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 extra pair paternity in birds, and there is evidence that females have control ov er who fathers their offspring. However, it is unclear which benefits femal e tl ee swallows obtain from mating with multiple males. Using microsatelli te DNA. fingerprinting, we studied extrapair paternity in relation to nesti ng success and male, female, and offspring characteristics. More than 70% o f all nests contained extrapair young, and more than half of all offspring were extrapair. Within broods, the extrapair young were often fathered by s everal males. Despite screening all resident and some floater males, we cou ld identify the biological father of only 21% of all extrapair offspring. A ll identified extrapair males were close neighbors. Extrapair males did not differ from within-pair males in any of the measured characteristics, exce pt that the former had larger cloacal protuberances than the latter Extrapa ir males were equally successful in gaining paternity in their own broods a s males that did not father extra young. In nests with mixed paternity, ext rapair young did not differ from within-pair young in body size or mass. Ho wever, nests with extrapair young had higher hatching success than nests wi thout extrapair young. All examined unhatched eggs were fertilized and thus hatch failure resulted from embryo mortality. The available data are in ac cordance with the genetic diversity and the genetic compatibility hypothesi s, but not with the good genes hypothesis.