Swifts(burung Walet) are the fastest 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. This flight arrangement could also have benefits for the bird's control and maneuverability in the air.
The swiftlets or cave swiftlets have developed a form of echolocation for navigating through dark cave systems where they roost. One species, Aerodramus papuensis, has recently been found to use this navigation at night outside its cave roost too.
Swifts occur on all the continents except Antarctica, and also not in the far north, in large deserts, and on many oceanic islands. The swifts of temperate regions are strongly migratory and winter in the tropics. Some species can survive short periods of cold weather by entering torpor, a state similar to hibernation.
Many have a characteristic shape, with a short forked tail and very long swept-back wings that resemble a crescent or a boomerang. The flight of some species is characterised by a distinctive "flicking" action quite different from swallows. Swifts range in size from the pygmy swiftlet (Collocalia troglodytes), which weighs 5.4 g and measures 9 cm (3.5 in) long, to the purple needletail (Hirundapus celebensis), which weighs 184 g (6.5 oz) and measures 25 cm (9.8 in) long.
The hardened saliva nests of the edible-nest swiftlet and the white-nest swiftlet have been used in Chinese cooking for over 400 years, most often as bird's nest soup. Over-harvesting of this expensive delicacy has led to a decline in the numbers of these swiftlets,especially as the nests are also thought to have health benefits and aphrodisiac properties. Most nests are built during the breeding season by the male swiftlet over a period of 35 days. They take the shape of a shallow cup stuck to the cave wall. The nests are composed of interwoven strands of salivary cement and contain high levels of calcium, iron, potassium, and magnesium.
In Indonesia the swifts have long been cultivated by farmers, by making a house for swift, Almost all regions in Indonesia many who cultivate this bird to take the nest
Swift cultivation is increasingly making Indonesia a nest exporter of this bird saliva
source Wikipedia Swift(burung walet)