Pigeons do produce milk, though not from mammary glands or anything resembling a nipple. Both male and female pigeons generate a thick, pale substance called crop milk inside a muscular pouch in their throat, and they regurgitate it directly into the mouths of their newly hatched chicks. The process is governed by the same hormone that triggers milk production in mammals, and the resulting substance shares striking functional similarities with mammalian milk. Yet crop milk is made in a fundamentally different way, from a fundamentally different tissue, which makes it one of the more unusual examples of convergent evolution in the animal kingdom.
Where Crop Milk Comes From
The crop is a thin-walled, expandable pouch that sits partway down a pigeon’s esophagus. In most birds that have one, the crop is simply a food-storage chamber, a place to hold seeds before they move to the stomach. Pigeons have a crop with two lateral pouches, and the lining of this organ changes dramatically when it is time to feed chicks. Under a microscope, the inner surface of the crop shows wavy folds studded with gland openings, and the density of those openings is higher in adult pigeons than in juveniles.1PubMed Central. Anatomical, histological, and scanning electron microscopic features of the esophagus and crop in young and adult domestic pigeons (Columba livia Domestica) During the last days of egg incubation, the epithelial cells lining the crop begin to multiply rapidly, thickening the crop wall until it is engorged with layers of fat-laden cells. Those cells eventually slough off in sheets, forming the crumbly, cottage-cheese-like substance known as crop milk.
Prolactin Drives the Whole Process
The hormone behind crop milk production is prolactin, the same hormone that stimulates milk production in human mothers. In pigeons, prolactin is released by the pituitary gland during the incubation period, and it acts on crop tissue in both sexes. Research has shown that prolactin induces a gradual increase in the weight and thickness of the crop, which reaches maximum size around the third day after hatching.2PubMed. Crop milk protein is synthesised following activation of the IRS1/Akt/TOR signalling pathway in the domestic pigeon (Columba livia) The synthesis of crop milk appears to be controlled by prolactin working in concert with insulin-activated signaling pathways.3PubMed Central. Chemical composition of pigeon crop milk and factors affecting its production: a review
Injecting prolactin into non-breeding pigeons can actually force the crop to start proliferating. Studies have demonstrated that subcutaneous prolactin injections promote rapid epithelial cell growth in the crop through specific cellular growth pathways.4PubMed Central. Prolactin promotes crop epithelial proliferation of domestic pigeons (Columba livia) through the Hippo signaling pathway This is how researchers confirmed that prolactin is the primary trigger rather than some mechanical cue from incubating eggs or contact with chicks. The hormone alone is enough to flip the switch.
How Cells Become Milk
The cellular process behind crop milk production is genuinely unusual. Unlike a mammalian mammary gland, which secretes milk from living cells that remain intact, the pigeon crop produces milk by destroying its own lining. Epithelial cells at the base of the crop lining divide rapidly, pushing older cells toward the surface. As those cells rise, they fill with lipid-rich droplets, cornify (harden with keratin, the same protein in hair and nails), and eventually die and detach. The result is a slurry of fat-packed dead cells.
Gene-expression studies have revealed that these cells belong to a keratinocyte lineage, and the fats they contain appear to arrive as very-low-density lipoprotein particles that the cells absorb from the bloodstream and repackage as triglyceride-filled vesicles.5PubMed Central. Histological and global gene expression analysis of the ‘lactating’ pigeon crop The rapid proliferation also triggers cellular stress responses and ramps up antioxidant gene expression, likely because pushing cells through such an accelerated life cycle generates damaging free radicals. In a sense, the pigeon is deliberately running its crop epithelium into overdrive, harvesting the casualties as baby food.
What Crop Milk Contains
Crop milk is rich in protein and fat but unlike mammalian milk contains little to no sugar. The composition shifts over the feeding period. In the first few days after hatching, crop milk is a nearly pure substance with very high fat and protein content. As the chicks grow, parents begin mixing partially digested seeds into the crop milk, gradually diluting it until the squabs transition fully to a grain-based diet.6PubMed Central. Early weaning in pigeons (Columba Livia domestica): effects on squabs performance and reproductive performance of parents The absence of lactose and other sugars is one of the starkest differences from mammalian milk, which in most species is loaded with carbohydrates to fuel rapid brain development. Pigeon squabs apparently get their energy primarily from fat.
Proteomics work has identified a range of bioactive proteins in crop milk, including IgA (an antibody important for gut immunity), growth factors, and transferrin, an iron-transport protein.7PubMed Central. Exploration of Proteomics Analysis of Crop Milk in Pigeons (Columba livia) during the Lactation Period The presence of immune proteins is part of what makes crop milk more than simple nutrition: it is also a delivery vehicle for the parent’s immunological experience.
