Nature Studies by Michael McCarthy: Sweet birdsong that's like blossom in sound

The cottage looks out over a wide sea loch and across to the mountains in the distance. When we arrived, all the hills were hidden in cloud, but towards evening the banks of grey started to lift and first Blaven appeared, a peak as bulky as a prize bull, and then, in the sunset, the Cuillin ridge itself – a great jagged sawblade lit from behind by a blast of yellow light which slanted down onto the loch and turned the flickering waters to gold.

You're never far from the grandiose on Skye. Move a mile and the vista shifts – new peaks, new islands on the horizon – but always the scale is epic. One day we took the boat from Elgol to the head of Loch Scavaig and walked up the river to Loch Coruisk, the lonely spear of water which is aimed into the Cuillin's heart, the horseshoe of pinnacled summits; and looking up into the ring of black mountains somehow felt like peering down into a bottomless chasm.

Funny then, that the strongest memory of a week on Skye should be something tiny and sweet, something more on the scale of an after-dinner mint. It's one of the signs of spring in the islands, which is running about a month behind southern England – the celandines are still out, as are the daffodils in the gardens, with the buds on the grey-green, lichen-covered oaks still locked up tight and the silver birches just beginning to unfold their leaves. (The birches fill every cleft in the hillsides, and at those blessed moments when the sun comes out strongly and the drama of mountain, blue sea and blue sky makes the Scottish islands fleetingly resemble Greece, the birches take the place of the olive trees.)

On the loch below us, where one morning an otter swam lazily past the cottage window, causing the children to choke on their scrambled eggs in the scramble to see it, eiders are displaying, the archetypal ducks of the North. The fat males are fabulous, striking studies in black and white, but they also have a stunning salmon-pink flush on their breasts, and they compete to show this off to the dowdy brown females by throwing their heads back and raising their bodies out of the water. Each time they do this they make a curious sound which is sometimes described as ah-oo, ah-oo, but listening to it drift across the loch it seemed to me more like a small engine being revved: not quite varoom, varoom, more vareem, vareem.

But it is another sound which has made the deepest impression on me, diminutive though it is. It is a silvery falling cadence, heard almost everywhere, heard now at this moment, even as I am writing this, from the birches which surround the cottage: it is the song of willow warblers. It is soft and it is sweet and if you walk over the hills it is coming from every isolated birch clump, adding a surprise touch of softness to the northern landscape's severity.

I find it captivating and I suppose that's partly because I know where it comes from: tropical Africa. The eiders have been in this part of the world all year, more or less, as have the ravens croaking on the ridge, and the oystercatchers whistling on the beach, and the gannet which saunters into the loch each morning from the open sea and breezes around looking for fish worth catching. But these matchbox-sized bundles of feathers were a month ago in Senegal, and The Gambia, and Guinea-Bissau; and they have just flown 3,000 miles over the Sahara, and the Mediterranean, and the Pyrenees, and the Channel, and the length of Britain, to get to the Isle of Skye and start singing.

We have known this for less than a century. It is a hundred years next month since the Staffordshire solicitor John Masefield, an enthusiast for the new-fangled practice of bird-ringing, ringed a swallow chick in his porch which 18 months later, turned up in Natal, South Africa, and thus revealed the scarcely-believable scale of bird migration. It is a scale incredible by numbers as well as by geography. About five billion birds are thought to fly north out of Africa every spring, to breed across Eurasia before returning south for the winter, and of these the willow warbler is one of the most numerous of all, with perhaps in excess of 100m birds making the journey, of which about four million are thought to come to Britain.

Yet in England willow warbler numbers are steadily dropping, and are down by a third since 1995. Nobody knows why; nor does anyone know why, curiously, the numbers in Scotland are definitely increasing, up by perhaps a fifth over the same period (it may be that the English birds and the Scottish birds migrate to different parts of West Africa with conditions varyingly favourable in different regions.)

So if you want to hear the tiny symphony which is willow warbler song in spring, an Easter visit to Scotland is as good a way as any. Never mind the red deer stags, and the golden eagles, and the otters in the sea lochs, not to mention the mountains and all the rest of the magnificence – come to Skye for the willow warblers.

It's not a phrase likely to make it onto a tourist poster. But up here in the harsh north, where cherry blossom is in short supply, there is a spring blossom of a different kind in that sweet, soft, silvery descending song – blossom in sound.

m.mccarthy@independent.co.uk; twitter.com/mjpmccarthy