Pigeons make "milk."
Of course, they do not make milk; only animals can do so. It is, nevertheless, fundamentally the same as milk in that it is a white liquid containing all of the antioxidants, lipids, minerals, and proteins that a newborn bird requires. Another thing it has in common with animal milk is that its synthesis is controlled by the hormone prolactin. This milk is produced by both male and female mature pigeons in their crop. The crop is a small part of a bird's esophagus used for food storage and, in this case, crop milk production. Pigeons are one of just a few birds that produce crop milk, along with doves, flamingos, and male emperor penguins.