Question: Do birds chew their food?
Answer: Yes, depending on one's definition of "chew."
Birds don't have teeth, so they don't chew with their bills. However, many species "chew" food in a second, muscular stomach, also called a gizzard. Food starts in the first or true stomach. Tough food is passed to the second stomach, the gizzard. The gizzard can grind the food and pass it back to the true stomach and vice versa.
Many birds fill their gizzards with small grit, stones, or gravel which serve the same function as teeth -- breaking down (chewing) hard food and thus helping digestion.
Gizzards have a tough inner membrane made of the protein keratin. That layer is surrounded by a muscular pouch which provides the grinding action.