Minecraft wiki entry for Hunger_management, associated text: Advantages:
Chickens are relatively easier to find[verify] in the world than pigs or cows, and sheep are also easy to find.
Chickens are easily killed, as they have only 4 health, making obtaining raw chicken both time effective, and food/saturation effective.
Supplies of raw chicken are easier to maintain, as chickens also drop eggs which can be hatched into chicks for mass production of raw chicken rather than being dependent on seeds or wheat for breeding.
Sheep can be sheared to get wool.
Chicken farming can be automated, providing an easy way of collecting eggs, raw chicken and feathers.