Sizes of Feeding Territories among Birds

This analysis deals with size variations in the breeding territories of land birds which obtain most or all of their food on the territory. For the species studied as a whole, territory size shows a strong positive relationship to body weight. Predators tend to have larger territories than omnivores or herbivores of the same weight, presumably due to the relatively denser food of the latter species. The home ranges of raptors inhabiting two areas were found to be significantly correlated with an index of the numerical density of their prey and in one area with raptor weight. Higher clutch size is not significantly associated with larger territories in any category of birds tested. The number of individuals defending the territory and the number feeding the young are probably not correlated with territory size. The exponential relationships between body weight of the consumer and three dependent variables–food biomass consumed per unit time, average prey weight and territory or home range size–are used to derive three predictions: a) Heavier predators take fewer individuals per unit time than lighter species; b) If certain restrictions are satisfied, the collective biomasses in a given large area increase as individual biomasses become larger for omnivorous species and decrease as individual biomasses increase for predators; c) For predators, the density of acceptable and accessible food in biomass per unit area decreases as the weight of the consumer increases. Territory or home range size increases more rapidly with body weight for predators than for omnivores or herbivores. This relationship holds true for both birds and mammals and presumably reflects a rapidly decreasing food density for predators of increasing weight. Since smaller predators do not feed over a wider range of food size than larger species, and since there are less species or individuals feeding on large food than on small, the predators of the areas studied probably consume food whose distribution of biomass with food size is declining. The habit of feeding on exclusive areas is considerably more widespread among predators than among omnivores and herbivores. The smaller spatial needs for omnivorous and herbivorous birds of a given biomass and perhaps the greater patchiness of their food when compared to predators are used to explain the higher occurrence of gregarious nesting in the former group. Implications of this study for the functions of feeding territories maintained during the breeding season are discussed.