Great tit hatchling sex ratios

The sex of Great Tit Parus major nestlings was determined using PCR RAPDs. Because this technique requires minute amounts of DNA, chicks could be sampled soon (0-2d) after hatching, before any nestling mortality occurred. The proportion of males among 752 chicks hatching in 102 broods (98.9% of those that hatched) increased with hatching date from 42.4% male in first broods (n=555 chicks) to 50.9% male in second broods (n=167). The proportion of males among hatchlings also decreased with increasing clutch size and increased with increasing hatching asynchrony. These three variables are intercorrelated and it was not possible to separate them statistically. Hatchling sex ratio was not correlated with the age, weight or wing-length of either of the parents, brood size at hatching or fledging, hatching or fledging success, mean chick weight, or position in the hatching sequence. The functional significance of the sex ratio variation found is unknown.