20 May 2021

Brown-headed cowbirds (Molothrus ater) are the blackbird we love to hate.

Well known as a brood parasite, the female cowbird lays her eggs in the nests of smaller birds. The hosts foster her eggs and chicks while their own nestlings die. It’s particularly sad when we see a warbler feeding a cowbird chick knowing that his own nestlings did not survive.

According to Birds of Stanford only 3% of cowbird eggs make it to adulthood but this is achieved by flooding the market with cowbird eggs.

An average female cowbird can lay 40 eggs per season, usually one per nest, from mid-or-late April to mid-July. She doesn’t have a physical boundary between clutches, no regression of ovaries to shut off egg laying between clutches, so she just keeps going. This has lead ornithologists to characterize female cowbirds as “passerine chickens.”

Considering her output the female brown-headed cowbird is the white leghorn chicken of songbirds. Fortunately she doesn’t lay as many eggs as a leghorn, 300 per year!

(photos from Wikimedia Commons; click on the captions to see the originals)