Guardian: They are the quintessential masters of the air, able to feed, sleep and even mate on the wing. Every year they travel on a round trip of more than 20,000km to and from southern Africa without once touching the ground. And they have long been the subject of myths and folklore, including the belief that their piercing screams are the cries of lost souls. They are the birds that signal the coming of summer to our towns and cities: swifts. But if the predictions of a recent report prove …