This is a great article!

Birds are well known for their parental care, patiently incubating their eggs and then bringing food to their young until they are old enough to look after themselves. However, certain birds, known as brood parasites, lay their eggs in the nests of other birds and do not provide any parental care for their own offspring. Care that the "hosts" provide to the young parasites is care denied to their own young. This often has a detrimental effect on the reproductive success of the hosts and may affect their population numbers as well.

There are two types of brood parasitism, non-obligate and obligate. Non-obligate brood parasites lay eggs in the nest of conspecifics (i.e. same species) and in their own nests. Examples include several colonial nesting species such as Bank Swallows or African Weavers. Obligate brood parasites lay eggs in nests of other species and have completely lost the ability to construct nests and incubate eggs. Examples include Brown-headed Cowbirds and European Cuckoos. About 1% of all known bird species are obligate brood parasites. Other obligate brood parasites include: all African Honeyguides, about half of the species of cuckoos, the Black-headed Duck in South America, Shiny Cowbirds, Screaming Cowbirds, Bronzed Cowbirds, and Giant Cowbirds.

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