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 be en postulated that island populations have a lower frequency of EPP than ma inland 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 mainla nd, but the difference was not statistically significant. There was neither any difference in genetic variability between the two populations, as asse ssed by band-sharing analysis of a random subset of breeding individuals. H owever, band-sharing between Fair mates was significantly higher in the isl and 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 neg ative effect on hatching success. The annual EPP frequency in the two popul ations was positively correlated with breeding density, but not with breedi ng 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 c onspecific nests within 100 m) fur nests with EPP than for nests without EP P. There was no indication that individual synchrony (the percentage of fem ales in the population that also were fertile when the focal female was fer tile) affected the likelihood of EPP. We conclude that there was no particu lar island effect on the frequency of EPP in our data set. The marginal var iation in EPP frequency observed across years and populations is more likel y associated with local variations in breeding density and the number of cl ose neighbours.