Parasitic Egg Ejection by Vireo gilvus (Warbling Vireo) in New England

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.

Automated summary

The Warbling Vireo, the smallest bird species in North America, has been found to eject Brown-headed Cowbird eggs from their nests. The geographic differences in behavior may be related to the distribution of two subspecies, and there is currently a lack of experimental studies on egg ejection in eastern populations.