Crimson-breasted Shrike females with extra pair offspring contributed more to duets

Many socially monogamous bird species follow a genetically promiscuous reproductive strategy. Duetting birds may be an exception, as they appear to exhibit very low levels of extra pair paternity—which is thought to be connected to duetting. Duets are predicted to function either in paternity guarding or as signals of commitment to the pair bond, reciprocally reducing extra pair mating in both sexes. In this study, we used four microsatellites to document paternity loss in the Crimson-breasted Shrike (Laniarius atrococcineus), a socially monogamous subtropical duetting species. Extra pair paternity was surprisingly high, with extra pair males siring 20 % of young in 30 % of broods. Furthermore, we compared the levels of extra pair paternity of each pair to certain characteristics of their duets to test the paternity guard and the commitment hypotheses. Our results offered no support that duets function in paternity guarding, although the fact that males which had been cuckolded answered more of their female’s calls could represent an adaptation to avoid future cuckoldry. We also found no evidence for duets functioning as signal of commitment. Female vocal behaviour was in fact the reverse of that predicted: Levels of infidelity amongst social females were positively related to female answer rate of their mates’ songs. These results indicate that females may use increased answer rates as a form of manipulation, in an attempt to avoid potential costs associated with their extra pair matings.