News & Articles

The Merriam-Wesbster Pocket dictionary defines parasite as “A plant or animal living in, on, or with some other plant or animal.” This definition truly fits the brown-headed cowbird, as it is North America’s only parasitic bird. It does not build a nest of its own, but occupies those of others.
The territory of this bird is about 10 to 50 acres and is used for mating and nesting. Depending on where the hosts’ nests are located, the cowbird may be territorial. In open areas, where nests are sparse and located near woods edges, small flocks of males tend to stay put and the females cruise, looking for nests to occupy. In thick wooded areas, where there are more nests, they are territorial. Females will compete with other females for territory during the morning, but have been seen feeding together later on in the same day.  
In the open areas, males court by flocking and displaying to one another, competing for the area. The females are looking for nests to occupy at this time and will pair up with the male that is dominant in that particular area. In wooded areas, males tend to roam and females stay locally. As a result, females may mate with more than one male.
Unlike most birds, the female’s goal is to locate, not build a nest. Over 150 different species’ nests have been observed with brown-headed cowbird eggs in them! Females, from a high vantage point, usually in the morning, look over the area for other bird activity to locate nests. Others fly noisily into wooded areas trying to scare up nesting birds. Still another method is to walk on the ground looking for signs of nesting birds.
Once a nest is located, the cowbirds lay, as a rule, only one egg per nest. These are brownish spotted eggs that are off-white in color. Incubation takes up to just shy of two weeks with the nestlings remaining in the nests for about 10 days. With a short incubation period and a very rapid early-onset growth rate, they have an excellent chance of surviving in the nest of another species. There is debate as to whether or not any of the hosts’ offspring survive once a cowbird lays an egg in their nest. Some experts say no and some say that since only one egg is laid, only one host’s fledgling dies.
Seeds consist of most of the cowbird’s diet (78%), and the rest is mainly insects and spiders (22%).
The brown-headed cowbird occupies all of Minnesota. These birds arrive in early March through May and although it is not known exactly, it is believed the majority leave the area in late June or July, with some leaving during the fall months. They winter in southern States and Central America, although some are seen in the southern parts of Minnesota.

LaPLant is a conservation officer based in Holyke, MN.