Brown-Headed Cowbird (Molothrus ater)
Male Brown-Headed Cowbird
Brown-headed cowbirds (Molothrus ater) are so named because the male has a brown head and these birds often follow cows to eat the insects that are stirred up on the ground by the cattle.
It is the female brown-headed cowbird that is most famous because of her nest brood parasitism behavior. It is due to this behavior that brown-headed cowbirds are not well-liked, although to their credit, these birds do eat lots of insects.
Female Brown-Headed Cowbird
Brown-headed cowbirds don't build nests and so they need another nest building bird species to care for their young.
So, the female brown-headed cowbird will find a bird's nest where eggs have recently been layed. Then, when the female leaves the nest, the female cowbird goes into the nest and lays her own eggs. The original female then comes back to the nest and takes care of the cowbird eggs and young along with her own. Sometimes the cowbird eggs are rejected, but often they are not and the host bird(s) often take care of the cowbird offspring as if it was their own.
Eastern Phoebe Nest with One Brown-Headed Cowbird Egg
The reason this is a problem is because there may not be enough food to go around for all of the young, especially if the cowbird young grow faster than the host young, which is often the case. To see the size difference that may result, see the photo below of an adult Wilson's warbler feeding a fledgling brown-headed cowbird. The cowbird is the larger of the two birds.
Wilson Warbler Feeding a Cowbird Fledgling
Because the cowbird fledgling is so much larger than the adult Wilson warbler, you know it is much, much larger than the Wilson warbler chicks, so it is very likely that these chicks may not be getting enough food. This is probably true of most chick species that are sharing their nest with brown-headed cowbird offspring.
Brown-Headed Cow Bird Distribution
Brown-headed cowbirds are found through North America and move south during the winter. See the distribution map below.
Distribution map of Brown-headed Cowbird (Molothrus ater).
Blue: breeding, green: year-round, ochre: nonbreeding.