When will rejection of parasite nestlings by hosts of nonevicting avian brood parasites be favored? A misimprinting-equilibrium model

It has been suggested that discrimination and rejection of the nestlings of avian brood parasites are most likely to evolve when the parasite nestling is raised alongside the host nestlings, for example, many cowbird-host systems. Under these circumstances, the benefits of discrimination are high because the host parents may save most of their brood. However, there is a general absence of nestling rejection behavior among hosts of nonevicting parasites. In a cost-benefit equilibrium model, based on the premise that host species learn to recognize their offspring through imprinting on first breeding, we show that nestling recognition can be adaptive for hosts of cowbirds, but only under strict conditions. Namely, when host nestling survival alongside the parasite is low, rates of parasitism are high and the average clutch size is large. All of these conditions are seldom simultaneously achieved in real systems. Most importantly, the parasite nestling, on average, does not sufficiently depress host nestling survival to outweigh the costs of nestling recognition and rejection errors. Thus, we argue that nestling acceptance behaviors by hosts of nonevicting brood parasites may be explained as an evolutionary equilibrium in which recognition costs act as a stabilizing selection pressure against rejection when most of the host's offspring survive parasitism. Copyright 2003.brood parasitism; cost-benefit equilibrium; cowbirds; nestling rejection; nonevicting parasites; probability tree model