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Why Do Cowbirds Leave Their Eggs In Other Birds’ Nests?
Posted on Tuesday, April 30, 2013 by eNature
Eastern Phoebe nest with Brown-headed Cowbird egg
Eastern Phoebe nest with Brown-headed Cowbird egg
© Galawebdesign

After reading a recent blog entry about brood parasitism in cliff swallows, a number of readers wanted to know more about the well-known sort of brood parasitism practiced by cowbirds.

If a vote were taken tomorrow to find the most popular bird in the country, it’s doubtful that the cowbird would win. That’s because the cowbird has the nasty habit of laying its eggs in other birds’ nests.

It all started back in the days when American Bison ruled the Great Plains. At that time a rather nondescript grayish or black bird followed the herds around, feeding on seeds in the abundant supply of buffalo excrement. Settlers on the plains came to calling these animals buffalo birds.

But since the birds depended on wandering herds for food, they needed to wander as well if they wanted to survive.

The problem, of course, is that wandering birds can’t tend their nests. So the buffalo birds decided to leave their young in the care of other birds, an arrangement that seemed to work, at least from the buffalo birds’ perspective.

Then, during the 1800s, the prairies and buffalo disappeared, replaced by pasture and cattle. But the birds remained and started keeping company with cows instead of buffalo, eating insects in the grass, ticks on the livestock, and seeds and grain. The buffalo bird eventually became known as the cowbird.

Today there are two native cowbird species in North America, the Bronzed Cowbird of the Southwest and the Brown-headed Cowbird common in most of the United States and Canada. Both species still lay their eggs in the nests of other birds, which is to say that both maintain the parasitic tradition of their ancestors, much to the dismay of bird lovers and conservationists.

The main reason people find the cowbirds’ behavior objectionable is that it threatens biodiversity.

Cowbirds as a whole lay their eggs in the nests of more than 200 other species of birds. And in most cases, because these birds tend to be smaller species, the young cowbirds come to dominate the nests, pushing out the other young or hoarding the food. The result is that the two cowbird species thrive at the expense of hundreds of others.

More about the Brown-headed Cowbird »

More about the Bronzed Cowbird »

Cliff Swallows practics brood parasitism in a different way. »



don’t hate them too much, they do what they do to survive. It’s not thier fault that that it’s instinctive behavior

Posted by emily on 5/4

We have a newly hatched cowbird (this morning) in a finch nest.  Several people told me to remove the egg before it hatched but after doing research, I found that it is actually illegal to remove the egg since the cowbird is an indigenous species.  You can obtain a permit to remove the eggs in certain regions where the parasitized (host) is threatened but otherwise, leave them alone.  It is natures way.  There were 5 eggs, 1 cowbird and 4 finch.  I plan on letting nature take its course but also documenting as much as I can without disturbing them too much.  It’s also a great teaching tool for my 3 and 6 year olds.

Posted by Mike Dunlap on 5/4

I’m glad to know what this brown-headed black bird is.  Does anyone know where they spend the winter?  I only see them in spring, and I feed my winter birds, so I know they are not year-rounders.  I live in Upstate New York, near the northeast tip of Lake Ontario. seen one or two walking around in spring, but then I don’t see them anymore.  I presume they are out in the woods laying eggs in other birds’ nests.  It’s one of the interesting ways we animals make a living.
Suggestion:  When you show photographs of birds could you show a group of different sizes of species at the same scale so we can see the relative sizes of the ones you show?  This cow bird is pretty big, a little bigger than a robin, but not as big as a dove.  I think it is slightly smaller than a blue jay, but much bigger than a chickadee.

Posted by Barbara Bernhardt on 5/5

I assume cowbirds no longer build nests at all.  Correct?

Posted by SpringerJ on 5/5

Let’s not vilify cowbirds as this seems to be the tone of this article. They do not threaten biodiversity. That’s what people do. Cowbirds are simply playing their role in the ecosystem along with all the other native birds, fish, mammals, reptiles and amphibians are doing. To say they threaten biodiversity is like saying wolves threaten the existence of buffalo herds. We are to small and ignorant to make such comments.

Posted by Nancy Daly on 5/11

I must admit, I really like these birds for some reason.  Something about their attitude just makes me giggle when I see them on my deck socializing with other birds.

Posted by Buggy on 5/11

Buffalo herds were eliminated by Buffalo Bill, not wolves.  Thanks to Nancy Daly for reminding us it wasn’t wolves.
To add to my earlier post about size.  I think they pick smaller birds’ nests because then their chick is the biggest, has the biggest yawning mouth for begging, and is loudest.  That one gets the most food.

Posted by Barbara Bernhardt on 5/12
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