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 float ers. It is usually assumed that floaters are lower-quality individuals that do not reproduce, but floater tactics and potential reproductive success h ave rarely been examined carefully. To assess the potential reproductive ro le of male floaters in Tree Swallows (Tachycineta bicolor), we compared the ir reproductive organs and morphology with those of resident males. Tree Sw allows show high levels of extrapair paternity, but two studies attempting to find the fathers of the extrapair offspring have been remarkably unsucce ssful. Floater males that father extrapair young would face intense sperm c ompetition. Theory predicts that under intense sperm competition, selection favors males that produce more sperm. Comparative studies in birds and oth er taxa provide evidence that the level of sperm competition influences rel ative 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 char acters we measured, including testes size, but floaters had significantly l arger cloacal protuberances. Thus, our results do not confirm the general a ssumption 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.