It is generally well known that birds lack teeth. What is not as well known is that ancient birds had them.
Several fossil bird species from the Jurassic Period (about 200 million to 145 million years ago) had a mouthful of dinosaur-like teeth. During the next period (the Cretaceous, about 145 million to 66 million years ago), toothed birds vanished.
The loss of teeth was most likely one of many evolutionary modifications for flight that included reducing the size and weight of the skeleton while maintaining strength. Bones were lost, some were fused together, and sometimes heavy, compact bone was replaced by lighter, spongy bone. Nowhere was this more prominent than in the skull. The resulting smaller, more delicate jaws meant that teeth had to go.
From a functional point of view, teeth were replaced by the gizzard, a thick-walled muscular chamber of the stomach that slowly churns, breaking up almost anything inside. More specifically, the gizzard replaced the molars. Not only did swapping the teeth and skull for a gizzard save weight, it moved the center of gravity under the wings, creating a more aerodynamic weight distribution.
The stomach of a bird consists of two parts: a small, thin-walled, glandular chamber that secretes digestive enzymes and acid — the true stomach — and, behind it, a larger, thicker gizzard. Some birds (fruit-eaters, for example) have a weakly developed gizzard. At the other extreme, seedeaters have a gizzard whose walls are thick masses of muscle that contract with tremendous force. Inside is a strongly keratinized layer consisting of numerous grooves and ridges. The lining is very tough and constantly being regenerated.
An unusual feature of some Old World birds known as flowerpeckers is that both the intestine and gizzard come off the front stomach. Fruit bypasses the gizzard, going straight into the intestine, while insects are diverted to the gizzard for grinding. A New World songbird of our southwestern states, the Phainopepla, feeds heavily on mistletoe berries. The outer covering of the berry is removed in the stomach, and the fruit moves right into the intestine. The outer coverings accumulate in the gizzard and at intervals are passed as fecal pellets.
Nuts and shellfish
Among birds with well-developed gizzards are turkeys, pheasants, pigeons and doves, ducks, and many finches. Turkeys and dabbling ducks, such as Mallards, eat hard-shelled nuts that are easily pulverized in their gizzards. Sea ducks such as eiders and inland divers such as scaup eat mussels and other shellfish that are readily ground up.
According to John K. Terres, turkeys can crush pecans — shell and all — in an hour. Rapid digestion is important because of birds’ high metabolic rate, which is another adaptation for flight. Terres also reports on a 17th-century experiment in which a worker introduced glass balls, lead cubes, and chunks of wood into a turkey’s stomach. On the following day, the glass was pulverized, the lead cubes flattened, and the wood badly worn. When a modern researcher tried to duplicate these feats with a vise, he needed to apply a force of 437 pounds.
Sometimes birds eat pebbles, which are kept in the gizzard to increase its pulverizing ability. Almost any kind of stone will work, although limestone will not withstand the strong acid in the stomach of most birds. The ingesting of stones has led to some fascinating situations. Ruby mines in India, for example, were reportedly discovered after a ruby was located in the gizzard of a pheasant. Similarly, a minor gold rush was spawned in western Oklahoma when gold nuggets were found in the gizzards of ducks. The flightless Moas of New Zealand, brought to extinction by Polynesians in the mid-1300s, varied in size. The largest was about six feet tall and had about five pounds of grit in its gizzard.
The tendency to pick up abrasives can have a downside as well. Ducks that swallow grit and seeds often ingest lead shot that has sunk to the bottom of marshes. Waterfowl are highly susceptible to lead poisoning, which causes considerable mortality. Efforts to replace lead shot with available alternatives has become a political issue. The NRA lobbies strongly to maintain lead shot for upland game hunting.
The gizzard has a different function in birds of prey. Many owls and hawks swallow small birds and rodents whole. The non-digestible feathers, bones, and fur remain in the gizzard and, after a time, are compacted into a pellet and regurgitated. Grebes ingest feathers (and feed feathers to their young), which settle in the bottom of the gizzard and prevent fish bones from moving into the intestine.
The loss of teeth and development of a powerful, muscular gizzard are critical adaptations for the success of birds. It might seem far-fetched to think of these as adaptations associated with flight, but nothing should be a surprise when it comes to the amazing lives of birds.
This article from Eldon Greij’s column “Amazing Birds” appeared in the November/December 2014 issue of BirdWatching.
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