The Common Cuckoo (Cucullus canorus) is a member of the cuckoo order of birds, the Cuculiformes, which also includes the roadrunners, the anis, the coucals , and the Hoatzin.

It is a widespead summer migrant to Europe and western Asia, and winters in Africa. It is a brood parasite, which lays its eggs in the nests particularly of Dunnocks, Meadow Pipits, and Reed Warblers.

Female Cuckoos are divided into gentes, that is populations favouring a particular host species nest and laying eggs which match those of that species' in colour and pattern.

The exception is in the case of the Dunnock, where the Cuckoo's egg has no resemblance to its hosts' blue eggs. This is thought to be because the Dunnock is a recent host, and has not acquired the ability to distinguish eggs. Male cuckoos breed with females without regard to gens. This results in gene flow between the gentes and maintains a common gene pool for the species.

The chick which hatches from the egg laid in the other species' nest methodically evicts all other occupants of the nest, a behaviour that was first described by Edward Jenner. This is necessary since it is a much larger bird than its hosts, and needs to monopolise the parents' food supplies.

This cuckoo is a greyish bird with a slender body, long tail and strong legs. The females only are sometimes brown, the “hepatic” phase. It looks like a small bird of prey in flight, although the wings stay below the horizontal.

It is a bird of open country. Its food is insects, with hairy caterpillars, which are distasteful to many birds, being a speciality.

The cuckoo group gets its English and scientific names from the call of the male Common Cuckoo, usually given from an open perch, goo-ko. The female has a loud bubbling call. In England, hearing the call of the Cuckoo is regarded as the first harbinger of spring, and The Times newspaper notoriously features correspondence every year reporting the first calls.

In Russia, there's a popular belief that a cuckoo can predict how many more years a person will live. If a person hears a cuckoo in the woods, he or she usually asks "Cuckoo, cuckoo, how long will I live?". It is believed that a person will live as many years as a cuckoo cuckooed.

The word "cuckold" derives from the Cuckoo's practice of tricking other birds into raising its young.