Variation in the frequency of extra-pair paternity in birds: A comparison of an island and a mainland population of blue tits

The frequency of extra-pair paternity (EPP), as revealed by multilocus DNA fingerprinting, was examined over three breeding seasons each in an island and a mainland population of blue tits Parus cacruleus in Norway. It has been postulated that island populations have a lower frequency of EPP than mainland populations, due to a reduced level of genetic variability. We found an EPP frequency of 5% (of offspring) on the island, and 13% on the mainland, but the difference was not statistically significant. There was neither any difference in genetic variability between the two populations, as assessed by band-sharing analysis of a random subset of breeding individuals. However, band-sharing between Fair mates was significantly higher in the island population, where some individuals seemed to mate with close relatives tone sibling pair was detected), nigh genetic similarity between pair mates on the island did not affect offspring survival, but had a significant negative effect on hatching success. The annual EPP frequency in the two populations was positively correlated with breeding density, but not with breeding synchrony. When the data were analysed on a per nest level, the mainland pl,population showed a tendency of a higher neighbour density (number of conspecific nests within 100 m) fur nests with EPP than for nests without EPP. There was no indication that individual synchrony (the percentage of females in the population that also were fertile when the focal female was fertile) affected the likelihood of EPP. We conclude that there was no particular island effect on the frequency of EPP in our data set. The marginal variation in EPP frequency observed across years and populations is more likely associated with local variations in breeding density and the number of close neighbours.