Black-capped Vireos (Vireo atricapilla) and White-eyed Vireos (Vireo griseus) are closely related and ecologically similar. Despite these similarities, the White-eyed Vireo is widely distributed and common, whereas the Black-capped Vireo has a restricted breeding range and is currently federally endangered in the United States and Mexico. Here we address this apparent paradox with a comparative ecological study of co-occurring Black-capped and White-eyed vireos. We studied vireos in shrublands and woodlands in central Texas, USA, in 2013 and 2014. We used point count surveys (n = 256) and nest monitoring (n = 144) to determine arrival dates, settlement patterns, nest site selection, and nest survival relative to temporal and habitat factors. Additionally, we conducted reciprocal playback trials (n = 16) to test for the presence of interspecific aggression. White-eyed Vireos arrived first and established territories in both habitats with equal probability. Black-capped Vireos arrived after White-eyed Vireos and settled in greater numbers in shrubland habitat. White-eyed Vireos initiated nests earlier than Black-capped Vireos and selected nest sites surrounded by taller, more mature, and more densely wooded vegetation. For both species, nest survival declined as the season progressed and was greatest in tall, mature vegetation. Lower nest success in Black-capped Vireos (27% vs. 40%), although equivocal, supports earlier findings and suggests that the flexibility demonstrated by White-eyed Vireos confers a reproductive advantage. Playback trials failed to detect evidence of interspecific aggression, suggesting that competition with White-eyed Vireos is not limiting the Black-capped Vireo.

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