Fertilization success and UV ornamentation in blue tits Cyanistes caeruleus: correlational and experimental evidence

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Cases where less ornamented males are favored through sexual selection are rare among birds. Here we show, based on data from 3 consecutive breeding seasons, that male blue tits with less ultraviolet (UV)-ornamented crown feathers sire more offspring. This pattern was mainly driven by the higher success of older, less UV-ornamented males at siring extrapair offspring. The reason behind this relationship is unclear although we hypothesize that being less UV-ornamented may enable adult males to intrude into nearby territories by mimicking juveniles. To test causality, we experimentally manipulated male crown coloration creating 2 groups, one with higher (UV(+) treatment) and one with lower UV reflectance (UV(−) treatment). Contrary to our expectations, UV(−) males were less likely to sire extrapair offspring than UV(+) males. The treatment had no effect on the likelihood of losing paternity in a male's own nest. Because the experimental evidence does not support the observational data, a direct effect of male crown color on extrapair success cannot be confirmed. However, potential pitfalls of this and other such color manipulation experiments, like fading of treatment with time and mismatches between behavior and coloration, call for new improved manipulation techniques and detailed behavioral observations to conclusively test for the effect of blue tit crown coloration on male extrapair success.

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