We saw them first under moonlight, scurrying in small groups across the banks of estuary mud, right on the edge of the receding waterline, in a stop-start way that soon would become familiar. They were the dunlins. They had arrived in the autumn, before we even knew, their black breeding plumage already faded as they moved almost indistinguishably between the various greys of water and shore at night. I rubbed my eyes as if doing so might make them disappear. There they were, still. By the end of November they were lost again, gone to Africa to overwinter. Nearly a whole year passed before we discovered whether they would return. This, we realised, was the meaning of duration.