Reproductive anatomy and indices of quality in male tree swallows: the potential reproductive role of floaters

Avian populations often consist of breeding residents and nonbreeding floaters. It is usually assumed that floaters are lower-quality individuals that do not reproduce, but floater tactics and potential reproductive success have rarely been examined carefully. To assess the potential reproductive role of male floaters in Tree Swallows (Tachycineta bicolor), we compared their reproductive organs and morphology with those of resident males. Tree Swallows show high levels of extrapair paternity, but two studies attempting to find the fathers of the extrapair offspring have been remarkably unsuccessful. Floater males that father extrapair young would face intense sperm competition. Theory predicts that under intense sperm competition, selection favors males that produce more sperm. Comparative studies in birds and other taxa provide evidence that the level of sperm competition influences relative testes size and sperm production. However, intraspecific adaptations to different levels of sperm competition have received far less attention. Floater Tree Swallows did not differ from resident males in any of the characters we measured, including testes size, but floaters had significantly larger cloacal protuberances. Thus, our results do not confirm the general assumption that floaters are lower-quality individuals that do not reproduce. Furthermore, floaters showed high variation in the volume of the cloacal protuberance (reflecting sperm numbers), which suggests that they engage in copulations. We conclude that floater male Tree Swallows invest heavily in sperm production (as do resident males) to exploit breeding opportunities through takeovers or extrapair copulations. Received 17 November 1998, accepted 5 May 1999.