The brown-headed cowbird is never going to win any popularity contests. It will never be chosen as a state bird, a town bird, or even a team mascot. It’s a parasite, a brood parasite.
The female cowbird never builds her own nest, never raises her own “kids” and often destroys the eggs and nestlings of the birds who are actually doing the work of raising her offspring. She just drops her eggs off in the nests of other species of birds and lets the foster moms do all the work! Most birdwatchers and conservationists dislike brown-headed cowbirds immensely for their parasitic behaviors. Biologists and researchers, on the other hand, view them with deep evolutionary admiration.
Brown-headed cowbirds were originally called “buffalo birds” or “bison birds”. In the early to mid-1800s, brown-headed cowbirds followed the bison across the Great Plains and ate the swarms of insects that these great beasts kicked up as they moved in massive numbers across the open grasslands. When European settlers began moving westward, clearing forests and establishing farms, the cowbirds started following the settlers’ cows and other grazing livestock, catching insects flushed out by the moving herds, just as they had with the bison. The early settlers called them “cowbirds”.
As a brood parasite, female brown-headed cowbirds have been known to lay their eggs in the nests of at least 220 different species of birds, including red-winged blackbirds, robins, song sparrows, yellow warblers, and red-eyed vireos. Some of these “host” birds recognize an unfamiliar cowbird egg in their nest, most do not.
The yellow warbler, for instance, is good at recognizing a cowbird egg, but is too little to get rid of it. It solves this dilemma by building a new nest over the old one, hoping the cowbird won’t come back with another egg! But it often does; forcing the warbler to rebuild its nest multiple times over each cowbird egg!
The American robin is also good at recognizing cowbird eggs in its nest and is big enough to throw the unfamiliar object over the edge. Birds that don’t recognize cowbird eggs, like the tiny red-eyed vireo pictured here, will just raise the chick as one of its own, much to the detriment of its own offspring.
Cowbird nestlings are at a distinct advantage as foster birds. For starters, brown-headed cowbird mothers tend to choose the nests of much smaller birds, like yellow warblers, red-eyed vireos, song sparrows, wood thrushes, and common yellow throats, to lay their eggs. The egg they lay, and it’s usually only one per nest, will hatch much sooner than the eggs of the host species which gives the hatchling a head start getting food from the parents. The cowbird nestling will also mature faster and grow bigger than its nest mates. This over-sized, demanding cowbird baby will then monopolize the food brought to the nest by one or both of the parents—leaving host’s chicks vulnerable to starvation.
As a much larger, stronger bird, the cowbird nestling is also capable of getting rid of its competition by either tossing the host’s eggs out of the nest, tossing the other hatchlings out or smothering them at the bottom of the nest! Interestingly, if the host bird tries to prevent all this mayhem in the first place by disposing of the cowbird’s egg as soon as it’s found, the female cowbird parent may retaliate by destroying all the host’s eggs or even its hatchlings. This act of revenge has been dubbed the “cowbird mafia”.
Some bird species have actually learned to make the best of a bad situation by just keeping the cowbird egg in the nest and raising the hatchling as its own. They do this knowing full well that some of their own chicks may not survive, but hedging their bets against losing all of them with an attack by the cowbird mafia if they had rejected the egg in the first place!
It comes as no surprise then, that nest parasitism often lowers the reproductive success of host birds. The brown-headed cowbird has been heavily implicated in the population decline of the Kirtland’s warbler. However, when experimental studies have been conducted where the population of brown-headed cowbirds was strictly controlled, the endangered Kirtland’s warbler population did not increase! Only when cowbird control was combined with habitat management for young Jack Pine forests did the warblers rebound!
Like many species that we now consider pests, the brown-headed cowbird didn’t become a problem until we altered its environment. Habitat fragmentation across the United States from deforestation, farming and urbanization gradually opened up more and more suitable habitat for the brown-headed cowbird and allowed it to expand its range eastward. These birds were once constrained to the Great Plains of central North America but can now be found in great numbers across our entire country.
This is a hardy, resilient bird that has learned to thrive either in spite of or because of all the changes we have made to its habitat over the years. Unfortunately, the cowbird’s success has come at the expense of many other birds.
Every change we make to our environment has an impact. We can’t always know what that impact will be, but it’s critical to evaluate any potential risks before ever moving forward.
We are all connected.
What affects them, affects us.
