DNA fingerprinting of an island population of blue tits and great tits in southeast Norway revealed that extrapair paternity accounted for 36% (17/47) and 27% (15/55) of broods and for 7% (31/466) and 8% (33/408) of young in the two species, respectively. Cuckolded males did not differ from noncuckolded males with respect to morphology, age, or survival. There was no seasonal pattern in the frequency of extrapair paternity, and males showed no individual consistency in paternity loss over multiple broods. Extrapair offspring did not grow faster, they did not fledge with a higher body mass, and they did not show a higher local survival rate than their half siblings. Hence, there was no evidence of any association between extrapair paternity and male phenotypic or genotypic quality. Extrapair offspring were randomly distributed among broods, with the only exceptions of one blue tit and two great tit broods in which all young (six to nine) were sired by an extrapair male. This pattern is best explained by a small proportion of males (2%–4%) being infertile and by most females performing a few extrapair copulations as insurance against laying infertile eggs. We conclude that the results suggest a role for fertility insurance but that alternative functional explanations to extrapair paternity in these populations cannot yet be ruled out.