Churrrrr... It must be high summer if the song of the reclusive nightjar fills the dark sky

They used to compare it to the sound of a spinning wheel, but these days they mention a motorbike. A small motorbike, distantly heard.

They used to compare it to the sound of a spinning wheel, but these days they mention a motorbike. A small motorbike, distantly heard.

It's one of the ancient sounds of late summer evenings in the countryside, and over the coming weeks a small army of patient listeners will be cocking their ears in the dusk, trying to catch it.

The name for it is churring. It's the sound of the nightjar, one of Britain's most attractive but least visible birds, and often it's the only practical way of telling that this master of camouflage is actually there.

It will be used this month and next by nearly 2,000 observers taking part in the national nightjar survey, a once- a-decade exercise to check on the bird's population status.

For 50 years in the mid 20th- century it was in serious decline in both range and numbers and by the early 1980s there were only about 2,000 pairs of nightjars left in Britain. But since then it has staged a remarkable comeback: the last survey, in 1992, discovered 3,400 churring males. (It is the males that do it, advertising their presence to female nightjars and warning rival males off their territory).

Hawk-like, with a dashing flight on long sharp wings, the nightjar is an easy bird to be fascinated by: its alternative name is the goatsucker, deriving from an old country legend that the birds used their wide gaping mouths to suckle milk. In reality, the gape is used for snapping up moths and other large flying insects that come out at night.

For the nightjar is nocturnal, or to be more precise - at least as far as churring is concerned - it is crepuscular, which adds to its charm. To see it or hear it, you have to be out in that magical half-hour of dusk between sunset and nightfall. (Thomas Hardy, in one of his loveliest poems, "Aftermath", christened it the "dewfall hawk".) Hardy's archetypal landscape of heathland is the nightjar's preferred habitat, and its population decline set in when much of Britain's lowland heath was planted with conifers in the 1920s and 1930s.

But paradoxically, it is forestry operations that seem to have spurred its recent comeback, because the areas where plantations have matured and been felled provide a perfect breeding ground for the bird. It nests on the ground, its brownish plumage camouflaging it perfectly, so the survey's method is to listen for and count churring males.

It is being organised by the British Trust for Ornithology (BTO), with help from the Royal Society for the Protection of Birds, English Nature, and the Forestry Commission. One evening last week The Independent joined a group of observers from all four organisations listening for nightjars in Thetford Forest, Norfolk, one of the bird's strongholds.

Nick Gibbons, a Forestry Commission officer and nightjar expert, found two and possibly three churring birds for us - it is sometimes hard to tell numbers because they move around to different calling posts. To try to attract them Mr Gibbons and Greg Conway, the BTO survey organiser, used an old trick of waving and throwing white handkerchiefs into the air, which in the half-light the birds sometimes mistake for the white patches on the male nightjar's wings.

We followed the churring and eventually saw the birds when they flew - ghostly shapes dashing through the gloaming, familiar to Thomas Hardy and his world, but a much rarer sight and sound in urbanised 21st-century Britain.