Posted by: atowhee | July 4, 2009

Invader v. locals

The Brown-headed Cowbird is NOT native to the Pacific Coast states.  Like many other weeds and invasive species the cowbird came west with industrial scale ranching and then spread south to north. They first appeared in southern California about a hundred years ago.  Now many western species are forced to cope with the cowbird’s nest parasitism with no evolutionary preparation.  Recently I saw a pair of Black-headed grosbeaks being followed about our garden by the young cowbird they had reared.  Their “adopted” chick had clearly managed to do in his nest rivals and get all the food.  Fortunately some of our local grosbeaks have managed to fledge their own youngsters this year.  I’ve seen half-dozen at one time crowding our feeders.

I had never noticed adult cowbirds around our garden.  The female must sneak in, lay the egg and depart. Teh callsand bubbling sound of adult cowbirds is unmistakable once you are alert to it.  They do not often feed in heavily forested areas, preferring areas where large quadrupeds churn up the turf.

In other years in other places I’ve seen Song Sparrows, Hutton’s Vireo and Common Yellowthroats feeding “their” baby cowbirds.

Cavity-nesting birds do not have to deal with cowbird eggs in their nests.  Some open nest birds like the American Robin can recognize the foreign egg and chuck it out.  Robins’ eggs are a distinct “robin’s egg blue.”  Cowbirds have been especially hard on small insect-eaters that nest along streams: vireos, warblers, even sparrows.

The Brown-headed Cowbird evolved its way of life alongside the American bison in ther Great Plains.  A female cowbird can lay dozens of eggs per summer, never having to build a nest, defend eggs or feed young.  Eat and lay, that’s her way.  Having evolved in the wide open prairie, the cowbird is a singularly storng flier for a songbird, covering long distances daily if necessary.  They’re commonly seen flying over the ocean far from shore, just taking s shortcut across the open water.

Here’s a good summary from National Audubon on cowbird parasitism and control efforts which vary and are not co-ordinated in the U.S.


  1. Interesting article!

    Do you know if anyone can help me identify bird sounds on a 30-second video clip which doesn’t actually show the birds? It’s shot in San Francisco.

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