For Brown-headed Cowbirds, the Road is No Place to Raise a Family

For nesting birds, this is the busiest time of year: establishing territories, finding a mate, nest building, defending their territory, egg laying, incubating, and feeding and protecting the young until they are on their own. Depending on species, this annual chore lasts anywhere from four weeks to five months.

For the brown-headed cowbird, a bird common in our area, there isn’t anything hard at all about nesting season. They don’t establish a territory, they don’t build a nest, they don’t incubate, and they have other birds feed and raise their young!

How did they pull that off?

The reason lies in the association cowbirds had with the American bison. Bison thrived in the vast grasslands ranging from the Rocky Mountains to the Appalachians with estimates of their historic numbers being in the tens, possibly even hundreds of millions.

When the herds of bison moved across the prairie, the cowbirds fed on the insects kicked up, including the ticks and flies that would otherwise irritate the bison. Because of the cowbirds dependency on the bison for food and the bison dependency on the cowbird as a pest removal service, they refer to these animals as having a mutualistic relationship, a symbiotic relationship that benefits both species. 

Due to their size and massive food requirements the bison would strip an area clean of vegetation in a short time, so they lived a nomadic lifestyle, moving constantly from place to place to find fresh grass. The cowbirds, dependent on the bison for food had no choice but to follow.

Being constantly on the road is not a way to raise a family. And as far as birds go, it’s impossible. You can’t spend six weeks going through the nesting process when your food supply is constantly moving. So the cowbird adapted by laying their eggs in the nests of other birds and letting them do all the work! Sounds like a winning situation for the cowbird!

The problem is that the cowbird may be the only winner. Market hunting all but wiped out the bison in the 1800’s, but non-nomadic domestic livestock took their place. The forests of the eastern United States were opened up for development and that allowed the cowbirds range to expand.

Once the cowbird lays their egg, their egg hatches sooner and the young grow faster. Ultimately the involuntary parents raise a healthy cowbird while its own young die of starvation. Cowbird wins, host bird loses. 

Today the scenario is that cowbirds are in places they never existed a century and a half ago, and with forests fragmented, bird species historically nesting far removed from cowbird parasitism no longer have that luxury. Cowbirds are one of the main causes in the decline and even endangered status of some of our songbirds.

Since I’ve learned the history of the cowbirds, I’ve gone from the “kill them all” mentality to at least a respect for them. Nevertheless, the problems cowbirds are causing are very real and yet another problem that biologists must deal with in trying to conserve our remaining songbird populations.