I have fond memories from my teenage years of long, lazy summer days in suburban Liverpool. Barbecues outside, evenings sitting on the bench on the patio sipping orangeade while the sky above me turned the same shade as the drink, and a constant background soundtrack of dozens of screaming Common Swifts. Parties of them would whip fast and low between the semi-detached houses and, as dusk fell, their screams would get fainter and fainter as they circled higher into the sky. I can remember, in the height of summer, raising my binoculars to see, in the fading late-evening light, a swirling group of 40 or 50, invisible to the naked eye but still just about audible.