Thursday, April 17, 2008

Here, but NOT Welcome!

Oh, the hated brown-headed cowbirds are back. They return in the spring just before the songbirds that they parasitize so thoroughly. Here's a male (right) and female hoping to fatten up on cracked corn in our side yard.

I won't recount the entire natural history of the brown-headed cowbird, except to say that they evolved following roving herds of bison, eating the insects these large mammals kicked up. Being constantly mobile, cowbirds could not afford to build a nest, incubate eggs, and raise young. So they adapted to let someone else do all the work. Female cowbirds lay their eggs in the nests of other birds. They are amazingly good at finding hidden bird nests and waiting until the coast is clear to slip in and make an unwanted egg deposit.

Two weeks later the "foster" parents are raising a youngster that is not their own (and probably arguing over whose side of the family the nestling's ugliness comes from).

If it weren't illegal and bad for the soul, I'd consider doing something to control this species. Cowbird control measures have resulted in increased reproduction by the endangered Kirtland's warbler in Michigan.

Every year here on our farm we see fledgling cowbirds and it makes me heartsick to think of the wood thrushes, blue-winged warblers, and indigo buntings that were fooled into raising them.

One species that never has to raise cowbirds to fledging is the American goldfinch. Their nests get cowbird eggs to be sure, but the nestling cowbirds cannot survive on the goldfinches' all-seed diet.

Instead of "offing" cowbirds, we'll quit feeding so much corn and mixed seed (which the cowbirds love) and we'll curse these parasites whenever we see them.

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