Friday: Cowbirds – Witness for the Prosecution

By Hugh Powell
August 8, 2008
View video of the Blue-winged WarblerBlue-winged Warbler

I’m a grudging admirer of cowbirds – the villainy of palming your kids off on other birds balances against the ingenious solutions to living a life of subterfuge and masquerade. But even I blanched when Peter Arcese confronted us with video evidence of so-called “nest farming” by cowbirds.

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The video footage, collected by Phil Elliott of Eastern Connecticut State University, is here. In just over a minute, we see a female Brown-headed Cowbird grab seven Blue-winged Warbler nestlings, one by one, and toss them over the side of the nest. It then hops out of the nest and finishes the job with swift pecks at each chick. (The violence occurs offscreen.) It then returns to the nest for a quick evaluation, and leaves.

Why sneak in, commit murder, and leave without laying an egg? Cowbirds won’t lay eggs in nests that are too far along – their chicks would never be able to compete with their older nestmates. So instead, cowbirds destroy the nest, then keep an eye on the territory. As the host bird furiously builds another nest, the cowbird watches, waits, and then drops in an egg at the prime moment.

It’s gruesome, but as Arcese told me, “It’s what they do. It’s like when you see a Cooper’s Hawk take out a bird at your feeder. It’s not pretty, but it is amazing.”

Cornell Lab of Ornithology

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