SCRAMBLE FEEDING IN JACKASS PENGUINS - WITHIN-BROOD FOOD DISTRIBUTIONAND THE MAINTENANCE OF SIBLING ASYMMETRIES

Sibling asymmetries in asynchronously hatching jackass penguin, Spheni scus demersus, broods allow more efficient resource use than in artifi cially synchronized broods. Behaviour that determines food allocation and maintains weight asymmetries between siblings was examined. Total time spent begging and feeding by each chick during a feeding episode was measured at seven two-chick and 43 singleton nests, with repeat ob servations spanning the entire fledging period. First-hatched (A) chic ks either obtained more feeding time, or begged less for the same feed ing time, than their siblings (B-chicks) in all but one age category ( 21-40 days). At this age increasing demands for food associated with r apid growth, coupled with increasing mobility, appeared to intensify c ompetition, and A-chicks had to beg longer for the same feeding time t han their siblings. By 41-60 days A-chicks were again gaining more fee ding time than B-chicks. A-chicks were better coordinated, using their weight advantage effectively during scramble feeding. A- and B-chicks generally spent more time begging for a given amount of feeding time than did singletons. A-chicks fed more at the beginning than at the en d of a feeding episode, while B-chick feeding rate remained the same, suggesting a drop in A-chick feeding rate due to satiation, which may prevent asymmetries from becoming too large. In this way A-chicks main tain their weight and competitive advantage, without risking B-chick s tarvation when food supplies are adequate for both siblings. (C) 1996 The Association for the Study of Animal Behaviour