Spawning
Chickens will spawn naturally on Grass Blocks. They can naturally spawn in any biome other than dessert and ocean biomes. As well as this, chickens can be spawned from a Chicken Spawn Egg which you can get from the creative menu. There is also a 1/8 chance of spawning a baby chicken when you throw and break an egg.
Drops
Drops are items which are 'DROPPED' by an animal. This usually happens when the animal is killed, unless specified.
Breeding
Two adult Chickens can breed to make a baby Chicken by giving some seeds to each of the adult Chickens.