Variation in extra-pair paternity in the polygynous Great Reed Warbler (Acrocephalus arundinaceus)

In five years (1992, 1994-97) we measured the frequency of extra-pair pater nity (EPP) in a Bavarian population of polygynous Great Reed Warblers (Fran kische Weiher region: 49 degrees 40'N, 10 degrees 51'E); these data were co mpared with corresponding findings in Sweden. Progeny from copulations with a male other than the partner (extra-pair young, EPY) were identified by m ulti-locus DNA fingerprinting with oligonucleotide probes. In 48 broods wit h 194 nestlings we found 19 EPY in 5 broods. The extra-pair fertilisation ( EPF) rate as a percentage of total juveniles (9.8 %) is higher, but that wi th reference to nests (10.4 %) is only insignificantly higher than in Swede n. In contrast to Sweden, neighbour males in our population were never involve d in EPF. For two EPF nests the genetic fathers of the EPY were identified; both were unpaired males several years old, with very distant territories (2.4 km away). Both had larger song repertoires than the pair males, and on e sang a Great Reed Warbler x Reed Warbler (Acrocephalus scirpaceus) mixed song. In the other cases we were unable to detect the genetic father, eithe r among the neighbouring males or the others tested. Presumably these EPY s pring from floater males or rapid mate switching. As in Sweden, EPY were ob served in nests of monogamous (2x) as well as polygynous (3x primary female s) males. However, in Germany the breeding density was lower and distributi on more patchy and the EPF broods were produced earlier (before the median egg-laying date) than those in Sweden. Our comparison supports the hypothes is that the EPF rate is higher in populations with greater genetic variabil ity than in those with less variability. The relatively low EPP rate in thi s species seems to be due to a close correlation between male and territory characteristics.