Dawn. A south London hill. Thin cloud, clearing. Harvest moon, aglow. The sun, just above the horizon, doing its bit for the rehabilitation of the colour orange. The kind of September day that could serve as an advert for autumn.

There are signs of life. An early jogger shuffles doggedly past. To my left, a wren scolds me for my presence. Screech of jay, flutter of robin, scamper of squirrel.

The locals, going about their business, doing no harm.

But while these familiars always give pleasure, there is more on offer than the daily doings of my local wildlife. Because this is migration season, and the chance of encountering passing travellers overhead has me scanning the skies even more than usual.

In autumn I might get up early, find somewhere with a good view of the sky, and sit and wait. And on a good day, there will be rich pickings. Sometimes a trickle, sometimes a surge, each bird with its own journey, destination unknown.

Or I might point a microphone at the sky, set it to record all night, then sift through the recordings the next day in the hope of hearing a “tswee” or a “graark” that will tell the tiniest part of a larger story, each sound in the headphones somehow both intrusion and privilege.

These aural and visual encounters can only ever be the smallest part of the picture, the briefest glimpse into lives mysterious and unfathomable. Tiny things; vast journeys. And each contact – sound in the ear, fluttering speck overhead – invites thoughts of the thousands unencountered. The silent, the invisible, living their lives beyond our reach.

It’s a quiet morning for migrants, but that’s the way it goes. There are wood pigeons – about 30 in a loose flock, barging their way through the air with belligerent purpose. And there’s the odd glimpse of species unknown – too high for identification, a reminder that birds don’t hang around us for our benefit.

As I leave, I get my reward. Three swallows – fast, nimble, flying with beginning-of-journey vim – flit over my head, away and out of my sight, taking summer with them once and for all.

And one day soon I’ll be walking home late at night, and I’ll hear the thin “tseep” of a redwing overhead, and I’ll know the year is on to its next stage.

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