Paternity analysis reveals opposing selection pressures on crown coloration in the blue tit (Parus caeruleus)

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Abstract
In socially monogamous species, extra–pair paternity can increase the variance in reproductive success and thereby the potential for sexual selection on male ornaments. We studied whether male secondary sexual ornaments are selected through within– and/or extra–pair reproductive success in the blue tit (Parus caeruleus). Male blue tits display a bright blue crown plumage, which reflects substantially in the ultraviolet (UV) and previously has been indicated to be an important sexual signal. We show that males with a more UV–shifted crown hue were less cuckolded, which probably resulted from female preference for more ornamented mates. By contrast, however, older males and males with a less UV–shifted hue sired more extra–pair young. This probably did not reflect direct female preference, since cuckolders were not less UV–ornamented than the males they cuckolded. Alternatively, a trade–off between UV ornamentation and other traits that enhance extra–pair success could explain this pattern. Our results might reflect two alternative male mating tactics, where more UV–ornamented males maximize within–pair success and less UV–ornamented males maximize extra–pair success. Since crown colour was selected in opposite directions by within–pair and extra–pair paternity, directional selection through extra–pair matings seemed weak, at least in this population and breeding season. Reduced intensity of sexual selection due to alternative mating tactics constitutes a potential mechanism maintaining additive genetic variance of male ornaments.