A keel or carina (plural carinae) in

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 capabl ...

is an extension of the

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sh ...

(breastbone) which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of the ribs. The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequate

lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...

age for

flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...

. Not all birds have keels; in particular, some

flightless bird Flightless birds are birds that through evolution lost the ability to fly. There are over 60 extant species, including the well known ratites (ostriches, emu, cassowaries, rheas, and kiwi) and penguins. The smallest flightless bird is the In ...

s lack a keel structure. Without a keel a bird will not be able to fly. Some flightless birds have a keel, such as the penguin; but in the penguin's case, its wings are too small for its body, so flight would require flapping its

wings A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expresse ...

too fast to be practical. Historically, the presence or absence of a pronounced keel structure was used as a broad classification of birds into two orders:

Carinatae Carinatae is the group of all birds and their extinct relatives to possess a keel, or "carina", on the underside of the breastbone used to anchor large flight muscles. Classification Definition Traditionally, Carinatae were defined as all birds ...

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

ratite A ratite () is any of a diverse group of flightless, large, long-necked, and long-legged birds of the infraclass Palaeognathae. Kiwi, the exception, are much smaller and shorter-legged and are the only nocturnal extant ratites. The systematics ...

s (from ''ratis'', "raft"referring to the flatness of the sternum), having a subtle keel structure or lacking one entirely. However, this classification has become disused as evolutionary studies have shown that many flightless birds have evolved from flighted birds. African Penguin Skeleton

References

* Cummins, Jim (April 1, 1996).
Anatomy of Flight
. Retrieved January 31, 2005. * Ramel, Gordon

. Earth-Life Web Productions. Retrieved January 31, 2005. Bird anatomy {{ornithology-stub

See also

References