Immune Protection and Gut Colonization
One of the more compelling parallels between crop milk and mammalian milk is the transfer of immunity from parent to offspring. Feral pigeons pass antibodies to their young through two routes: prenatally in the egg yolk and postnatally in crop milk. Research on feral pigeons has found that crop milk has positive effects on offspring humoral immunity and could have a similar immune role to mammalian milk.8Functional Ecology. Prenatal and postnatal parental effects on immunity and growth in ‘lactating’ pigeons This makes intuitive sense: a squab hatching into an environment full of pathogens benefits enormously from receiving pre-made antibodies tuned to the local disease landscape.
Crop milk also appears to play a critical role in seeding the chick’s gut with beneficial microbes. A study on White King pigeon squabs found that the colostrum, the earliest crop milk produced right after hatching, serves as the most significant driver for establishing the intestinal microbiota in newborn squabs.9PubMed Central. Establishment and maturation of gut microbiota in White King pigeon squabs: role of pigeon milk This mirrors what happens in mammals, where breast milk shapes the infant gut microbiome in ways that affect immune development and digestion for months or years afterward. The pigeon version appears to accomplish something analogous in a much shorter window.
Both Parents Lactate
Perhaps the most unusual feature of pigeon lactation is that both the male and female produce crop milk. In mammals, lactation is exclusively female. In pigeons, prolactin levels rise in both sexes during incubation, and both develop the thickened crop lining needed to generate milk. Research has tracked prolactin and insulin levels in breeding pairs, collecting crop tissue samples from both males and females across the incubation and lactation periods.2PubMed. Crop milk protein is synthesised following activation of the IRS1/Akt/TOR signalling pathway in the domestic pigeon (Columba livia) The chicks typically receive regurgitated crop milk from whichever parent is on duty at a given feeding.
This biparental investment is expensive. During lactation, both sexes shift into a catabolic state, burning through body reserves to sustain crop milk production. Males experience a continuous decline in body weight, dropping to an average minimum around 551 grams, while females lose weight early in lactation and then gradually recover.10PubMed Central. Comparative Analysis of Metabolic and Developmental Changes Across Reproductive Stages in Breeding Pigeons The transition from incubation to lactation also brings heightened oxidative stress as the body redirects resources. Female pigeons begin recovering sooner partly because their ovaries are already developing for the next reproductive cycle, a biological priority that seems to pull resources back from lactation earlier than in males.11The Journal of Poultry Science. Comparative Analysis of Metabolic and Developmental Changes Across Reproductive Stages in Breeding Pigeons
Why Squabs Cannot Survive Without Crop Milk
Pigeon chicks, called squabs, are what biologists term superaltricial. They hatch blind, nearly featherless, and completely unable to feed themselves. They depend entirely on crop milk for their first week or more of life. Under traditional breeding conditions, squabs typically remain with their parents for about 21 to 28 days before they are fully weaned.6PubMed Central. Early weaning in pigeons (Columba Livia domestica): effects on squabs performance and reproductive performance of parents
Attempts to wean squabs at hatching, bypassing crop milk entirely in favor of commercial replacers, result in significantly reduced body weight, poorer weight gain, lower feed intake, and higher mortality compared to squabs left with their parents for even just the first week. Interestingly, squabs weaned at seven days showed no significant differences in body weight or mortality compared to those kept with parents for the full 28 days, suggesting that the first week of crop milk provides most of the critical developmental boost.6PubMed Central. Early weaning in pigeons (Columba Livia domestica): effects on squabs performance and reproductive performance of parents The gap between commercial replacers and real crop milk likely comes down to the complex blend of immune factors, microbiome-seeding bacteria, and the specific protein and fat profile that no artificial formula has yet replicated.
Experiments that pharmacologically disrupted the cellular machinery responsible for crop milk protein synthesis in breeding pigeons confirmed the link from another direction: when the protein and amino acid levels in crop milk dropped, the average daily weight gain of squabs declined and mortality increased significantly.12PubMed Central. Targeted disruption of TORC1 retards young squab growth by inhibiting the synthesis of crop milk protein in breeding pigeon (Columba livia)
Functional Convergence with Mammalian Milk
The similarities between pigeon crop milk and mammalian milk go well beyond coincidence. Both are produced under the control of prolactin. Both are rich in fats and proteins. Both deliver immunoglobulins to immunologically naive offspring. Both seed the infant gut with microorganisms. And both are essential for early survival in ways that artificial substitutes have struggled to match.
