Milk is produced by mammals in order to provide nutrition to their growing young. It's pretty special stuff, as not only does it provide all the nutrients and energy needed to fuel a growing baby (consider that for at least six months a human infant drinks nothing but milk) it also aids in the development of both the immune system and the baby's microbiotica - the bacteria that develop in its gut and stomach.
Although milk is exclusively a mammalian production, some birds, such as pigeons, penguins and flamingos, produce a milk-like substance which provides similar benefits to their young. Both female and the male pigeons produce it in their crop, and like mammalian milk production is controlled by the hormone prolactin. It contains protein (60%), fat (32–36%), carbohydrate (1–3%), minerals (calcium, potassium, sodium and phosphorus), IgA antibodies (important for the immune system) and assorted non-pathogenic bacteria.
In order to explore the benefits of pigeon milk on the young, researchers fed the milk to baby chickens to see how they benefited from it. They found that after 7 days, the milk-fed chickens were around 12.5% heavier than the non-milk-fed control group and were also expressing different genes in the gut associated lyphoid tissue (where the immune system and the gut come into contact). Milk fed chickens had a much greater expression of the immunoglobulin IgA than the non milk fed, as shown in the graph below:
As well as affecting the immune system, feeding pigeon milk to the chickens also changed the composition of bacteria within the chicken cecum (a part of the gut). Pigeon milk fed chickens had a much greater bacterial diversity, at both phylum and genus levels. Unsurprisingly, milk fed chickens contained bacteria found within the pigeon milk (originally produced to go inside little pigeons) although not all of the bacteria present in pigeon milk had managed to survive in the chickens.
The reason chickens were used for this experiment rather than pigeons (which would seem more natural) is that pigeon milk contains so many benefits required by baby pigeons that they end up dead or very sickly if you try to bring them up without it. The fact that pigeon milk provides significant growth advantages for chickens is also interesting for the farming industries. Nobody is particularly fussed about making bigger pigeons, but bigger chickens can lead to more money, although the current lack of formula pigeon-milk means that this is not a viable strategy for chicken farming.
Credit link for image 1
Reference 1: Gillespie MJ, Stanley D, Chen H, Donald JA, Nicholas KR, et al. (2012) Functional Similarities between Pigeon ‘Milk’ and Mammalian Milk: Induction of Immune Gene Expression and Modification of the Microbiota. PLoS ONE 7(10): e48363. doi:10.1371/journal.pone.0048363