Photo by Herb Amex (SC)
The cowbird has quite the interesting genus, but today I am going to write about one species in particular, Molothrus ater. Commonly known as the Brown-headed Cowbird, this life of this little winged bastard is quite the interesting and sordid tale of lies, deceit, and outright fratricide.

The more you know about Molothrus ater, the more you feel like they are some sort of crazy animal villain in the story of life above the treeline. They have all of the traits of a Dickensian thug, and while you do have to admire their methods, you have to wonder if the world would be better off if they ended up like old Bill Sikes. It is tough as a biologist to think objectively about species like the Brown-headed cowbird. Being Brood parasites, they have evolved a spectacularly effective if not despicable means of reproduction. Let’s dig in, shall we.

What is a Brood Parasite species?
Brown-headed cowbirds don’t usually get a lot of press, they can thank their more well known brood-parasitic compatriots, the Common Cuckoo Cuculus canorus. Brood Parasite bird species have evolved a method of reproduction and growth that greatly minimizes the parent’s investment in their offspring. Sounds great right? How do you reproduce, lay an egg, and have a little bird without doing any of the hard work? Easy. Have some other sucker do it all for you. Brood Parasites have developed the cunning scheme of laying their eggs in the nests of other birds. When their little chicks hatch, the host birds are none the wiser and feed the little alien nest-guest as one of their own. The little chick often will eat so much that the other birds, the true offspring of the hosts, die of starvation. Little Brown-headed Cowbird chicks have even been known to give their pseudo-siblings a little push out of the nest as well. Get rid of the competition, genius! Cowbird eggs have been found in hundreds of bird species nests throughout the U.S. Even in the nests of some large birds of prey. Talk about the deception of the century! This is one dirty, nasty, and mean way to raise your kids. Have you seen “The Good Son”, yeah, scary.

Image courtesy of

Sticks out like a sore thumb

Why don’t the host birds just kick out the intruding chick?
Well, the short answer is, most of them do just that, but a few don’t seem to notice the egg that can be significantly different in size and shape of their own. Cowbirds sometimes even remove an egg from the host nest before they lay their own, to throw off any suspicion over the change in number of eggs. I don’t think there is a generally agreed upon reason why some bird species don’t notice the alien egg in their nest, but I would think that most birds don’t even give it a thought. Why would they? It is their nest, those eggs must be theirs, and it would be crazy to think otherwise. Some species seem to notice the eggs though, and dispose of the intruder in a myriad of ways. As you can see in the photo above cowbird eggs are easy to pick out from other species in many cases, that is because the brown-headed cowbird is bad at what they do. The cuckoo has evolved a very unique way of dealing with the problem of their eggs getting spotted by their hosts. Over time the cuckcoo species have developed different “gentes”, which is to say their females can specialize in laying particular eggs that mimic the color and patterns of their host animals. Certain female cuckoos can mimic a particular species of bird egg, to blend in with the host’s own brood. It is very genius and quite diabolical. Cowbirds cannot mimic host eggs, and they employ a “shotgun approach” to egg laying. Cowbirds can lay many (30+) eggs in one season, spreading their eggs amongst many nests and species of birds. If they lose a couple of eggs to some smarter host birds, no big deal, there are plenty others. They absolutely do not “Keep their eggs all in one basket”. (I apologize profusely for that line)

You are not fooling anyone, you intruding giant

That sounds crazy, why did this happen?
One of the hardest parts of being a wild animal is caring for your young. It takes time, energy, and it is dangerous, every time you leave the nest you are in danger from many predators. It makes biological sense to raise your young this way, you get to pass on your genes and not deal with the trouble of raising your young. Quite an argument for the nature side of the whole nature/nurture debate. Evolution can be quite a scary thing.

I get what you are saying, but what is wrong with this?
Nothing really. These birds have developed a novel way of dealing with the problem of chick-rearing. While this is fine and dandy for the cowbird, it really can take a toll on the species of other birds around them. These birds are native to the U.S. and have been around for centuries. The cowbird did, however, benefit greatly from the settlement of the Midwest. Before the Europeans moved in and created roads and farmlands, the cowbirds were relegated to the edges of the vast forests. The Brown-headed Cowbird is a species that sticks to the edge of the tree line, this is to better scout out host nests and stay close to the insect cornucopia of the open prairie. Now that the midwest has been settled, there are far more edges in the forests. Roads, fences, lawns, farms, these are all man-made fields for the cowbird to settle in. We are making their job of finding hosts easier, and this is not good for the other birds. In some areas of the country other species of birds are becoming endangered due to the increase in cowbird activity.

Photo by Lee Duer

2 males, plotting...

Surely there isn’t anything else terrible that these birds can do?
Well… sorry to burst your bubble, but yeah the Brown-headed Cowbird has one more trick up its sleeve. Remember those birds that discover the intruder egg laid by the cowbird, and dispose of it? Cowbirds don’t like that very much, and in some cases have been known to raid the nest of the host that destroyed their egg. They can destroy host’s other eggs and even the nest itself. Why would they do this? Is it purely out of spite? Surely these birds aren’t just watching the poor host birds return to an empty nest, laughing and twisting their little bird mustaches… There is a good reason to destroy those other eggs and host nest. When the host bird makes a new nest or lays a new clutch of eggs, they can put another of their eggs in with the new batch, and try again.

Hi, I am an asshole!

What can I do to help those poor host birds?
Nothing really. Sorry. I have seen some people take up arms against the little buggers, pop em with a BB gun or other air-powered rifle. I don’t condone this level of violence, but if there were any bird that deserves a couple of pot-shots from a Red Rider, it is certainly the Brown-headed Cowbird. Not only do they mess up the other bird species, they are also not even that good at being a brood parasite. The Common Cuckoo is a much better cuckold. So, when you see a cowbird, at least give them a dirty look. They know what they did.

A Song “Rid of you”(To the tune of “Close to you” by the Carpenters)
Why do cow-birds suddenly appear,
and lay their eggs around midyear,
just like me, they long to be
rid of you….

They lay their spawn in that nest in the sky,
causing their adopted siblings to die,
just like me, they long to be
rid of you….

On the day they were born their host mother gathered them food,
And decided to care for them as one of their own,
little did they know this parasitic brood,
would leech of them till they’re full grown….

That is why all the birds in town
hate you…
all around…

Just like me, they long to be…
rid of you….

Thank you and goodnight.