Pigeon Crop Milk

演化之聲
Mar 13
3 min read

Updated: 18 hours ago

Crop milk (also known as pigeon milk) is a nutritive substance secreted from the crop or upper esophagus of certain birds during the breeding period. In pigeons (order Columbiformes), crop milk is produced by the crop and appears as a pale yellow or whitish paste-like material. Newly hatched squabs are unable to feed themselves for several weeks and grow more slowly than many other birds; during this early stage they rely entirely on crop milk provided by their parents as their sole source of nutrition. Both male and female pigeons are capable of producing this secretion.

Location of the pigeon crop（Courtesy of Olympic Peninsula Audubon Society）

The crop is an extension of the digestive tract formed by an enlargement of the esophagus on the left side of the thoracic cavity. It connects to the main esophageal tract and ultimately leads to the stomach. Outside the breeding season, the inner lining of the crop consists of a thin keratinized stratified squamous epithelium. During the breeding period, however, endocrine regulation involving hormones such as prolactin and insulin stimulates rapid proliferation of basal epithelial cells in the crop. As a result, the thickness of the crop wall can increase to more than four times its normal state.

Anatomy of the pigeon crop (thickening changes) and crop milk（Image source：Jin CL et al， CC BY-NC-ND 4.0 ）

Prolactin is the principal hormone that induces crop milk synthesis. Through signaling pathways such as JAK2-STAT5, it promotes the proliferation of crop epithelial cells and the synthesis of milk proteins. Insulin also participates in the process by assisting the synthesis of certain proteins. Crop milk is produced through a holocrine mode of secretion: during secretion the entire epithelial cell breaks down, and both the cell membrane and cytoplasmic contents become incorporated into the crop milk. This process results in a secretion rich in lipids. A similar holocrine mechanism also occurs in mammalian sebaceous glands.

Signaling pathways（Image source：Jin CL et al， CC BY-NC-ND 4.0 ）

The composition of crop milk is highly complex and includes a wide range of proteins, such as caseins (αs1-casein, αs2-casein, and β-casein), phosphatases, galactase, leucine aminopeptidase, immunoglobulins IgA and IgG, as well as various free amino acids. Among these components, immunoglobulins contribute to enhancing the immune protection of squabs during early development.

Crop milk also contains numerous symbiotic microorganisms, including members of the genera Lactobacillus, Bifidobacterium, Enterococcus, and Veillonella. These microbes help establish the initial gut microbiota of squabs and support intestinal protection and immune system development. Studies have shown that squabs fed with crop milk develop more stable and diverse intestinal microbial communities.

Beyond pigeons, certain other birds produce milk-like secretions to nourish their young. Flamingos (family Phoenicopteridae) and male emperor penguins (Aptenodytes forsteri) are notable examples. Penguins do not possess a crop; their milk-like secretion is produced directly from the epithelial lining of the esophagus and has the consistency of a thick white fluid. Flamingo milk is distinctive because it contains astaxanthin, giving it a reddish coloration that may provide antioxidant and immune-regulatory functions. However, it remains uncertain whether flamingos possess a crop structure, and therefore the exact anatomical site of milk secretion in flamingos is still unclear.

Although these bird “milks” share functional similarities, their biochemical composition and mechanisms of secretion differ among lineages. Because these traits evolved independently and are not inherited from a common ancestral system, they are regarded as examples of convergent evolution. The complex nutritional and immunological support that parents provide to their offspring through such secretions illustrates the remarkable evolutionary innovation of parental care strategies among non-mammalian animals.