Now that bluebird nesting season is here once again, be on the lookout for Brown-headed Cowbirds. Brown-headed Cowbirds are parasitic birds that do not build their own nests. They lay their eggs in nests of other bird species. Usually one or two Cowbird eggs are deposited per nest. If undetected by the parents of the host species, the Cowbird egg hatches and the cowbird nestling is raised along with the young of the host species. The female Cowbird waits until the some of the eggs of the host species are laid and then surreptitiously lays her egg in the same nest sometimes removing a host species egg to make room for hers. Female Cowbirds are very prolific and can lay as many as 36 eggs in a season.
The host species parents may notice the Cowbird egg and react in different ways. Sometimes the nest is abandoned. Other times, the Cowbird egg is buried under nesting material. In some cases, the Cowbird egg is removed from the nest. Some host species may even eject the Cowbird nestling from the nest.
Brown-headed Cowbirds periodically check their eggs and young after they have deposited them. Removal of the parasitic egg may trigger a retaliatory response from the Cowbirds such as ransacking the host nest or complete destruction of the nest to force the host species to build a new nest and start over. The female Cowbird will observe the nest-building activity and deposit her egg in the new nest at the appropriate time.
You may have difficulty in spotting female Brown-headed Cowbirds. However, the male Brown-headed Cowbird is quite distinctive and easy to identify. Remember if you see a male Brown-headed Cowbird, the female is close by.
If you spot Brown-headed Cowbirds in your yard, keep a close watch on all of your nest boxes. Contact your state Wildlife Service if you see a Cowbird egg in one of your nest boxes.