Crop milk

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A rock pigeon feeding squabs. RockPigeonkids.jpg
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 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 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 apocrine secretion. [5] Crop milk contains both fat and protein, as with mammalian milk, but unlike mammalian milk, it contains no carbohydrates. [5]

Contents

Pigeon milk

Crop milk bears little physical resemblance to mammalian milk, though in pigeons it is 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 Rosaflamingo-Kuken - Fuetterung mit Kropfmilch.jpg
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]

Related Research Articles

<span class="mw-page-title-main">Milk</span> White liquid produced by the mammary glands of mammals

Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest solid food. Immune factors and immune-modulating components in milk contribute to milk immunity. Early-lactation milk, which is called colostrum, contains antibodies that strengthen the immune system and thus reduce the risk of many diseases. Milk contains many nutrients, including calcium and protein, but also lactose and saturated fat.

<span class="mw-page-title-main">Poultry</span> Domesticated birds kept by humans for their eggs, meat, or feathers

Poultry are domesticated birds kept by humans for the purpose of harvesting animal products such as meat, eggs or feathers. The practice of raising poultry is known as poultry farming. These birds are most typically members of the superorder Galloanserae (fowl), especially the order Galliformes. The term also includes waterfowls of the family Anatidae but does not include wild birds hunted for food known as game or quarry.

Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Mechanical digestion takes place in the mouth through mastication and in the small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small compounds that the body can use.

<span class="mw-page-title-main">Flamingo</span> Family of birds

Flamingos or flamingoes are a type of wading bird in the family Phoenicopteridae, which is the only extant family in the order Phoenicopteriformes. There are four flamingo species distributed throughout the Americas, and two species native to Afro-Eurasia.

<span class="mw-page-title-main">Rock dove</span> Species of bird

The rock dove, rock pigeon, or common pigeon is a member of the bird family Columbidae. In common usage, it is often simply referred to as the "pigeon", although this is the wild form of the bird; the pigeons most familiar to people are the domesticated form of the wild rock dove.

<span class="mw-page-title-main">Prolactin</span> Protein family and hormone

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 secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation and nursing. It is secreted heavily in pulses in between these events. Prolactin plays an essential role in metabolism, regulation of the immune system and pancreatic development.

<span class="mw-page-title-main">Mammary gland</span> Exocrine gland in humans and other mammals

A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word mamma, "breast". The mammary glands are arranged in organs such as the breasts in primates, the udder in ruminants, and the dugs of other animals. Lactorrhea, the occasional production of milk by the glands, can occur in any mammal, but in most mammals, lactation, the production of enough milk for nursing, occurs only in phenotypic females who have gestated in recent months or years. It is directed by hormonal guidance from sex steroids. In a few mammalian species, male lactation can occur. With humans, male lactation can occur only under specific circumstances.

<span class="mw-page-title-main">Breast milk</span> Milk produced by the mammary glands in the breast of a human female

Breast milk or mother's milk is milk produced by the mammary glands in the breast of human females. Breast milk is the primary source of nutrition for newborn infants, comprising fats, proteins, carbohydrates, and a varying composition of minerals and vitamins. Breast milk also contains substances that help protect an infant against infection and inflammation, such as symbiotic bacteria and other microorganisms and immunoglobulin A, whilst also contributing to the healthy development of the infant's immune system and gut microbiome.

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The domestic pigeon is a pigeon subspecies that was derived from the rock dove or rock pigeon. The rock pigeon is the world's oldest domesticated bird. Mesopotamian cuneiform tablets mention the domestication of pigeons more than 5,000 years ago, as do Egyptian hieroglyphics. Pigeons were most likely domesticated in the Mediterranean at least 2000–5000 years ago, and may have been domesticated earlier as a food source. Research suggests that domestication of pigeons occurred as early as 10,000 years ago.

<span class="mw-page-title-main">Imperial pigeon</span> Genus of birds

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<span class="mw-page-title-main">Squab</span> Food; young domestic pigeon, typically under four weeks old

In culinary terminology, squab is an immature domestic pigeon, typically under four weeks old, or its meat. Some authors describe it as tasting like dark chicken.

<span class="mw-page-title-main">Lactation</span> Release of milk from the mammary glands

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<span class="mw-page-title-main">Oscar Riddle</span> American biologist

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References

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  9. Mysteries of pigeon milk explained, archived from the original on 2011-09-24
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  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.