Researchers have tested whether the functional overlap extends to molecular-level immune effects. When chickens, which do not normally receive crop milk, were fed pigeon crop milk, they showed significantly enhanced expression of immune-related gene pathways and interferon-stimulated genes in their gut-associated lymphoid tissue.13PubMed Central. Functional similarities between pigeon ‘milk’ and mammalian milk: induction of immune gene expression and modification of the microbiota The crop milk didn’t just provide passive antibodies; it actively switched on immune gene programs in the gut of a completely different bird species. That finding suggests the bioactive components of crop milk are broadly functional rather than pigeon-specific, which is exactly what you would expect if this system evolved convergently with mammalian lactation to solve the same fundamental problem: how to keep a helpless newborn alive and protected.
Pigeons are not the only birds that produce crop milk. Flamingos and emperor penguins also feed their young a crop-derived secretion, though the exact composition and mechanism differ. In flamingos, the substance is sometimes bright red due to the carotenoid pigments in the adult diet. The existence of crop milk in these distantly related bird lineages suggests that the trait has evolved independently more than once, making it a case study in how similar ecological pressures can generate remarkably similar biological solutions.
What Crop Milk Can Also Transmit
The intimacy of regurgitative feeding comes with risks. Because crop milk is produced from the parent’s own tissue and secretions, it can carry more than just nutrients and protective antibodies. Researchers have detected antibodies to the pathogen Chlamydia psittaci in the crop milk of about 4% of young pigeons sampled in the first ten days of life, and antibodies to Coxiella burnetii in about 2% of crop milk specimens from a similar age range.3PubMed Central. Chemical composition of pigeon crop milk and factors affecting its production: a review The presence of pathogen-specific antibodies confirms that crop milk is a live immunological channel between parent and offspring, capable of transmitting evidence of disease exposure along with the protective response.
This dual nature, protective and potentially transmissive, again mirrors mammalian milk, which can carry both beneficial immune components and, in some cases, infectious agents. For pigeon breeders and researchers working with pigeons in urban environments, the pathogen-transmission dimension of crop milk is a practical concern. The close physical contact during feeding, with the squab inserting its bill into the parent’s mouth, creates an efficient transfer route for anything present in the parent’s crop lining.
Challenges of Artificial Rearing
The pigeon industry, particularly squab farming for meat production, has long grappled with a bottleneck created by crop milk dependency. A breeding pair can typically only raise two squabs at a time, and the parents are physiologically occupied with lactation for weeks. This limits the rate at which a farm can produce birds. If artificial rearing could replace crop milk reliably, breeding pairs could return to laying eggs much sooner.
Developing a synthetic crop milk substitute has proven difficult. Early work on hand-feeding formulas found that incubator-hatched squabs could be sustained on slurries made from isolated soy protein, soybean oil, glucose, and mineral supplements, starting at very dilute concentrations and gradually thickening the mixture over the first week. But “sustained” is different from “thrived.” The nutritional profile of these formulas does not replicate the immune components, the microbiome-seeding function, or the precise lipid packaging that natural crop milk delivers. The weaning studies discussed earlier reinforce this point: squabs deprived of even just the first week of parental crop milk fare measurably worse, even when given the best available commercial replacers.6PubMed Central. Early weaning in pigeons (Columba Livia domestica): effects on squabs performance and reproductive performance of parents
The practical upshot is that pigeon farming remains unusually constrained by the biology of parental care. Unlike chickens, which can be reared in large batches with formulated feed from day one, pigeon squabs need their parents for at least the first several days if mortality is to stay low and growth rates are to remain acceptable. Research into the signaling pathways that govern crop milk production has partly been motivated by this economic bottleneck, with the hope that understanding the biochemistry might eventually enable better artificial substitutes or methods to enhance natural production.
Why “Milk” Is the Right Word
There is sometimes resistance to calling crop milk “milk” at all, since the term is so strongly associated with mammary glands and mammals. But the word has been used in the scientific literature for well over a century, and the functional case for it has only grown stronger as research has accumulated. Crop milk is produced by a parent, induced by prolactin, rich in fat and protein, loaded with immunoglobulins, essential for early survival, and delivered mouth-to-mouth in a way that seeds the offspring’s gut microbiome. The tissue of origin is different, the cellular mechanism is different, and the lack of sugars is a real compositional divergence. But the job description is the same, and convergent evolution has produced a substance that, in the ways that matter most to a helpless newborn, functions remarkably like the mammalian version.