The first indications of feather tracts appear during the fifth day of embryonic development when the feather papillae appear. The Skeleton of the fowl is compact, lightweight, and strong. Another interesting feature of chicken bones is called medullary bone. This bone fills the narrow cavity with a readily available source of calcium for eggshell formation when calcium intake is not sufficient. The chicken has a simple digestive system, with few to no microorganisms living in the digestive system to help digest food like in ruminants such as cattle.
Chickens depend on enzymes to aid in breaking down food so it can be absorbed, much like humans.
The beak of the bird replaces the mouth and lips. The crop is a pouch formed to serve as a storage area for the food until it can be passed along for digestion in the gizzard and intestines. The gizzard is the oval organ composed of two pairs of thick red muscles. These muscles are extremely strong and are used to grind or crush the food particles. This process is aided by the presence of grit and gravel picked up by the bird.