Crop milk is a secretion from the lining of the

crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. In other words, a crop is a plant or plant product that is grown for a specific purpose such as food, Fiber, fibre, or fuel. When plants of the same spe ...

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 The emperor penguin (''Aptenodytes forsteri'') is the tallest and heaviest of all living penguin species and is Endemism in birds, endemic to Antarctica. The male and female are similar in plumage and size, reaching in length and weighing fr ...

, suggesting independent

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

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 Prolactin (PRL), also known as lactotropin and mammotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secr ...

, which is the same hormone that causes lactation in mammals. Crop milk is a holocrine secretion non- sex specific in response to

brooding Broodiness is the action or behavioral tendency to sit on a clutch of eggs to Egg incubation, incubate them, often requiring the non-expression of many other behaviors including feeding and drinking.Homedes Ranquini, J. y Haro-García, F. Zoogen� ...

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

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

ian

milk Milk is a white liquid food produced by the mammary glands of lactating mammals. It is the primary source of nutrition for young mammals (including breastfeeding, breastfed human infants) before they are able to digestion, digest solid food. ...

, though in pigeons it is compositionally similar. Pigeon milk is a semi-solid substance somewhat like pale yellow cottage cheese. It is extremely high in

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

and

fat In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers specif ...

, containing higher levels than cow or human milk. Unlike mammalian

, which is an

emulsion An emulsion is a mixture of two or more liquids that are normally Miscibility, immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloi ...

, pigeon crop milk consists of a suspension of protein-rich and fat-rich cells that proliferate and detach from the lining of the

. Also unlike mammalian milk, the substance does not contain carbohydrates (sugars). 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. Crop milk has been shown to contain carotenoids such as xanthophylls, which may be important in immunological functions. 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 to be able to provide the squabs (baby pigeons and doves) with crop 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. 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

. 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.

References

External links

Article on Bird Milk from stanford.edu

Article on the relationship between crop milk and clutch size in Mourning Doves (.pdf)

Smithsonian National Zoo article on Common Crowned Pigeon, includes paragraph on crop milk
{{Birds Bird anatomy Bird behavior Milk by animal de:Kropf#Kropfmilch