Intrusion Of The Cowbird!

Last week it was a red katydid outbreak. This week we have a cowbird intrusion. Nature is endlessly fascinating and this is another example.

When we checked our nestbox #4 last week, it was empty except for a lone bluebird egg which had not developed from the previous brood. We only recently learned that you should remove the old nest so we had left the nest and egg alone. This week, during a bird species survey with my friends and bird experts, Jesse Huth and Jaci Kroupa, we checked the box as we hiked past. The old blue bird egg was gone and in its place were five small, whitish eggs. Jesse identified them as cowbird eggs. He removed the eggs and destroyed them. We looked around the box to see if we could find the blue bird egg, but weren't able to.

Brown-headed cowbirds (Molothrus ater) are nest parasites. They construct no nests of their own, but instead lay their eggs in the nests of other bird species. This behavior evolved because the cowbirds evolved to follow the large bison herds that roamed the prairies and would feed in the short grass left behind by the herbivores. The adult cowbirds would not remain in any area for long and left their eggs behind to be raised by others. The cowbird young hatch sooner and are larger than the host species young and either actively expel the other chicks or out-compete them for food.

The other bird species adapted to this periodic cowbird parasitism, but now the scenario is very different. The great bison herds are gone, replaced by fenced-in cattle which remain in one area. The cowbirds are doing what they’ve always done, but their constant presence is now a significant threat to other bird species.

Texas Parks and Wildlife has a cowbird trapping program which tries to address the problem. We may have to put some cowbird traps on our property if the situation gets worse.

In this case, the cowbirds did no harm, since the blue birds had already fledged and only an old egg remained. We may not be so lucky next time.