Last week I witnessed a Yellow Vented Bulbul scolding a Female Asian Koel in a nearby tree and wondered what the relationship was, after looking into it it appears that the Koel is a "Brood Parasite" that lays it's eggs in the nests of other birds. The fascinating thing about the Koel in Singapore is that it usually lays it's eggs in the nests of crows, however as crows are considered pests here (and therefore their nests are destroyed on sight), it has adapted to laying in the nests of much smaller (non-pest) birds. It's interesting to see that the host parents still feed these much larger adopted children - as is photographed in the article.
A more in depth study is available here: https://www.jstor.org/stable/23034196?seq=1