Keel (bird anatomy)

In bird anatomy, the keel or carina (pl.: carinae) is an extension of the sternum (breastbone) which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of the ribs.^[1] The keel provides an anchor to which the supracoracoideus (the muscle responsible for the upstroke) and pectoralis (the muscle responsible for the downstroke) attach, thereby providing adequate leverage for flight.^[2] Not all birds have keels; in particular, some flightless birds lack a keel structure. Some flightless birds have a keel, such as the penguin; this is because penguins still require strong wing muscles to power underwater movement.^[3]

Historically, the presence or absence of a pronounced keel structure was used as a broad classification of birds into two orders: Carinatae (from carina, "keel"), having a pronounced keel; and ratites (from ratis, "raft" – referring to the flatness of the sternum), having a subtle keel structure or lacking one entirely.^[4] However, this classification has become disused as evolutionary studies have shown that many flightless birds have evolved from flighted birds.^{[citation needed]}