproductive biology responded to differences in available prey. Differences in available prey in the vicinity of a red-tailed hawk's nest were inferred rather than measured. The estimate of populations of two important prey species of red-tailed hawks (rabbits and snakes) is difficult (Fitch and Bare, 1978; Fitch et al., 1946). There is evidence that different Corresponding address: 2267 N. Yale, Wichita, Kansas 67220. This content downloaded from 157.55.39.112 on Wed, 07 Sep 2016 05:24:28 UTC All use subject to http://about.jstor.org/terms VOLUME 91, NUMBERS 3-4 97 habitats contain different amounts of available prey. Red-tailed hawks delivered more prey to nests located in certain habitats compared with others (Luttich et al., 1970; McInvaille and Keith, 1974). Nests of red-tailed hawks located near water or pasture habitats, which were presumed to have more prey, had larger clutches than those in other habitats (Adamcik et al., 1979; Howell et al., 1978). In Swainson's hawk (Buteo swainsoni), a congeneric species, the amount of searchable space available varies with habitat and so does the amount of prey and the development of the nestlings (Bechard, 1982, 1983). It was assumed that there were more prey available in a pasture than crop habitat. Weather conditions differed markedly for the three years of this study. A severe drought occurred during 1980 and much wetter conditions occurred during the next two years. An increase in microtine populations was reported in several areas of the state between 1980 and 1982 (Slade, pers. comm.; Terman, pers. comm.) and more microtine carcasses were found in the nests during 1982 than in either previous year (Cress, 1982). More prey were assumed available during 1982 than 1980. Growth rates, asymptotic body sizes, nestling mortality and biomass fledged per nest of red-tailed hawks reared in different habitats and during different years were compared to see which of these parameters responded to potentially different amounts of available prey.