High fidelity on islands: a comparative study of extrapair paternity in passerine birds

It is commonly assumed that the intensity of sexual selection is lower in island populations. Extrapair paternity (EPP) is widespread within passerine birds and is indicative of sexual selection. A conservative analysis of the levels of EPP in island and equivalent mainland populations of passerines reveals that insular populations are indeed characterized by low levels of EPP. This supports the idea that the intensity of sexual selection is lower on islands. This relationship has previously been predicted, based on the assumption of low levels of genetic variation for fitness in such populations. The evidence from this analysis suggests that this is just one of several nonmutually exclusive hypotheses that may explain the high fidelity of island-living females.