SOCIAL AND SPATIAL SYSTEMS IN A TEMPERATE NONBREEDING BIRD COMMUNITY

Abstract

Field work to determine possible causes of variation in avian social behavior in a nonbreeding community of 42 species was undertaken during two winter seasons in oak-juniper woodland of the Santa Rita mountains of southern Arizona. Transect observations are used to calculate monthly densities, frequency of social foraging, and size and composition of social groups for all species. Additional data taken to quantify the foraging logistics of 22 species includes foraging substrate, time spent at a foraging station, and distance travelled between foraging stations. These data are used to generate indices of logistic similarity and interspecific association for species pairs in order to test the hypothesis that species which are similar in their foraging logistics are more likely to associate than those which are not. Causes of solitary behavior and variation in intraspecific social behavior are also examined. Exclusively solitary foraging behavior is strongly associated with species that use sparse, unpredictable prey too small to share. These species, referred to as "extensive foragers," appear to be constrained from both intraspecific and interspecific association by being logistic mismatches with all but their closest competitors, with which the potential for interference competition is high. Evidence is presented to refute the hypothesis that solitary behavior is due to low vulnerability to predators. Species which are intraspecifically solitary tend to use evenly distributed resources, and gregarious species use patchy and abundant resources. Exceptions are most frequent among permanent residents, which may continue to maintain social and spacing systems used in the breeding season. Among gregarious species, permanent residents are more likely to maintain stable group sizes than winter residents. A correlation exists between similarity of foraging logistics and likelihood of interspecific association, but it is subject to considerable variance. Causes of this variance include the tendency of transient migrants not to associate with other species, and the tendency of certain species to form temporary associations, regardless of logistic similarities. Why some species form temporary ("opportunistic") associations while others do not remains unclear.