Any of some 138 species of tree-dwelling and terrestrial birds of the family Cuculidae. They are found worldwide in temperate and tropical regions but are most diverse in the Old World tropics. New World species are sometimes classified as a separate family (Coccyzidae) and include the roadrunner. Cuckoos range from 6.5 to 36 in. (16 to 90 cm) long. Most are drab gray, but a few are partially or completely brightly coloured or iridescent. Aside from the European cuckoo's familiar two-note call, cuckoos are best known for their habit of brood parasitism (see cowbird); their eggs resemble those of the host species (egg mimicry), and the adult cuckoo removes one or more host eggs to ensure that the substitution is not detected. The newly hatched cuckoo may also eject eggs or nestlings.
Cuckoo (Cuculus).—Graeme Chapman/Ardea London
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