Quality of breeding territory mediates the influence of paternal quality on sex ratio bias in a free-living bird population

Sex allocation theory predicts that females will invest more in offspring of the sex that offers a higher fitness potential. Male offspring should be favored where direct genetic benefits from the father or indirect benefits from the parents or environment are likely to result in higher than average quality offspring. In birds, sex allocation has previously been linked to paternal quality and sexual attractiveness, but the potential importance of breeding territory quality has been largely overlooked. Using a wild population of a territorial bird species, the blue tit (Cyanistes caeruleus), we show that variation in sex ratio is associated with paternal quality, but that the relationship hinges on the quality of the breeding territory. Among broods in high-quality territories, there is a positive association between primary sex ratio and male mate condition, which is not found among broods in low-quality territories. In turn, we find that paternal quality as opposed to maternal quality is associated with chick recruitment and that the condition of the chick is predicted by an interaction of parental and territory quality, reflecting the trend in primary sex ratio bias observed in our system. These results suggest that sex allocation is influenced not only by variation in the quality of the father but also by differences between territories in the resources available for breeding, which in turn suggests an interaction between genetic and environmental effects.