March 13, 2010
March 15 – Molothrus ater
One of these eggs is not like the other…one of these eggs just doesn’t belong. We can see this very obviously, but for some songbirds, not noticing this different egg can have very negative consequences. The different egg is that of a Brown-headed Cowbird, Molothrus ater, a brood parasite. These birds range widely across North America and as adults, they feed primarily on insects, often following grazing animals to eat the insects that they stir up and attract. These brood parasites have been documented using over 200 different species of birds as hosts. Though they’re lazy parents, they are very demanding parasites and check back on their eggs' progress. And, these birds can also demonstrate a very nasty retaliatory behavior. If the host bird recognizes the Cowbird egg and kicks it out, the Cowbird will often trash the host nest, punishing the discriminating host. You can read more about this here.
See photos of the adult birds here.