Partial Rejection of Immaculate Foreign Eggs by Yellow-Breasted Chats

Abstract

Egg ejection has been documented in numerous songbirds in response to natural or artificial brood by Brown-headed Cowbirds (Molothrus ater; Rothstein 1975a, Finch 1982, Rich and Rothstein 1985, Regosin 1994). Ejection of foreign eggs is thought to occur because birds learn to recognize their own eggs through an imprinting-like process and remove eggs that differ substantially from their own (Rothstein 1974, 1978; Lotem et al. 1992, 1995). Selection pressure from brood appears to maintain egg recognition and response to foreign eggs (Davies and Brooke 1989, Soler and M0ller 1990, Briskie et al. 1992, Dufty 1994, Soler et al. 1994). The Yellow-breasted Chat (Icteria virens) is a common host of the Brown-headed Cowbird (Friedmann 1929, 1963; Berger 1951, Nolan 1963, Young 1963) and is considered an accepter of (Rohwer and Spaw 1988). However, varation in frequencies among populations has been noted, and Friedmann (1963) speculated that parasitism intolerant chat populations explained locally low frequencies. Moderate to high frequencies of should exert pressure on chats to evolve responses to parasitism, but responses to cowbird eggs may not evolve rapidly in chats because of close resemblance between the two species' eggs (Rothstein 1975a). In 1992, we noted a parasitized chat nest where a cowbird egg apparently was ejected and remained beneath the nest. We proposed that chats are capable of ejecting cowbird eggs, but generally fail to do so because cowbird and chat eggs are difficult to distinguish from one another. We predicted that chats would eject dissimilar eggs frequently, but would reject normal, maculated cowbird eggs, if at all, in proportion to their divergence from their own clutch. We tested these predictions by evaluating the response of Yellow-breasted Chats to white-painted cowbird eggs and normal cowbird eggs added to their nests. Methods.-We searched for Yellow-breasted Chat