Chickens don't have teeth to chew on their food. They peck on their food, gets stored in their crop. The strong muscles of the gizzard grind the food who's nutrients get to the system.
You can't see these in caged chickens, but for newly released pastured chickens, notice how they go to pebbles and eat them? Don't be alarmed, it is normal :) They will need the small stones to grind the food, with the help of the strong muscles of the gizzard.
This is the gizzard of a newly slaughtered Sunshine Chicken. Obviously pastured well into time. Gizzard is big, strong and matured with age. The stones found inside have sort of polished as they worked and ground food that the chicken ate :)