The impact of learning foster species' song on the evolution of specialist avian brood parasitism

Obligate interspecific avian brood parasites do not build nests of their own but lay their eggs in the nests of other species. It has been proposed that a flexible song learning mechanism (copying the heterospecific songs of the foster species) facilitates the evolution of brood-parasitic behavior. Some sort of song copying is common to all songbirds; hence, to better understand the evolution of brood parasitism it is important to study the role of song learning. The proposed hypothesis does not take into account that flexible song learning might make mate acquisition more difficult because males that are preferred by brood-parasitic females would be initially rare. We examine this by means of two population dynamic models. By using a recurrence equation model of brood parasites competing with their nestbuilding ancestors, we show that flexible song learning is indeed an obstacle to the evolution of brood parasitism. Results from a more realistic, individual-based model, in which the brood-parasitic trait can evolve more gradually, confirm this finding. However, we also show that the obstacle of flexible song learning can be overcome quite easily when males also are carriers of the brood-parasitic trait. This is probably because brood parasitism is a neutral trait in males, which increases the number of mutants carrying genes for brood parasitism, and thus makes the female task of finding suitable partners easier.