Perhaps the most extraordinary biological feature of pigeon nesting season is the production of “crop milk” or “pigeon’s milk.” Unlike true milk from mammals, this is a semi-solid, curd-like substance sloughed off from the lining of the adult’s crop (a pouch in the esophagus). Both male and female parents produce it, triggered by the hormone prolactin.

Nevertheless, mortality during nesting season is high. In urban environments, nest predation by crows, gulls, and rats claims up to 40% of eggs and young squabs. Human disturbance—from building maintenance, window cleaning, or simple curiosity—is another major cause of nest failure. A parent flushed from the nest for more than 15 minutes in cold weather can spell doom for the eggs or hatchlings. Additionally, the very ledges that pigeons favor often lack railings, and clumsy fledglings frequently fall to their deaths.