This moment in September is an odd time, a kind of limbo in the natural world. Everything here is still green, although the greenness has taken on a rather tired, dusty aspect, as if the very chlorophyll is getting creaky after a long, dazzling season. Despite a gloomy weather forecast of low skies and mean cloud, the Scottish sun is dancing about the place as if there is life in the old girl yet. My swallows have gone, but I saw a gang of swifts yesterday who have still not set off on their epic journey south.

Yet, there are some subtle signs of the shifting of the season. I wake to improbably golden sun, but when Stanley the Dog and I step outside it is properly cold for the first time. It is put on your gloves and scarf cold. It is almost wear a hat cold. In these northern parts we have not yet seen the first frost, but there are heavy, silvery dews, a gentle harbinger of the freezes to come. The beech hedge is still verdant, but the very first leaves are beginning to turn, and some, jumping the gun a little, have already begun to fall when the wind blows.

tan 01The limes just starting to turn.

It was a glorious, long, hot summer. It was a real treat, after the endless cold and rain of last year, when summer never really pitched up and all the farmers looked fraught and sad as they battled to get in the harvest and cut the hay. Even the cows looked demoralised. (Cows, it turns out, really object to the wet.) There is a faint melancholy to see it go, and it is going, for all the late sunshine. At the same time, I have that back to school impatience for the season to turn properly. I want the metaphorical sense of sharpening my pencils for the new term.

tan 02Meanwhile, up at HorseBack UK, just along the river, the Deeside hills are still as vivid as ever in the September light.

This morning, down at the paddock with my mare, I was mooching about with her when suddenly her head went up and her ears pricked and she was at once on full alert. Horses, being prey animals, are amazingly sensitive to the slightest noise or movement, often ones which are completely beyond human senses. I looked about, trying to see what she could see. I heard nothing. The field seemed quiet. The woods were drowsing in the morning light and everything was still. She gazed fixedly to the north. There was something there.

tan 03View of the set-aside this morning, with a happy pair of mares enjoying a free graze. You can see the greenness just starting to fade. The two equines, however, care nothing for that as they still find the good grass.

Finally, I heard it. It was a distant call and chatter, as if someone were having a cocktail party over the other side of the hill. It couldn't be, I thought, not so early. I scanned the sky. (We must have made an odd picture – the horse and the human, both staring up into the blue.) It was. It was the geese. The very first skein of the season, making their migratory path out of the north-west. They tracked a sure line, flying on a perfect diagonal to the south-west, calling as they went. I wondered, as always, at the twenty-seven avian mysteries. What do those calls mean? How do they organise their clever rotas, so that everyone takes a turn at the sharp end? How do they keep that wonderful formation? Where do they hone their astonishing navigational skills?

The mare, having ascertained that the noise was not that of a predator, lost interest and fell to grazing. I, still fascinated, oddly delighted, as happy as if I were greeting old friends, went on staring at the sky, watching the miraculous group until it went out of sight. There we are, I thought. That's it. Autumn is official. I can go back to school.