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Cowbirds make other species raise their chicks, neglecting their own

A female brown-headed cowbird will lay her eggs in other birds’ nests.
A female brown-headed cowbird will lay her eggs in other birds’ nests.

It’s an odd sight; a small bird like a Carolina wren, song sparrow, house finch or Eastern phoebe is seen feeding a much larger begging bird. I have seen this a few times lately, and so have some of you. The chick is a brown-headed cowbird, a unique species among our passerine avifauna.

Cowbirds lay their eggs, usually only one per nest, in the nests of other species of birds and accept no responsibility for the rearing of their chicks. Most of the time the parasitized species is much smaller or less aggressive than the cowbird, a key to the success of the cowbird’s reproductive strategy. The cowbird egg hatches first, giving the chick a head start in growth. Already larger than the host chicks, the cowbird then out-competes them for food the host adults bring to the nestlings.

Songbird parents instinctively stuff food down the widest mouth they see. When a gape is much larger than the others, that mouth gets fed. I have never seen a host bird feeding anything more than a single cowbird chick. The host chicks just don’t make it. I have seen estimates that a single female brown-headed cowbird can produce up to 40 eggs in a season.

The cowbird practice of brood parasitism can, and has, had serious implications for other native birds. Cowbirds form huge flocks in the late summer and winter. It is staggering to know that each individual adult brown-headed cowbird represents a failed nesting of another species. Some endangered or threatened species populations have been adversely affected by cowbird parasitism to the point where cowbird trapping programs have been implemented. Indications are the trapping programs have been successful in greatly diminishing the parasitism rate on Kirtland’s warblers in the upper Midwest and black-capped vireos in Texas.

Since brown-headed cowbirds are native species, they are a protected species under the Migratory Bird Act. Only when it is demonstrated that the cowbirds are adversely affecting a threatened or sensitive species are they allowed to be removed.

I write frequently in spring about the calls and songs of various birds I am excited to hear as they arrive for the nesting season. One call that makes me cringe a bit is the high-pitched, clear whistle or dry rattling call of arriving cowbirds to my neighborhood.

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

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