Brown-headed Cowbird: Pest or Unique Species?

Have you ever seen a cardinal feeding a small brown bird.  If you have, you likely saw it feeding a brown-headed cowbird.  The young cowbird was not willingly adopted by the cardinal, its eggs were laid in the cardinal nest and then the young cardinal eggs were removed by the mother cowbird.  The cardinal unknowingly incubated the eggs and raised the cowbird young.  This happens to tens of millions of unsuspecting songbirds each year.

Throughout North America songbird numbers are declining. While there is no one single reason for these declines, one major contributing factor is the spread of the brown-headed cowbird. These brown birds which are about 8 inches long were once limited to grasslands in the United States where they followed the herds of buffalo, feeding on the insects stirred up by the movement of herds as they moved from place to place. Buffalo herds were constantly on the move and they did not allow the cowbirds which were dependent on them to kick up food to develop a normal nesting territory as other birds.  As a result, they developed the unique ability to lay their eggs in other birds nests (parasitize) while they kept up with the roving buffalo herds. 

Today, there are no roving buffalo herds, but there are stationary livestock herds everywhere and this highly adaptive bird has found a “gold mine” and has spread throughout North America. This is a problem because of the reproductive strategies the species employs. The cowbird is what is referred to as a brood parasite. This means the female lays her eggs in the nests of other birds, abandoning them to the care of foster parents. The foster birds unknowingly raise the cowbird chick to the detriment of their own young. Because the female cowbird can lay as many as 70 eggs per season, susceptible species such as cardinals and other songbirds can be severely impacted in areas with high cowbird numbers.  Some species of birds are able to resist parasitism by cowbirds, but many are not. Those that are resistant tend to be the birds that evolved with cowbirds in the Great Plains. Cowbirds have been known to successfully parasitize more than 225 species of birds in the United States.

Those birds that have not evolved with cowbirds tend to be birds that are found in tree-covered, forested areas. The most common to be parasitized are small woodland songbirds. What is important to understand is that these parasitized nests are not unsuccessful nests just because no baby songbirds have been raised. They are successful nests. The problem is they are producing the wrong “product”. They are producing baby cowbirds, not baby songbirds.

Therefore, trapping of cowbirds to reduce their numbers becomes an important option to consider if we are to prevent some declines in songbird populations.  Trapping of cowbirds locally is done by the Texas Organization of Wildlife Management Association in cooperation with the Texas Parks & Wildlife Department.  The traps are checked daily to make sure any non-target species of bird (any bird that is not a cowbird) can be quickly released. Female cowbirds are removed and humanely killed by cervical dislocation. Some of the males are banded and released to help learn more about cowbird movements.

If you would like to contact your local biologist, see our website at; http://www.tpwd.state.tx.us/wildlifebiologist.