"Every morning during a bout of hot weather at the end of August a few years ago, Ian Jackson looked out of the window of his fifth-floor apartment and saw literally hundreds of House Martins clustered on a nearby roof. They must have come from near and far. The black slates of that roof face the rising sun and absorb the warmth of its weak rays, and the martins were standing with their backs to the sun, sunbathing with their wings spread."