The Brown Headed cowbird (Molothrus ater) is a parasitic bird of North America. They used to follow the bison around and consume the insects associated with them. With the expansion of cattle in North American, the brown-headed cowbird populations have also expanded which has raised some interesting questions. Female cowbirds lay their eggs in the nests of other birds and allow those birds to raise their young. This type of parasitism is called brood parasitism. Because the cowbird is typically larger than these birds their young die in the process of raising the cowbird. Because the cowbird was not tied to a nest this allowed them to follow the bison which was a food rich source. It is unknown if they evolve this strategy to follow bison or were able to follow bison because of this strategy. They also are able to produce more eggs in a season than a typical bird, up to 3 dozen. Bird populations of the birds they parasitize have suffered as a result of this and efforts have been made to try to reduce this. Using feed designed for small birds, not spreading seed on the ground, and avoid feeding sunflower seeds, cracked corn, and millet will reduce the number of cowbirds in your area. It is important to note that the birds are covered by the Migratory Bird Act so it is illegal to remove their eggs from a nest or harm them in any other way without a permit. In some states, if they threaten an endangered species you can obtain a permit to trap the birds.