We shall read from the Book of Wikipedia, Article 254453666, Revision 50.

Crop milk, also known as pigeon's milk, is a secretion from the lining of the crop of pigeons and doves with which the parents feed their young by regurgitation.
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Crop milk bears little resemblance to mammalian milk, being a semi-solid substance somewhat like pale yellow cottage cheese.

The article points out that various commercial substitutes for pigeon milk are sold on the market.