Nest Parasitism and the Brown-Headed Cowbird

By Dave Hanks

There is a bird, a black bird, that is becoming a major problem for other smaller bird species. The male is black but has a brown head that blends in with his black body. The brown is sometimes hard to distinguish. The female is totally a pale brown. This is a species that, in years past, lived on the Great Plains and could be viewed riding on the backs of Bison. In that environment, it was an advantage to the bird to lay as many eggs as possible – more than two parents could care for. Therefore, the answer was to secretly lay an egg in another species nest and let them raise the chick. Since the arrival of Europeans to this continent - forests have diminished and open areas have increased. Thus the cowbird’s range has increased dramatically. A problem that once was kept within bounds has now intensified.

It would not be a problem if the host species could also raise their own chicks along with the intruder. The adoptive parents can not seem to recognize the different egg and throw it out of their nest. That egg hatches first, the chick grows faster than the others, and it soon kills (“Cain-ism”) its siblings and throws them out of the nest. The adoptive parents continue to raise the chick until it fledges. This nest parasitism is especially hard on warblers and other small species that historically nested in woodlands.

There are only two nest parasites in the USA. Both are cowbirds, with the Brown-Headed the most common. Besides open areas, untrimmed grass and livestock attract this bird. Letting others raise your chicks frees one up to forage – on the ground or the backs of ruminants, and the female has an amazing reproductive capacity. She may lay up to 80 eggs over a two month period. The eggs are white, with either a bluish or greenish tint, and are speckled reddish-brown. The Brown-Headed Cowbird’s song is a series of liquidly, gurgling sounding notes, which they make with their heads and beaks pointing straight up.

We have observed that they will readily come to bird feeders and are present in our yard during spring and summer months. That makes me wonder how many Yellow Warblers, Wilson Warblers, and Song Sparrows are being displaced on our property. We have seen a Song Sparrow feeding a baby that was twice as large as the sparrow was.

(Foraging on the ground in our yard’s vegetation))