Sizes of Feeding Territories among Birds

Abstract

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 h…