Is variation in brood sex ratios adaptive in the great tit (Parus major)?

Life-history theory predicts skewed offspring sex ratios in a range of situations in which the costs and benefits of producing the two sexes differ. In recent years, many studies have demonstrated biased sex ratios in a variety of bird species. However, many of these investigations have been based on small sample sizes, on data from a single year, or both. Using a recently developed polymerase chain reaction-based molecular DNA technique, 912 great tit (Parus major) nestlings from 118 broods in 5 different years were sexed. As found in a number of previous studies on the same species, there were significant predictors of offspring sex ratio in individual years. However, there were no consistent trends across years, and none of the measured variables significantly predicted sex ratio over all years combined. Furthermore, brood sex ratio of the population did not depart from the expected binomial distribution. Although there are theoretical advantages to manipulating the sex ratio in this and other species, the physiological mechanism by which it is achieved in birds remains obscurer We argue that data from several years are needed to confirm whether facultative sex ratio manipulation is a consistent breeding strategy used by birds.