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 clu tch size optimization, when birds can lay more eggs than their optimal clut ch size. The results show that once intraspecific parasitism has appeared ( i.e., females start to spread their eggs over their own and other nests) th e evolutionarily stable number of eggs laid in its own nest decreases. Two possible ESSs exist: (1) either the evolutionarily stable number of eggs la id 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) eit her the evolutionarily stable number of eggs laid in its own nest is zero a nd all eggs are laid parasitically Since all females lay parasitically, thi s could favor the evolution of obligate interspecific brood parasitism. The key parameter allowing the shift from intraspecific to obligate interspeci fic parasitism is the intensity of density-dependent mortality within brood s (i.e., nestling competition). Strong nestling competition, as in altricia l species, can lead to an ESS where all eggs are laid parasitically: Altric ial 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 de velopment. Different nestling survival functions provided similar findings (i.e., obligate brood parasitism is more likely to evolve in altricial spec ies), suggesting that these results are robust with respect to the main ass umption of the model.