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Capitol Hill Aviary | Deadbeat Hill birds abandon chicks

It’s breeding season, so most bird species around Capitol Hill are busy building nests, brooding eggs, or rearing chicks. But one Hill resident, the brown-headed cowbird, doesn’t do any of those things. Instead, cowbirds lay eggs in other birds’ nests and abandon them. Biologists call this behavior brood parasitism.

Male brown-headed cowbirds have shiny black bodies and dull brown heads. Females are dull gray-brown all over. Cowbirds look a bit like red-winged blackbirds, but they have shorter, more wedge-shaped bills. Male cowbirds lack the male blackbirds’ red wing patch, and the females are solid in color, unlike the streaky blackbird females.

Cowbirds are not the only birds that practice brood parasitism. Females of many other species, including mallards and cliff swallows, occasionally dump an egg in a nest belonging to a neighbor of their own species. Biologists think of this behavior as a sort of genetic insurance policy: if a female’s own nest gets destroyed, there’s a chance one of her offspring will survive.

Brown-headed cowbirds, in contrast, cannot build nests, incubate eggs, or rear chicks on their own. A female may lay up to 40 eggs in a season (far more than most other species), each of which she leaves in a different nest. Though individual female cowbirds tend to favor a single host species, cowbirds as a group are known to have left eggs in the nests of 220 different kinds of birds. At least 144 of these species have successfully reared cowbird chicks.


Brown-Headed Cowbird, originally uploaded by kptrease.

Some host species don’t seem to recognize the difference between cowbird eggs and their own. Others do, and they sometimes peck or eject the egg that doesn’t belong. But this is a dangerous prospect. They can damage their own eggs in the effort, and even if they don’t, female cowbirds sometimes return and destroy the whole nest if they find their egg missing.

For host birds as large as cowbirds or larger, the fledgling cowbird’s presence in the nest is not devastating—unless the female cowbird removes or damages the other eggs, which does sometimes happen. Even so, large birds sometimes successfully rear their own chicks alongside a cowbird. In contrast, smaller host birds usually lose their own broods to starvation as they hustle to feed a large, aggressive cowbird baby.

Historically, cowbirds lived mainly in the Great Plains, where they followed the buffalo and fed on insects in the grasses disturbed by the herds’ movements. Today cowbirds tend to live on the edges of any field where people raise cattle, grow grass, or cut down forests.

Cowbird parasitism can exacerbate the effects of habitat destruction on a variety of endangered and threatened songbird species. But here on the Hill, the cowbirds’ main host species are probably red-winged blackbirds and song sparrows, both of which have healthy, abundant populations.

If you’d like to see (or perhaps shake your fist at) a brown-headed cowbird on the Hill, look for them in grassy fields near flocks of blackbirds. They sometimes hang out in the meadows around the Washington Park Arboretum or Portage Bay.

Interested in Learning More?

    • Check out the brown-headed cowbird’s page at the Cornell Lab of Ornithology.
    • You can hear the brown-headed cowbird’s song, call, chatter, and flight whistles here.
    • This video shows a female brown-headed cowbird depositing an egg in the nest of a northern cardinal. Note how she removes one of the cardinal eggs and pecks at the other, possibly damaging it.

More Capitol Hill Aviary

Melissa Koosmann is a freelance writer and resident of Capitol Hill. She writes about education, culture, and nature — and, sometimes, birds for CHS.

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5 Comments
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Jacob Mendenhall
Jacob Mendenhall
10 years ago

This article is for the birds.

Melissa K
Melissa K
10 years ago

The picture on this post is a mistake. It’s some kind of sparrow instead of a cowbird. We’ll fix it as soon as we can.

Andreas
10 years ago
Reply to  Melissa K

Based on a google image search, it looks like that could easily be a female and/or juvenile brown-headed cowbird.

Earnest_Tee_Bass
Earnest_Tee_Bass
10 years ago

The closest egg looks like it has writing on it. Chinese? May be something to design a tattoo from.

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[…] are probably too small to successfully rear the most common brood parasite in the neighborhood, the brown-headed cowbird. There are a handful of recorded instances of cowbird eggs being found in kinglet nests, but none […]