Comparison To Mammalian Milk
Crop milk bears little resemblance to mammalian milk, being a semi-solid substance somewhat like pale yellow cottage cheese. It is extremely high in protein and fat and contains more of it than cow or human milk. It has also been shown to contain anti-oxidants and immune-enhancing factors. Like mammalian milk, crop milk contains IgA antibodies. It also contains some bacteria. Unlike mammals, where the milk is an emulsion, pigeon crop milk consists of a suspension of protein rich and fat rich cells that proliferate and detach from the lining of the crop. In pigeons and flamingoes, both male and female adult produce crop milk and share in the feeding and care of the young. In penguins only the males lactate. Lactation in birds is also controlled by prolactin, the same hormone in mammals.
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