I know this may be an unappetizing topic for some, but being a physician I find the comparative anatomy and physiology of avian digestion fascinating. Don’t confuse my title and posting with the venerable and recommended periodical “Bird Watchers Digest”, mainly for their sake. Check it out at http://www.birdwatchersdigest.com.
I reckon that a bird spends the majority of his life eating or hunting for food. Even the apparent sedentary perching owl or hawk is likely planning his next attack and contemplating the next meal. And this is time well spent since the survival of these warm-blooded, active birds, with very high metabolic rates requires a constant source of energy. Reproduction (breeding, nest building, and rearing of the young) along with migration are also time consumers, but take a back seat to eating and daily survival.
Given the requirements of flight, birds do not have the luxury of storing heavy layers of fat or foods internally, with the one exception being the preparation for migration. For this some songbirds increase their body weight by 40% and need every last ounce and calorie for the rigors of migration. But generally most birds need a steady and constant inflow of food and energy to survive. This is even more critical in the cold of winter.
Luckily birds have evolved a rapid and efficient digestive system, able to cope with a varied diet. For some birds and food types the transit from beak to cloaca can be as rapid as 30 minutes. The beak and toothless mouth are for stabbing, carrying, crushing, and tearing the food, quickly sending it downstream to the tubular esophagus. Fortunately, given their diet, birds have a small tongue with few, if any taste buds.
Many have a widened area in the mid-esophagus called the crop. This is the site of short-term parking for a big meal as is often demonstrated by the tell tale neck bulge of the heron who recently swallowed the large fish, always head first.
The bird stomach is very different than ours. It is a two-part affair with a glandular first sac called the proventriculus. Strong acid, enzymes, and mucus start the digestive process here, before transporting the food to the second part called the gizzard.
The gizzard is a thicker muscular sac with a rough sand-like lining, perfect for grinding and mixing. Some contain sand and stones further aiding the process. Pellets containing the non-digestible waste such as bone fragments, hair, shells, and feathers are passed and often mark the roosting sites of owls and other birds-of-prey.
The actual absorption of nutrients occurs in the small intestine where food is mixed with the enzymes from the pancreas. Birds-of-prey have a relatively short small bowel, whereas herbivorous birds have a longer one, needed for the slower digestion of the tougher cellulose-rich food. Multiple small sacs off the small bowel are called caeca and harbor beneficial bacteria, further aiding digestion.
A bird’s colon or large bowel is short, just serving as a conduit to the final cavity, the cloaca. The cloaca empties to the outside world via the vent, sometimes onto the unsuspecting birder. As you know the cloaca is the common chamber for both sexes receiving the products of the gastrointestinal, urologic, and genital tracts. The close and rapid contact of the vents and cloacae is when and where the genetic material is exchanged.
The Cattle Egret below was finally fed up with his diet of insects and mice and got in the drive-thru lane at McDonalds thinking that they might offer a better menu. I’m not so sure.