Bird wing in the context of "Moa"

Birds are a group of warm-blooded theropod dinosaurs constituting the class Aves, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. There are over 11,000 living species and they are split into 44 orders. More than half are passerine or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology.

Birds evolved from earlier theropods, and thus constitute the only known living dinosaurs. Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Birds are descendants of the primitive avialans (whose members include Archaeopteryx) which first appeared during the Late Jurassic. According to some estimates, modern birds (Neornithes) evolved in the Late Cretaceous or between the Early and Late Cretaceous (100 Ma) and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non-ornithuran dinosaurs.

Falcons (/ˈfɒlkən, ˈfɔːl-, ˈfæl-/) are birds of prey in the genus Falco, which includes about 40 species. Some small species of falcons with long, narrow wings are called hobbies, and some that hover while hunting are called kestrels. Falcons are widely distributed on all continents of the world except Antarctica, though closely related raptors did occur there in the Eocene.

Adult falcons have thin, tapered wings, which enable them to fly at high speed and change direction rapidly. Fledgling falcons, in their first year of flying, have longer flight feathers, which make their configuration more like that of a general-purpose bird such as a broadwing. This makes flying easier while still learning the aerial skills required to be effective hunters like the adults.

Flight feathers (Pennae volatus) are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (/ˈrɛmɪdʒiːz/), singular remex (/ˈriːmɛks/), while those on the tail are called rectrices (/ˈrɛktrɪsiːz/ or /rɛkˈtraɪsiːz/), singular rectrix (/ˈrɛktrɪks/). The primary function of the flight feathers is to aid in the generation of both thrust and lift, thereby enabling flight. The flight feathers of some birds perform additional functions, generally associated with territorial displays, courtship rituals or feeding methods. In some species, these feathers have developed into long showy plumes used in visual courtship displays, while in others they create a sound during display flights. Tiny serrations on the leading edge of their remiges help owls to fly silently (and therefore hunt more successfully), while the extra-stiff rectrices of woodpeckers help them to brace against tree trunks as they hammer on them. Even flightless birds still retain flight feathers, though sometimes in radically modified forms.

The remiges are divided into primary and secondary feathers based on their position along the wing. There are typically 11 primaries attached to the manus (six attached to the metacarpus and five to the phalanges), but the outermost primary, called the remicle, is often rudimentary or absent; certain birds, notably the flamingos, grebes, and storks, have seven primaries attached to the metacarpus and 12 in all. Secondary feathers are attached to the ulna. The fifth secondary remex (numbered inwards from the carpal joint) was formerly thought to be absent in some species, but the modern view of this diastataxy is that there is a gap between the fourth and fifth secondaries. Tertiary feathers growing upon the adjoining portion of the brachium are not considered true remiges.

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. The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequate leverage for flight. Not all birds have keels; in particular, some flightless birds lack a keel structure. 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 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 (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. However, this classification has become disused as evolutionary studies have shown that many flightless birds have evolved from flighted birds.