AKA pigeon's milk. This is a thick, usually white, milk-like substance produced by some birds, which is used to feed their young.

Pigeons:
Both male and female pigeons produce crop milk, which they feed to their hatchlings during the first week of life. Pigeon milk contains more protein and fat then does either cow or human milk. Pigeon milk is made in the crop, hence the name crop milk. When the hatchlings are about one week old, the parents start regurgitating seeds with the crop milk; eventually the seeds will replace this milk completely as the chicks' main food source.

Emperor penguins:
After the female penguin lays her egg, she heads out to sea to hunt. The male stays to watch over the egg, holding it off the ice on the tops of his feet. He will fast until the mother returns, in about two months. If the egg hatches before the mother returns, the father will feed the chick with a milky substance produced in his esophagus. Once the mother returns, the chick is raised on regurgitated food (not milk) from both parents. In the case of the Emperor penguin, the crop milk is a back-up safety measure, which may never be used if the mother returns in time.

Greater flamingos:
Young flamingos are fed milk for their first couple months, while their filter-feeding mouth-parts develop. The flamingo crop milk is actually produced by a number of glands spread though the entire upper digestive tract. This milk contains more fat and much less protein than does pigeon milk. Flamingo milk starts out a dark red colour, as it contains both red and white blood cells. Both male and female birds produce milk.

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