I look at nature in a way that was beyond the scope of the finest nature writers that ever saw a swallow and felt a joy. Gilbert White, Charles Darwin, John Clare: none of them ever looked at the sky in the last week of April and felt as I do. The dreadful anxiety is the unique privilege of 21st-century humans.

This sense of the world's fragility is not something that troubled White, as he marvelled his way around Selborne, or Darwin, when every living thing he saw provided another question of bottomless profundity. But it troubles me: and this last week in April, the troubles become acute.

Regular readers of this space will be anxious for news about my back, damaged last week in the course of a newt hunt. Well, it's much better, thanks, but I'm still pretty much tied to home. I need the wildlife to come to me. But that is something I need in this week before May every year: the week before the great explosion of life.

But will it explode? That's what bothers me. I am scanning the skies and fretting. And every now and then: look, there really is a swallow, no, two. Nothing flies like a hirundine; you can tell them from miles off. Most birds go from A to B. Swallows go round and round.

So every time I see a bird going round and round, I give a small, relieved smile. The other night there were two of them, not chasing insects but swallows, goosing each other in the air and swapping perky double-note calls. That's what I need to see: a bit of passion, a bit of life.

I need to hear the swallows chattering in the stables as they remake last year's nests; I need to see the whitewash on the saddles in the tackroom; I need to duck when I enter the stables and a parent swallow swoops indignantly out. There's time enough for all that, I know: but I need reassurance. I need to know that the great processes of life are under way.

No martins yet. Not a glimpse. They nest under the eaves of the house; perhaps they can trace their ancestry directly to the birds who nested here when the house was first built out of sticks and cowshit in the 16th century. But they're not here yet; and this year, as every year, I find myself wondering if this time it won't happen: if this time, it's a year too far, and after five centuries the house will be martin-less.

Then I hear the merry farting call from the sky and see them going round and round and skimming under the eaves, seeking out last year's nest, perhaps where they raised young, perhaps where they were hatched. But there were fewer last year than the year before: and even that was well down on the peak of a few years back.

Still too early for swifts, of course. Ted Hughes caught the anxiety that Clare never knew: “They've made it again,/ Which means the globe's still working...” But for how much longer? The life of the long-distance migrant exists on the far edge of the possible: that is the glory of it, that is the fearfulness.

Well no doubt they'll be back in a few days, this year anyway, and I'll be back to hymning their glories in a world of cacophonous Maytime song beneath a sky crowded to bursting point with swifts and swallows and martins. But today I feel nothing but the world's fragility: and that is every bit as real as its glories.

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