The widespread occurrence of extra-pair paternity (EPP) in birds adds rich complexity to our understanding of sexual selection and mating system evolution. Extra-pair matings are typically cryptic so for most species we lack the detailed behavioral observations needed to fully determine whether both sexes benefit from EPP and, when trait correlations with EPP are found, whether they reflect female choice or male intrasexual competition. Here we examine behavioral and morphological correlates of EPP in the lark bunting (Calamospiza melanocorys), a grassland songbird where EPP-seeking behaviors of males are unusually overt and conspicuous. Males sought EPP by closely following receptive pairs, often in groups. Virtually all observed extra-pair copulation attempts involved male aggression, were resisted by females and larger females had a lower rate of extra-pair paternity in their nests. Male plumage traits predicted both male effort in seeking EPP (number of mating groups joined) and fitness gained through EPP; these same male traits have previously been linked to male-male competition but were not linked to consistent mate choice in lark buntings. There was no trade-off between investment in seeking EPP and fitness at a male’s own nest, indicating that fitness from EPP is likely a potent driver of sexual selection in lark buntings, one that may not entirely be driven by female fitness interests.

Behavioral observations of color-banded birds in the field. We collected measurements during banding. The genetic data were obtained from blood samples, microsatellite loci were obtained as described in the paper and the program CERVUS was used to establish parentage.

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