On July 11, I saw a song sparrow feeding a young bird that was much larger than it was.
It was a funny scene to see with the young bird fluttering its wings and begging from the adult bird, but the implications were anything but funny.
It is likely the sparrow's own young were dead. The young bird was a brown-headed cowbird, and its biological mother had laid an egg in the sparrow's nest, leaving the baby to be raised by the smaller bird.
Brown-headed cowbirds are brood parasites, which means they build no nest of their own and make no effort to raise their progeny. They typically parasitize the nests of smaller, less aggressive species so the baby cowbird can have every competitive advantage over its nestmates.
Brown-headed cowbirds are birds of open country, rarely venturing into extensive wooded areas. Unfortunately for woodland species, the amount of that habitat is dwindling so the cowbirds are able to have an impact on species that they once rarely encountered.
The steep decline of wood thrushes has been attributed to this forest fragmentation and cowbird parasitism. In some parts of the country, cowbird control programs have been implemented to lessen their impact on some endangered songbirds. The programs have been successful but are controversial because cowbirds are a native species.