Brown-headed Cowbirds

Brown-headed Cowbird


Carolyn Gritzmaker Indian Trail Master Naturalist

The cardinals in our back yard are hard at work feeding their second brood of the season. One of the young birds they are feeding is not a cardinal at all, but a brown-headed cowbird(Molothrus ater). This year, as in years past, they are raising one cowbird along with their own young.

When other birds are busy building their nests, the female brown-headed cowbird sits quietly by and watches them work. She makes no nest of her own, instead laying her eggs in the nests of other birds and relying on them to raise her young.

By watching the other species within her breeding territory, she locates several nests and will check on them periodically when the other bird is gone. In this way she knows in advance where she will lay her eggs, and usually times that to coincide with the host bird. Early in the morning when the host is out she will sneak into the nest and lay an egg, generally only one egg in each nest. If there are two or more eggs of the host in the nest, the cowbird will probably remove one.  It is not known for certain, but is thought that she will lay 10 or 12 eggs per year. Some sources put this number as high as 40 eggs, but because of the host species’ defenses and predators, only one or two survive to become an adult.

More than 200 species of birds have been parasitized by the cowbird. Some of these hosts will accept the egg and nestling as their own, while others will not. Robins, for instance, will throw the strange egg out. Others, such as the yellow-breasted chat, will usually desert a nest if a cowbird egg is laid in it. Tanagers and flycatchers have been known to bury the egg in the bottom of the nest so it will not hatch. Some warblers will actually build another nest on top of the first nest which contains a cowbird egg.

Many believe that if a cowbird is in a nest, none of the brood of the host will survive. And indeed, parasitism by the brown-headed cowbird has had serious effects on the populations of some species. In our area, the Bell’s vireo has declined considerably due to this. This is not always the case, though. Careful studies have shown that nests in which cowbirds are raised successfully generally also produce successful fledglings of the host, though usually at the cost of one of their own.

So once the cowbirds have fledged, why don’t they identify with the host that raised them? How do they know they are cowbirds, and to associate with others of their species? I don’t know.

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