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 milk
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. In pigeons and flamingoes, both the male and female adult produce crop milk and share in the feeding and care of the young. In penguins, only the males lactate. 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 squabs
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. Soy based formula, nutritional value->
The soy based formula must not contain lactose because birds can not digest it. The infant formula mix is used in the same way(Part powder to part water) as Nutribird A21, depending on age of the bird.
Overfilling the crop may lead to fermentation and souring of the crop milk substitute. The crop should still feel *squishy* after a feed, not like a full water balloon or hard like a ball. If the content has once turned to fermentation, the crop needs to be emptied by a vet or experienced carer to avoid death. 
When the crop empties slower than normal and fermentation may occur, adding enzyme powder to weak fennel tea and feeding it to the bird will aid digestion and cause a faster emptying of the crop.
Crop milk substitute can be feed again after the crop has emptied fully and feeding has paused for two hours to minimize the risk of bacterial residues in the crop which might restart the fermentation.
Birds who suffered from fermenting crop content once, are prone to reoccurring bouts of faulty digestion. Problems can be minimized by adding more enzymes to the crop milk substitute to aid proper digestion.
Using boiled, cooled down water to mix the formula, keeping feeding equipment sterile and the bird properly warm, is the main key to prevent problems with the crop. Especially very young birds who have problems to keep their body temperature by themselves are in danger of suffering from sour crop if their body temperature drops below the normal range.
Baby birds can only properly digest their food if their normal range of body temperature is ensured.
- 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).
- Levi, Wendell (1977). The Pigeon. Sumter, S.C.: Levi Publishing Co, Inc. ISBN 0-85390-013-2.
- Silver, Rae (1984). "Prolactin and Parenting in the Pigeon Family" (PDF). The Journal of Experimental Zoology 232 (3): 617–625. doi:10.1002/jez.1402320330. PMID 6394702.
- 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.
- Ehrlich, Paul R.; Dobkin, David S. and Wheye, Darryl (1988) Bird Milk. stanford.edu
- "Mysteries of pigeon milk explained" Retrieved September 22, 2011
- Gillespie, M. J.; Stanley, D.; Chen, H.; Donald, J. A.; Nicholas, K. R.; Moore, R. J.; Crowley, T. M. (2012). Salmon, Henri, ed. "Functional Similarities between Pigeon 'Milk' and Mammalian Milk: Induction of Immune Gene Expression and Modification of the Microbiota". PLoS ONE 7 (10): e48363. doi:10.1371/journal.pone.0048363. PMC 3482181. PMID 23110233.
- Gillespie, M. J.; Haring, V. R.; McColl, K. A.; Monaghan, P.; Donald, J. A.; Nicholas, K. R.; Moore, R. J.; Crowley, T. M. (2011). "Histological and global gene expression analysis of the 'lactating' pigeon crop". BMC Genomics 12: 452. doi:10.1186/1471-2164-12-452. PMC 3191541. PMID 21929790.
- Vandeputte-Poma, J.; van Grembergen, G. (1967). "L'evolution postembryonnaire du poids du pigeon domestique". Zeitschrift für vergleichende Physiologie 54 (3): 423–425. doi:10.1007/BF00298228.
- Blockstein, David E. (1989). "Crop milk and clutch size in mourning doves". The Wilson Bulletin 101 (1): 11–25. Retrieved 25 March 2016.
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.