Odd Man In

"The Brown-headed Cowbird (Molothrus ater) is a brood parasite, meaning that it lays its eggs in nests of other species. A female cowbird quietly searches for female birds of other species that are actively laying eggs. Once she has found a suitable host, the cowbird will sneak onto the resident bird’s nest when it is away, usually damage or remove one (or more) egg, and replace that egg with one (or more) of her own (watch a cowbird laying an egg in a Northern Cardinal nest on NestCams). The foster parents then unknowingly raise the young cowbirds, usually at the expense of their own offspring. Cowbird eggs require a shorter incubation period than most other songbirds and thus usually hatch first. Cowbird nestlings also grow large very quickly. These advantages allow them to command the most food from their foster parents, usually resulting in reduced nesting success of the host species."

from the Cornell Lab of Ornithology's Net Watch