Behavioral Defenses to Brood Parasitism by Potential Hosts of the Brown-Headed Cowbird

Abstract

An understanding of avian brood parasitism requires investigation of a complicated system of co-evolutionary adaptations and interactions between host and parasite. Selection favors parasites which can best utilize their hosts while at same time favoring hosts which best avoid parasitism. Brownheaded Cowbird (Molothrus ater) hosts have several behavioral adaptations for decreasing parasite success. Both nest desertion and nest reconstruction in which a cowbird egg and usually a host egg are buried involve loss of host's eggs and energy required to build a new nest (Friedmann 1963). Ejection of a cowbird egg by a host, which requires accurate discrimination between parasite and host eggs, is probably an energetically preferable defense (Rothstein 1970). Yet all of these anti-parasite defenses operate after parasite egg is laid and consequently do not prevent loss of host eggs removed by parasite. Rothstein (1970:133) pointed out that the advantageous form of host adaptation is to avoid being One way to avoid parasitism is to guard nest site with aggressive behavior. Host aggression towards brood parasites has been recognized clearly in European Cuckoo (Cuculus canorus) (Rothschild and Clay 1952) and several African cuckoos (Friedmann 1948). Hosts frequently mob cuckoos and may attack when nest is approached too closely. Edwards et al. (1949) and Smith and Hosking (1955) conducted field experiments on English songbirds using models of European Cuckoos. In general they found that models were attacked violently by regular host species but not by birds seldom parasitized. Antagonistic behavior towards Brownheaded Cowbird has not been investigated previously, and only a few reports of host aggression appear in literature. Friedmann (1963:33) stated that On whole, majority of American species of passerine birds do not act as if they recognize an enemy in However, in an earlier work (Friedmann 1929:195) he stated most birds are so vigilant of their nests that often a laying cowbird must be subject to considerable attack, or a least be witness of many intimidation displays on part of victim. Seland r and LaRue (1961) reviewed li era ure on host aggression to cowbirds. Aggressive host-parasite interactions have been reported for Song Sparrow (Melospiza melodia) by Nice (1943). She observed a few instances when Song Sparrows actually att cked a female cowbird. The American Red-