Sibling negotiation

Current discussions of offspring begging typically assume either that it is a signal directed at parents or that it represents a form of scramble competition to gain access to them. However, offspring might also display to inform nest mates that they will contest the next food item to be delivered; in other words, begging (possibly in the absence of parents) might serve purely as a form of negotiation among siblings. Here, we develop a game-theoretical model of this possibility. We assume that offspring vary in their need for food, which influences how intensely they compete for access to parents. Before parental arrival, however, young may exchange signals informing each other of their state, and these signals may influence their subsequent competitive behavior. We focus on the possibility that a costly display during the "negotiation" phase can serve to inform rivals of a chick's need for resources and thereby deter them from competing intensely when the parent arrives. We show that this form of negotiation is more likely to prove stable when the food delivered by parents is indivisible, and when it is hard for one chick to monopolize access to resources. Investment in negotiation (as opposed to competition) is predicted to increase with nestling relatedness; in addition, all other things being equal, hungrier chicks are expected to invest relatively more effort in displaying to their rivals, whereas weaker or smaller chicks are expected to invest less.