Laying order and offspring sex in blue tits Parus caeruleus

According to sex allocation theory, females may benefit from a differential investment in sons and daughters, where fitness returns from male and female offspring vary. One way in which investment may be differentiated is by assigning male and female offspring to eggs of different sizes, and/or eggs laid early or late in the laying order. Here, we investigate whether such sex-specific adjustments occur in blue tits Parus caeruleus . We found that the proportion of males changed non-linearly with laying order showing the increase in early-laid eggs. Egg size increased with laying order, but this pattern differed between female and male eggs. Female eggs increased initially in size and became smaller later in the laying sequence, while male eggs exhibited constant increase in mass. However, egg size did not differ between genders. Egg size and laying order was found to interact in moulding sex of the embryo. Thus, females seem to simultaneously adjust the sex of the offspring in relation to their investments into eggs and the position of the egg in the laying sequence.