Feathered Foster Kids

Found my first Brown-headed Cowbird (Molothrus ater) of the year today. Looked just like this one that I photographed last year:

Brown-headed Cowbird (Molothrus ater),
Illinois Beach SP, Lake Co, IL  5/20/2012

This is a male; females are a drab brown color. Last September, today's bird might have looked like this:

Brown-headed Cowbird (Molothrus ater),
Illinois Beach SP, Lake Co, IL  9/20/2011

This is, of course, a young male molting out of juvenile plumage into his first adult feathers.

And before that, he may have looked like this:

Common Yellowthroat nest, Spring Bluff FP, Lake Co, IL 5/16/2012

This isn't a cowbird nest, although the majority of the eggs are from cowbirds. The owners of this nest are a pair of luckless Common Yellowthroats (Geothlypis trichas):

Common Yellowthroat (Geothlypis trichas),
Gander Mt. FP, Lake Co, IL 7/31/2012

What are cowbird eggs doing in a warbler nest? The answer to that is the reason why so many birders have issues with cowbirds. They are what's known in the trade as brood parasites. They lay their eggs in someone else's nest, and let those birds raise the young cowbirds. Baby cowbirds are larger than most of their adopted siblings, faster to develop, and noisier, all of which means that they get the majority of the food that the parents deliver. Some parents will even work themselves to death trying to support the changelings. Relatively few of the host parents successfully raise their own young when they've got cowbird babies to handle.

Brown-headed Cowbirds are native to the Great Plains, and many of the species around here have developed techniques for dealing with the threat. Yellow Warblers in the Midwest, for example, will actually build a second nest on top of the one with the eggs, sacrificing their own eggs for a chance at a brood they can successfully raise. But Brown-headed Cowbirds do very well in human dominated landscapes, and have expanded their range dramatically. Many species that haven't had to deal with brood parasites don't have those defenses, and in some cases the Cowbirds have seriously impacted populations. Least Bell's Vireo (Vireo bellii pusillus) in California and Kirtland's Warbler (Setophaga kirtlandii) in Michigan are examples of species that are actually managed, in part, by aggressive action to reduce Cowbird numbers in their known breeding locations.

The impacts of cowbird parasitism appear to vary both by habitat fragmentation and location: Hoover, et. al. found that forest patch size didn't impact parasitism rates in Wood Thrushes (Hylocichla mustelina) in Pennsylvania (although it did impact predation on nests), (1) while Trine found that it did have impacts in Illinois. (2) She also found that patches in Illinois were population sinks for thrushes at sizes that are consistently population sources in Pennsylvania. In the West, where cowbirds are relatively recent immigrants, Airola found that parasitism rates increased with human disturbance, at least in parts of the Sierra Nevada. (3)

Cowbirds aren't really an invasive species - they haven't been moved into those new areas. Instead, they've moved in on their own as we've altered place after place to suit our own needs. Unfortunately, that's the sort of change that has ramifications beyond anything we intended at the time.


(1) Hoover, J. P., Brittingham, M. C., & Goodrich, L. J. (1995). Effects of forest patch size on nesting success of Wood Thrushes. Auk, 112(1), 146-155.

(2) Trine, C. L. (1998). Wood thrush population sinks and implications for the scale of regional conservation strategies. Conservation Biology, 12(3), 576-585.

(3) Airola, D. A. (1986). Brown-headed Cowbird parasitism and habitat disturbance in the Sierra Nevada. The Journal of wildlife management, 571-575.