Last week’s column discussed how to tease apart the fur and feathers in bird pellets to uncover the bones, teeth, claws and, in some cases, entire skulls of the prey caught and swallowed by hawks and owls. For many years, I assumed that these two groups of birds were the only ones that coughed up pellets. But most bird species are capable of producing this interesting wildlife “sign,” a result of their unique digestive systems.Birds use up energy quickly, and carrying around extra weight in the form of undigested food is a liability for flying. To address this, they have evolved a digestive system that can break down food items fast, get energy and key nutrients into the bloodstream quickly, and eject indigestible material as soon as possible.
Fruit can be completely digested in a half hour, shellfish and crabs in 45 minutes. Owls take six hours to digest small mammal prey and regurgitate pellets. In comparison, some solid foods take six hours to pass through the stomach and small intestines of humans, and it’s another 40 hours before the indigestible material passes through the large intestines and is eliminated—for a 46-hour average transit time. (There is much variability in this, but even the faster digestion times are much slower than that of birds.)
Birds lack teeth, so food items are generally swallowed whole and passed quickly from the mouth to the esophagus or gullet. (Although I have most of my teeth, I’ve been accused of bypassing the chewing process and swallowing my food whole; I attribute that feeding strategy to being one of nine mouths at the family dinner table while growing up!)
Some birds—for example, hawks, vultures, cormorants, and gulls—have a gullet that can stretch to hold large food items. I once watched a herring gull swallow an entire chicken drumstick; it looked quite painful but didn’t seem to faze the gull at all.
Others, including many of the ground-dwellers (grouse, quail and turkey), have a pouch-like appendage to the gullet, called the crop, where seeds and nuts can be stored and digested later when the bird reaches the safety of its nighttime roost.
The gullet leads to the first of two stomachs, called the proventriculus, where gastric juices begin breaking down the food. The second stomach, the ventriculus, more commonly called the gizzard, is a very interesting component of the system. It’s essentially a muscle capable of exerting 400 pounds of pressure per square inch. Lined with horny plats and ridges, and often aided by sand, small stones and other grit ingested on purpose, it crushes and grinds hard food items such as shells, acorns, beetles and fish.
One of my references mentions that the “grit” swallowed by ostriches, a large bird that doesn’t have to worry about getting airborne, includes 1-inch-diameter stones!
Some waterbirds have developed the bad habit of ingesting lead shot picked up on the bottoms of lakes and ponds. Apparently, it is quite abundant and the perfect-sized “grit.” Many of them eventually die of lead poisoning. This is a major cause of mortality among loons, and led to a federal law banning the use of lead shot over water bodies in 1991. Various substitutes, including steel shot, are now used.
Once the grinding process is complete, indigestible material, including feathers, fur, bones, teeth, nails, and scales, are separated out and held, while the rest of the food is moved into the small and large intestines for further digestion and absorption of nutrients and water.
The indigestible material remaining in the gizzard is compressed into a pellet, covered with mucous and eventually regurgitated, or “coughed up,” before feeding again. It’s a process I’ve witnessed an osprey chick perform on a nest cam, and it does not look like a fun thing to do.