Extrapair copulations in coal tits (Periparus ater): female insurance against male infertility?

The evolutionary causes of female extrapair mating in socially monogamous birds are poorly understood. Adaptive hypotheses focus on direct benefits, i.e. fertility and paternal care, or indirect, genetic benefits, but the empirical support is generally sparse and mixed. Here we report a study of extrapair paternity in a Norwegian population of coal tits (Periparus ater), a species in which previous studies have reported a high frequency of extrapair paternity, but with no clear support for any female benefits. We confirmed a high occurrence of extrapair paternity (32% of offspring) and found that most extrapair sires were breeding neighbours. Extrapair offspring were heavier than their withinpair nestmates during the early nestling period, indicating that they were among the earlier eggs in the laying sequence. We evaluate various hypotheses for female extrapair mating and conclude that a direct benefit in the form of insurance against mate infertility is a plausible explanation to our results. There is empirical evidence for a low, but presumably biologically significant rate of infertile males in several passerine species, including other tit species. Fertility insurance may have a wider relevance as a direct benefit of female extrapair copulation than previously acknowledged, and male infertility warrants more scrutiny in wild birds in general. Significance statement Nearly one third of all offspring in coal tit broods were sired by a male neighbour. Data on early nestling body mass hierarchy indicate a paternity skew in the egg-laying order. The results are consistent with the fertility insurance hypothesis, which states that female birds engage in extrapair copulations before the start of egg laying to safeguard against a potentially infertile mate. However, direct evidence for male infertility is lacking and further studies are needed.

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