Brown-headed Cowbird Dilemma
by Don H. Wolfe
A female and male cowbird.
Across most of North America, many species of songbirds have shown declines in numbers. Although countless factors have no doubt contributed to these declines, a lot of birders have been quick to accuse Brown-headed Cowbirds because of their habit of laying eggs in nests of other birds. To many people, this is at best considered the height of laziness.
At worst, cowbirds are considered despicable creatures that would best be eradicated. In the case of some endangered species, diligent cowbird control efforts have been shown to increase productivity. This has included such measures as capturing and killing cowbirds, and removal of cowbird eggs from hosts' nests.
In actuality, such efforts may be likened to taking aspirin for a broken back; short term, localized relief may be felt, but the problem still exists.
In 1993, a symposium/workshop on cowbird biology and management was organized to address the cowbird dilemma. About 200 biologists and concerned birders converged on Austin, Texas, to attend this meeting. Probably the biggest news to come out of this consortium was that, even though cowbirds had been studied quite extensively, little was actually known about the supposed impacts on many host species.
Cowbirds landing on cattle in foggy weather.
It was decided that a second symposium/workshop in about three years would be appropriate, allowing researchers time to address some of the issues of impact on hosts. The second meeting was held in Sacramento, CA. At this meeting, many researchers showed that the assumed impact cowbirds were having on most host species was greatly overestimated.
Brown-headed Cowbirds are a species native to North America. Historically, they lived entirely in the Great Plains and followed migratory bison herds across the landscape, feeding on insects stirred up by the feet of the big ungulates.
This transient lifestyle made it difficult to take up house-keeping in any one locale. Consequently, cowbirds developed a breeding strategy known as "brood parasitism," in which they would lay their eggs in the nest of another species, allowing this "host" species to raise the cowbird offspring. This strategy has worked well for cowbirds for thousands of years.
If a host species’ population were severely affected by cowbirds, this strategy would not have worked over the long term (who would raise the cowbird young?). Even today, brood parasitism affects productivity considerably less than many other factors such as predation, weather, habitat changes etc. In fact, researchers have shown that even endangered species such as
Cowbirds on the backs of cattle.
Least Bell’s Vireos and Southwestern Willow Flycatchers can have brood parasitism rates of 50% or greater without severely affecting their seasonal reproductive efforts. In our own studies in Oklahoma, the species that show the highest rates of brood parasitism have nest success rates for parasitized nests equal to or higher than unparasitized nests. Some examples are: Orchard Orioles, which are parasitized 38% of the time, have 67% success rates for parasitized nests, but only 32% success for unparasitized nests.
Red-winged Blackbird nests here are parasitized about 22% of the time, but parasitized nests have a success rate of 28%, while unparasitized nests have a success rate of 29%. Dickcissel nests are parasitized about 19% of the time, but parasitized nests have a success rate of 30%, while unparasitized nests have a success rate of 33%. The species that do seem to be adversely affected by the act of brood parasitism are parasitized much less frequently.
Examples include: Grasshopper Sparrows, which have a success rate of only 5% for parasitized nests compared to 33% for unparasitized nests, and are only parasitized about 7% of the time. Eastern Meadowlarks have a success rate of 13% for unparasitized nests compared to 24% for parasitized nests, but are only parasitized about 6% of the time.
There are definitely cases, especially in areas where cowbirds have expanded their range due to deforestation, where host species have not developed a strategy to deal with parasitism. The birds in these areas may be affected to some degree.
But, the question remains, should we be quick to put all the blame on cowbirds, especially when it was our (human) actions that have allowed cowbirds to expand the range north and east? And, considering that possibly billions (!) of birds are killed by house cats or feral cats in North America each year (another effect created by humans; for more information about cats and birds see our Cats Indoors! page), maybe the lowly cowbird should not be portrayed as such a menace to the well-being of more charismatic bird species. In fact, maybe those students of bird behavior should look at cowbirds and their breeding strategy with greater interest and less contempt.