Factors Affecting Interspecies Variation in Home-range Size of Raptors

Abstract

Considerable work has been devoted to examining factors that affect interspecies variation in homerange size. Home ranges for most vertebrates increase with body mass and decrease with increased habitat productivity (e.g. McNab 1963, Schoener 1968, Harestad and Bunnell 1979, Lindstedt et al. 1986). Large species have high energetic requirements and presumably must occupy large home ranges to obtain sufficient food, but even small species occupy relatively large home ranges when the availability of food is low. Interspecific variation in home-range size also has been attributed to differences in age and sex (Harestad and Bunnell 1979, Schoener and Schoener 1982), dominance status (Schoener and Schoener 1982), and trophic status (McNab 1963, Schoener 1968). For example, McNab (1963) showed that mammalian hunters have larger home ranges than similar-sized croppers and suggested that this difference was due to less biomass of food per unit area being available at higher trophic levels. Raptors provide valuable insights into the factors that affect an animal's home-range size because they use many different hunting tactics and take many different kinds of prey (Snyder and Wiley 1976, Marti et al. 1993, Korpimaki and Marti 1995). Newton (1979:63) has shown that home-range size of raptors increases with body mass, and Schoener (1968) has shown that predatory birds have larger home ranges than similar-sized nonpredatory birds. Here, I extend Newton's and Schoener's analyses by asking the question: Does the way in which a raptor exploits its prey base influence the size of its home range? Assuming that raptor home ranges are determined, in part, by food availability (e.g. Marquiss and Newton 1981, Village 1982), at least four predictions can be made about the size of a raptor's home range. First, invertebrates generally occur in higher densities than mammals for a given body size, whereas mammals generally occur in higher densities than birds for a given body size (Greenwood et al. 1996, Silva et al. 1997). Hence, one would expect bird-eating raptors to have larger home ranges than mammal-eating raptors, which in turn should have larger home ranges than invertebrate-eating raptors. Second, predators specializing on a narrow prey base should have to range over a relatively large area in order to encounter sufficient prey, whereas gen-