Minecraft wiki entry for Egg, associated text: When thrown by a dispenser or by pressing the use button, an egg has a 1⁄8 (12.5%) chance of spawning a chick. If this occurs, there is a 1⁄32 (3.125%) chance of spawning three additional chicks (on average, 1 out of every 256 eggs spawns 4 chicks). In other words, whenever an egg is thrown, there is a 31⁄256 chance of spawning 1 chick and a 1⁄256 chance of spawning four chicks.