Fitness Consequences of Avian Coloniality

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9613638 BROWN This project seeks to understand why animals live in groups. Many different organisms, including humans, show different degrees of group-living: some animals live in tiny groups or solitarily, whereas others in the same population may reside in enormous colonies. To understand why some individuals prefer living in large groups and others in small groups requires knowing the reproductive consequences of such choices. In what way do certain animals benefit from groups of one size while others apparently benefit from groups of another size? This question is addressed in this research, which focuses on a colonially breeding bird, the cliff swallow (_Hirundo pyrrhonota_), that breeds on the sides of steep cliffs and canyons and underneath the eaves of bridges and buildings throughout much of western North America. The objectives are to determine how different birds sort themselves among different sized nesting colonies and on what basis; whether there is a single colony size that is best for all birds (where reproductive success is greatest); whether a swallow's preference for a particular sized nesting colony is genetically based and thus inherited from its parents; and how the birds' nesting success is affected if their nesting colony is changed in size. The research uses primarily mark-recapture methods that involve bird banding: birds are banded at a particular site and their subsequent activity and lifespan is determined by recatching them at that site or others in later years. This study builds on 15 years of past work on this cliff swallow population. This project will further our understanding of why animal group sizes vary in nature. Group size affects many aspects of animal behavior: for example, the spread of disease, extent of competition for resources such as food, type of mating strategy, and extent of cooperation and altruism among individuals are all influenced heavily by the size of a social group. We can better understand these important conseque nces of group life if we know what causes groups to vary in size in the first place and if we know whether different groups are composed of different kinds of individuals. The cliff swallow provides a well-studied animal model whose social behavior parallels that of other social species, and it will yield insight into fundamental processes that govern group formation in many kinds of organisms.