A rock pigeon feeding squabs.

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,[1][2][3] suggesting independent evolution of this trait.[4] Unlike in mammals where 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 by prolactin, which is the same hormone that causes lactation in mammals.[6][5] Crop milk is a holocrine secretion, unlike in mammals where milk is an exocrine secretion.[5] Crop milk contains both fat and protein, as with mammalian milk, but unlike mammalian milk, it contains no carbohydrates.[5]

Pigeon milk

Crop milk bears little physical resemblance to mammalian milk, though in pigeons it's compositionally similar.[4] 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.[7] A 1939 study of pigeon crop milk showed, however, that the substance did not contain carbohydrates (sugars), unlike mammalian milk.[8] It has also been shown to contain anti-oxidants and immune-enhancing factors which contribute to milk immunity.[9] Like mammalian milk, crop milk contains IgA antibodies. It also contains some bacteria.[6] 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.[10]

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.[11] 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.[12]

Other birds

A greater flamingo chick in Zoo Basel is fed crop milk

Crop milk evolved independently in flamingos and the male emperor 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 of canthaxanthin.[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]

References

  1. 1 2 Levi, Wendell (1977). The Pigeon. Sumter, S.C.: Levi Publishing Co, Inc. ISBN 0-85390-013-2.
  2. 1 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. Archived from the original (PDF) on 13 September 2016.
  3. 1 2 Eraud, C.; Dorie, A.; Jacquet, A.; Faivre, B. (2008). "The crop milk: a potential new route for carotenoid-mediated parental effects" (PDF). Journal of Avian Biology. 39 (2): 247–251. doi:10.1111/j.0908-8857.2008.04053.x.
  4. 1 2 3 Ornithology, British Trust for (2012-08-22). "Crop milk". BTO - British Trust for Ornithology. Retrieved 2023-04-30.
  5. 1 2 3 4 5 6 7 Ann M. Ward, Amy Hunt, Mike Maslanka, and Chris Brown, Nutrient Composition Of American Flamingo Crop Milk (PDF){{citation}}: CS1 maint: multiple names: authors list (link)
  6. 1 2 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. Bibcode:2012PLoSO...748363G. doi:10.1371/journal.pone.0048363. PMC 3482181. PMID 23110233.
  7. ↑ Ehrlich, Paul R.; Dobkin, David S.; Wheye, Darryl (1988), "Bird Milk", stanford.edu
  8. ↑ Davis, W.L. (1939). "The Composition of the Crop Milk of Pigeons". Biochem. J. 33 (6): 898–901. doi:10.1042/bj0330898. PMC 1264463. PMID 16746989.
  9. ↑ Mysteries of pigeon milk explained, archived from the original on 2011-09-24
  10. ↑ 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.
  11. ↑ Vandeputte-Poma, J.; van Grembergen, G. (1967). "L'evolution postembryonnaire du poids du pigeon domestique". Zeitschrift für vergleichende Physiologie (in French). 54 (3): 423–425. doi:10.1007/BF00298228. S2CID 32408737.
  12. ↑ Blockstein, David E. (1989). "Crop milk and clutch size in mourning doves". The Wilson Bulletin. 101 (1): 11–25. JSTOR 4162684. 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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