FOOD ALLOCATION AMONG NESTLING STARLINGS - SIBLING COMPETITION AND THE SCOPE OF PARENTAL CHOICE

Parental provisioning for nestling birds is generally considered to be an interactive, conflictive process because the optimal provisioning rate differs between parents and young and because nestlings are engaged in intersibling competition. Understanding the evolution of communication in such a situation presents unusual problems because the scope for parental strategies may be limited by competitive behaviour of the chicks. We substantiate this view by studying parent-offspring feeding interactions between chicks and provisioning adults in the European starling Sturnus vulgaris in relation to chick state and intersibling competition. The state of one target chick in each nest was manipulated in the field by temporarily placing it in enlarged, reduced or normal-sized broods before returning it to its original nest. Conditions in the original nests were standardized during manipulation by using substitute chicks. Once returned to its original brood, the probability of the target chick being fed increased if it increased its begging intensity and/or it positioned itself closer to the entrance of the nest. Both begging intensity and position were functions of the treatment previously experienced, with target chicks begging more and attaining positions closer to the nest entrance after they had spent time in larger broods. We postulate that these factors must be included in theoretical analyses of the evolution of food-solicitation signalling because, although the effect of begging on feeding probability may be mediated by parental choice, the effect of position depends on between-chick dynamics, and the parents apparently accept the outcome of these interactions.