Mate quality affects offspring sex ratio in blue tits

Questions concerning the occurrence of adaptive sex ratio manipulation have a long research tradition in evolutionary biology, but evidence from birds is scarce. It has been suggested that birds are unable to adaptively determine the sex of their young because of the constraints of their sex determination systems. Here we report results from a study of variation in offspring sex ratio in the blue tit, Parus caeruleus, a small passerine bird. Our data suggest that females are able to manipulate their primary sex ratios away from the equal sex ratio predicted from simple chromosomal segregation at meiosis. Contrary to the situation in some other birds, offspring sex ratio variation in this species was not influenced by timing of breeding or brood size. We show that females mated to males with high survival prospects bias the hatching sex ratio of their broods in favour of sons. Although it has previously been proposed that females should produce an excess of sons when mated to high-quality males, our data provide the first evidence that the phenotypic quality of fathers contributes to the sex ratio variation in a natural population.