The Brown Headed Cowbird is not the sentimental favorite of many birders. It's habit of not building a nest, rather laying it’s eggs in another birds’ nest, using those birds to hatch and raise their young, started eons ago. This bird was originally found in the great plains, following the vast herds of Bison. These large animals kicked up many insects as they moved lazily along their way. These insects, along with the seeds from native grasses and weeds, comprised a great source of easy food for the Cowbirds as they sat and foraged not only on the ground, but atop these great beasts. As the Bison herd numbers fell, they were replaced by equally large herds of free roaming cattle. Now you see where the name Cowbird originated from. But as the fences started going up, this natural food source began to diminish. Thus the Cowbird began it's eastward expansion into areas unknown to this species before. It now covers most of the US and much of Canada.
The birds habit of following these roving herds, seems to be the reason for it's parasitic nature. If the food source moves on, so must the birds. It usually lays one egg in a host birds nest and may or may not remove one or more of the host birds eggs. The young Cowbirds will hatch a day or two before the parent bird’s eggs and thus will demand more of the food, growing faster than the other chicks in the nest. Cowbird nestlings may receive as much as 100% of the food brought to the nest as the others slowly starve. They may even scoot around the nest in an attempt to eject the other nestlings. About 50% of the host birds will reject this effort in one way or another. Some Robins and Catbirds will puncture the shell and throw the egg out of the nest. Yellow Warblers, Phoebes and Cardinals may build another nest on top of the infected one. The rest of the host birds will not recognize the difference in the eggs or chicks and raise them as their own. If you do see Cardinals feeding a strange looking gray youngster, it is probably a Cowbird. Many think some of our forest birds are being adversely affected by the Cowbird while others argue they are not. However birds that nest in very large forested tracts seem to fare better than birds in small woodlots or fragmented forest areas. Some believe the Cowbird will not penetrate more than 100 yards into a canopied forest Think of the size of many wooded areas. Most are over 200 yards deep each direction. The female Cowbird is very stealthy about her egg laying and will quickly dart in to deposit her egg» when the nest is unguarded. They lay up to 40 eggs each nesting season. Southern Florida has a tropical species called the Shiny Cowbird, while Texas and Arizona have the Bronzed Cowbird who’s spreading its range.
Contributed by Mr. Weir Nelson of Wildlife Habitat, Cedar Rapids, Iowa