High levels of extra-pair paternity in an isolated, low-density, island population of tree swallows (Tachycineta bicolor)

Molecular genetic studies have suggested that apparently nonbreeding males ('floaters') may account for a significant proportion of extra-pair paterni ty (EPP) in avian populations. Attempts to determine the influence of breed ing density on EPP are therefore confounded by the presence of a subpopulat ion of floaters whose numbers are difficult to estimate. To study EPP in a tree swallow (Tachycineta bicolor) population with few floaters, we chose a nestbox grid on an island with an excess of available breeding sites and v ery few floaters. We obtained DNA samples from 13 complete families and per formed DNA profiling on them using four microsatellite loci. For comparison , we also obtained a sample of 58 extrapair young (EPY) from a mainland pop ulation typed at five microsatellite loci. Paternity assignments among resi dent males in both populations were made using the microsatellite profiles and a likelihood-based statistical method. Of the 67 island nestlings typed , we found 37 (55%) nestlings from 11 (85%) different nests that were EPY. The proportion of nestlings that were EPY and the proportion of broods cont aining EPY did not differ significantly between island and mainland populat ions studied previously. There was no significant difference between island and mainland populations in the proportion of extra-pair paternities assig ned among neighbouring resident males. Male breeding density does not appea r to affect the ability of female tree swallows to obtain extra-pair fertil izations, at least over the range of densities studied so far. The rate of EPP has remained remarkably consistent over many years, studies and populat ions implying an important role of active female choice in determining EPP.