Chocolate Birds are, as the name implies, small birds made out of chocolate that act like real birds.
In a flashback of Willy Wonka's first candy store, Charlie's grandfather tells Wonka that they are out of Wonka bars and chocolate birds. Wonka then takes a small egg and puts it in Grandpa Joe's mouth. He says, "Open!", and when he opens his mouth, a small chocolate bird is seen on his tongue.