Sex allocation from an owl perspective: clutch order could determine brood sex to reduce sibling aggression in the Eagle Owl Bubo bubo

Abstract

Hatching asynchrony in altricial bird species occurs if incubation starts before clutch completion. It determines differential within-brood growth, which may result in a competitive and developmental hierarchy among siblings.As a consequence, asymmetric sibling competition frequently results in post-hatching mortality of the last-hatched offspring. Because nestling hierarchy is not necessarily adaptive, species that are able to raise large broods should have evolvedmechanisms to reducewithin-broodmortality due to sibling competition. One mechanism to reduce offspring mortality could be to favour the occurrence of the larger sex among late-laid chicks. We explored this possibility by studying the sex allocation of 349 Eagle Owl (Bubo bubo) chicks from 137 broods, as a function of clutch size and order. In broods of four chicks, parents seemed to invest in the less energetically expensive sex (male) at the third position, and females (the larger sex) as the last, and therefore smallest, chick in the brood. This strategy should reduce asymmetric sibling competition: the relative position of chicks of each sex within the brood is likely to be crucial for maximizing the success of clutches.