The crop is a pouch just under the skin at the front of the chest/neck over to their right side that chickens fill up with food through the day and it slowly releases the food into their digestive tract overnight.... a bit like an enclosed hopper or funnel. In the morning it should be pretty much empty until they start filling it again. Chicks about 5-6 weeks of age seem to go though a phase when they really stuff it full of food and it protrudes from the chest like they swallowed a golf ball, but obviously feels slightly softer and granular, because of the feed it contains.