Why Can a Pigeon Eat So Fast? A Bag Before the Stomach

The structure is an expandable widening of the gullet, sitting at the base of the neck, which holds food temporarily before it passes into the stomach. That storage solves a specific problem, since feeding exposes a bird to predators and competitors while digestion takes time, so being able to swallow a great deal quickly and process it later in a safe place is a substantial advantage. A bird with a full one is visibly bulging at the throat. It also softens and moistens what it holds, and in some species begins chemical breakdown, so the food entering the stomach is already partly prepared. Not all birds have one, and its size and form vary enormously with diet.

The same structure serves several different purposes across the birds that have one:

• •Rapid bulk feeding, then digesting in cover, which is the basic function
• •Carrying food back to nestlings, regurgitated on arrival
• •Softening hard seeds before they reach the grinding part of the stomach
• •Storing grit and small stones in some species
• •Producing a nutritious secretion to feed young, in pigeons and a few others
• •Displaying, where the inflated pouch is part of a courtship performance

Pigeons and doves produce a substance from the lining of this organ that is genuinely comparable to milk, and it is one of the more surprising convergences in biology. Cells lining the pouch proliferate and are shed as a thick pale material rich in protein and fat, higher in both than cow or human milk, and it is produced by both parents rather than only by females. Squabs are fed on nothing else for their first days and on a mixture thereafter, and they cannot survive without it. Production is controlled by the same hormone that governs lactation in mammals. Flamingos and male emperor penguins produce comparable secretions from different tissues, which makes this an independent solution to the same problem.

A substantial number of birds lack the structure entirely and what they do instead explains why. Owls have none, swallowing prey whole and processing it in a stomach that separates the digestible parts and compacts the rest into a pellet for regurgitation. Geese have only a slight widening, and they compensate by feeding almost continuously on low quality plant material, since a store would be pointless when the bird eats all day anyway. Penguins hold fish in the stomach itself and can suspend digestion for weeks while carrying food back to a chick. Insect eaters that feed constantly on small items gain nothing from storage. The structure is therefore associated with feeding in concentrated bouts rather than with any particular group.

Anybody keeping birds learns about this organ mainly through its failures, which are common enough to be a standard veterinary topic. Food can sit in it and ferment rather than passing on, producing a sour swollen condition caused variously by infection, by blockage further down, by eating something indigestible or by feeding a chick food that is too cold. Hand-reared chicks are particularly vulnerable, since the temperature and consistency of what they are given determine whether it empties properly. Sharp objects can puncture it. Chronic dilation leaves it permanently stretched and unable to empty. In wild birds, plastic and fishing tackle recovered from this organ is a recognised indicator of the debris in an environment.

An expandable pouch at the base of the gullet lets a bird swallow far more than it can immediately digest, then process it somewhere safer, and it softens and sometimes begins breaking down the contents. It carries food to nestlings, stores grit and features in courtship display. Pigeons produce a protein-rich secretion from its lining, controlled by the same hormone that governs mammalian lactation.