Producing milk to nourish young is usually associated with mammals. However, a small number of avian species independently evolved a unique and effective adaptation to feed their hatchlings a highly nutritious liquid or semi-solid substance.
The Primary Example: Crop Milk in Pigeons and Doves
The most widely recognized examples of this parental feeding strategy are found within the family Columbidae (pigeons and doves). The specialized substance they produce is known as “crop milk.” Both male and female parents participate equally in generating this secretion to feed their young.
The source of this substance is the crop, a muscular pouch near the lower end of the esophagus used by most birds for temporary food storage. In pigeons and doves, the crop lining undergoes a dramatic structural change before the eggs hatch. This transformation allows the parents to provide the sole source of nourishment for the nestlings, called squabs, during their initial days of life.
The parents feed the squabs by regurgitation; the young stimulate the milk’s release by inserting their beaks into the adult’s mouth. This method ensures the rapidly growing squabs receive an easily digestible, energy-dense diet. As the young birds mature, the parents gradually introduce softened adult food mixed with the crop milk until the squabs are fully weaned.
Composition and Production of Avian Crop Milk
Avian crop milk is fundamentally distinct from mammalian milk, as it is a semi-solid, curd-like material, not a fluid secretion. It is classified as a holocrine secretion, meaning the entire epithelial cells lining the crop wall are shed to form the substance. These detached cells are exceptionally rich in stored nutrients, giving the milk a consistency often described as resembling pale-yellow cottage cheese.
The nutritional profile is noteworthy for its high concentration of fat and protein. On a dry matter basis, pigeon crop milk contains approximately 50 to 60 percent protein and 35 to 45 percent fat, exceeding the levels found in cow and human milk. This dense composition provides the necessary fuel for the squabs’ rapid growth rate during their first week.
The entire process of cellular proliferation and shedding is stimulated by the hormone prolactin, the same hormone that governs milk production in mammals. Prolactin acts on the crop lining, triggering cellular growth and lipid accumulation that precedes the final detachment of the cells into the crop’s lumen. The substance also contains antioxidants, immune-enhancing factors, and antibodies, which contribute to the squab’s early immunity.
Other Birds That Feed Young Secretions
Beyond the pigeon and dove family, a few other bird species feed their young specialized secretions. Flamingos, including the Greater Flamingo, produce a form of crop milk synthesized by secretory glands lining the entire upper digestive tract, not just the crop. Both male and female parents produce this substance.
Flamingo crop milk has a different appearance, sometimes exhibiting a bright red color. This is due to the presence of the pigment canthaxanthin, derived from the parents’ diet of microscopic algae and crustaceans. The transfer of this pigment means parents can temporarily lose some of their characteristic pink coloration while feeding their chicks.
Another unique example is the Emperor Penguin, where the male parent alone produces a highly specialized secretion to sustain the chick. This secretion is produced in the esophagus and stomach. It provides a survival ration, allowing the chick to survive for an extended period until the female returns with a meal.
Key Biological Differences From Mammalian Milk
The term “milk” is used for avian secretions because of the shared function of nourishing the young, but the biological distinction from mammalian milk is profound. Mammalian milk is an emulsion, a liquid suspension of fat globules and proteins produced by specialized mammary glands. Conversely, avian crop milk is a suspension of sloughed epithelial cells derived from the lining of the digestive tract.
The mechanism of production also differs at a cellular level. Mammalian milk is an apocrine secretion, where parts of the secreting cell are lost along with the product. Avian crop milk is a holocrine secretion, involving the complete disintegration of the cell to release its contents. This fundamental difference results in the avian product being a semi-solid curd rather than a liquid.
The most significant compositional difference is the carbohydrate content. Mammalian milk contains the sugar lactose, which is a primary energy source for the young. Avian crop milk, however, is virtually carbohydrate-free, focusing energy delivery almost entirely through its high protein and fat components.