The sky is not just the space above our heads. It’s home. If not for us, then for many. In the warmer months, the sky above this country is alive: a staggering and complex community of beings. For the forces of life, the sky is just one more opportunity, and when life sees an opportunity, life doesn’t hang about.

It remains one of my favourite lines in a seriously intended work of science: under “Sky”, Dylan Aspinwall writes in A Field Guide to Zambian Birds not found in Southern Africa: “This is the most widespread habitat in Zambia.” It is pretty widespread here, too. That’s the sky for you: not a space but a place. A habitat.

It begins with little specks and flecks of life: bacteria, pollens, spores, seeds. It goes on with arthropods, that massive group that includes insects and spiders. And they fill the air, to a degree beyond our imaginings. It has been estimated that if you take a square mile of sky and start counting in the box that lies between 20ft feet and 500ft above the ground, you will, on a fine June day, find 32 million arthropods.

Spiders may be wingless, but they too love the sky, and deliberately seek the heights by ballooning. They climb to a reasonably eminent branch-end or grass tip, and let out quantities of silk until the wind takes them. The whole mass of living stuff is called aerial plankton, and if you are uncomfortable at the idea of all those living things above your head, then remember that without this stuff, we would be without some of our best-loved creatures as well.

Swallows and swifts travel from Southern Africa for this annual bonanza, favouring us with their presence because of the great booty of the air. But you have to be pretty good to take advantage of the floating larder of the sky. You can’t just drift about with your beak open and hope for the best. You have to be a master of the air: seriously fast, and devastatingly manoeuvrable. I have just described the requirements of a fighter plane: and, not by coincidence, that is the model used by swifts and swallows: slim, swept-back wings that allow them to corner at such staggering speeds.

They’re not closely related, swallows and swifts: they have just come up with the same solution for the same problem, in the most efficient way imaginable. It’s called a convergence — bats fly and dragonflies fly, but they aren’t related at all. Swallows and swifts have reached the same optimum solution by different routes.

There was a fledging-out of swallows on the morning I wrote these words, the two hard-labouring swallows suddenly becoming seven or eight, whizzing round the house, calling to each other with that sweet double note. Now the young ones must learn to fend for themselves: howling on swept-back wings through the thin speckles of plankton, and making those fighter-plane swerves, banks and shimmies to catch the juiciest.

Swallows love in particular that belly-brushing, crop-spraying flight: low to the grass, picking up creatures that have only just decided to join the air, or who thought it was time to rejoin the earth.

Round the house the swallows go again: making a summer.

All the same, I’d be a swift if I had the choice. If we define our relationship with birds by our envy and joy in their powers of flight, then swifts are the ultimate birds.

“With a bowing power-thrust to the left, then to right, then a flicker they tilt into a slide, a tremble for balance, then a lashing down disappearance.” That’s Ted Hughes.

There was a screaming party howling down the road in front of the pub the other day: a dozen young swifts revelling in their youth by making a terrible din and showing off: down the street they went, chimney-pot high, power-thrusts and flickers and all, screaming at the tops of their voices. It’s the easiest mind-boggler in the birding life: just explain to anyone mystified by your passion that these young swifts have been flying constantly since they left the nest, and they won’t come out of the air until they are ready to breed themselves: say, next summer.

That’s two years of uninterrupted flight: eating on the wing, sleeping on the wing, travelling to Africa and back, and then again, and then back here again to mate — and yes, they do that on the wing as well.

Swifts are flight. It’s as if everything remarkable about the flight of birds were put into a single body. The sky is not their backdrop but their home: and no bird lives in a more intimate relationship with the sky than the black sickles of summer.

“They’ve made it again, which means the globe’s still working . . . ” Hughes again. And every year the wait for them gets more fraught.

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