Photo © 2005 Jim Jung and licensors. All rights reserved.

Cowbirds

The Brown-Headed Cowbird is a parasitic species. They arrive every spring from their wintering grounds in Mexico and the southeastern US, court and seek out suitable nests - other birds nests - to lay their eggs.

Brown-Headed Cowbirds were originally birds of the Great Plains. For thousands of years they followed the migrating bison herds snapping up insects that the bison flushed from the prairies. And since the bison were nomadic the cowbirds had to be nomadic as well. However this wandering habit made it difficult to raise a family so the Cowbirds' solution was to lay their eggs in other birds' nests and let them raise their young.

Cowbird eggs hatch earlier than their nestmates and Cowbird chicks push out all other eggs and young that they find in their nest leaving only the Cowbird chick to be raised by its foster parents. Other bird varieties that lived on the prairie and savanna and that had evolved with the Cowbirds developed strategies for coping with this parasitism which forced the Cowbirds to the forest edges where they sought out nests of less sophisticated species - the neotropical songbirds: warblers, vireos, tanagers and others - as surrogate parents. This strategy worked well enough since only those neotropical species on the forests' edge were affected.

But with the coming of our civilization and large-scale forest clearance the once vast woodlands were fragmented into small pockets of trees. Worse, we replaced the bison with domestic cattle and suddenly the Cowbirds found that new Cowbird habitat had opened up all over eastern North America allowing Cowbirds access to territory formerly out of their reach.

But the most insidious part of this story is that the fragmented, remnant woodlands were now open to marauding Cowbirds who could easily fly through entire tracts of woodland and deposit their eggs in the nests of species that had never had to deal with this threat before and were completely unable to cope with this new state of affairs. As a result Cowbird populations have soared while the populations of their host species have dwindled to an alarming extent. In most areas of the United States the neotropical songbirds are now raising cowbird chicks instead of their own and some species seem doomed to local extinction across great swaths of America.

Ornithologists investigating this phenomenon are at a loss for a solution to this problem. And while we're responsible for this state of affairs there doesn't seem to be any easy solution. We can only hope that our dwindling neotropicals learn to cope with this new threat before extinction solves the problem for them.

Offsite Links:

• USGS Brown-Headed Cowbird page
• Cornel Lab of Ornithology Cowbird page, with more on Cowbird parasitism
• North American Cowbird Advisory Council
• From Buffalo Birds to Modern Scourge: Smithsonian/National Zoo Cowbird page with more detail on habitat.
• Partners in Flight - Missouri Conservationist online

Related Link:

• Neotropicals also impacted by techniques of coffee production, From the Smithsonian/National Zoo

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