Extra-pair copulations can insure female blue tits against male infertility

Mating outside the pair-bond occurs frequently in socially monogamous
birds, but the benefits that females gain from this behaviour remain
debated. One hypothesis is that females engage in extra-pair copulations
(EPCs) to ensure that their clutch is fertilised in case their own mate is
infertile, but evidence for this idea is scarce. We report on a case of an
infertile male blue tit that bred in three successive years with three
different females. In the first year, all eggs were sired by an extra-pair
male whereas in the second year all eggs were unfertilised and contained
no sperm. In the third year, the female produced two clutches that were
fertilised by an extra-pair male, but - unlike ‘normal’ clutches - sperm
numbers on the perivitelline membrane decreased rapidly over the laying
sequence. Our findings show that blue tit females mated to an infertile
male can escape reproductive failure by engaging in EPCs and support
previous suggestions that EPCs in blue tits cease after the onset of egg
laying.