The Bird’s Digestive System
The digestive system is based on a tube which runs through the bird from its mouth to cloaca. Along this tube various glands secrete the chemicals which break down the food into substances which the body cells can assimilate. Some birds have saliva glands which lubricate the food but others, including most waterbirds, are entirely without them; for instance, on the Island of Skokholm the Great Black-backed Gulls took their food to the sea to eat. From the base of the neck the oesophagus or gullet which is a thin-walled tube opens out into a balloon-like crop. This crop varies in size from almost nothing in some meat-eaters to a huge sack-like store in grain-eaters like the Woodpigeon. Here the food is stored and moistened before passing on to the stomach. The pigeon’s crop is particularly interesting in the breeding season, as it secretes a substance known as pigeon’s milk which is sloughed off the side of the gullet and regurgitated for the young.
Leaving the crop, the food moves down into the stomach where it is first treated with enzymes and then subjected to the grinding action of the muscles of the gizzard combined with the abrasive effect of small stones which collect there and which some species pick-up deliberately.
The metabolism of birds is so high that energy is used quickly. Carnivorous birds digest large quantities of food very rapidly. Seedeaters take longer to assimilate the greater quantities of less nourishing food, indeed, small birds seem to be constantly swallowing at one end and ejecting at the other.