Crop milk is a secretion from the lining of thecrop of parent birds in some species that isregurgitated to young birds. It is found among allpigeons and doves where it is also referred to aspigeon milk. Crop milk is also secreted from the crop offlamingos and the maleemperor penguin,[1][2][3] suggesting independentevolution of this trait.[4] 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.[5]
Lactation in birds is controlled byprolactin, which is the same hormone that causes lactation in mammals.[6][5] Crop milk is aholocrine secretion non-sex specific in response tobrooding unlike in mammals where milk is anapocrine secretion.[5] Crop milk contains both fat and protein, as with mammalian milk, but unlike mammalian milk, it contains no carbohydrates.[5]
Crop milk bears little physical resemblance tomammalianmilk, though in pigeons it is compositionally similar.[4] Pigeon milk is a semi-solid substance somewhat like pale yellowcottage cheese. It is extremely high inprotein andfat, containing higher levels than cow orhuman milk.[7] Unlike mammalianmilk, which is anemulsion, pigeon crop milk consists of a suspension of protein-rich and fat-rich cells that proliferate and detach from the lining of thecrop.[8] Also unlike mammalian milk, the substance does not contain carbohydrates (sugars).[9] It has also been shown to contain anti-oxidants and immune-enhancing factors which contribute tomilk immunity.[10] Like mammalian milk, crop milk containsIgA antibodies. It also contains some bacteria.[6] Crop milk has been shown to contain carotenoids such as xanthophylls, which may be important in immunological functions.[11]
Pigeon's milk begins to be produced a couple of days before theeggs 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.[12] 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.[13]
Crop milk evolved independently inflamingos and the maleemperor penguin.[1][2][3][4] In flamingos, crop milk produced in the first weeks is a bright red, relatively thin liquid.[5] Though it resembles blood in color, it contains no red blood cells; the red color is from the presence ofcanthaxanthin.[5] 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.[5]
{{citation}}: CS1 maint: multiple names: authors list (link)The fact that none of the nearly 300 species of Columbiformes has a clutch size larger than two eggs suggests that there is limited plasticity in crop-milk production.
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