The Eurasian cuckoo (Cuculus canorus) (formerly European cuckoo) is a member of the cuckoo order of birds, Cuculiformes, which includes the roadrunners, the anis and the coucals. This species is a widespread summer migrant to Europe and Asia, and winters in Africa. It is a brood parasite, which means it lays eggs in the nests of other bird species, particularly of dunnocks, meadow pipits, and reed warblers. Although its eggs are larger than those of its hosts, the eggs in each type of host nest resemble the host's eggs. The adult too is a mimic, in its case of the sparrowhawk; since that species is a predator, the mimicry gives the female time to lay her eggs without being seen to do so.
The common cuckoo is 32–34 centimetres (13–13 in) long from bill to tail (with a tail of 13–15 centimetres (5.1–5.9 in) and a wingspan of 55–60 centimetres (22–24 in). The legs are short. It is greyish with a slender body and long tail and can be mistaken for a falcon in flight, where the wingbeats are regular. During the breeding season, common cuckoos often settle on an open perch with drooped wings and raised tail. There is a rufous colour morph, which occurs occasionally in adult females but more often in juveniles.
A study using stuffed bird models found that small birds are less likely to approach common cuckoos that have barred underparts similar to the Eurasian sparrowhawk, a predatory bird. Eurasian reed warblers were found more aggressive to cuckoos that looked less hawk-like, meaning that the resemblance to the hawk helps the cuckoo to access the nests of potential hosts. Other small birds, great tits and blue tits, showed alarm and avoided attending feeders on seeing either (mounted) sparrowhawks or cuckoos; this implies that the cuckoo's hawklike appearance functions as protective mimicry, whether to reduce attacks by hawks or to make brood parasitism easier.
Hosts attack cuckoos more when they see neighbors mobbing cuckoos. The existence of the two plumage morphs in females may be due to frequency-dependent selection if this learning applies only to the morph hosts see neighbors mob. In an experiment with dummy cuckoos of each morph and a sparrowhawk, reed warblers were more likely to attack both cuckoo morphs than the sparrowhawk, and even more likely to mob a certain cuckoo morph when they saw neighbors mobbing that morph, decreasing the reproductive success of that morph and selecting for the less common morph.
This picture was taken at the Mahananda Wildlife Sanctuary situated at Darjeeling district of West Bengal.