Common Cuckoo Cuculus canorus

Common Cuckoo adult male
Common Cuckoo
The Cuckoo, known as the Common Cuckoo outside the UK to distinguish from other birds in the cuckoo family, is well-known for the call after which it is named, for its summer migration to Europe/Asia from its wintering grounds in Africa/South Asia and for being a brood parasite (laying its eggs in the nests of other birds). The adult male, above left - photo by Alexandra Makhnina - is grey with hawk-like grey bars on its front. The adult female is also usually grey, the hawk-like appearance helping her to access the nests of small birds.
Common Cuckoo hepatic female
Common Cuckoo adult female hepatic
Common Cuckoo hepatic phase female
Rarely, the adult female Common Cuckoo is rufous brown, known as "hepatic phase" after the colour of liver. The hepatic phase brown Cuckoo above is overlooking a reed bed, waiting for the resident Reed Warbler to leave the nest long enough for the Cuckoo to lay an egg beside the host's eggs. The tiny Reed warblers, together with Dunnocks and Meadow Pipits, are favoured hosts.
Common Cuckoo
Common Cuckoo
Common Cuckoo
The feet are yellow, as are usually the base of the bill and the eye-ring. Juvenile Cuckoos can also be brown, as shown above, but look very different to the hepatic phase adult female.