CONTAINS SPOILERS

The key to the Cat Shine locked in the bird cage can be found by bouncing your way up Pounce Bounce Isle to the area where the Boomerang Bros first appear. It's located on top of the wall facing the slide. The key is only revealed once the objective "Key to the Cat Shine" appears. After picking up the key, make sure to not attempt to swim, otherwise you'll have to retrieve it again.