On the right September morning, you can stand on the Upper Rock and watch the sky come apart. It begins with a few dark specks turning in the same patch of blue. You blink and there are forty of them, then a hundred, all wheeling together in a slow rising column until they are flecks of pepper against the cloud. Then, as if some signal has passed between them, they peel off the top one by one and pour south in a long unbroken thread, pointed straight at the brown hills of Morocco fourteen kilometres across the water. Multiply that single column by the whole of an autumn and you have something close to 300,000 birds of prey passing the mouth of the Mediterranean, hunted by no one, watched by a handful of people who happen to be looking up. The Rock of Gibraltar sits at the eastern jaw of the narrowest gap between two continents, and for a few golden weeks each year it becomes one of the front-row seats for the greatest raptor crossing in the Western Palearctic. Most travel writing about this spectacle points across the bay to Tarifa, the windswept Spanish town beside the very narrowest pinch of the Strait. That is fair, because Tarifa catches a great deal of the actual crossing, and serious birders rate it highly. Gibraltar has its own strong claim, though. The Rock is a 426 metre limestone wall standing right at the lip of the channel, it throws up the warm rising air that soaring birds live by, and it has been a centre of careful bird study for half a century. If you want to watch the great raptor river while sleeping in a proper bed, cooking your own breakfast and walking ten minutes to dinner, Gibraltar makes the better base. Large birds of prey are heavy, broad-winged creatures built for gliding rather than flapping. To cross a continent without starving they ride thermals, the columns of warm air that rise when the sun heats the ground. A bird climbs inside a thermal almost without a wingbeat, then peels off and glides for miles, losing height slowly until it finds the next thermal and climbs again. It is an astonishingly efficient way to travel hundreds of miles on a near empty stomach. The catch is that thermals form over land, not over open sea. Water does not heat the air in the same way, so a raptor that strikes out across a wide stretch of ocean is committed to costly flapping flight with no lift to rescue it. Many simply cannot make a long sea crossing and live. So the birds streaming south through Iberia in autumn do not fan out across the Mediterranean. They funnel, drawn towards the one place where the gap between Europe and Africa shrinks to almost nothing. At its narrowest the Strait of Gibraltar is only about fourteen kilometres wide. For a soaring bird that is the difference between a manageable hop and an impossible voyage, so the birds converge on it from across half of Europe. The result is one of the great migration bottlenecks on the planet, a corridor you can see clear across on a bright day, with millions of birds squeezed through it each year. The Rock stands at the eastern edge of that corridor, the last great landmark before the leap. Birds gather over the high ground and spiral upward to bank as much height as they can, then glide out over the water hoping to reach the African shore before they run low. On a good day from the Upper Rock the whole drama plays out above your head, and you can trace the departing flocks all the way out towards the blue mountains of the Rif. The word every raptor watcher uses is kettle. A kettle is a swirling, rotating mass of birds all riding the same thermal at once, turning together like leaves caught in a slow whirlwind. Dozens, sometimes hundreds, of honey buzzards or black kites stack up inside a single rising column, circling higher and higher until they are specks, then streaming off the top in a long glide. The name borrows from the way the spiralling birds churn and boil like water in a pot. > Stand under your first kettle of several hundred honey buzzards turning slowly against a September sky, and you understand at once why people travel across the world to see it. Kettles are the reason timing your visit to a thermal-rich part of the day matters. Early in the morning the air is cool and the birds sit tight in the trees. As the sun warms the land the thermals build, the kettles form, and the sky fills. Patience is the only skill the day really asks of you. The Gibraltar Ornithological and Natural History Society was founded in 1976 to study and protect the wildlife of the Strait. Its Strait of Gibraltar Bird Observatory monitors the migration of seabirds, raptors, storks and small birds, and it holds a database of records reaching back to 1964. The ringing station at Jews Gate sits on the Upper Rock Nature Reserve, in a pocket of Mediterranean scrub dominated by wild olive and lentisc, and the society counts raptors and storks from well established points on the high ground: looking north over the isthmus during the southward autumn pas