Crop milk

Crop milk is a secretion from the lining of the crop of parent birds in some species that is regurgitated to young birds. It is found among all pigeons and doves where it is also referred to as pigeon milk. Crop milk is also secreted from the crop of flamingos and the male emperor penguin,  suggesting independent evolution of this trait. Unlike in mammals where typically only females produce milk, crop milk is produced by both males and females in pigeons and flamingos; and in penguins, only by the male. Lactation in birds is controlled by prolactin, which is the same hormone that causes lactation in mammals. Crop milk is a holocrine secretion, unlike in mammals where milk is an apocrine secretion. Crop milk contains both fat and protein, as with mammalian milk, but unlike mammalian milk, it contains no carbohydrates.

Pigeon milk
Crop milk bears little physical resemblance to mammalian milk, though in pigeons it's compositionally similar. Pigeon milk is a semi-solid substance somewhat like pale yellow cottage cheese. It is extremely high in protein and fat, containing higher levels than cow or human milk. A 1939 study of pigeon crop milk showed, however, that the substance did not contain carbohydrates (sugars), unlike mammalian milk. It has also been shown to contain anti-oxidants and immune-enhancing factors which contribute to milk immunity. Like mammalian milk, crop milk contains IgA antibodies. It also contains some bacteria. Unlike mammalian milk, which is an emulsion, pigeon crop milk consists of a suspension of protein-rich and fat-rich cells that proliferate and detach from the lining of the crop.

Pigeon's milk begins to be produced a couple of days before the eggs are due to hatch. The parents may cease to eat at this point in order to be able to provide the squabs (baby pigeons and doves) with milk uncontaminated by seeds, which the very young squabs would be unable to digest. The baby squabs are fed on pure crop milk for the first week or so of life, or about 10-14 days. After this the parents begin to introduce a proportion of adult food, softened by spending time in the moist conditions of the adult crop, into the mix fed to the squabs, until by the end of the second week they are being fed entirely on softened adult food.

Pigeons normally lay two eggs. If one egg fails to hatch, the surviving squab gets the advantage of a supply of crop milk sufficient for two squabs and grows at a significantly faster rate. Research suggests that a pair of breeding pigeons cannot produce enough crop milk to feed three squabs adequately, which explains why clutches are limited to two.

Other birds
Crop milk evolved independently in flamingos and the male emperor penguin. In flamingos, crop milk produced in the first weeks is a bright red, relatively thin liquid. Though it resembles blood in color, it contains no red blood cells; the red color is from the presence of canthaxanthin. After the first few weeks, the color fades gradually; flamingos can produce crop milk for up to the first 6 months of their young's life.