Pigeons and doves produce a highly nutritious, milk-like substance to feed their young. This secretion is fundamentally different from mammalian milk, as it does not come from mammary glands, and it is known as “crop milk.” This remarkable biological function represents a unique form of parental care in the avian world.
The Specific Answer: Pigeons and Crop Milk
Crop milk is a specialized secretion produced in the crop, a muscular pouch located near the esophagus, which birds normally use to store food before digestion. Pigeons and doves are the most prominent examples of this phenomenon in the bird kingdom. The substance is regurgitated by the parents directly into the mouth of the newly hatched squab, acting as the sole source of nourishment for the first week of life. This process is entirely non-mammary. A few other bird species, such as flamingos and the male emperor penguin, have independently evolved a similar nutrient-rich secretion to sustain their chicks.
The Biological Mechanism of Production
The preparation for crop milk production begins shortly before the eggs hatch, driven by a surge in the hormone Prolactin. This is the same hormone that stimulates milk production in mammals, demonstrating a shared regulatory mechanism across different classes of animals. Prolactin stimulates the epithelial cells lining the crop sac to rapidly multiply and thicken, causing a noticeable enlargement of the pouch. Both the male and female parent participate equally in this physiological change and subsequent feeding process.
The final production involves a process called cellular sloughing. The newly proliferated cells become laden with fat and protein, then detach or shed from the crop wall into the center of the pouch. This sloughed material, which appears as a thick, pale yellow, semi-solid substance, is the actual crop milk. The parents then feed this nutrient suspension to their young through regurgitation.
Nutritional Makeup and Role in Young
On a dry weight basis, crop milk is exceptionally rich, consisting of approximately 60% protein and 30% fat, making it far more concentrated than cow’s milk. This high concentration provides the energy necessary for squabs to grow at an accelerated rate.
Unlike true mammalian milk, the avian secretion contains no sugars, such as lactose, and is also low in calcium. Instead of being an emulsion, like dairy milk, crop milk is a suspension of shed epithelial cells. Beyond primary nutrients, the milk contains immune-enhancing factors, including IgA antibodies and antioxidants, which help support the squab’s undeveloped immune system. After the first week, the parents begin to gradually introduce softened grains into the crop milk mixture, which eventually weans the young birds onto an adult diet.