Why Do Puffins Have Colorful Beaks? Seasonal Bill Plates

A beak built for the breeding season

The Atlantic puffin has one of the most striking bills of any North Atlantic bird. In summer it is broad and triangular from the side, with an orange-red tip, a slate blue-gray base and a yellow ridge between the two. A wrinkled yellow rosette sits at each corner of the mouth, and small horny plates above and below each eye give the face its triangular look.

Much of this decoration is temporary. The colorful parts are hard outer sheaths and fleshy ornaments that grow over the true bill underneath. They develop in late winter and spring, reach full color during the breeding season and are shed afterward in a partial molt.

What happens to the puffin’s beak in winter?

When the chicks have fledged and the adults head out to sea, puffins shed the outer plates at the base of the bill, the eye ornaments and the yellow rosettes. The bill becomes narrower, its tip less bright and its base darker gray. At the same time the pale face feathers are replaced by darker ones, so the whole head looks much plainer.

Few people ever see this winter look, because puffins spend autumn and winter far out on the North Atlantic. In spring the plates regrow, and the birds arrive back at their colonies in full color.

Why the bright colors help puffins

Bright, costly ornaments that appear only in the breeding season usually serve as signals to other members of the species. For puffins, the bill seems to work in several ways:

• Attracting a mate. Both sexes have colorful bills, and young birds looking for a first partner can judge one another by their bills and displays.
• Pair bonding. Mated puffins regularly greet each other by billing, swinging their heads from side to side and rattling their bills together, often surrounded by watching neighbors.
• Signaling condition. A large, bright bill takes energy and nutrients to grow. It is thought to show that a bird is healthy and well fed, though how closely color tracks quality in puffins is still being studied.
• Showing maturity. Adults have deeper, brighter bills than young birds, which helps separate breeders from immature visitors at a crowded colony.

The red and orange colors in bird bills are typically produced by carotenoid pigments that come from food. For a fish-eating seabird, these pigments ultimately come from plankton passed up the food chain.

The puffin beak’s ultraviolet glow

In 2018, British ornithologist Jamie Dunning reported that parts of the puffin’s bill glow under ultraviolet (UV) light. Under a UV lamp, the yellow ridge and the yellow rosettes at the corners of the mouth fluoresced brightly. This kind of glow, in which a material absorbs UV and gives off visible light, is called fluorescence.

Most birds, unlike people, can see ultraviolet light, so the glow may be part of what other puffins notice when they look at a mate or a rival. Its exact role is still unknown, and so far it is best described as an interesting finding that needs further study rather than a proven signal. Fluorescent bill parts have also been reported in related species such as the rhinoceros auklet.

Can you tell a puffin’s age from its beak?

To some extent, yes. A newly fledged puffin has a narrow, dark bill with a dull yellowish-brown tip and no bright plates. Over the next few years the bill grows deeper, its upper edge becomes more curved and the colors brighten, so immature birds visiting a colony often look slimmer-billed than breeders.

Adults also develop grooves across the red part of the bill as they age, starting with one and adding more over the years. Researchers use the number of grooves together with bill depth as a rough guide to age in younger adults, but the method is imprecise for older birds, so ringing remains the only reliable way to know a puffin’s exact age.

A beak that works as well as it looks

The puffin’s bill is not only decoration. Its hinged jaws, serrated edges and spiny palate let a parent carry a whole row of small fish back to its chick, as described in How many fish can a puffin carry? Puffins also use the bill to dig and clear out their burrows and as a weapon when fighting over nest sites.

Puffins’ Pacific relatives, the tufted and horned puffins, also grow brighter bills and ornaments in the breeding season and shed them afterward, and the related rhinoceros auklet grows a pale horn at the base of its bill that is shed in the same way. Where Atlantic puffins spend their colorful summers and their plain winters is covered in Where do puffins live?