The evolution of obligate interspecific brood parasitism in birds

We present a simple analytical model to investigate the conditions for the evolution of obligate interspecific brood parasitism in birds, based on clutch size optimization, when birds can lay more eggs than their optimal clutch size. The results show that once intraspecific parasitism has appeared (i.e., females start to spread their eggs over their own and other nests) the evolutionarily stable number of eggs laid in its own nest decreases. Two possible ESSs exist: (1) either the evolutionarily stable number of eggs laid in its own nest is larger than zero, and a fraction of the total number of eggs is laid parasitically (i.e., intraspecific parasitism); and (2) either the evolutionarily stable number of eggs laid in its own nest is zero and all eggs are laid parasitically. Since all females lay parasitically, this could favor the evolution of obligate interspecific brood parasitism. The key parameter allowing the shift from intraspecific to obligate interspecific parasitism is the intensity of density-dependent mortality within broods (i.e., nestling competition). Strong nestling competition, as in altricial species, can lead to an ESS where all eggs are laid parasitically. Altricial species are, therefore, predicted to evolve more easily toward obligate interspecific parasitism than precocial species. These predictions fit the observed distribution of brood parasitism in birds, where only one species out of 95 obligate interspecific parasites exhibits a precocial mode of development. Different nestling survival functions provided similar findings (i.e., obligate brood parasitism is more likely to evolve in altricial species), suggesting that these results are robust with respect to the main assumption of the model.