Extrapair paternity and egg hatchability in tree swallows: evidence for the genetic compatibility hypothesis?

Tree swallows (Tachycineta bicolor) show one of the highest levels of extrapair paternity in birds, and there is evidence that females have control over who fathers their offspring. However, it is unclear which benefits female tree swallows obtain from mating with multiple males. Using microsatellite DNA fingerprinting, we studied extrapair paternity in relation to nesting success and male, female, and offspring characteristics. More than 70% of all nests contained extrapair young, and more than half of all offspring were extrapair. Within broods, the extrapair young were often fathered by several males. Despite screening all resident and some floater males, we could identify the biological father of only 21% of all extrapair offspring. All identified extrapair males were close neighbors. Extrapair males did not differ from within-pair males in any of the measured characteristics, except that the former had larger cloacal protuberances than the latter. Extrapair males were equally successful in gaining paternity in their own broods as males that did not father extra young. In nests with mixed paternity, extrapair young did not differ from within-pair young in body size or mass. However, nests with extrapair young had higher hatching success than nests without extrapair young. All examined unhatched eggs were fertilized and thus hatch failure resulted from embryo mortality. The available data are in accordance with the genetic diversity and the genetic compatibility hypothesis, but not with the good genes hypothesis.