The correct answer is option 2: Birds. Pigeon milk refers to a substance produced by some species of pigeons and doves to feed their newborn hatchlings. It is not produced by crops or mammals. Pigeon milk is a secretion produced by both male and female pigeons in their crop, a part of their digestive system. This secretion is a nutritious, milky substance that contains fats, proteins, and other nutrients that are essential for the growth and development of the baby pigeons. It is similar to mammalian milk in its function of nourishing the young, but it is produced differently. While mammals produce milk in their mammary glands, pigeons and doves produce pigeon milk in their crop.