Earlier this week I noticed a plump gray bird begging food from a smaller bird about two-thirds its size. While the smaller bird searched for insects and worms in the leaf litter, the larger bird would stay just a few inches behind it, showing its gaping mouth and giving harsh cries. The smaller bird was a song sparrow and the baby bird was indeed being raised by it, but it was not a song sparrow. It was a juvenile brown-headed cowbird.
It’s a sad story, really. Brown-headed cowbirds are brood parasites, meaning the adults build no nest, instead opting to lay their eggs in other birds’ nests and leaving the parenting duties to them. Usually the “parasitized” species is a much smaller species, which gives the young cowbird a competitive advantage in the sibling rivalries.
Songbirds of most species feed the chick that is able to open its mouth first or the widest. When a parent sparrow, warbler, thrush or flycatcher returns to the nest with food, the larger, more aggressive cowbird gets the food. This is repeated over and over again until the true chicks die. Cowbird chicks also tend to crowd out the other chicks and push them out of the nest way too early. They don’t survive.
Birders are often conflicted when they find a nest with a cowbird egg in it. Cowbirds can negatively impact populations of native species, so there is a temptation for us to remove it. But brown-headed cowbirds also are a native species and are doing only what they evolved to do.
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And it’s not all the cowbird’s fault either. Humans have a hand in it. Cowbirds are birds of open country, not deep woodlands. The fragmentation of our landscape has resulted in patches of woods being much reduced in size, allowing the cowbirds access to woodland species that did not evolve having to deal with the cowbird threat.
To get an idea of the impact cowbirds can have on other native species, consider that cowbirds form huge flocks in the winter of thousands of individuals. Each individual cowbird represents one failed nesting of another native species. I find that realization staggering.