Brown-headed Cowbirds lay their eggs in other birds' nests. And these birds raise the cowbird chicks - often at the expense of their own young. This beautiful Yellow Warbler is a frequent target of the cowbird's unwelcome eggs. But it has developed a way to reject the role of foster parent. When a cowbird lays its egg in the warbler's nest, the warbler may weave another layer - or more - of grasses over the top of the cowbird egg, preventing its incubation. One Yellow Warbler nest in Ontario grew six layers deep!
Cowbirds and Yellow Warblers
Written by Bob Sundstrom
This is BirdNote!
[Brown-headed Cowbird song & call]
It seems that Nature doesn’t always play fair. One notorious avian example: the behavior of Brown-headed Cowbirds. [Brown-headed Cowbird song] Cowbirds, which are found throughout much of North America, lay their eggs in other birds’ nests…and these birds raise the cowbird chicks – often at the expense of their own young.
[Brown-headed Cowbird call]
The beautiful Yellow Warbler is a frequent target of the cowbird’s unwelcome eggs. But it has developed a way to reject the role of foster parent. [Yellow Warbler song] When a cowbird lays its egg in a Yellow Warbler’s nest – a tidy, compact cup woven of plant fibers – the warbler weaves another layer of grasses over the top of the cowbird egg, preventing its incubation. Sometimes a cowbird returns and lays another egg in the same nest – and now the Yellow Warbler covers over the second egg. Amazingly, one Yellow Warbler nest in Ontario grew six layers deep.
[Yellow Warbler song]
Cowbirds developed their habit of palming off their eggs on other birds, because they followed migrating herds of bison. Cowbirds couldn’t stay in one place long enough to raise their own young. [Editor's note: Well..l.l... this is not exactly so. Here's more info on that: https://birdnote.org/blog/2015/05/cowbird-story-revisited.]
[Brown-headed Cowbird call] Today, it’s still a problem that even the hardest working Yellow Warbler is sometimes challenged to overcome.
[Yellow Warbler song]
Writers for BirdNote include Bob Sundstrom, Dennis Paulson, Ellen Blackstone, Frances Wood, and Todd Peterson. For BirdNote I’m Michael Stein.
Sounds of the birds provided by The Macaulay Library of Natural Sounds at the Cornell Lab of Ornithology, Ithaca, New York. Song and call together 47953 recorded by D.S. Herr; song of Yellow Warbler 125226 recorded by M.J. Anderson.
Producer: John Kessler
Executive Producer: Chris Peterson
© 2013 Tune In to Nature.org June 2013 Narrator: Michael Stein