The function of extrapair paternity in blue tits and great tits: good genes or fertility insurance?

DNA fingerprinting of an island population of blue tits and great tits in s outheast 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 noncucko lded males with respect to morphology, age, or survival. There was no seaso nal pattern in the frequency of extrapair paternity, and males showed no in dividual consistency in paternity loss over multiple broods. Extrapair offs pring 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. H ence, 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 extrapa ir male. This pattern is best explained by a small proportion of males (2%- 4%) being infertile and by most females performing a few extrapair copulati ons as insurance against laying infertile eggs. We conclude that the result s suggest a role for fertility insurance but that alternative functional ex planations to extrapair paternity in these populations cannot yet be ruled out.