Crop milk is a secretion from the lining of the crop of parent birds that is regurgitated to young birds. It is found among all pigeons and doves where it is referred to as pigeon milk. An analog to crop milk is also secreted from the esophagus of flamingos and some penguins.
Comparison to mammalian milkEdit
Crop milk bears little physical resemblance to mammalian milk, the former being 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. It has also been shown to contain anti-oxidants and immune-enhancing factors. 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. Lactation in birds is also controlled by prolactin, which is the same hormone that causes lactation in mammals.
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. 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 adequately feed three squabs, which explains why clutches are limited to two.
Hand-rearing pigeon squabsEdit
The very high nutrient concentration of pigeon's milk makes it difficult to devise an artificial substitute for hand-rearing squabs. Various proprietary mixes are available. A good home-made mix can be made from equal quantities of dehydrated dried pure soy protein (with no added salt, flavorings, preservatives etc.) and soy-based dairy-free butter-substitute (ordinary dairy butter is not suitable, as young squabs' digestive systems cannot cope with dairy products). This is made into a runny paste with a little water, a tiny crumb of a multivitamin tablet and a tiny pinch of chalk (which is necessary for bone development to avoid splay-leg). The crop should be allowed to empty completely between feedings. With care and gentleness, it is possible to raise a pigeon on this mix from the moment of hatching.
Soy based infant formula can be used as a substitute when other options are not available. The content of vitamins and minerals is very similar to crop milk. 
Overfilling the crop may lead to fermentation and souring of the crop milk substitute. If the content has undergone fermentation, the crop needs to be emptied by a vet or experienced caretaker to avoid death. 
- In Animal Crossing: Wild World for the Nintendo DS and Animal Crossing: City Folk for the Wii, a character named Brewster uses it as an ingredient when serving coffee to long-time customers.
- In the reality TV series Flavor of Love during Season 2, the father of Deelishis requests a carton of pigeon milk at dinner.
- Ptasie mleczko (Polish: bird milk) is the name of Polish candy, which variations are also popular in Eastern Europe countries as Ptichye Moloko (Russian: птичье молоко), ptashyne moloko (Ukrainian: птaшине молоко).
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- Eraud, C., Dorie, A., Jacquet, A. and Faivre, B. (2008). "The crop milk: a potential new route for carotenoid-mediated parental effects". Journal of Avian Biology. 39 (2): 247–251. doi:10.1111/j.0908-8857.2008.04053.x.
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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.
- http://www.versele-laga.com/en-NL/nutribird/Products/nutribird-a21. Retrieved 21 July 2017. Missing or empty
- "Archived copy". Archived from the original on 2012-03-17. Retrieved 2014-02-21.