Once the nest is built, a female vireo lays one egg everyday for 3 to 4 days, but the nest is susceptible to being parasitized by the Brown-headed Cowbird (Molothrus ater). While the adult vireos are away foraging, a female cowbird will fly in and lay one of her own eggs in the vireo nest. Sometimes she will even destroy the eggs or kill any nestlings present. This behavior is the primary reason the Black-capped Vireo is listed as Federally endangered. Although cowbirds are native to the US, their population has exploded in recent years due to land cover changes altered by human activity, including ranching.
This is inside the nest of a Black-capped Vireo. Inside are two eggs. The plain egg at the bottom of the photo is a vireo egg, while the dark speckled egg is that of a Brown-headed Cowbird. The vireo pair abandoned this nest, and their next nest was also parasitized by a cowbird. But this pair of vireos persevered, and had a successful nesting attempt on their third try!
Lat: 31.22, Long: -97.74
Spotted on Apr 26, 2012
Submitted on Aug 7, 2012