No evidence for illegitimate young in monogamous and polygynous warblers

Abstract

IN animals with internal fertilization, paternity is uncertain. In birds, the occurrence of copulations outside the pair-bond has been documented in a number of species^1,2, but the extent to which these result in illegitimate young is largely unknown, and constitutes a major deficiency in our understanding of avian mating systems^3–5. The analysis of tandemly repeated sequences (minisatellites), has enhanced our ability to make individual identifications and paternity determinations^6–11. Here we describe the use of a bird minisatellite DNA probe in assigning paternity in natural populaá-tions of the monogamous willow warbler Phylloscopus trochilus and of the polygynous wood warbler Phylloscopus sibilatrix. In both species this probe detects a multiple locus pattern and a single locus that exhibits a variable number of tandem repeats12. Although we observed intrusions by non-resident males into the territories of paired males and extra-pair copulations, no illegitimate offspring were detected among 176 young from 32 families of both species, implying that extra-pair copulations have little or no genetic impact.