Although the British Cuckoo is notorious for its habit of parasitising the nests of others, most species in the family are not so disruptive; more than half rear their own young. The cuckoo family is a diverse one, though all are united by similarities in anatomy. Many species resemble small hawks with long wings and a down-curved bill. Most species occur in tropical regions. Although the sexes generally look alike, in many species (including the British one) the sexes have different calls.
Amongst the parasitic cuckoos, the nest in which a female will lay is genetically determined (that of the same species in which she was raised) and there is a continual 'arms-race' between the cuckoos and the host: by the cuckoo to mimic the pattern and colouration of the host's eggs, and by the host to spot the impostor. Some species such as the Pied Wagtail, have evolved the knack of spotting the cuckoo egg and will reject it immediately, at least in countries where they have experience of cuckoos; in Iceland, where cuckoos do not occur, Pied Wagtails are still naive. Others, such as the Reed Warbler, have not yet learned and are still frequently parasitised.