Cowbirds hatched a survival plan

  • Updated: July 30, 2013 - 3:59 PM

When their bison-filled habitat changed, cowbirds solved their food shortage by foisting their eggs on other birds.

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This pair of brown-headed cowbirds won’t tend their own young, instead laying eggs in another species’ nest.

Photo: Photos by Jim Williams • Special to the Star Tribune,

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Brown-headed cowbirds are plain Janes in the birding world, brown on brown.

They don’t look like much, but cowbirds are nonetheless impressive. They found a clever solution to a serious problem.

Once known as buffalo birds, flocks of cowbirds followed the huge herds of bison long since gone. The birds ate insects disturbed by bison feet, and grain found in the prairies.

But the grocery store kept walking away. So cowbirds became brood parasites, laying their eggs in the nest of other birds. In fact, cowbirds have been called North America’s most successful brood parasites. (Cuckoos do it, too.)

Lay eggs in an available nest. Let someone else raise the baby while we move on. Simple as that.

While the cowbird has solved its problem, in turn it has left unsolved problems, particularly for the host birds. We’re one of them.

We took away the buffalo but substituted other herbivores. From day one we have altered the landscape to widely extend suitable cowbird habitat. Cowbirds now have access to the nests of bird species previously unknown to them.

On the prairies, other grassland birds co-evolved with cowbirds, evening their survival odds. Today, cowbirds are known to use the nests of more than 200 North American bird species. Most of them play a defensive game of catch-up.

Cowbirds perch and watch and wait. When a nest is located and its owner is out for a moment, the cowbird swoops down to lay an egg.

Because it doesn’t incubate eggs, a female cowbird can lay them by the dozen, like a chicken. A hen cowbird can produce up to 40 eggs a season.

Overkill? Not quite. Host birds, in many cases, have learned to recognize cowbird eggs, almost always of a different color and size than those of the nest owner. Cowbird eggs are rolled out of the nest, they’re punctured, they’re buried beneath a nest rebuilt.

One researcher found a yellow warbler nest that had been rebuilt six times, seven layers of nest containing 11 cowbird eggs.

The edge still goes to the cowbird. Its eggs need a shorter incubation period than most host birds. Cowbirds hatch first, almost always larger than the host’s hatchlings. Young cowbirds eat more. They can monopolize food delivery, starving host hatchlings. Cowbirds will push out nest mates.

In two years, a female cowbird might produce 80 eggs. Because host birds are learning to combat this intrusion, and because nature always takes its toll, only 3 percent of those 80 eggs are likely to become adult birds.

That comes to 2.4 birds raised per cowbird pair. Two is the replacement number. The annual fraction is equal to one additional cowbird every two years. Cowbird numbers grow. Numerous and invasive beyond their historic habitat, they’re a problem for other species, reducing nesting success.

In the case of Kirtland’s warbler, a species found only in Michigan and Wisconsin, the issue is critical. Kirtland’s has highly specific jack pine habitat needs. Loss of habitat combined with cowbird parasitism threatens this species. Cowbirds in those nesting areas are trapped and eliminated.

Other host species are not so fortunate. They fight on their own, many of them slowly learning to live successfully with cowbirds, a species that millions of years ago started solving its own survival problem.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at www.startribune.com/wingnut.



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