9.1 Host learning as a defense against avian brood parasitism

Author: Mary Mao

Introduction

Seven taxa comprising approximately 80 species of birds around the world reproduce as obligate interspecific brood parasites , which means that they must make use of the nests of other species of birds in order to reproduce (Sorenson and Payne 2002). Such behavior is detrimental to the fitness of the hapless hosts, frequently passerines or perching birds, who sacrifice part or all of their own brood when deceived by parasitic species. The high cost of parasitism has spurred an evolutionary arms race, with hosts developing mechanisms by which to recognize and reject parasitic eggs and hatchlings, and parasites developing techniques such as mimicry, or protective and deceptive imitation, and bullying to thwart such defenses. Evolution is based on costs and benefits; hosts will only learn to recognize or reject hatchlings when the potentially beneficial fitness gains outweigh costs such as erroneously rejecting the wrong eggs. One important consequence of selectively rejecting eggs, for example, is that certain parasitic birds will return to depredate such nests in what has come to be called the “ Mafia hypothesis ” (Hoover and Robinson 2007). This system of costs and balances is one of the most robust examples of coevolution , where related parties exert selective forces on each other to spur evolution, known to occur in the natural world.

Since many factors are involved in the success of learned behaviors, the method by which a host is parasitized greatly affects the resulting behavior; some birds are more likely to simply accept the parasitic eggs and cut losses, while others are far more discriminatory (Rothstein 1975). We focus on host responses to parasitism by the brown-headed cowbird ( Molothrus ater ) and the common cuckoo ( Cuculus canorus ). The brown-headed cowbird is a generalist and has been known to parasitize over 200 species of birds (Marchetti, H. Nakamura, H. L. Gibbs 1998; Friedmann and L.F. Kiff 1985). On the other hand, the common cuckoo is more of a specialist ; although it has been known to parasitize a handful of species, each female acquires specificity to only one (Vogl et al. 2002). Selection for learned host recognition and rejection behavior is determined by a number of factors including whether host chicks remain in the nest; extent of mimicry of parasitic eggs, hatchlings, or begging; and the experience of the host breeder. We will hope to elucidate these factors for a better explanation of how host learning has adapted to selection pressure from brood parasites.