Extra-pair paternity and the reproductive role of male floaters in the tree swallow (Tachycineta bicolor)

In many avian species, a part of the population is present at the bleeding grounds but does not breed. Current theories generally assume that floaters are younger or lower-quality individuals, and empirical data confirm this. However, floating could also arise as an alternative strategy to breeding, if floaters are able to reproduce via extra-pair copulations. Until the pr esent study, there has been no evidence that floaters father offspring. We studied a population of tree swallows (Tachycineta bicolor), a species with one of the highest levels of extra-pair paternity known in birds. Using mi crosatellite markers, we determined the biological fathers of 65% of the ex tra-pair young. OF a total of 53 extra-pair young (52% of all offspring), 4 7% were fathered by local residents, 6% by residents breeding elsewhere (up to 2 km from the focal grid), and 13% by floaters. Residents seemed to be more successful and they were also more likely to return as territory holde rs in the next breeding season compared to floaters. Extra-pair males were on average in better condition than the within-pair males they cuckolded. I nterestingly, resident males that disappeared (possibly to float) during th e fertile period were heavier than males that stayed, and floaters were hea vier than residents, but not different in any other characteristic. Althoug h alternative interpretations of the data are possible, we propose that flo ating might be a conditional strategy in tree swallows whereby males in goo d condition gain more paternity via extra-pair copulations, whereas males i n worse condition are more successful by providing parental care.