Do Birds Have Teeth? Serrated Bills and Spiny Tongues

What birds have instead of teeth

A bird’s bill is a bony jaw covered with a tough layer of keratin, the same protein that forms our fingernails. The sharp cutting edges of the bill are called the tomia (singular: tomium). In many birds, these edges are shaped to grip, cut or strain food in ways that look surprisingly like teeth:

None of these are real teeth. They are made of keratin, not enamel and dentine, and they are not set in sockets in the jaw.

Do geese have teeth?

Geese are the birds most often mistaken for having teeth. Look closely at a Canada goose or a domestic goose with its bill open and you will see rows of small, sharp, pointed ridges along the edges of the bill. The tongue is also fringed with stiff, backward-facing spikes.

These serrations work like a pair of garden shears. A grazing goose grips a blade of grass between the upper and lower edges of its bill and tears it with a quick jerk of the head. The spikes on the tongue help hold food and move it back toward the throat. They also explain why a goose bite can pinch painfully.

Ducks: built-in strainers

Dabbling ducks such as the mallard have rows of thin plates called lamellae along the inside edges of the bill. When a mallard dabbles, it takes in water and mud, then pushes the water out through the lamellae with its tongue. Seeds, insects, snails and other small food items are trapped inside, much as a whale’s baleen traps krill.

Ducks also have a hard “nail” at the tip of the bill, useful for digging and prying food loose. In fish-eating ducks such as mergansers, the bill is long and narrow, with sharp, backward-pointing serrations that give these birds their nickname, “sawbills”.

Penguins: spiny tongues

Penguins swallow their prey whole, often underwater. To grip slippery fish, squid and krill, the tongue and the roof of the mouth of a penguin, such as the emperor penguin, are lined with stiff, backward-pointing spines made of keratin.

The spines act like a one-way ratchet: food can slide toward the throat, but it is hard for it to slip back out. Photographs of a penguin’s open mouth often look startling, but these spines are an elegant solution for an animal with no teeth.

How birds digest food without chewing

Since birds cannot chew, they swallow food whole or in large pieces. Many birds store food first in the crop, a pouch in the throat. The food then passes through the glandular stomach, where digestive juices begin breaking it down, and on to the gizzard.

The gizzard is a thick, muscular stomach that grinds food. Many seed-eating and plant-eating birds, including geese, chickens and pigeons, swallow small stones or grit, which collect in the gizzard and help crush tough seeds and plant fibers. In effect, the gizzard is a bird’s set of molars, tucked safely inside the body.

Birds with teeth: the fossil record

Birds did not always lack teeth. Birds evolved from small, feathered theropod dinosaurs that had teeth, and the earliest birds kept them, which is one reason scientists say that birds are living dinosaurs (see are birds dinosaurs?).

• Archaeopteryx, which lived about 150 million years ago in what is now Germany, had small, sharp teeth in its jaws.
• Hesperornis, a large flightless diving bird of the Late Cretaceous seas, and Ichthyornis, a tern-like flier, both had teeth.
• Enantiornithines, a large group of Cretaceous birds, mostly had teeth too.

These toothed groups died out with the non-bird dinosaurs about 66 million years ago. Much later, the giant pelagornithids, or “bony-toothed birds”, evolved tooth-like spikes of bone along their bills. These were not true teeth, and the last of these huge seabirds disappeared around 2.5 million years ago.

Why did birds lose their teeth?

A large genetic study published in 2014 compared the genomes of dozens of bird species. It found that all living birds share the same disabling mutations in genes needed to make enamel and dentine. This suggests that the common ancestor of modern birds lost its teeth around 116 million years ago.

Why it happened is still debated. The main ideas are:

• Saving weight for flight, by replacing heavy teeth and jaw muscles with a light bill. Many scientists doubt this was the main reason, because many toothed birds flew well.
• Changing diet, as some early birds switched to seeds and plants and relied on a gizzard to grind food.
• Faster hatching. Teeth take a long time to develop in an embryo, and losing them may have helped shorten the time chicks spent in the egg.

Hidden tooth genes

Birds still carry some of the genetic machinery for making teeth. In 2006, researchers studying chickens with a rare mutation called talpid2 found that the embryos developed simple, cone-shaped tooth-like structures in their jaws. The mutant embryos did not survive to hatching, but the discovery showed that the ancient instructions for teeth have not completely vanished.

So the old saying “as rare as hen’s teeth” is almost exactly right: modern birds have no teeth at all, only very clever substitutes.