The Asian koel is a brood parasite, and lays its single egg in the nests of a variety of birds, including the jungle crow, and house crow.
In Sri Lanka before 1880 it was only known to parasitize the jungle crow, later shifting to the house crow.
A study in India found 5% of Corvus splendens and 0.5% of Corvus macrorhynchos nests parasitized.

In Bangladesh, they parasitise long-tailed shrike (Lanius schach), common myna (Acridotheres tristis) and house crows (Corvus splendens) at about 35.7, 31.2 and 10.8% rates respectively.
Host nests at low heights and nearer to fruit trees tended to be preferred by koels.
In southern Thailand and the Malay Peninsula, koels have shifted host from crows to mynas (Acridotheres sp.)
as the latter became more common in the late 1900s.
In the Indian Subcontinent they have sometimes been found to parasitize the black drongo, the European magpie and possibly the black-headed oriole.
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.
The koel is not known to lay eggs in an empty host nest and a study in Pakistan found that the first koel eggs were laid, on average, within one and half days of the laying of the host's first egg.
The chicks of the koel hatched about 3 days ahead of the host chicks.
Koels usually lay only an egg or two in a single nest but as many as seven to eleven eggs have been reported from some host nests.
A female may remove a host egg before laying.
Eggs hatch in 12 to 14 days.
The young koel does not always push out eggs or evict the host chicks, and initially calls like a crow.
The young fledge in 20 to 28 days.
Unlike some other cuckoos, the young do not attempt to kill the host chicks, a trait that is shared with the channel-billed cuckoos which are also largely frugivorous as adults.
It has been suggested that koels, like some other brood parasites do not evict the host chicks due presumably due to the higher cost of evicting nestmates.
A small parasite may not be able to evict large host eggs or chicks from a deep Corvid nest without risking starvation and possibly accidental self-eviction.
An alternative hypothesis that retaining host chicks might benefit the koel chicks did not gain much support.
Adult female parents have been known to feed young koels in the nests of the hosts, a behaviour seen in some other brood parasitic species as well.
Adult males have however not been noted to feed fledglings.

The Asian koel is omnivorous, consuming a variety of insects, caterpillars, eggs and small vertebrates.
Adults feed mainly on fruit.
They will sometimes defend fruiting trees that they forage in and chase away other frugivores.
They have been noted to be especially important in the dispersal of the sandalwood tree (Santalum album) 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.
They have a large gape and are capable of swallowing large fruits including the hard fruit of palms such as Arenga and Livistona.
They have been known to occasionally take eggs of small birds.

They feed on the fruits of Cascabela thevetia which are known to be toxic to mammals.

A number of parasites of the species have been described, including malaria-like protozoa, lice and nematodes.
The Asian koel is a brood parasite, and lays its single egg in the nests of a variety of birds, including the jungle crow, and house crow. In Sri Lanka before 1880 it was only known to parasitize the jungle crow, later shifting to the house crow. A study in India found 5% of Corvus splendens and 0.5% of Corvus macrorhynchos nests parasitized.
In Bangladesh, they parasitise long-tailed shrike (Lanius schach), common myna (Acridotheres tristis) and house crows (Corvus splendens) at about 35.7, 31.2 and 10.8% rates respectively. Host nests at low heights and nearer to fruit trees tended to be preferred by koels. In southern Thailand and the Malay Peninsula, koels have shifted host from crows to mynas (Acridotheres sp.) as the latter became more common in the late 1900s. In the Indian Subcontinent they have sometimes been found to parasitize the black drongo, the European magpie and possibly the black-headed oriole. 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. The koel is not known to lay eggs in an empty host nest and a study in Pakistan found that the first koel eggs were laid, on average, within one and half days of the laying of the host's first egg. The chicks of the koel hatched about 3 days ahead of the host chicks. Koels usually lay only an egg or two in a single nest but as many as seven to eleven eggs have been reported from some host nests. A female may remove a host egg before laying. Eggs hatch in 12 to 14 days. The young koel does not always push out eggs or evict the host chicks, and initially calls like a crow. The young fledge in 20 to 28 days. Unlike some other cuckoos, the young do not attempt to kill the host chicks, a trait that is shared with the channel-billed cuckoos which are also largely frugivorous as adults. It has been suggested that koels, like some other brood parasites do not evict the host chicks due presumably due to the higher cost of evicting nestmates. A small parasite may not be able to evict large host eggs or chicks from a deep Corvid nest without risking starvation and possibly accidental self-eviction. An alternative hypothesis that retaining host chicks might benefit the koel chicks did not gain much support. Adult female parents have been known to feed young koels in the nests of the hosts, a behaviour seen in some other brood parasitic species as well. Adult males have however not been noted to feed fledglings.
The Asian koel is omnivorous, consuming a variety of insects, caterpillars, eggs and small vertebrates. Adults feed mainly on fruit. They will sometimes defend fruiting trees that they forage in and chase away other frugivores. They have been noted to be especially important in the dispersal of the sandalwood tree (Santalum album) 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. They have a large gape and are capable of swallowing large fruits including the hard fruit of palms such as Arenga and Livistona. They have been known to occasionally take eggs of small birds.
They feed on the fruits of Cascabela thevetia which are known to be toxic to mammals.
A number of parasites of the species have been described, including malaria-like protozoa, lice and nematodes.
Asian koel