Brown-headed Cowbirds (Molothrus ater) are brood parasites that lay their eggs in the nests of host species. Cowbird chicks typically grow up in mixed broods consisting of one or more cowbird chicks and one or more host chicks (as seen in this photo). In contrast, chicks of some brood parasitic species (e.g. European Cuckoo, Cuculus canorus) always push the hosts’ eggs or chicks out of the nest, ensuring that the parasite receives the full attention of the host adult. Such behavior is generally not known to occur among cowbirds.

The video below depicts a 6-day-old Brown-headed Cowbird nestling ejecting a 6-day-old Indigo Bunting nestling (Passerina cyanea) from the nest. This previously undocumented behavior was recorded on videotape by Don Dearborn at the University of Missouri’s Thomas Baskett Wildlife Research Area on 15 July 1995 and discussed in this paper: Dearborn DC. 1996. Video documentation of a Brown-headed Cowbird nestling ejecting an Indigo Bunting nestling from the nest. Condor 98(3): 645-649.

The video caused quite a stir among people who study brood parasitism, in part because a broader dataset presented in that same paper showed that host chicks disappeared from parasitized nests more often than from unparasitized nests, even when they were not losing weight (in those excluded cases, the host chicks might have starved to death due to competition with cowbird nestmates and then been removed by the host adult as a form of nest sanitation).