Long-distance migration is evolution as an extreme sport. It is a device that stretches the notion of survival mechanism to an almost absurd degree. No wonder proto-naturalists of the 18th century were inclined to think swallows spent the winters at the bottom of ponds: it's a sight more likely than flying all the way to Cape Town and back.

I have been at both ends of the migration routes of our long-distance travellers. “Not European swallows,” I have been told. “African swallows that happen to breed in Europe.” I have seen the savannahs as swallow-skimmed as an English meadow: birds sweeping over the grasses at tummy-tickling height. I have seen them feeding on insects disturbed by elephants; I have seen them feeding on insects disturbed by the horses in the field behind my house.

But in a way, it is understandable with swallows. You can't look at a swallow, or for that matter at a martin or a swift, without saying to yourself: “That bird is awfully good at flying.” They are aeronauts, flying is their business, and so, if you think about it, you'd have to agree that they are obviously well suited to extreme life-choices involving flight.

All the same, my house martins still haven't turned up, and so far, at least around my place, the annual tide of migrants seems no more than a trickle. I am bothered by this, really more than a trifle spooked by the rhythms of this unfamiliar spring. So I took a walk in one of those strange Suffolk places where woodland, scrub and desolate open reedbeds all come together. I wanted to see if they were having any better luck around here.

Well, the joyful sounds of spring were all around us: but the chaffinches and wrens and robins and tits and reed buntings are all our own non-travelling birds. No disrespect, as footballers say, but I wanted to see how the travellers were getting on. Or rather, I wanted to hear how they were getting on. My ears were peeled, as it were, agog for the sound of a traveller.

I didn't have much luck, and I was inclined to be a little despondent. But then, out of the merry din of the residents, a tiny silvery little trickle of notes, lisping sweetly down the scale. It stopped almost as soon as it started, but it was a brief and gloriously poignant moment: one of those marvellous dramas played to an audience of one, of the kind that the wild world delivers so often.

It was a willow warbler. I have heard willow warblers in Suffolk; I have heard them in the Luangwa Valley in Zambia. This bird, a favourite bird, links my two favourite places in the world. And willow warbler is a bird to worry about even more than most migrants: numbers are dwindling in England, so that you wonder if the extreme sport of migration is still a viable option for a long-term future.

Willow warblers are not sleek jet fighters like swallows. Each one is nothing more than a scrap of feathers. You wouldn't fancy them to fly across Suffolk: but they fly across the world and they do it twice annually. It is one of the greatest miracles in all of life: and there before me, unseen, singing tentatively from a tangle of scrub, the thin lovely lisping song. Not the fully monty just yet: this was a kind of throat-clearing, a celebration of arrival, a prelude. It was a sweet shaft joy in a troubling couple of weeks: an suggestion that the extreme way is still just about possible. This year, anyway.

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