From the Super Mario Wiki
Peepers are bird enemies in Yoshi's Story. They are mostly found at the highest parts of levels. They first appeared in 3-2, The Tall Tower, where they tried to stop Yoshi from reaching Inviso. If a Yoshi were to eat one, it will not heal the Yoshi but instead it will give him an egg to help him along the way. When a Peeper is shot down, a puff a smoke will appear and the Peeper will lose its feathers and fall away. If Yoshi eats a Peeper when they lose their feathers, will completely heal him.