Why is pigeon’s milk important for newly hatched squabs?

Pigeon’s milk is a highly proteinaceous, cheeselike substance produced by the sloughing off of the lining of the adult pigeon's crop, a process stimulated by the hormone prolactin.123

This substance is vital for newly hatched squabs because it provides a source of nutrition that allows these seed- and fruit-eating birds to be independent of animal-based food sources during the critical early stage when the young require maximum nutrition for development. Because pigeons are typically limited by the production of this crop milk, it also serves as a biological factor that regulates the number of offspring a pair can successfully rear, which is why most pigeons lay no more than two eggs.2