Crop milk is a secretion from the lining of the crop of parent birds in some species that is regurgitated to young birds. Wikipedia

secretion used by some birds to feed their young Wikidata

A semi-solid substance, somewhat like pale yellow cottage cheese, secreted from the lining of the crop of (both male and female) pigeons and doves that is fed to their young (squabs) through regurgitation. An analog to crop milk is also secreted from the esophagus of flamingos and some penguins. Wiktionary

“milk” from a bird’s crop. Wiktionary (translation)