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Why Do Cowbirds Lay Eggs In Other Birds’ Nests?
Posted on Thursday, May 17, 2012 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. »



This hypothesis is BS (pun intended). After a herd of bison passed by, there would be abundant BS on the ground for months of feeding by cowbirds. They could raise their young and then catch up with the herd later.

Posted by Terry L Todd on 5/18

SO how to cowbirds know they are cowbirds and how do they find other cowbirds?

Posted by Katbird on 5/18

Bull is right!
Cowbirds are NOT hatched near Cowbirds, they are NOT raised as Cowbirds, they are NOT fed by other Cowbirds, they are NOT taught to fly nor find food by Cowbirds, so… How in He_ _, did these birds, “decide to leave their young in the care of other birds, an arrangement that seemed to work, at least from the buffalo birds’ perspective.” WHAT perspective? They were reared by birds NOT Cowbirds! How could they possibly have a ‘perspective’ as a Cowbird?
Leave it to an evolutionist to invent so much non-sense!

Posted by Dee Barker on 5/18

I agree w/Dee Barker. If you haven’t known that there is proof that nothing just evolved, I pray you’ll find the Lord. HE is the one that programmed how birds [& everything else] was going to nest etc and live. Evolution is being proved wrong every day. look around & listen is my advice…

Posted by Helen on 5/18

Have no idea why some of you have taken off the subject of cowbirds to discuss religion.  I’d still like to know how birds raised as warblers find out they are cowbirds.  Evolution is beside the point.

When I click on more about cowbirds I get a grackle…

Posted by Katbird on 5/19

Why don’t they just tell us the truth? the cowbirds don’t want to take the time to build a nest, all the rest of that is BS its just someone’s best guess, they goto collage to learn and this is what they are trying to teach all the others.

Posted by Warren on 5/19

If you don’t want to take advantage of the scholarly studies that have been done on nest and brood parasites, why do you bother reading this website?

Posted by Katbird on 5/19

Link fixed— thanks for pointing that out!

eNature Staff

Posted by eNature Admin on 5/19

I agree with Katbird.  I’m aware of the many studies that have investigated cowbird behavior and have, unfortunately got to watch cardinals raise cowbird young and I’ve always been curious how the young cowbirds eventually start to behave as adult cowbirds.

Posted by Sandi Wheeler on 5/19

I find your info great!

Posted by dorothy greaney on 5/19

Thank you.  I accidentally clicked on the line that said I did not want notifications, so this is just a correction.

Posted by Katbird on 5/19

I have NEVER seen vitriol on this site before now.  What is so infuriating about the habits of cowbirds, or rather the discription of them?  And what on earth has college got to do with it?

I would have thought the evolutionary advantage of getting your genes out there and letting some other bird feed them would be obvious, especially if they displace the other birds.  By the way, I don’t see that they decrease biodiversity by this habit, however successful it may be.  The birds whose nests are parasitized are numerous throughout the woodlands, but the cowbirds only occur near the edges, where woodlands merges into grasslands. 

Anyhow, why be so judgmental?  Nature is how it is, and we have the privilige and good luck of being smart enough to study it.  It would be interesting, however, to find out how they “learn to be cowbirds”.  Has any work been done on this?

Posted by Barbara Bernhardt on 5/19

Warren+Dee Baker…this is a very nice ,easy going website.Where in your education(?)did you
learn to reduce a conversation with such petty analysis(?)?Learn some lesson inlife and be a little more civil in your expression.AND,they in fact could be correct!!

Posted by John D Hudson on 5/19

@katbird: I do not view this information as a scholarly study. It is merely speculation based on an assumption that the cowbirds developed this behavior for survival purposes. The assumption may not be warranted.

Posted by Terry L Todd on 5/20

It obviously has something to do with their genetic makeup.

Posted by Vicki on 5/20

Maybe it’s not a learned behaviour, but a genetic trait that they are born with?

Posted by Victoria on 5/20

It’s genetic.  They don’t have to learn to be cowbirds, it’s instinctive.  They are born knowing how to be cowbirds.

Posted by Tom on 5/21

Yet many birds learn their song and so forth from parents.  So when black birds gather in late summer, they instinctively go with them (and find each other before the bison herds move on in spring)?  Instinct must keep them from leaving with the warblers rather than the grackles.

