Chick Begging Strategies in Relation to Brood Hierarchies and Hatching Asynchrony

Altricial offspring solicit food by begging, and their parents feed them according to begging intensity, which has been shown to be positively related to offspring need. Parent-offspring genetic conflict calls for analyses of evolutionary stability, and various theoretical models have shown that stability is possible in the framework of handicap theory. The models predict that a negative relationship exists between offspring condition and begging and that offspring in poorer condition should be fed preferentially. However, these predictions depend on two unsatisfactory assumptions. First, they assume a monotonically decelerated relation between condition and fitness (this function is more likely to be sigmoid); second, they ignore physical competition between siblings, which is known to be important. We examined the significance of these issues by manipulating hatching asynchrony in broods of starlings Sturnus vulgaris, thus controlling competitive asymmetries between nest mates. We created broods with senior (older) and junior (younger) chicks and control broods with synchronous chicks. In field and laboratory experiments, we found that seniors begged less than juniors and controls, whereas juniors did not differ significantly from controls. However, seniors received more food from their parents and grew better than juniors or controls (hence, they were in better condition). These results violate the predictions of available theoretical models and, together with limitations in the universality of their assumptions, indicate that fundamental aspects of parent-offspring communication are not yet understood.