Cowbird is the common name for a group of birds found in North and South America. The best known species is the brown-headed cowbird, which lives from southern Canada to Mexico.
They are songbirds that are also known as brood parasites. This term means that the female cowbird lays her eggs in the nests of other smaller birds and then leaves the eggs and resulting young to be raised by the foster parents. It has been suggested that they became parasitic and derived their name because they followed moving herds of bison, cows and other large herbivores and had no time to stop and nest. They tend to follow large animals in order to eat the insects that they stir up as they walk through grassy areas to feed.
The brown-headed cowbird is 6 to 8 inches long and has a finch-like bill. The male has a brown head and a shiny black body while the smaller female is plain gray in color. The female lays four to five white eggs that are lightly speckled with brown. The eggs are laid one at a time in different nests using the nests of more than 200 other species of songbirds. They usually pick out the nest of a songbird species that is smaller than themselves. This trickery by the cowbird takes place while the owners of the nest are absent.
After the female cowbird lays her egg in the nest, she flies off and does not return. Sometimes the host species recognizes that the egg is foreign and ejects the unwanted egg from the nest. Other host birds may detect the foreign egg and not allow it to hatch.
For example, a yellow warbler may cover the egg and build another nest on top of the old one. However, most birds will incubate the egg along with their own eggs and rear the newly hatched cowbird as one of their own. The young cowbird is larger than the other nestlings and usually receives most of the food. The foster parents do not seem to realize that they are raising an outsider and treat it as one of their own. T
he young cowbird grows quickly at the expense of the host's own young who may become undernourished or starve to death. Sometimes the young, larger cowbird actually ends up pushing the smaller young of the host out of the nest to their death. There are several other species of cowbirds that live in the Americas. Most of them lay their eggs in the nests of other species.
Larrie Stone is a retired Dana College science professor.