One remarkable adaptation in birds is the production of a substance known as “pigeon milk.” This unique biological phenomenon allows certain bird species to nourish their young with a milk-like secretion. The process is functionally similar to lactation in mammals, yet achieved through entirely different means. This specialized form of nourishment is crucial for the early development of their offspring.
Defining Pigeon Milk
Pigeon milk, more accurately termed crop milk, is a nutrient-rich, semi-solid substance produced by the parent birds. It differs significantly from mammalian milk, as it is not a secretion from mammary glands but rather a suspension of cells. This yellowish-white, curd-like material forms in the crop, a pouch-like enlargement of the esophagus used for food storage. While all pigeons and doves produce crop milk for their young, this feeding strategy also appears in other bird species, including flamingos and male emperor penguins. The term “pigeon milk” is colloquial due to its functional resemblance to mammalian milk.
How Pigeon Milk is Produced
The production of crop milk involves significant physiological changes in the parent bird’s crop. Specialized cells lining the crop proliferate rapidly and fill with lipids and proteins. These nutrient-laden cells then slough off into the crop’s lumen, forming the semi-solid crop milk. Both male and female parent pigeons produce this substance, sharing the responsibility of feeding their offspring.
The process is stimulated by the hormone prolactin, which also regulates milk production in mammals, highlighting a shared hormonal pathway despite different anatomical structures. This preparation begins a few days before the eggs are expected to hatch, ensuring the milk is ready when the squabs emerge. Once produced, parents regurgitate the crop milk directly into the mouths of their young.
Nourishing the Young
Pigeon milk provides a highly concentrated source of nutrition for rapidly developing chicks. Its composition is notably rich in protein, making up around 60%, and fat, typically ranging from 32% to 36%. It also contains various minerals such as calcium, potassium, sodium, and phosphorus. Additionally, crop milk includes important immune-enhancing factors like IgA antibodies, antioxidants, and beneficial bacteria, which contribute to the squabs’ immune system development and gut health.
Newly hatched squabs are fed exclusively on pure crop milk for their first week to ten days of life, as their digestive systems are not yet capable of processing solid food. This period of exclusive feeding supports their initial burst of growth, allowing them to double their birth weight rapidly. As the squabs mature, typically after the first week, parents gradually introduce softened adult food mixed with the crop milk. By about two weeks of age, the young birds are primarily consuming softened adult food, transitioning away from their milk-based diet.