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 engag ed in intersibling competition. Understanding the evolution of communi cation in such a situation presents unusual problems because the scope for parental strategies may be limited by competitive behaviour of th e chicks. We substantiate this view by studying parent-offspring feedi ng 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 manipulat ed in the field by temporarily placing it in enlarged, reduced or norm al-sized broods before returning it to its original nest. Conditions i n the original nests were standardized during manipulation by using su bstitute chicks. Once returned to its original brood, the probability of the target chick being fed increased if it increased its begging in tensity 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 lar ger broods. We postulate that these factors must be included in theore tical analyses of the evolution of food-solicitation signalling becaus e, although the effect of begging on feeding probability may be mediat ed by parental choice, the effect of position depends on between-chick dynamics, and the parents apparently accept the outcome of these inte ractions.