McNab (1963) plotted home-range size (HR) against body weight (BW) for a variety of mammal species, and concluded that home-range size was determined by an animal's energetic needs and by the density of available food. Using a regression model, he fitted an allometric relationship of the form HR = a * BWb. The value of the exponent (b) was close to 0.75, the value which relates basal metabolic rate to body weight (Kleiber 1961). McNab (1963) also divided the species in his sample into two dietetic categories, "hunters" and "croppers," and showed that the allometric coefficient (a) was higher among the hunters, a finding he attributed to the relatively low density of their preferred food items. Since then, similar relationships have been revealed in other taxa (lizards: Turner et al. 1969; birds: Schoener 1968; Armstrong 1965; mammalian carnivores: Gittleman and Harvey 1982; primates: Milton and May 1976; Clutton-Brock and Harvey 1977; Harvey and Clutton-Brock 1981; and mammals in general: Harestad and Bunnell 1979) where variation in both a and b have been noted. In particular, exponents tend to be higher than McNab originally documented. Jenkins (1981) has recently summarized some of these data in an attempt to compare the scaling of home-range on body weight for various bird and mammal taxa. However, simple comparison of exponents may be rather uninformative since correlations for most of the relationships are weak, and confidence limits for the slopes therefore wide. If, as McNab (1963) suggested, home-range size is determined by energy needs, then consistent differences in the metabolic requirements of similar-sized birds and mammals should lead to differences in the scaling relationships of home-range size and body size in the two groups. In this paper, metabolic rate, home-range size, and dietary data from bird and mammal species are used to address the following questions: (1) Does the relationship between daily energy expenditure and body weight differ between birds and mammals? (2) Does daily energy expenditure directly determine home-range size?