Question: What is the adaptive significance of extra-pair parentage? Theoretical approach:We view parentage as a ‘transaction currency’ for exchanges of ecological benefits. We develop a multi-player cooperative game, using the core and the Nash bargaining solution as solution concepts. Model assumptions: Birds can negotiate about who pairs with whom. Parentage can be exchanged between individuals as a result of negotiations. Number of offspring fledged from a nest depends on the experience and situation of the social parents and not on their genes (i.e. only direct benefits, no genetic benefits). Predictions: We predict extra-pair parentage to occur when individuals with higher breeding capability are paired to individuals with lower breeding capability. Social interactions between males are predicted to precede the occurrence of extra-pair paternity. We give an example experiment to test our model.