Every year in late August, I receive phone calls from readers who want to know about a large brown baby bird that is being fed by a much smaller parent. This is usually a chipping sparrow or a gray-headed junco locally, but it may be almost any smaller bird. Brown-headed cowbirds arrive in the spring about March 1 and are the only bird in America that does not build its own nest. They come to feeders regularly because they prefer millet to all other grain. They watch the other birds to see where they are building nests, and when the rightful owner of the nest is away, they deposit an egg. If the eggs are similar enough, the rightful owner just accepts it as her own, but occasionally if it is too different, she will throw it out or build a second nest on top of it, laying a new set of her own eggs.
Thus, small birds such as chipping sparrows, warbling and solitary vireos, yellow warblers and gray-headed juncos are parasitized by the irresponsible cowbirds, which means these small songbirds decline in numbers and the cowbirds increase.
It is believed that cowbirds evolved in Mexico and worked their way north into the central plains, where they found a perfect habitat among the herds of bison. Then, with settlers clearing forests and introducing domestic cattle, they found more and more open habitat, with grazing animals becoming much more abundant, and extended their range both eastward and westward. No one knows when or why they ceased building their own nests, for they are members of he blackbird family, all of whom build nests. Some, such as orioles, build exceptionally beautiful woven nests of amazing durable structure.
If a young cowbird is born into a nest full of smaller birds, it soon dominates the feeding so that the smaller babies starve to death, or it shoves the eggs and young out of the nest. The result is always the same: The host bird ends up raising only one big baby, which is a cowbird. Lawrence Walkinshaw did some research on cowbirds some years ago in Michigan and discovered that one female cowbird laid 25 eggs between May 15 and July 20. This means that any one cowbird is capable of having 25 of her babies raised at the expense of 25 other nests being destroyed. Many people feel that cowbirds should be removed from the protected bird list, but as usual they don’t like to make exceptions for fear of too many other species being killed by mistaken identity. However, in Mio, Mich., on the sanctuary for Kirtland’s warblers, the scientists have a special permit to remove cowbird eggs or young from the warbler nests, and this has brought the Kirtland’s warbler back from the brink of extinction.
Early September is a good time to study blackbirds. After the young of most blackbirds leave the nest, they gather together in large flocks prior to migration. These flocks move about during the day looking for food and return to their swamp or marsh to spend the night. Thus, you can often find large flocks of mixed blackbirds feeding on lawns, meadows and barnyards during early fall, often in areas where you can just pull off the road and study them from the comfort of your car. These flocks, locally, may contain such diversity as red-winged blackbirds, brewer’s blackbirds, rusty blackbirds, common grackle and possibly great-tailed grackle. Occasionally they may also contain a few meadowlarks, bobolinks and brown-headed cowbirds, which are also all blackbirds. It’s nice to be able to sit and look for the fine details that separate these very similar species. It is also an opportunity to view these birds in their different plumage. Be sure you have Sibley or some good field guide with you to refresh your memory. Have a nice fall.