cowbirds 101

November 30, 2005

in Birds, Science

The other day I introduced the topic, are Kirtland’s Warblers really in jeopardy?  I posed the question because funding cuts may reduce the number of cowbird traps used in the recovery of this endangered warbler.  Many people have an immediate negative reaction to cowbirds, and it’s often based on a lack of understanding about their ecology and impact on other birds.  Let’s start out this series of posts by getting to know cowbirds.  (A list of references and resources will follow the last post in the series.)


Male Brown-headed Cowbird, photo by Trisha Shears.

Cowbirds (Molothrus spp.) are New World blackbirds .  Five of the six species are obligate nest parasites:  they lay their eggs in the nests of other birds and abdicate parental care to the hosts.  The most common cowbird in North America is the Brown-headed Cowbird (M. ater). Henceforth, “cowbird” will refer to this species.

Nest parasitism has evolved in various species and families of birds due to different selective pressures. In Brown-headed Cowbirds, it is an adaptation to their formerly nomadic lifestyle.  Throughout most of their history, these birds followed herds of large mammals; they were once so closely associated with bison that they used to be called “buffalo birds.”  Moving with these animals, which could travel many miles a day, offered little opportunity for cowbirds to stop and raise young, so they began laying their eggs in the nests of other birds, leaving the parenting to the hosts.

Cowbirds parasitize many species of songbirds.  They lower the reproductive output of their hosts in a number of ways, but primarily through two behaviors.  First, female cowbirds often remove host eggs from host nests, although this varies among individual cowbirds.  Second, the cowbird chicks, which may be larger and louder than the chicks of the host, out-compete the host young for food.

Host species may be classified as rejectors or acceptors, with some species having intermediate reactions to parasitism that vary depending on the individual or population.  Rejectors react to parasitism by abandoning their nests (often renesting elsewhere if it is early enough in the season), ejecting the cowbird eggs from their nests, or building a new nest floor over the cowbird eggs (and any of their own) and starting a new clutch.

Acceptors, on the other hand, incubate cowbird eggs and raise cowbird young as if they were their own.  Usually, broods begin as a combination of both host eggs and a cowbird egg or eggs. Cowbird eggs require less incubation than most host species, and their eggs often hatch first.  This also gives the cowbird chick a competitive advantage.

Myths and misconceptions
Prior to European settlement, cowbirds were not found in the forested eastern U.S.  As land was cleared and agriculture spread, cowbirds expanded their range. As new frontiers opened up to cowbirds, they found many new hosts that had not co-evolved with them, and consequently had not developed defenses against nest parasitism. Many of these host species were therefore “acceptors.”  Today, cowbirds are found over most of the U.S. and southern Canada.

For generations, as people began to notice the habits of cowbirds, their behavior was not seen as the unique adaptation that it is, but as somehow evil.  Cowbirds have an undeserved poor reputation as being lazy or immoral.  Of course, the attitude that birds, or any other animal, can or should follow human expectations of ethical or moral behavior is illogical and unreasonable.

Armed with this misconception, people blamed cowbirds for declines in songbird populations, in particular the decline of migratory songbirds that has been the source of much concern the last few decades.  Reasons for declines in bird populations, particularly migratory songbirds, are varied and complex. While cowbirds do have an impact on the reproductive success of their hosts, as explained above, but they are not the dire threat they were once thought to be.

One of the world’s leading authorities on cowbird ecology, Catherine Ortega, has said that “the assumption that parasitized nests are destined to failure is unequivocally incorrect.” I reviewed multiple studies covering over two dozen cowbird hosts presented in Ortega’s book, Cowbirds and Other Brood Parasites. In parasitized nests, the percent of host young fledged per egg is about 21%.  For cowbirds, it is around 28%.  In other words, cowbirds are not much more successful than their hosts.  Not all, nor even most, individuals of a host species are parasitized by cowbirds, and not all parasitized nests fail to produce host young.

Despite the fact that they have expanded their historical range, cowbirds are not, as is commonly thought, increasing in numbers. Populations have remained stable, or in some regions steadily and significantly declined, over the last 50 years. Cowbirds do not, contrary to popular belief, pose an increasing threat to songbird populations.

Humans facilitated the spread of cowbirds into new habitats, introducing them to new and vulnerable hosts.  We continue to provide them with plenty of edge habitat, abundant food in vast agricultural areas and feedlots, and increasing edge habitats with opportunities for them to penetrate into shrinking forested areas. Our alteration of landscapes has helped cowbirds while simultaneously eliminating, reducing, or degrading habitat, the primary reason many species of songbirds have declined.  We have been left in a situation where cowbird management has been deemed necessary to assist some species of birds.  That will be the subject of the next post.

Peter Bryant November 30, 2005 at 12:22 pm

Very interesting post, as always. I've been peripherally aware of this issue over the years, at least regarding programs to promote the recovery of Black-capped Vireos and Golden-cheeked Warblers in Texas, but have never before examined the details.

I did not know that cowbird populations were roughly stable or declining. But could this stability be, at least in part, due to the numerous cowbird control programs in place around the country?

I have some other questions as well, but no time to ask now.

Wisecrow November 30, 2005 at 3:13 pm


A lot of the info in the new AOU Monograph on this topic was a little bit of a shock to me. I had always assumed the worst, especially with species that had not been exposed to parasitism. I still believe that, with species with as small a population as Kirkland's Warbler, you have to do everything possible to help. Expanding habitat, etc. is a great idea, but not always possible. Maybe the bird will do ok despite fewer cowbird traps. I say "Why take a chance?", especially with the small amount of money required (although if you believe everything in the monograph, cowbird control has no effect on Kirkland's population). As the last article in the monograph mentions, my state (Texas) has really been aggressive with cowbird control – maybe too aggressive. Goes with the rancher/cowboy culture I guess. But, my only experience with this is finding an active Black-capped Vireo nest with one vireo egg, one Brown-headed Cowbird egg, and one Bronzed Cowbird egg. Sure, we need more vireo habitat, but I would rather err on the side of caution (not Ortego 2000 caution, however), at least with really vulnerable species. Anyway, great topic. Can't wait to read the next installment. Thanks.

Pamela November 30, 2005 at 8:25 pm

So glad to that you posted on this topic. I was shocked when I first became aware of the terrible reputation of the cowbird–just for making its way in the world the best it could, like everyone else. Sudden changes to habitat put things out of whack–not the cowbird's fault, but as you point out, as usual, a result of human activity.

I spent some time this past breeding season thinking about differences in behaviour in the vicinity of the nest by different warblers, and became interested in the different rates at which they are parasitized by cowbirds.

I'm looking forward to reading future instalments.


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