Low frequency of extrapair paternity in the polygynous great reed warbler, Acrocephalus arundinaceus

We carried out DNA fingerprinting on 553 young (130 broods) great reed warblers (Acrocephalus arundnaceus) in 1987–1991. In the study population, where 40% of the males become polygynous, there was a low frequency of extrapair fertilizations (EPF). When data from all five years were pooled, 3.1% of the young were sired by extrapair males (EPF-males) and 5.4% of the broods contained extrapair young. We found no cases of extrapair maternity; young with 6–17 mismatched DNA bands (n= 17) had high band sharing with their putative mothers (range = 0.52–0.72) but low band sharing with their putative fathers (range = 0.24–0.40). In broods exposed to EPF, on average 53% of the young were sired by EPF-males. We found the genetic father to each of the illegitimate young. In all cases the same EPF-male sired all extrapair young in a brood. Broods containing EPF-young tended to be initiated late during the breeding season. Breeding attempts were rather evenly distributed over two months, thus this breeding asynchrony would have facilitated EPFs. There was no difference in EPF frequency between broods where the pair males had left their females unguarded during parts of their fertile periods and broods where males guarded throughout the fertile periods. Nests with extrapair young had significantly shorter mean distance to the closest male neighbor and more male neighbors within 100 m than nests without extrapair young. We found no indication that females engaged in EPF to get parental care from the EPF-males, or because they were forced to copulate with extrapair males. The low frequency of EPF suggested that females did not seek genetic diversity to their brood. We cannot rule out the possibility that females engaged in EPF to insure fertility. However, data supporting this hypothesis were weak. Instead, our data support the conclusion that females engaged in EPF to increase the genetic quality of their offspring, and that females may have used male song repertoire size as a cue when choosing EPF partners.