The loggers also showed that the birds' flight activity appeared lower during the day than at night, probably because they spend their days near-effortlessly soaring on warm air currents known as "thermals."
The common swifts ascend at dawn and dusk, reaching altitudes close to 10,000 feet. As they fly, they feast on any insects that they can capture.
Their life on land moves, as their name suggests, swiftly. The birds go to Sweden in late May/early June, where they breed and raise one to three chicks. Breeding normally takes place in natural tree holes, like those left behind by woodpeckers. But these days, the birds have been seen nesting in buildings and under roof tiles. Their chicks are fed "insect balls" that the adult birds store in a muscular pouch near their throat.
Here's what a typical flight plan looks like for the common swift: In the autumn, they travel via the Iberian Peninsula in West Africa and, from there, further east into the Congo Basin of Central Africa. This becomes part of their six-month wintering period.
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"Spring migration starts in April," he said, "when they again pass via West Africa, where they spend about 10 days in Liberia to fuel up, before the rapid spring migration across the Western Sahara and Europe to return back to their breeding site."
Given all of the birds' time in the air, they can't afford to shed many flight feathers. So they molt extremely slowly, replacing their feathers one by one over a six-month or so period to avoid any dangerous gaps. The prep seems to work, as the birds can live to advanced ages. Some have been documented as reaching 20 years old, and they can probably live even longer than that if a predator, such as falcon, doesn't grab them.
The rest of their body is a lean, mean natural flying machine. Their wings are long and narrow, their legs are short and lightweight (so not very good for walking around on land), and their entire frame has a streamlined shape.
Hedenström said that the bird's flap-and-glide mode of flight is extremely energy efficient. Their shape, anatomy and airborne moves may inspire future micro air vehicles (MAVs) meant for very long-distance flights.
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