Field Notes: BROWN-HEADED COWBIRD
The male Brown-headed Cowbird is a black bird with a brown head. The female is a light gray-brown. They are common throughout North America and will sometimes be seen in flocks with other blackbirds. Brown-headed Cowbirds are fascinating for how they deal with their young. They never build their own nests. Instead, they parasitize smaller birds nests, laying their eggs and leaving them for the owner of the nest to raise. I have seen Brown-headed Cowbirds being raised and fed by sparrows. The parasitized species doesn’t seem to know the difference. Unfortunately, they are so common their nesting habits are threatening some of the species they parasitize. When the mother drops her egg in a nest if that nest already contains eggs from the other species, she will sometimes eat or push out one of the eggs. And when all the eggs hatch, the Cowbird, being a larger bird, can crowd out the other hatchlings and out-complete them for food.
I was lucky enough to see a female Brown-headed Cowbird inspecting a nest to determine if it was suitable for her eggs. In one of the photos she can be seen sticking her head into the nest. I knew that Brown-headed Cowbirds leave their eggs in the nests of smaller birds, but I hadn’t expected them to look for birds this small. The nest in the photos is still being built by a pair of Blue-gray Gnatcatchers, which are around 4.5 inches. The Brown-headed Cowbird is around 7.5 inches. I don’t know whether the Cowbird laid any eggs in this nest, but unfortunately, no hatchlings were ever raised from it. The entire branch the nest was built on was downed in a storm that passed through not that long after it was built.