Baby pigeons that are being raised by their parents eat a substance called "crop milk" for the first three days of their lives. This is produced from the lining of the adult pigeon's crop.
The baby puts its beak in the parent's mouth, and the parent pumps the contents of its crop into the baby's mouth. For hatchling pigeons that are being hand-raised by humans, there is a variety of commercially available foods to substitute for the crop milk. They are ideal to use because they contain special enzymes that the baby pigeon needs to properly digest food during the first three days of life.