Bay Journal

Brown-headed cowbird follows her instincts even if they aren’t maternal

  • By Michael Burke on November 08, 2013
A female brown-headed cowbird lays up to 36 eggs a year — in nests of other species.  (Lee Karney / U.S. Fish & Wildlife Service)

Standing on the dirt road 100 feet in front of us was a plain brown bird. It was slightly smaller than a robin and the wings seemed to be a bit darker than the body. The bill was stout, almost finch-like.

The late autumn sun was sliding farther below the equator, leaving us with shorter days and premature dusk. It was not yet 5 p.m., and the weak light made it difficult to see any subtle markings that may have aided in identifying the bird. Binoculars helped a bit. We could see a pale eye stripe and throat patch. The breast was a streaked light brown.

As a rule, birders love birds. The female we were watching may be the exception. It’s not just her Plain Jane appearance. The brown-headed cowbird’s (Molothrus ater) behavior is what makes it a bird that’s hard to love.

The male cowbird looks like a different species, with a milk-chocolate head atop an iridescent black body. Cowbirds belong to the Icterid family, which includes blackbirds, orioles and meadowlarks. They are about 7 inches from beak to tail and weigh an ounce and a half. Cowbirds eat seeds and insects, like many other species. So, what’s not to like?

Brown-headed cowbirds are brood parasites, the only ones in North America. They build no nests and incubate no young. Nevertheless, their expanding population is now estimated at 56 million birds.

Females manage this reproductive magic by laying their eggs in the nests of other birds, which end up caring for the orphaned cowbird. Almost any nest will do.

Ornithologists report that cowbirds have laid eggs in at least 220 different species’ nests. The range of nests includes the ridiculous (e.g., a ruby throated hummingbird) to the commonplace (e.g., vireos and warblers). Many of these efforts

fail, but the rate of success is surprisingly high.

When the female cowbird spots an active but temporarily vacant nest, she zooms in. She usually pushes one of the host’s eggs out of the nest before depositing her own. That’s it for her maternal duties, and off she goes, never to return.

Cowbird eggs are typically larger than the other eggs and often differently colored. Nevertheless, most host birds don’t seem to notice the difference. Cowbirds have a quick incubation period, so they tend to hatch first. Parents take to feeding

the intruder right away, giving it a crucial head start.

The aggressive young cowbird outcompetes its fellow chicks with brutal efficiency. Often, the bird will push one or more eggs out of the nest. Its bigger size and bullying behavior result in the cowbird getting the majority of food brought to the nest by parents. Many host chicks die of starvation during this critical time.

Cowbirds have successfully completed this coerced adoption with 144 different species.

These birds are following a successful adaptation that has helped assure their reproductive success. That’s cold comfort to the birder who finds a favorite songbird’s nest overrun by this large intruder.

Scientists speculate that cowbirds evolved alongside the great herds of bison that blanketed much of the Central Plains of North America. Cowbirds couldn’t stop for a few weeks to lay eggs and rear young. They needed to stay with the constantly moving herd and the seeds, a vital food source, left in the bisons’ wake. Female cowbirds put all of their reproductive energy into laying eggs in other species’ nests. A female brown-headed cowbird can lay up to 36 eggs a year.

The species is promiscuous, with birds freely mating with multiple partners during the breeding season.

The loss of our great bison herds would seem to remove the pressure to parasitize nests. Without a counterbalancing force to make nest construction and brooding more successful than parasitism, though, the ingrained behavior remains intact.

Today, cowbirds can be found year-round from the Chesapeake Basin to southern California and down into Mexico. In the spring, breeding birds fan out across the lower 48 states and up into most Canadian provinces.

They can be found where cattle graze, as well as in meadows, agricultural fields, parks and suburban lots. Any place with an ample supply of open fields and their essential seeds will do. The forest fragmentation that has occurred with suburban developments over the last several decades has opened up lots of new habitat for these adaptive creatures.

Biological imperatives don’t always neatly align with human value systems. I call cowbirds “bullies.” In truth, the birds are simply doing their best to survive and thrive. Ill will doesn’t motivate these creatures, biology does.

I should withhold my harsh judgment of brown-headed cowbirds. I would do better to use the opportunity to evaluate the role of biology in my own life. An enlightened view is likely to be more interesting — and complicated — than the dim light that quick judgments often yield.

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About Michael Burke

Mike Burke is an amateur naturalist who lives in Cheverly, MD.

Read more articles by Michael Burke

Comments

Mike Schaffer on November 13, 2013:

I have never liked cow birds because of their parasitic behavior. Now I really don't like them after I learned that they lay as many as 36 eggs a year. Even if a quarter of their eggs hatch, that is a lot of new cow birds added each year. I wonder what their mortality rate is? What is going to happen when they breed themselves out of other nests to put their eggs in? I guess if the other species fail (become extinct) they will too. Excellent article Mike.


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