Brood parasitism is extremely rare in the vertebrate world. It occurs in a few kinds of fish and in some species of ants, bees, wasps, beetles, and butterflies, but no reptiles or mammals. In the bird world, just 1 percent of species, around one hundred, are so-called obligate parasites, dependent on this way of reproducing. The strategy is risky, but for some birds, it seems to work. It has evolved independently at least seven times in different bird lineages: in ducks, honeyguides, finches, and cowbirds, and three times in different lines of cuckoos.
