How Does a Brown-headed Cowbird Know It’s a Cowbird?

For the past several days, the whistle and jingle of a pair of Brown-headed Cowbirds has been common around our yard, and I’ve seen the pair several times, including at the birdbath. They’re an unwelcome presence, and their dark coloring – the males are black with brown heads – encourages me to think of them as sinister. But there’s one question about Brown-headed Cowbirds that intrigues me.

Brown-headed Cowbirds are brood parasites. They do not make nests of their own. They lay their eggs in the nests of other species – such as Yellow Warblers, Red-eyed Vireos, Eastern Phoebes, and more than 200 other host species, including some whose populations are in serious decline. The eggs of a Brown-headed Cowbird are incubated by the host species and the young are fed along with the young of the host species. In most cases, the Brown-headed Cowbirds are larger and fed more often, so the young of the host species may fail to survive.

Once they leave the nest, Brown-headed Cowbird juveniles continue to be fed by the host species, apparently until they are old enough to become independent and feed on their own.

So my question is: How does a Brown-headed Cowbird know it’s a cowbird? If it’s hatched, fed and raised by Red-eyed Vireos, why doesn’t it grow up to behave like a Red-eyed Vireo? When and how does it discover it’s a Brown-headed Cowbird?

In late summer and early fall, Brown-headed Cowbirds begin to gather in large flocks, so by that time, the young have somehow figured out where they belong. It’s sort of the ugly duckling story in real life, with a few unpleasant twists.

I’ve so far failed to find an answer to the question of how this actually works, but in the process of looking, have been somewhat surprised to find accounts of Brown-headed Cowbirds’ behavior and natural history much more interesting and more complex than I had expected – and ended up spending far more time on it than I meant to, and feeling considerably less antagonistic toward them than when I started.

While parasitism by Brown-headed Cowbirds is generally condemned as sneaky and especially harmful to some declining songbird species, I think it’s worth remembering that habitat loss, fragmentation and degradation are the fundamental problems – and we’re the ones who created that situation and who continue to make it worse. We’re the ones who created the kind of habitat in which Brown-headed Cowbirds thrive and many other songbird species suffer.

I still don’t like having Brown-headed Cowbirds around, but I’m not sure it’s fair to blame them.

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