Genetic monogamy in the absence of paternity guards: the Capricorn silvereye, Zosterops lateralis chlorocephalus, on Heron Island

We investigated the genetic mating system of a socially monogamous passerin e bird, the Capricorn silvereye Zosterops lateralis chlorocephalus, on an i sland of the Great Barrier Reef. There were no cases of extrapair paternity (EPP) among 122 offspring from 53 broods detectable by minisatellite or mi crosatellite DNA fingerprinting. Behavioral observations of paired birds sh owed that this was not a consequence of efficacious paternity guards and th at females did not engage in extrapair copulation (EPC). Frequency of intra pair copulations was also low, with only 14 cases observed during 199 hours of observations of the 11 focal pairs in the fertile periods of females, a nd this was consistent with anatomical features of the cloacal protuberance in males. In this population, young birds form life-time pair bonds soon a fter gaining independence but females are obviously not attempting EPC poss ibly to redress this early mate choice. This is despite the fact that they breed in high density with a synchronous start and asynchronous spread of l aying in a protracted season and males do not positively exhibit mate guard ing behavior when females are fertile. Our results support high fidelity of socially monogamous birds on islands and are consistent with the hypothesi s that sexual selection is reduced where genetic variation in fitness is li mited.