Druridge Bay is enveloped in the dispiriting coastal fog of a typical Northumberland sea fret, but as we wander across the beach to meet the water, coils of cloud break apart and a milky sun breaks through. I wade into the shallows and a small flounder suddenly appears from its perfect sandy camouflage, as if decloaking from an invisibility shield, and darts rapidly away. Beyond the shore are lively flocks of black-headed gulls, oystercatchers and gannets, some only heard in the fog; but we do spot a small flock of the world’s greatest travellers. The arctic terns (Sterna paradisaea) work their way along the coastal shallows, raiding the water for prey in quick, stabbing dives. It is both baffling and humbling to think that these birds will soon be abandoning our autumn to set off on the longest self-powered migration of any species: a meandering route all the way to the edge of the Antarctic. The vastness of this round trip means that some arctic terns travel as many as 44,000 miles a year, and they live for up to three decades; in one lifespan they might fly the equivalent distance of a trip to the moon and back three times. Swapping ends of the Earth, they bask in two polar summers every year, seeing the most daylight of any animal. In terms of their capacity for roving, arctic terns are the freest of free birds. But even so, they haven’t been spared by this year’s outbreak of highly pathogenic avian flu; as elsewhere, it has taken a horrible toll on Northumberland’s wild seabirds, including many terns. The spread of this disease is thought to have been fostered, in large part, within populations of captive commercial poultry. Among the thousands of birds known to have been killed amid the world-significant colonies of the Farne Islands, 20 miles from here, was an eight-year-old arctic tern that would have flown over the course of its lifespan to Antarctica and back eight times, covering 144,000 miles. The wildest of lives terminated by a disease bred in captivity; it seems an especially grim way for an endless summer to end.