The effects of habitat density and the spatial distribution of food on the social behaviour of captive wintering American Tree Sparrows

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Abstract
Two artificial habitats containing 15% (low density) and 40% (high density) coniferous ground cover, and three feeding patterns representing different levels of spatial food distribution were used to investigate the effects of factors influencing individual spacing on the behaviour of captive, wintering American Tree Sparrows (Spizella arborea). Specifically, three aspects of social behaviour were examined: the frequency of agonistic encounters, the percentage of interactions initiated by dominants, and nearest-neighbour distances. Temperature effects were also considered. Interaction rates were lowest when food was most dispersed, and at higher temperatures. More interactions were recorded in the higher density habitat, as compared with the low density habitat, but the trend was reversed when food was highly dispersed. Nearest-neighbour distances decreased with increasing habitat density and food dispersion and were shortest in the high density habitat. Dominant birds initiated most encounters (70.7%), but less frequently when food was abundant or temperatures high, contradicting an assumption of previous models of optimal flock behaviour. It is suggested that these models, which rely on predicted changes in the allocation of available time by dominant birds to predict flock size, could be improved by considering the effects of habitat density and food dispersion, as well as differences in the social optimization strategies of different flocking species.