SciTech

Inside NC Science: Cowbirds are feathered freeloaders

Brown-headed cowbird: These drifters are called brood parasites because they trick other species into raising their offspring.
Brown-headed cowbird: These drifters are called brood parasites because they trick other species into raising their offspring. US Fish and Wildlife

Long ago, I gave up using my alarm clock in the spring. The songbirds wake me far too early, and there is no dial to reset the timing of the dawn chorus. The songbirds are justified, though, because they have a lot to do in a relatively short window of time.

Migratory songbirds have two homes. One in Central or South America for winter, and one in North America for spring and summer. During breeding season, most songbird species defend a territory, build a nest, lay eggs and raise their young, while still leaving enough time to prepare for their journey south.

They aren’t that different from us. Their territory is marked not by a white-picket fence but by their repeated visits to singing posts. Their nest is not an entire house, but just the crib to hold their young. They spend lots of time feeding their children and cleaning up after them. They head south every year after the kids are grown, just like many of us do in retirement.

Of course, not all people choose to establish a home base – and the same is true for birds. There is a nomadic bird species, a drifter that never settles down. Roaming from place to place like Jack Kerouac, these birds never stay put long enough to adopt the traditional avian routine of building a nest, laying eggs, and raising young. Like a beatnik, they fully embrace the life of a vagabond. That’s harmless enough for people (and so we’ll stop the analogy here), but these birds need to reproduce, so how do they do it?

These drifters are called brood parasites because they trick other species into raising their offspring.

The most common one in the Piedmont is the brown-headed cowbird. Cowbirds stalk songbirds to find their nests and patiently, quietly wait until they can lay their eggs without being detected by the unwitting host. Brown-headed cowbirds used to live their carefree days roaming with the buffalo. Now they roam with cows. Modern landscapes are mosaics where forests are interspersed with cow pastures. Consequently, cowbirds are most likely to trick birds nesting on the forest edge, and many songbird nests contain an additional hungry mouth to feed.

Cowbirds are one of many threats faced by songbirds, particularly those living in woods within agricultural landscapes. Given the songbirds’ challenges, I appreciate each day that begins with my neighborhood’s feathered alarm clocks.

Caren Cooper is assistant director of the Biodiversity Research Lab at the N.C. Museum of Natural Sciences.

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