Doves with young develop a layer of cells lining their crops. These cells are richer in fat and protein than cow's milk, and they slough off to produce a very nutritious food for baby doves. Unlike mammals, both parents produce crop milk. At first, the adults don't eat, so the crop milk will not be contaminated by other foods. As the babies grow, they eat more and more seeds, which soften and are fed to the young along with the crop milk.
Greater Flamingos and Emperor Penguins have the ability to produce similar food for their young.