Offspring sex ratio is related to male body size in the great tit (Parus major)

Sex allocation theory predicts that the allocation of resources to male and female function should depend on potential fitness gain realized through investment in either sex. In the great tit (Parus major), a monogamous passerine bird, male resource-hording potential (RHP) and fertilization success both depend on male body size (e.g., tarsus length) and plumage traits (e.g., breast stripe size). It is predicted that the proportion of sons in a brood should increase both with male body size and plumage traits, assuming that these traits show a father-offspring correlation. This was confirmed in our study: the proportion of sons in the brood increased significantly with male tarsus length and also, though not significantly, with the size of the breast stripe. A sex ratio bias in relation to male tarsus length was already present in the eggs because (1) the bias was similar among broods with and without mortality before the nestlings' sex was determined, and (2) the bias remained significant when the proportion of sons in the clutch was conservatively estimated, assuming that differential mortality before sex determination caused the bias. The bias was still present among recruits. The assumption of a father-offspring correlation was confirmed for tarsus length. Given that both RHP and fertilization success of male great tits depend on body size, and size of father and offspring is correlated, the sex ratio bias may be adaptive.