Only hummingbirds can hover for any length of time without the help of a headwind. Their wings work in a unique way. They routinely beat 25 times a second, making the humming sound that gives them their name. Hummingbird wings work by producing down-drafts. Their wings beat by making a figure of eight, and flick over on the backstroke. Unlike the wings of other birds, they are symmetrical in cross-section, so work equally well to propel the bird in any direction, depending on the steer of its tail. This flight takes a lot of energy so they have to refuel very frequently. But flowers close at night, so what then? This is a particular problem in the Andes where the nights can be very cold indeed. As evening approaches, the Andean hillstar hummingbird heads to a cave to roost and goes into a state of torpor at night to save energy. Its heart - which beats at 1000 times a minute in flight - now slows to almost nothing. Its body temperature falls dramatically and its breathing seems to stop altogether. Basically it is doing what a hedgehog does every winter, but a hummingbird has to do this every evening of the year. The following morning it will start up all its motors once more.