Negotiation over offspring care?--a positive response to partner-provisioning rate in great tits

Game theoretical models of biparental care predict that a change in work rate by one parent should be met by incomplete compensation by its partner. However, in empirical studies on biparental birds, there has been some inconsistency in the direction and extent of the response, and the mechanism behind it has so far been unclear. Parents could be responding directly to partner work rate or indirectly via chick begging. In this study of great tits (Parus major), the work rate of one parent was increased experimentally by augmenting the begging of the chicks with playback of extra begging calls whenever the parent visited the nest. The playback had no effect on the chicks' begging behavior, so any change in the focal parent's behavior was a direct response to its partner's work rate over a short timescale. An experimental increase in care by either male or female parent led to an increase (to a lesser extent) in the work rate of its partner, which is counter to the decrease predicted by partial compensation models. This seemingly paradoxical result may reflect decisions made exclusively over a short timescale and is in keeping with new theoretical work, which takes into account the information content of partner work rates. Copyright 2006.