Pigeon milk production is a four hour cycle. Pigeon milk is produced by proliferation of the crop germinal epithelium (proliferating cells of the germinal epithelium are stained red with an antibody to proliferating cell nuclear antigen) which results in the accumulation of the differentiated lipid and protein containing cells (counterstained blue with hematoxylin) every 4 hours during lactation. (A) The crop ready to regenerate pigeon milk after it has been regurgitated to the young. (B) Pigeon milk (blue) has been produced and is nearly ready to separate from the germinal epithelium. (C) The pigeon ‘milk has started to separate from the germinal epithelium, ready for regurgitation to the young. Scale bar = 100 μm.