BARN SWALLOW (HIRUNDO RUSTICA) SOCIAL BEHAVIOR: NEST ATTENDANCE, AGGRESSION, AND SEXUALLY SELECTED INFANTICIDE (ALTRUISM, HELPING, SOCIOBIOLOGY)

The relative importance of direct and indirect components of fitness on the evolution of social behavior is an unresolved issue in evolutionary biology. Since many recent studies have focused on groups of relatives, in which both individual and kin selection can occur, controversy has arisen concerning what level of selection is necessary to explain particular classes of behavior. I chose to study the social behavior of the colonial barn swallow (Hirundo rustica) because colony members are not closely related, and thus kin selection is not likely to have played a role in the evolution of barn swallow behavior. I concentrated my attention on three behavioral categories, nest attendance, aggression, and infanticide, in an effort to determine whether barn swallow behavior is shaped primarily by selfishness, mutualism or altruism.;Historically, helpers that aid parents in raising offspring have been reported in barn swallows. Although both sexes attend nests other than their own occasionally, my results indicate that such nest attendance is primarily a competitive strategy by which males gain access to breeding sites and females. During a three year study (1981-1983) at the Cranberry Lake Biological Station of the State University of New York, there were no significant differences in the reproductive success of attended and unattended nests. Attendants showed little in the way of "helping" or "cooperative" behavior, more often behaving in ways that were obviously detrimental to one or both breeders (e.g., attempting copulation with female, infanticide). This major difference between barn swallow nest attendants and attendants in cooperative breeders probably stems from differences in relatedness within groups. In groups of related individuals, indirect benefits can be gained, even if the primary function of helping behavior is selfish.;The pattern of aggression I documented is consistent with an interpretation of barn swallow behavior being of a primarily competitive nature. Young male barn swallows were involved in aggressive incidents more frequently than were other classes of individuals, and intrasexual chases occurred more commonly than intersexual chases. The pattern is similar to the "territoriality" of other colonial swallows, with males (and to a lesser degree, females) defending nest sites, nests, and perhaps mates from "intruders." (Abstract shortened with permission of author.).