Bird Milk, Reptile Milk, Fish Milk

Crop Milk 


In the middle is a discus-fish parent infested with its beloved fry.



It’s moderately well known that several families of birds (pigeons and doves, flamingos, and penguins) secrete a kind of milk (called “crop milk”, though penguins don’t have crops) for their young. This is real milk,  secreted by a special gland – not regurgitated, half-digested food from the parent’s stomach. (Regurgitators put their bills into the chick’s mouth to deliver food, whereas in milk-providing species the chicks put their bills inside the parent’s mouth to nurse. ) The three families evolved this capacity independently, for three different reasons. With penguins it seems to be an emergency food substitute in case the feeding parent (usually the female) doesn’t return to the nesting parent in time. With flamingos it’s apparently because the chicks aren’t able to handle normal food, and perhaps because the flamingo’s bill makes the regurgitation method awkward.  In the case of pigeons, it allows them to raise several small broods per season (only one or two eggs each), since their well-fed squabs grow extraordinarily quickly. (This is probably also why squabs are so fat, juicy and tasty.)


It's less well known that at least one species of fish, the discusfish, feeds its young with a secretion that might as well be called milk. (Mammary glands are skin glands too, in the big picture of things). The tiny fry live off their egg-sacs for several days, and then migrate and attach themselves to one of the parents, where they feed off the fish-milk.


The capacity to produce milk is thought to trace back to the therapsids -- Triassic links between reptiles and the mammals and birds. (The discus-fish are different, unrelated story.) But regardless of everything, mammals will continue to be called mammals, and pigeons ain't mammals. Don't let your biology teacher fool you with a trick question. 


The most interesting thing that came out of my quest for exotic milk is that the hormone prolactin seems to be associated with parenting activities by either sex --  not only in mammals and birds, but even in fish (and by conjecture and extrapolation, even in therapsids). I'm less narrow-minded than most, and I don't have any difficulty with the idea that human, mammalian, and avian parents all feel similiar emotions. Proto-mammals, maybe. But fish? Cold-blooded things? The mind boggles..




Discus-fish fry, now old enough to separate from parents


Pigeons and miscellaneous:

“Thus three very different groups of birds have evolved the capacity to produce milk as solutions to very different problems: the need for protein and fat in the pigeons, which feed very little animal material to the squabs; the need for liquid food consumption during the development of the specialized feeding apparatus of the flamingos (which would make any other form of food difficult for the chicks to ingest); and the need for a convenient food supplement when breeding on the barren Antarctic ice shelf favored by penguins.”


Crop-milk production allows the rapid production of multiple broods in a nesting season, but only one or two young per brood:


Like mammalian colostrum, crop milk gives resistance to disease:


Recipe for fake crop milk:

Everything you want to know about the intestinal tract of birds:


Bird digestion in great detail:


Many PubMed articles on crop milk (pay site):





Quick flamingo sketch:


Flamingo traits in detail:


Flamingo chicks are unable to filter-feed until they are three months old…. Flamingo crop milk is bright red due to the presence of canthaxanthin, and feeding parents can lose the red color in their feathers…. Parents can recognize their own young by its call, and will not feed others in the flock:


Fake flamingo mommy:


Flamingo males and females share child-raising approximately equally, but tend to specialize in particular tasks. Sometime a second female helps out:





Everything you want to know about discusfish (fantastic site)


Discusfish breeding cycle with lots of photos:


More detail on the discusfish breeding cycle:


A Romanian point of view on discusfish:





“Lactation appears to be an ancient reproductive trait that predates the origin of mammals. ….. Mammary patch secretions were co-opted to provide nutrients to hatchlings, but some constituents including lactose may have been secreted by ancestral apocrine-like glands in early synapsids. Advanced Triassic therapsids, such as cynodonts, almost certainly secreted complex, nutrient-rich milk, allowing a progressive decline in egg size and an increasingly altricial state of the young at hatching. This is indicated by the very small body size, presence of epipubic bones, and limited tooth replacement in advanced cynodonts and early mammaliaforms. Nipples that arose from the mammary patch rendered mammary hairs obsolete, while placental structures have allowed lactation to be truncated in living eutherians.”


“The secretion of nutrient rich milk probably began in therapsids, such as cynodonts”:


Therapsids as a missing link between reptiles and mammals:




Non-parental helping behavior in woodpecker nesting is associated with presence of prolactin and is apparently adaptive.


Below studies of a dozen or more bird, fish, and mammal species show that prolactin is associated with, and possibly a cause of, paternal behavior by males. (It was already known to be associated with maternal behavior by females). Three versions of about the same paper, two of them pdfs.



I am emersonj at gmail dot com.

Original materials copyright John J Emerson

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