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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteMammalian milk is made by mammary glands; crop milk is a nutrient-rich secretion made by specialized cells in parts of certain birds’ digestive tracts. Both feed dependent young, but they come from different tissues. Many animals use neither: they deliver prey or pass along food they have eaten.
How the feeding methods differ
| Feeding method | Where the food comes from | Examples | Key distinction |
|---|---|---|---|
| Mammalian milk | Mammary glands | Mammals, including monotremes such as the platypus | A mammalian secretion; its composition and delivery vary by species. |
| Crop milk | Specialized cells in the crop sac | Pigeons and doves | Cells proliferate and are shed into food given to chicks. |
| Other milk-like bird secretions | Crop or other upper digestive tract tissues, depending on the bird | Flamingos and emperor penguins | They serve a similar feeding role, but should not be assumed to have identical anatomy. |
| Regurgitated food or prey transfer | Food a parent has eaten or prey it has caught | Many birds | The parent transfers food rather than producing a dedicated milk-like secretion. |
The useful questions are where the food is produced, what tissue produces it, and whether the parent makes a secretion or transfers food gathered elsewhere. A similar purpose—nourishing offspring—does not mean the feeding methods are biologically the same.
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What mammalian milk is
In the strict biological sense, milk is a secretion of mammary glands, a defining feature of mammals. Its nutrients and proportions vary considerably among mammal species; the Smithsonian National Zoo’s Milk Repository studies milk samples from many species in connection with animal care and research.
Mammals do not all deliver milk in the same way. Most feed young from nipples, but monotremes—including platypuses—lack nipples. Their milk is secreted onto skin, where the young lick it. The Animal Diversity Web’s overview of mammary glands describes this variation; a Smithsonian document also gives the platypus as an example.
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What crop milk is—and how pigeons make it
In pigeons and doves, crop milk is produced in the crop, a food-storage pouch in the upper digestive tract. Under the influence of prolactin, specialized cells in the crop lining proliferate and are shed. Parents pass this cell-rich material to their newly hatched chicks, typically by regurgitation. Both male and female pigeons can produce it; the behavior is not shared by all birds. The mechanism and parental behavior are described in a review indexed by PubMed and in an open-access study of crop-milk composition.
One study, Gillespie and colleagues’ 2012 paper, reports that pigeon crop milk contains 60% protein, 32–36% fat, and 1–3% carbohydrate. Those figures refer to pigeon crop milk as reported in that paper; they are not values for all bird secretions or mammalian milk. The authors compare functional similarities between pigeon crop milk and mammalian milk, but similarity in nutritional role does not make crop milk a mammary secretion.
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Which other birds produce milk-like secretions?
Crop milk is most closely associated with pigeons and doves. The British Trust for Ornithology also reports milk-like feeding in flamingos and emperor penguins. These examples are often grouped under the everyday label “bird milk,” but the label can obscure anatomical differences: secretions may arise from different portions of the upper digestive tract, not necessarily the crop itself. An older Smithsonian-hosted review discusses this distinction; the named examples should not be read as a complete inventory of bird species or feeding strategies.
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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Flamingo feeding is also observed in zoo settings. A Smithsonian National Zoo account of caring for nine flamingo chicks describes those observations; it is useful context, not a rule that establishes one universal schedule or diet for every flamingo population.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How food transfer differs from milk production
Many parents feed young without making a specialized secretion. A bird may regurgitate food it has eaten, or bring a captured prey item directly to its offspring. In both cases the parent is transferring food, rather than producing milk from mammary glands or specialized crop cells. Regurgitation alone does not establish that the material is crop milk: in pigeons, the distinction is that crop-lining cells contribute to the secretion.
These approaches can overlap with other foods as young develop. The examples here do not establish one universal period of dependence or diet pattern across pigeons, flamingos, penguins, mammals, or birds generally.
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A quick way to tell the terms apart
- Mammalian milk: made by mammary glands.
- Pigeon or dove crop milk: a secretion containing shed cells from the crop lining.
- Other bird “milk”: a useful functional comparison for certain secretions, with source tissue varying among birds.
- Regurgitated food or delivered prey: food passed to offspring, not necessarily a specialized secretion.
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