"Coo Coo" (The Larks' Melody) is a song from "Sleepyhead". The larks sing a song high in a tree and Flick sings loudly but Pinch refuses them so they sang a soothing lullaby called Oodelee, Oodelay to make Walter Raccoon go to sleep.
Flick (Horribly singing):
Cuckoo. Cuckoo! I'm so cuckoo when I'm with you-you!!!