Vireo gilvus (Warbling Vireo) is the smallest bird species in North America known to eject Molothrus ater (Brown-headed Cowbird) eggs from their nests. Populations eject parasitic eggs in central North America and the Great Lakes region but not in western North America. The geographic differences in behavior may reflect the range distributions of 2 subspecies. However, experimental studies of egg ejection in eastern populations are currently lacking. To address this gap, I experimentally parasitized 3 Warbling Vireo nests in Massachusetts using real and model cowbird eggs. At all 3 nests, Warbling Vireos ejected the parasitic egg. To my knowledge, this is the first documented account of parasitic egg ejection by Warbling Vireos east of the Great Lakes region.