Thursday, June 28, 2007

Baby Huey and the European settlers

Today a Chipping Sparrow came to our yard with its "kiddo" in tow: a big, ungainly juvenile Brown-headed Cowbird. The young cowbird stumbled around in the grass, begging continually. The Chipping Sparrow, about half the weight of Baby Huey, hopped around frantically, diving after insects in an attempt to satiate the gaping, hungry mouth.

To the uninitiated, this would be a surprising sight. You might wonder if the tiny Chipping Sparrow had adopted the baby cowbird out of the goodness of its heart. Well, the Chipping Sparrow is certainly raising the cowbird as its own, but not by choice. The female cowbird is a brood parasite—she removes an egg from the nest of another species and replaces it with her own. Then, Ma Cowbird leaves the residents of the nest to incubate the eggs and eventually care for her progeny. When the egg hatches, the young cowbird usually dominates its smaller siblings for food and space. Despite a striking contrast in appearance and, often, size, the host parents continue to care for the cowbird... and within about 10 days Baby Huey leaves the nest and follows his adoptive parents, continuing to take whatever he can from them until he's ready to fly off on his own to make more little cowbirds.

I sat watching this scene through my living-room window as my own kiddo nursed. An image flickered through my mind of attempting to breastfeed a baby 50% heavier than me. I've felt sorry before for parent birds parasitized by the Brown-headed Cowbird—but this time I felt true empathy.

The unfortunate thing about Brown-headed Cowbirds is that they are only doing what they have evolved to do, but sweeping habitat changes brought on by—you guessed it—human beings have made it even easier for the cowbird to survive. Naturalists speculate that oodles of years ago, the cowbird followed the bison herds across the sweeping prairies, dining upon insects attracted to and flushed out of the grass by the grazing animals. Because the bison were nomadic, the cowbirds found it necessary to evolve the same kind of nomadic ways. As any nomad knows, however, it's downright tough to find time to pick out territory, defend that territory, build a nest, lay eggs, incubate eggs, and raise young. That could take weeks. By the time the young were raised, the bison herd could be many miles away. So, the cowbird evolved a better plan: let someone else raise the kids, and once they were full grown, they could catch up with the herd.

As with all things in nature, that tactic worked as long as a delicate balance was maintained. Balance, though, was not something that the European settlers excelled at. As the forests in the east fell to logging, the cowbirds took notice. Before, the cowbirds would only visit the nests of woodland species that utilized the edge habitat of forests. But today, most tracts of woodland are heavily fragmented. You can't walk too far into the forest before you're walking back out the other side. Cowbirds gained access to what used to be the deeper parts of the woods, and began to use the nests of some more delicate songbirds. According to Ken Kaufman's Lives of North American Birds, a female Brown-headed Cowbird is known to have used the nests of more than 220 species of birds, and she can produce up to 40 or even 70 eggs in a single season.

That's a lot of displaced songbirds. And that means the Brown-headed Cowbird is looked upon by many as a villain. But for Baby Huey's bio-mom, it's the only method of survival for her species.

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