Well, you've seen my post on Vanya - a white headed babbler's adventures. After a babbler, now its time for a Koel.
First things first, the very unique behavior of Koel - it doesn't build a nest. Its a blood parasite. It lays its eggs in the crow's nest. Before it does, it pushes out a couple of crow's eggs from the nest, so the count remains the same when the crow comes back to its nest. Since its appearance its black, like that of a crow, when the eggs open up and the little koels come out, the crow thinks, they're its own and feeds the Koels!!!
Infact, the young koels even have a sound similar to that of crow. Once the koel is considerably older, its original voice, comes out. Only then the crow realizes that the little one is not its own and pushes it out of the nest.