Brown-headed Cowbirds have been around at least 500,000 years, based on fossil records, spending almost all of that time following herds of large hoofed animals on North American prairies. Buffalo were their mainstay, but in our neighborhood, horses will do. The birds eat seeds knocked loose by grazing quadrupeds and on insects stirred to action. It was all buffalo to begin with, and then we came along. Our cattle and horses (above) perform just like buffalo.
Once restricted to the Great Plains for the obvious reason, as we civilized the continent we made more and more land attractive to these birds. Large hoofed animals are not absolutely essential. Cowbirds now cover the continent, from Mexico to northern Canada. As nest parasites, that means that now impact many more bird species than before we began to settle.
Cowbirds lay their eggs in the nests of other species, letting the host hatch and raise the youngster. A female cowbird can lay as many as 40 eggs per season, usually placing just one per nest as it looks for surrogate parents. It finds host nests by careful observation, sometimes landing roughly in a tree or bush to see if a nest is revealed when the occupant flushes.
In its history, the cowbird was doing what was necessary: you can’t follow a moving herd of buffalo while incubating eggs. Nowadays, the bird is simply doing what it does, often to the detriment of other bird species.
Cowbirds eggs usually hatch before host eggs, and the hatchling is larger, demanding so much food that its nest-mates sometimes starve. Many bird species have learned to combat this by removing cowbird eggs or renesting over them. Many haven’t made that adaptation; over 220 species have been reported as being cowbird victims. Yellow Warblers and Song Sparrows top the list of species most often used as hosts. The bill-pointing you see in this photo (the two males behind, a female in the foreground) is a way that males showing dominance over subordinates.