Ungrateful Birds: No Good Deed Indices

There's a hidden bonus stage with a koi pond, cats, and a bonus umbrella that's very easy to pick up. On the third world map, walk east (right) of the stage called "Downstream" to get there. 

On the second world map, go south (press the down key) to follow a hidden path to a bonus stage entitled "Legendary Eggcarina." The Legendary Eggcarina item can be picked up at the top of the stage. 

Once you have the Eggcarina, it can be played in stages to instantly release all birds. It can only be used three times. If you pick up the item and clear the game without ever using it, you get awarded bonus stars on your save file.