Egg rejection behavior is more likely to develop in avian populations that experience high levels of heterospecific brood parasitism than in populations where parasitism exerts little selection pressure to recognize and respond to foreign eggs (Davies and Brooke 1989, Brown et al. 1990, Soler and Moller 1990 [but see Zufiiga and Redondo 1992], Briskie et al. 1992). Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) are not preferred hosts of the brood-parasitic Brownheaded Cowbird (Molothrus ater; hereafter cowbird) and rarely are parasitized (Willson 1966, Ortega and Cruz 1988, Neudorf and Sealy 1994 [Table 1], L. Beletsky and G. Orians, pers. comm.; but see Lincoln 1920). Consequently, low rates of heterospecific brood parasitism exert little selection pressure on Yellow-headed Blackbirds to recognize parasitic eggs, and individuals from populations with low parasitism rates do not reject nonmimetic eggs placed in their nests (Ortega and Cruz 1988). However, if a population of Yellow-headed Blackbirds were to experience significant amounts of brood parasitism, then one might expect to see evidence of egg rejection. I investigated this hypothesis in a population of this species that is heavily parasitized (1991: 7/33 nests [21.2%]; 1992: 5/29 nests [17.2%]) by cowbirds (Dufty, in prep.). I noted responses to the presence of foreign eggs by replacing single eggs in Yellow-headed Blackbird nests with an egg of the Red-winged Blackbird (Agelaius phoeniceus).