The Witch's House
The bird eats the Greenery that is blocking the way to the Skull Puzzle Room and Poisonous Hall. You won't be able to open the door to the birdcage. To get it out, you need to give the White Powder to the person in the cell. They will then be angry for not having a Jade Pipe and will accidently knock over the cage making acces to let the bird out.
In Extra Mode you need to have a Green Candle to burn the rope to open the cage, but the bird won't leave. To get it out, you need have it eat the Green Candle.
Once it has eaten the greenery you have access to the rooms that had their paths blocked.