Brood parasite is a term specifically applied to birds or insects that leave their eggs in the nests of other birds or insects to be raised. This relieves the parent parasites from the investment of rearing young, enabling them to feed only themselves and to lay more eggs. They usually only lay one egg per nest. Typically, the young of the brood parasites are larger than the young of the nest parent, and can out-compete them for feeding from the parent. Often, the brood parasite young will actually kick the host young out of the nest to their deaths, so that the parasite young becomes the only young in the nest.
Brood parasitic birds include the old-world cuckoos, cowbirds, wydahs and the honeyguides.
The cuckoo bees are a group of brood-parasitic insects, laying their eggs in the hives of other bees.
Most brood parasites are of only minor consequence for their hosts, but the Brown-headed Cowbird has become a serious invasive pest due to its habitat preference. It feeds in open areas adjacent to woodlands, and will marginally penetrate the forest fringe to lay its eggs in the nests of songbird species. As the North American forests have become fragmented, this has resulted in the Cowbird being a significant part of a negative set of edge effects, since they have gained access to a greater percentage of songbird nests.