Posted by Katbird on 5/21

Same bizarre comments as last year. I’m not sure I understand getting offended and hating a bird because of it’s nature. If there’s issues with biodiversity, it’s probably because other species are also under too much stress from habitat loss and other problems, and therefore can’t keep up with the Cowbirds.

Posted by Taryn on 5/21

Thank you Vicki and Tom for pointing out the obvious about how Cowbirds know who they are.  It IS instinctive.  I’m no expert, but it’s sort of the same way Monarchs who are born here know instinctively where to go during their fall migration.  Mother Nature is amazing.

Posted by Donna on 5/25

I hate these birds; last year I was putting out black sunflowere seeds to support a family of Cardinals…the only ones on our ranch, and I found a few weeks later that their little ones are dead and there were Cowbirds being raised by the Cardinals.  With that, I set up the traps and paid my kids to shoot them all.  Last year we were able to exterminate about 150 and this year we have gotten rid of about 75 so far…and the cardinals actually were able to raise their own this year.  I know the tree huggers will be crying, but let ‘em!  Hopefully there won’t be any to kill next year!

Posted by Troy on 6/1

Isn’t that illegal? They’re protected under the Migratory Bird Treaty.

Posted by Vicki on 6/1

Actually, Brown-headed cowbirds, house sparrows, and European Starlings aren’ t protected under the Migratory Bird Treaty Act .  At least in the state of Texas though you are supposed to go through a training course before you use a cowbird trap to control them.

Posted by Sandi Wheeler on 6/1

No matter how they find each other (instinct?), I saw a brown-headed cowbird male out courting a female today, June 1. He was strutting after her and raising the feathers on his neck- or so it appeared to me.  I wondered if they were starting late or early- sure not out following the buffalo.

Posted by Katbird on 6/1

I don’t know what state that person lives in, but they are protected under the migratory bird treaty. Brown headed cowbirds can be trapped by wildlife officials who will then relocate them. But he shouldn’t be shooting them or trapping them, unless he has a permit to trap them, but other then that I’m pretty sure it’s illegal.

Posted by Vicki on 6/1

Yes, they are protected by the Migratory Bird Act, but are a threat to some endangered species of songbirds, so controlled trapping programs, such as the one in Texas are allowed (not shooting at random).

It is illegal to kill protected species without special permits and, as in the case of Texas, special training.

Posted by Katbird on 6/1

To Troy on 6/1:
So sad you fill yourself with hate when you could be enjoying ALL of God’s creatures, not just some of them.  Do you know why He made Cowbirds?  Neither do I.  But saddest of all, you are teaching hate to your childrenc, crowding out the space for love in their hearts.

Posted by Barbara Bernhardt on 6/2

Whoa- I don’t hate any of God’s creatures!

Posted by Katbird on 6/2

I’m not filled with hate, I love life and almost everything about it, and don’t bring God into this, you don’t want to go there.  Have you ever been bitten by a mosquito and cursed it, or worse yet, swatted it and killed one?  Have you ever sprayed poison to kill ants, spiders or other insects?  I’m sure you have and yet they are God’s creatures.  What’s the difference between those and birds?  I’m sure someone will come up with some clever answer to justify their point of view, but if I have to kill 100 cowbirds to enable one family of cardinals or other birds to survive and raise a family, then it’s worth it.  And I shouldn’t say I hate Cowbirds, poor choice of words, I just hate what they do.  If they built their own nests and raised their own, I’d have no issues with them.  Thanks for all the opinions out there, love diversity:-)

Posted by Troy on 6/3

To Katbird:  My comment was addressed to Troy.
To Troy:  I was just responding to your words.  If you didn’t mean them, good.  I do spray things.  Big paper wasps’ nests, when they are built near where my grandchildren play.  But I spray them with regret, not curses. Also cockroaches, termites and poison ivy when I lived where they do.  I vaccinated my children against bacteria and viruses that could have harmed them, and I use mouthwash and brush my teeth.  I think we should all realize that only plants make their own food.  All the rest of us take what they make, one way or another.  Of course we all like some animals better than others.  But that’s not a reason to exterminate cowbirds.  After all cardinals murder many, many baby plants by eating them in the seed stage.  How ridiculous does that sound?

Posted by Barbara Bernhardt on 6/3
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