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Brown-headed cowbirds leave their eggs in other species’ nests

A brown-headed cowbird laid the speckled egg in this blue grosbeak nest. The cowbird chick will push out or out-compete the chicks of the host bird.
A brown-headed cowbird laid the speckled egg in this blue grosbeak nest. The cowbird chick will push out or out-compete the chicks of the host bird.

Several people reported a strange sight at their feeders this past week. In all cases, a small bird was busily feeding a much larger begging bird. The smaller bird was indeed the parent. Unfortunately, the begging bird was the progeny of a brood parasite, a brown-headed cowbird.

Brown-headed cowbirds lay their eggs in other birds’ nests, leaving the hard work of raising their chicks to the unknowing hosts. In almost all cases the cowbirds pick on species that are much smaller or less aggressive than they are. I’ve seen Carolina wrens, house finches and song sparrows feeding fledgling cowbirds. The photo today is of a blue grosbeak nest with a cowbird egg. The cowbird egg is the speckled one.

The strategy of a female cowbird laying an egg in another species’ nest is devastatingly effective. Young cowbirds see to it that all competition for food and host parent’s attention is eliminated. The cowbird usually hatches first, enabling the chick to roll out the other species’ eggs. If all the eggs hatch at the same time, the cowbird will attempt to roll the other chicks out. Even if the other chicks remain in the nest, the cowbird, by its larger size and aggressive nature, is able to out-compete for all of the food brought to the nest. Even more efficiently, the adult cowbirds lay only one of their eggs in a nest so their offspring do not compete with themselves. You will never see a host species feeding its own brood plus the cowbird. Only the cowbird will survive.

In winter, brown-headed cowbirds can form huge flocks of hundreds to thousands of individuals. Each of those adult birds represents a failed nesting of another species.

Species most vulnerable to the ways of the brown-headed cowbird have been birds of open country and field edge. Cowbirds do not go deep into forest to parasitize nests, so woodland species were safe. As land has been cleared and forests have been fragmented, cowbirds have been able to affect species that haven’t learned how to deal with the threat they pose. It has been theorized that declines in the wood thrush population can be at least partly attributed to brown-headed cowbird parasitism. Their impact on some species, such as the endangered Kirtlands warbler in Michigan, has even necessitated a brown-headed cowbird control program, which has proven to be successful..

Taylor Piephoff is a naturalist with an interest in the birds and wildlife of the southern Piedmont: PiephoffT@aol.com.

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