Mysteries of pigeon milk explained

Pigeon mothers produce 'milk' that has similar nutrients to the milk mammals produce, new research shows.

A one-day-old pigeon chick. Mother pigeons feed their chicks a type of milk that is similar to mammal milk. (Susanne Wilson)

LACTATION MAY SEEM  a uniquely mammalian trait, but some birds are more like us than you think.

Australian researchers have found that pigeons produce a type of 'milk' in their gut that is similar to the milk that mammalian mothers produce to feed their young.

"We looked at the genes involved in the production of pigeon milk and found that it contains antioxidants and immune-enhancing factors," says Deakin University and CSIRO research fellow Dr Tamsyn Crowley. "This suggests that, like mammalian milk, it plays a key role in enhancing the immune system of the developing baby."

Pigeon throat pouch like a mammary gland

Both male and female pigeons produce the nutrient- and fat-rich substance in their crop (a pouch found in the bird's throat), which they regurgitate to their chicks, known as squabs.

Meagan Gillespie, Deakin PhD student and co-author of the study, says that a pigeon's crop would usually be used to store food. "However, in the pigeon, the crop changes prior to 'lactation' - in response to hormones - and returns to its 'non-lactating' state at the end of the lactation period, a bit like the mammary gland."

Milk production is found in all pigeon and dove species, as well as flamingos and male emperor penguins, she told Australian Geographic. While the phenomenon is still not well understood, in the case of emperor penguins it is believed milk is only produced by males, and only when their partners take too long to return to the nest with food.

A mother pigeon keeps her one-day-old chick warm. She will feed it
milk from her throat pouch, called a 'crop'. (Credit: Susanne Wilson)

Pigeon milk mystery

"This work has provided an insight into the process of pigeon milk production, by studying the genes expressed in the lactating crop," Meagan says. But many questions still remain about how the process has evolved.

The similarity between this process and lactation in mammals is striking, but is a case of convergent evolution, where unrelated species independently evolve similar traits. "Even these bird species that produce milk are not directly related to each other, so the best we can say is that this trait must have evolved separately in each one," says the CSIRO's Tamsyn.

Dr Nelson Horseman a physiologist at the University of Cincinnati in the US, says that a kind of milk production is found in many species. "In some fishes the 'milk' that is fed to their young is a mucous secretion on the skin. Certain amphibians shed actual skin cells that are fed to the young. [But the] milk of pigeons and doves is the most highly specialised of any non-mammalian milk."

Farming pigeon milk

"Pigeon's milk is extremely nutritious, to be sure, so understanding how it can provide a jump-start to the immature squabs is important,"  she says. A study where pigeon milk was fed to chickens showed that it increased the growth rate of chicks by 38 per cent.

"I would like to have more answers about this question," agrees Tamsyn. "For the moment, though, we have more questions about how this process evolved and what it may be used for than we have time to research."

The results of the study are published this week in BioMed Central's journal BMC Genomics.