How do birds digest their food in the absence of teeth?

They have a gizzard to grind the food.

Explanation:

Birds cannot chew since they have no teeth. However, the beak allows them to split very large seeds or swallow them directly.

For their digestion they have a stomach where digestion enzymes are released. Also they have a gizzard that is an organ that grinds the food mechanically. This gizzard usually has small stones inside that were previously ingested and that helps to digest the food.

its the cell wall and the they are unique to plant

the generally accepted parts of modern cell theory include: all known living things are made up of one or more cells. all living cells arise from pre-existing cells by division. the cell is the fundamental unit of structure and function in all living organisms.