Which is the fastest bird in the world ?

The swifts are a family, Apodidae, of highly aerial birds. They are superficially similar to swallows, but are not closely related to any of the passerine species. Swifts are placed in the order Apodiformes, which they share with hummingbirds. The treeswifts are closely related to the true swifts, but form a separate family, the Hemiprocnidae.

Resemblances between swifts and swallows are due to convergent evolution, reflecting similar life styles based on catching insects in flight.

The family name, Apodidae, is derived from the Greek ἄπους (ápous), meaning "footless", a reference to the small, weak legs of these most aerial of birds.The tradition of depicting swifts without feet continued into the Middle Ages, as seen in the heraldic martlet.

Some species of swifts are among the fastest animals on the planet, with some of the fastest measured flight speeds of any bird.

Taxonomy

Taxonomists have long classified swifts and treeswifts as relatives of the hummingbirds, a judgement corroborated by the discovery of the Jungornithidae (apparently swift-like hummingbird-relatives) and of primitive hummingbirds such as Eurotrochilus. Traditional taxonomies place the hummingbird family (Trochilidae) in the same order as the swifts and treeswifts (and no other birds); the Sibley-Ahlquist taxonomy treated this group as a superorder in which the swift order was called Trochiliformes.

The taxonomy of the swifts is in general complicated, with genus and species boundaries widely disputed, especially amongst the swiftlets. Analysis of behavior and vocalizations is complicated by common parallel evolution, while analyses of different morphological traits and of various DNA sequences have yielded equivocal and partly contradictory results.

The Apodiformes diversified during the Eocene, at the end of which the extant families were present; fossil genera are known from all over temperate Europe, between today's Denmark and France, such as the primitive swift-like Scaniacypselus (Early - Middle Eocene) and the more modern Procypseloides (Late Eocene/Early Oligocene - Early Miocene). A prehistoric genus sometimes assigned to the swifts, Primapus (Early Eocene of England), might also be a more distant ancestor.

Species
There are around 100 species of swifts, normally grouped into two subfamilies and four tribes.

Description
Swifts are the most aerial of birds. Larger species are amongst the fastest fliers in the animal kingdom, with the white-throated needletail having been reported flying at up to 169 km/h (105 mph).Even the common swift can cruise at a maximum speed of 31 metres per second (112 km/h; 70 mph). In a single year the common swift can cover at least 200,000 km.

Compared with typical birds, swiftlet wings have proportionately large wingtip bones. By changing the angle between the wingtip bones and the forelimb bones, they are able to alter the shape and area of their wings, maximizing their efficiency and maneuverability at various speeds.Like their relatives the hummingbirds, and unlike other birds, they are able to rotate their wings from the base, a trait that allows the wing to remain rigid and fully extended deriving power on both upstroke and downstroke.The downstrokes produces both lift and thrust, while the upstrokes produces a negative thrust (drag) that is 60% of the thrust generated during the downstrokes, but simultaneously it contributes with a lift that is also 60% of what is produced during the downstroke.