Bird of the Week: Brown-headed Cowbird


Though it was not a robin egg being eaten by this female Brown-headed Cowbird (background), the male American Robin made multiple attacks during this episode at Deschutes River State Park, Sherman County, Oregon on 5 June 2011.

During a recent birding trip Jay Withgott, Shawneen Finnegan and I watched an adult female Brown-headed Cowbird consume the small white-shelled egg of some other songbird. Since cowbirds parasitize the nests of other birds, I presumed that the egg was removed in order to make room for one of its own. I knew that cowbirds cast eggs out of the nests of host species, but neither Jay nor I knew that they often eat the eggs, so I set out to learn more.

Part of my research involved an e-mail exchange with Dan Gleason, author of Birds! From the Inside Out. Dan can always be counted on to provide interesting factoids about birds and he didn't disappoint. He also shared the scathing italicized account below, which appeared in Frank Chapman's  Birds of Eastern North America (published in 1902). It should be noted that is was Chapman, a highly-respected ornithologist who ultimately ascended to being the Curator of Birds at the American Museum of Natural History, who is credited with inventing one of the most popular birding activities that we engage in...the Christmas Bird Count.

"The Cowbird is an acknowledged villain, and has no standing in the bird world. English Sparrows, either because they are not aware of the customs of New-World bird life, or because of a possible and not unlikely affinity, associate with him; but no self-respecting American bird will be found in his company.

“As an outcast he makes the best of things, and gathers about him a band of kindred spirits who know no law. There is an air about the group which at once tells the critical observer that their deeds are evil. No joyous song swells the throat of the male. His chief contribution to the chorus of springtime is a guttural bubbling produced with apparently nauseous effort. ... They build no nest, and the females, lacking every moral and maternal instinct, leave their companions only long enough to deposit their eggs in the nests of other and smaller birds. I can imagine no sight more strongly suggestive of a thoroughly despicable nature than a female Cowbird sneaking through the trees or bushes in search of a victim upon whom to shift the duties of motherhood."

Clearly, Chapman had no affection for the Brown-headed Cowbird and many still take a dim view of this bird. But a closer look reveals a fascinating creature. While on the surface brood parasitism (the laying of eggs in the nests of other species) evokes a negative human response, in an evolutionary sense it is magnificent adaptive response. 

This unassuming looking bird is an adult female Brown-headed. In addition to drawing the wrath of Frank Chapman, this bird is widely blamed for declining populations of some songbirds. This bird was photographed at Deschutes River State Park in Sherman County, Oregon. This species is abundantly common in this part of Oregon, where it would have been a shocking rarity three centuries ago. 

Prior to European settlement, which opened up avenues for expansion, the range of the Brown-headed Cowbird was essentially limited to the Great Plains in the middle of the North American continent. Since cowbirds are evolved to eat the insects stirred up by buffalo and other constantly-moving herds of ungulates, they were highly nomadic. Their wandering ways precluded them from setting up and defending breeding territories during the nesting season. Instead they deposited eggs in the nests of other birds (host species) as they moved about.

Typically, the host species are smaller birds, which ensures that the cowbird nestling will be the largest bird in the nest with the biggest mouth to feed when the adults return with food. A study that involved several host species demonstrated that feeding rates for cowbird nestlings are always higher than those for the host species' nestlings (Woodward 1983). Additionally, larger host species are more capable of removing cowbird eggs from their nests, while smaller birds typically can't cast them out (Dan Gleason pers. comm.). According to Lowther (1993), 220 species have been documented to host cowbird eggs, with Yellow Warbler being the most common.

When I looked into why cowbirds eat the eggs (shell and all) of host species the answer made perfect sense. Egg production requires lots of calcium.  For cowbirds, this creates huge calcium deficits because they lay far more eggs than other songbirds.  They are known to lay up to 40 eggs during a single breeding season (Lowther 1993). In controlled captive environments, a female has been recorded to lay 44 eggs in 40-day period (Jackson and Roby 1992). Since eating egg shells is a quick way to replace lost calcium, cowbirds eat the eggs and shells that are removed from the nests of host species.

The fact that cowbirds have expanded beyond their ancestral home on the range is widely attributed to alterations humans have made to the landscape. Brown-headed Cowbirds are not a bird of dense woodlands. Thus prior to clear-cutting and the conversion of woodland to more open agricultural lands, there were no avenues giving them access to host species that evolved in an environment devoid of cowbirds. These new host species are not endowed with natural defense mechanisms to brood parasitism.

While it is hard to overlook the negative impacts that cowbirds have on some of their host species, it's important remember that their actions are instinctual and hard-wired, not malicious or with the intent to harm other bird species. If we don't like the results, perhaps we best look in the mirror, rather than casting aspersions towards a bird that is simply doing what it has evolved to do.

Sources Cited:

Jackson, N.H. & D.D. Roby. 1992. Fecundity and egg-laying patterns of captive yearling Brown-headed Cowbirds. Condor 94:585-589.

Lowther, Peter E. 1993. Brown-headed Cowbird (Molothrus ater), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

Woodward, P. W. 1983. Behavioral ecology of fledgling Brown-headed Cowbirds and their hosts. Condor 85:151-163.

I can’t believe the thesis of habitat change explains the range expansion of the Brown-headed Cowbird. The eastern US was not

a homogenous forest at the time of European contact. There was
an endemic prairie chicken on the eastern seabord, and bison as
well. A thriving trade in their hides provided cheap (leather) armor
for Europe during the century prior to actual attempts at colonization.
Large tracts of Kentucky and surrounding states were grassland,
also with bison.
In the west, most of the landscape was tree-less. Fire suppression in the 20th century has resulted in more extensive
forests. Clearcutting in the Oregon Coast Range and Cascades
have clearly created cowbird habitat on a local level. But the species
entered the state from the southeast, where the semi-arid grassland
is a continuum of the Great Plains. Seen from the air, the Rocky
Mountains of Wyoming, Colorado, New Mexico, etc, are a series
of forested islands, greatly overwhelmed by the grass and sagebrush
around them.
Eastern Bison were exterminated by the time of Wilson, Audubon,
and other early ornithologists. Wood Bison were native to Nevada,
Oregon, Idaho and Utah. They mysteriously died out in the late 18th
century, only decades before the arrival of European observers. A few
were still in southern Idaho in the first decades of the 1800s. What
is the history of modern animal husbandry in the west? My guess is
that cattle were fairly rare in the nineteenth century. Sheep may have
greatly outnumbered them in the Rockies and inter-mountain west
until the 20th Century. But are cowbirds NOT commensal with sheep?
To what extent are native species of open country such as Brewer’s and Sage Sparrows subject to parasitism by cowbirds?
The widespread habitation of the tree-less west by humans has
led to islands of trees and shrubs which in turn host numerous
species of song birds that were previously only migrants there.


Thank you for the great article. It was enjoyable to read. Your second picture is terrific. I’ve recently struggled with the ID of this species and your picture was very helpful.


Great post! (lol!) It is nice to see pictures with your reenct entries. Interest in the program is building and I am sure your visitors are telling others to come as well. I hope I can make a visit this month, weather permitting.


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