IN animals with parental care, parents rearing offspring of variable ages are typically assumed to exert less effort than those rearing even-aged offspring. This is because spaced births spread out peak loads in the combined food demands of all offspring(1-3). Creating a mixed-size sibship also helps establish a hierarchy among the young which reduces the costs of sibling rivalry(4) and can help efficient elimination of young if food becomes short(5,6). We manipulated hatching spread within broods of the blue tit Parus caeruleus and studied postbreeding survival rate of the adults. We report here that, contrary to current,theory, female parents suffer less when the young are even-aged than when they are of variable ages, whereas the opposite result was found for male parents. Apparently, the male contributes more in synchronous broods, thus lightening the female’s total investment burden. In blue tits this sexual conflict over hatching pattern is won by the female because she alone incubates. By delaying incubation until most eggs have been laid, she reduces hatching span.