A bird's beak is bone wrapped in ever-growing keratin, with hidden tricks
A kiwi breathes through nostrils at the very tip of its bill, sniffing out food in the soil. Gannets have no outer nostrils at all and breathe through their mouths. Beaks look simple, but they are layered, living tools that keep growing, and some birds even shed parts every year.
Every beak is built the same way underneath: an upper and a lower jaw of bone, stiffened inside by a lattice of tiny struts, and sheathed in a thin layer of keratin called the rhamphotheca. That sheath grows continuously in most birds, and in some it changes colour with the seasons. Puffins shed pieces of it after breeding, and certain pelicans grow a temporary horn on the bill for the breeding season. Albatrosses and emus wear a sheath made of several plates, which appears to be the ancestral design; the seamless beak of most living birds came from those seams fading away over evolutionary time. A flexible hinge where the upper jaw meets the skull lets many birds lift the top half as well as drop the bottom.
The cutting edges reveal diet. Seed eaters have ridges for splitting husks, and mergansers carry saw-like serrations for gripping slippery fish. Falcons have a pointed tooth on the upper edge that fits a notch below, backed by bone, whereas a shrike's similar tooth is keratin alone. Birds in around 30 families line their edges with short bristles that add friction when gripping snails or hard-shelled insects, and serrated bills turn up in 23 percent of hummingbird genera, possibly helping some pierce flowers to rob nectar.
Beaks also carry messages. The red spot near the tip of a large gull's lower bill prompts chicks to peck at it, and that pecking makes the parent bring up food. Nestlings flash bright mouths, some patterned in ultraviolet that birds can see and people cannot. In barn swallows the vividness of a chick's gape tracked the strength of its immune response, and several experiments show red gapes encourage feeding. Some cuckoo chicks exploit this with wing patches that mimic their hosts' gape colour.
Even the nostrils vary. Ptarmigans have feathered ones that warm incoming air, and woodpeckers' feathered nostrils keep sawdust out.
Source: Beak
