Facultative sex ratio adjustment in response to male tarsus length in the Varied Tit Parus varius

During the last decade, evidence from a number of studies has suggested systematic deviations from a 1:1 primary sex ratio in birds, in spite of the fact that birds have chromosomal sex determination systems; the mechanism of sex allocation is not fully understood. However, it still remains uncertain whether adaptive manipulations of primary sex ratio occur, especially in Parus species. We studied sex ratio variation in the Varied Tit Parus varius, which is a socially monogamous species similar to the Great Tit P. major and the Blue Tit P. caeruleus. In total, 362 chicks that hatched from 72 broods over 3 years were sexed. Of all nestlings, 51.9% (188/362) were male. The nestling sex ratio did not differ significantly from unity. However, the proportion of sons in each brood was significantly and positively related to the father's tarsus length. This corresponds with our predictions, given that larger males have higher resource holding potential if tarsus length is a heritable character between fathers and sons.