They swoop to the ground and perform breathtaking displays of aerial acrobatics before soaring to dizzying heights above the clouds. Now scientists believe they know how swifts are able to fly so fast and for so long.
A series of experiments with a wind tunnel has shown just how it is possible for a small bird such as a swift to spend so much of its life in the air - eating, sleeping and even mating on the wing.
The secret is their ability to change the shape of their wings so that they get the best performance for the smallest amount of energy - scientists call it "wing morphing".
Swifts can often be seen and heard on summer evenings when they fly at high speed in screeching flocks, but it is not widely known that they spend almost all of their lives in flight, rarely landing except to lay eggs and to rear their young.
Scientists estimate that over the course of its lifetime a swift will cover 2.8 milion miles, a distance equivalent to six round-trips to the Moon or 100 times around the Earth.
They catch up to 20,000 insects a day and have been monitored by radar at night flying at a height of almost a mile where they spend much of the time "roosting" in flight.
A team of Dutch and Swedish scientists has found that the swift is able to take to a life of flight with such apparent ease because it is able to "morph" its wing into a range of shapes to get the best aerodynamic performance with the smallest energy costs.
"During flight, they continually change the shape of their wings from spread wide to swept back," said David Lentink, a zoologist at Wageningen University in The Netherlands.
"When they fly slowly and straight on, extended wings carry swifts 1.5 times farther and keep them airborne twice as long. To fly fast, swifts need to sweep back their wings to gain a similar advantage," Dr Lentink said.
A study published in the journal Nature monitored the swift's flight using a wind tunnel and the dismembered wings of swifts that had been brought dead or dying to a number of Dutch bird sanctuaries.
The scientists compared the amount of "lift" and "drag" on each wing according to its shape and the speed of the wind, which could be varied by up to 30 metres per second to mimic different flight speeds.
At slower speeds, outstretched wings gave the best flight efficiency. However, for flying fast and straight, it was better to sweep the wings backwards into a "V" shape.
"Swept wings are also better for fast and tight turns, but this time swept wings are better because they do not break as easily as extended wings," Dr Lentink said.
"The main findings are that by changing wing shape, swifts can fly up to 60 per cent further, 100 per cent longer in terms of time, and their turns can be up to three times better, sharper and faster," he said.
At night during sleep, swifts have their wings outstretched so that they fly at slower speeds of between 8 and 10 metres per second. At these speeds, swifts fly with maximum efficiency, with more gliding and less flapping to maintain altitude, Dr Lentink said.
Wing morphing in swifts and other birds is being studied by scientists at the US space agency Nasa who are interested in adapting the techniques in the design of new aircraft and aerial-surveillance robots.
Dutch scientists are attempting to incorporate the latest findings into the design of micro-airplanes that can fly like swifts, Dr Lentink said.