Functional infertility in a wild passerine bird

Information about male infertility in free‐living bird populations and its underlying causes are poorly documented in the literature. Here, we assessed sperm quality and infertility among males in a wild population of Blue Tits Cyanistes caeruleus, a socially monogamous passerine with frequent extra‐pair mating. One of 30 males (3.3%) studied across two breeding seasons had morphologically abnormal and nearly immotile spermatozoa in combination with zero annual fertilization success. Demonstration of male functional infertility suggests the possibility that socially monogamous females of the study population could obtain a direct fitness benefit by mating polyandrously to increase fertilization success, which could contribute to selection on extra‐pair mating behaviour.