Brown-headed cowbirds used to follow the huge buffalo herds on their migrations across the North American prairies. Because the herds continually moved, the cowbirds were not in one place long enough to build their own nests, incubate eggs, and raise young. So, they laid their eggs in the nests of other birds.
Now that the herds of buffalo are gone, cowbirds have adapted to a less transient lifestyle, though they still lay their eggs in the nests of other birds.
A female brown-headed cowbird will watch potential host birds, such as song sparrows, chipping sparrows, phoebes, yellow and chest-sided warblers, and cardinals, as they build nests and begin laying their eggs. In the middle of the hostís egg laying period, the female cowbird will sneak into the nest late in the afternoon, remove a hostís egg, and then replace it with one of her own at dawn the following morning. Some hosts are hit more than once by cowbirds removing eggs and replacing eggs of their own.
The host birds, apparently unaware that one or more of their youngsters is an alien cowbird chick, will continue the rearing process. Sometimes, the cowbird chick grows faster than the hostís own youngsters, and the larger chick may push the smaller chicks out of the nest.
In the end, the host parents feed the baby cowbirds as if they were their own, and will protect them against predators until they are large enough to be on their own.
George H. Harrison