A keel or carina in bird anatomy

Bird anatomy

Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply,...

 is an extension of the sternum

Sternum

The sternum or breastbone is a long flat bony plate shaped like a capital "T" located anteriorly to the heart in the center of the thorax...

 (breastbone) which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of the rib

Rib

In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

s. The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequate lever

Lever

In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...

age for flight

Flight

Flight is the process by which an object moves either through an atmosphere or beyond it by generating lift or propulsive thrust, or aerostatically using buoyancy, or by simple ballistic movement....

. Keels do not exist on all birds; in particular, some flightless bird

Flightless bird

Flightless birds are birds which lack the ability to fly, relying instead on their ability to run or swim. They are thought to have evolved from flying ancestors. There are about forty species in existence today, the best known being the ostrich, emu, cassowary, rhea, kiwi, and penguin...

s lack a keel structure.

Historically, the presence or absence of a pronounced keel structure was used as a broad classification of birds into two classes: Carinatae

Carinatae

This article is about bird taxonomy; for the topic in pottery and glassware design, see Carinate.The Carinatae are, in phylogenetic taxonomy, the last common ancestor of the Neornithes and Ichthyornis , and all its descendants...

 (from carina, "keel"), having a pronounced keel; and ratite

Ratite

A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis...

s (from ratis, "raft" — referring to the flatness of the sternum), having a subtle keel structure or lacking one entirely. However, this classification has fallen into disuse as evolutionary studies have shown that many flightless birds have evolved from flighted birds. The current definition of Carinatae now includes all extant birds.