Consider the cowbird - Nature’s only true nest parasite
—by Steve Lekwa
A friend recently asked me if he could have seen a flock of hundreds of Brown Headed Cowbirds. Huge flocks of blackbirds are normal in spring and fall, but I couldn’t recall seeing a huge flock of cowbirds before. I checked the site he mentioned and saw only a scattering of starlings on the wires. I happened by the site a day later and there they were; hundreds, perhaps more than a thousand cowbirds in a single dense flock feeding in an area of cut grass.
That flock of cowbirds made me stop and think. Every one of those birds meant that another bird’s nest failed to produce its own young. Cowbirds are brood parasites, meaning that they never rear their own young. Some background reading revealed that cowbirds provide for their young’s survival in a variety of devious and downright amazing ways, though. The scientific name for the cowbird genus is Molothrus. The word goes back to a Greek word for vagabond or tramp. That pretty much describes their original life style of following wandering herds of bison. These herds seldom stayed in one area more than a few days before moving on to fresh grazing. Staying with the herd meant that there was no time to nest and rear young cowbirds. Leaving their eggs in the nests of other species as they moved on worked well.
Being a nest parasite may sound simple, but it’s not. A female cowbird constantly monitors the activity of other birds in her area. She learns when and where the other birds are making nests. She waits to lay her egg until there are at least two eggs in a host nest. She’ll often remove one of the host’s eggs before adding her own. She must lay her egg before the host bird starts to incubate her clutch. Cowbird eggs are programmed to hatch a day or two ahead of the host bird’s; giving the young cowbird a head start and competitive advantage.
Baby cowbirds grow more quickly by getting more food than any nest mates that survive. Most of their foster brothers and sisters don’t survive. Little cowbirds instinctively push against anything that touches their back and often push eggs or nest mates out of the nest. In addition, the lining of their beaks is bright hot pink or red, ranging to almost electric purple. The little cowbird’s bright mouth, larger size, and more aggressive begging for food means that any surviving nest mates may starve.
Sometimes the female cowbird will eat the egg she removes. She needs the energy because, unlike most other birds, she’ll continue to lay eggs for up to two months and can lay up to 40 eggs per season, mating with whatever males are around to insure fertility. This means that she can parasitize a wide variety of species over a long time. Some birds appear to recognize cowbird eggs and will abandon the nest, remove them, or build another nest on top of the parasitized nest. The female cowbird monitors nests she has left eggs in for a few days and will sometimes ransack and destroy the nest if her egg is not cared for. That forces the host birds to re-nest and gives mamma cowbird a chance to lay another egg in the new nest. Cowbirds have been documented as laying eggs in the nests of some 220 other species. Not all are good foster cowbird parents, though. Imagine a cowbird egg in a humming bird, hawk, or sandpiper nest. All have been documented. They may occasionally pick the wrong nest, but 144 species have been known to successfully raise young cowbirds.
One study estimated that only 3% of cowbird eggs become adult cowbirds. It’s enough, however, that cowbirds have radically expanded their range and numbers over the past century. Originally found only on the Great Plains, they now breed from coast to coast and well up into Canada. When humans began cutting and opening up eastern and western forests, cowbirds moved into the newly created habitat. They began parasitizing birds that had never been exposed to nest parasitism and had no defense against it. When the wandering herds disappeared cowbirds became resident birds. That meant that host populations were constantly exposed to their parasitism year after year rather than the old hit and miss pattern of their wandering days. A single cowbird egg can eliminate reproduction for a host pair of birds for that year. Cowbirds, habitat loss, increased predation by introduced house cats, in addition to other human caused factors has resulted in large declines in populations of a number of once common species across the continent.
Cowbirds somehow know how to be cowbirds even though they are raised by many other species. They are not very well thought of, but you have to admire Nature’s creative ways of insuring the survival of North America’s only pure nest parasite.