The Asian koel is a large member of the cuckoo family and like many of its related cuckoo kin is a brood parasite that lays its eggs in the nests of crows and other hosts, who raise its young. The male Asian koel is glossy bluish-black, with a pale greenish grey bill, the iris is crimson, and it has grey legs and feet. The female is brownish on the crown and has rufous streaks on the head. The back, rump, and wing coverts are dark brown with white and buff spots. The underparts are whitish but are heavily striped. The upper plumage of young birds is more like that of the male and they have a black beak.
Asian koels are mainly resident breeders in southern Asia from Iran, India, Bangladesh, and Sri Lanka to southern China and the Greater Sundas. They inhabit light woodlands, shrubby areas with scattered trees, gardens, plantations, and urban areas.
Asian koels are shy birds that are usually more often heard than seen. They spend their time singly or in pairs hiding in the dense canopy. Asian koels feed by day taking fruits directly from trees; they may sometimes defend favored trees and chase away other frugivores. Asian koels are very vocal especially during the breeding season, making a range of different calls. The familiar song of the male is a repeated 'koo-Ooo' and the female makes a rhythmic 'wook-wook' and shrill 'kik-kik-kik...' call.
The breeding season of Asian koels varies depending on location. In the Indian Subcontinent, it usually occurs from March to August. These birds don't build a nest and lay their eggs in the nests of a variety of birds, including the jungle crow, and house crow. They typically choose host nests that are located at low heights and nearer to fruit trees. Males may distract the hosts so that the female gets a chance to lay an egg in the nest. More often, however, the female visits the nest of the host alone. She usually lays only 1 egg or 2 in a single nest but as many as 7 to 11 eggs have been reported from some host nests. A female may remove a host egg before laying. The eggs of the koel hatch in 12 to 14 days, about 3 days ahead of the host chicks. The young koel does not always push out eggs or evict the host chicks, and initially calls like a crow. It usually fledges in 20 to 28 days and unlike some other cuckoos, the young koel does not attempt to kill the host chicks.
There are no major threats to Asian koels at present.
According to IUCN, the Asian koel is locally common throughout its range but no overall population estimate is available. The national population sizes of the species include around 10,000-100,000 breeding pairs and around 1,000-10,000 individuals on migration in China and less than 50 individuals on migration and around 100-10,000 breeding pairs in Taiwan. Currently, the Asian koel is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.
Asian koels are especially important in the dispersal of the sandalwood tree in India. Large seeded fruits are sometimes quickly regurgitated near the parent tree while small seeded fruits are ingested and are likely to be deposited at greater distances from the parent tree. This way koels contribute in shaping the ecology and evolution of tree populations.