Two Brown-headed Cowbird fledglings have been hanging out at the feeder lately. This species is a parasitic nester, laying eggs in the nests of other species. Historically, Brown-headed Cowbirds were common in the Great Plains, where flocks would follow wandering herds of American Bison. Since the bison herds didn’t stay in one place long enough for cowbirds to nest, it makes sense that these birds would “adopt” their offspring out to other birds. As forests in the east and west were cleared, creating edge habitat, Brown-headed Cowbirds greatly expanded their range, which now covers most of North America. This has led to the decline of many eastern songbirds, as young cowbirds displace the young of the host species.
The two birds coming to my feeder were being fed by a Song Sparrow, one of the more common hosts of Brown-headed Cowbirds. Cowbirds have been known to lay eggs in the nests of over 200 different species, including Blue-winged Teal, Ferruginous Hawk, and Ruby-throated Hummingbird. These species do not successfully raise young cowbirds, but enough eggs end up with compatible hosts to create a strong cowbird population. (Female Brown-headed Cowbirds can lay up to 40 eggs in a year.)