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| Reed Warbler raising a Common Cuckoo, a brood parasite. |
Brood parasitism, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism.
After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Brood parasites may be either
obligate brood parasites, which must lay their eggs in the nests of other species because they are incapable of raising their own young, or
non-obligate brood parasites, which sometimes lay eggs in the nests of conspecifics to increase their reproductive output even though they could have raised their own young. One hundred bird species, including honeyguides, icterids, estrildid finches and ducks, are obligate parasites, though the most famous are the cuckoos.
Some brood parasites are adapted to hatch before their host's young, which allows them to destroy the host's eggs by pushing them out of the nest or to kill the host's chicks; this ensures that all food brought to the nest will be fed to the parasitic chicks.
