The cowbird is a brood parasite. This means that they will lay their eggs in the nests of other bird species. A female cowbird will quietly scope out an available nest where the other bird species is actively laying eggs. Once she has found a site, she will sneak into the nest when the host is away, usually remove or damage eggs in the nest, and replace it with 1 or 2 of her own eggs.
The host birds will then raise the young cowbirds. The incubation period is typically shorter with cowbirds than any other songbird species, meaning that these young birds will hatch first and grow very large quickly. This makes it easy for them to demand all of the food from their foster parents and reduces the chances of brooding success for the other nesting birds.