Crop milk

Lore & Background

Crop milk is produced by the lining of the crop, a part of the digestive tract. In pigeons, the crop wall thickens from 0.15 mm to up to 3 mm during breeding periods. Stimulated by prolactin, the crop becomes more vascularized, allowing basal cells to divide rapidly. These cells differentiate, accumulate lipid droplets, and slough off into the crop cavity, forming a secretion fed directly to squabs. Pigeon milk is a semi-solid substance resembling pale yellow cottage cheese, containing a suspension of protein-rich and fat-rich cells, unlike mammalian milk which is an emulsion. It contains no carbohydrates but has antioxidants and immune-enhancing factors, including IgA antibodies and carotenoids such as xanthophylls.

Reader's Guide

Crop milk is significant as a form of avian lactation that evolved independently in pigeons, flamingos, and emperor penguins, demonstrating convergent evolution. It is essential for the survival of altricial young, such as pigeon squabs, which cannot digest seeds or grains. The production of crop milk is controlled by prolactin, the same hormone that triggers lactation in mammals, yet the secretion mechanism differs: it is holocrine rather than apocrine. The composition—high in protein and fat but lacking carbohydrates—supports rapid growth, with pigeon squabs doubling their body weight within two days. The limited supply of crop milk explains why pigeons typically lay only two eggs, as a pair cannot produce enough for three squabs. In flamingos, the red color of early crop milk comes from canthaxanthin, not blood. This substance provides a unique example of parental care and physiological adaptation across bird species.

Did You Know?

• Crop milk is produced by both male and female pigeons and flamingos, but only by male emperor penguins.
• Unlike mammalian milk, crop milk contains no carbohydrates (sugars).
• Flamingo crop milk is bright red in the first weeks due to canthaxanthin, not red blood cells.