Female birds frequently engage in extra-pair mating behaviour and may thereby increase the genetic quality of offspring in terms of either viability or sexual attractiveness.

In contrast to the first idea, extra-pair offspring did not differ in lifespan from within-pair offspring in the socially monogamous coal tit. Moreover, in contrast to expectations, male within-pair offspring reared more social offspring over their lifetime than male extra-pair offspring.

The crucial task for future studies is to measure extra-pair fertilisation success of male offspring to test whether genetic benefits of extra-pair matings may lie in a higher sexual attractiveness of male extra-pair offspring.

Proceedings of the Royal Society B: Biological Sciences

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