THIS is the time to hear, or even see, nightjars. As their name suggests, these are birds of the night-time hours, especially dusk, and because they are usually sighted as silhouettes against the darkening sky, they are seldom recognised.

Some of us may perhaps believe they are large swifts or even a hawk of some kind. They spend the spring and summer with us, chiefly in the south and west of England, although they can be seen and heard in this region before October, when they return to South Africa. Several species inhabit most countries of the world where they are generally known as night-hawks, although in this country they were called either goatsuckers or goat-milkers. For centuries, people wrongly believed these curious birds sucked milk from goats.

So how would we recognise one? The nightjar tends to favour open spaces such as moorland, sand dunes and open areas in or near woods and forests. That is where you are most likely to see one – rarely would you see one in your garden. If you do see one, it will probably be flying at dusk with lots of gliding and some strange flight manoeuvres. It flies in this way because its chief aim is to catch moths whose movements are very erratic – what you would be witnessing would be the nightjar’s hot pursuit of a moth.

The silhouette of a nightjar can be likened to that of a large swift or small hawk. The bird is slightly larger than a blackbird – ten-and-a half inches or so (27cm) – and it has long pointed wings with distinct “elbows” close to its body. The slow movement of its wings is also characteristic.

The tail is long and shaped rather like an axe-head. It is these features that provide its hawk-like silhouette and have probably led to it being mistaken for a bird of prey.

The colours of its plumage include warm browns and greys and these serve to make it very difficult to see when it squats on the ground among dead leaves and bracken. The male has some distinctive white patches on his upper wings and tail.

If you manage to achieve a close-up sighting of a nightjar in flight, it will have a short but wide-open beak with tiny hairs at each side of its gape. This enables it to catch moths and other insects on the wing, a skill at which it is particularly efficient.

The nightjar spends most of its daytime life squatting on the ground in a well-concealed place such as a clump of bracken or pile of dead vegetation where it is almost invisible. At night it flies and will take short rests on tree branches where it has a curious habit. It sits lengthways along a branch, not across it in the manner of other species. This make it difficult to see because its colours blend so well with the background.

It is while the nightjar is sitting like this that you might hear its strange song. Various attempts have been made to describe it in writing. Some authorities say it sounds like a small motor engine or a spinning wheel being revved up for spells of around four or five minutes because it does rise and fall in its pitch. Others describe it as being like a sewing machine in use.

I’ve also heard it described as a rather quiet drill of some kind, perhaps a smaller one used by a dentist or jeweller. But even if you hear this song on the edge of a stretch of woodland, you’ll have difficulty locating the bird as it squats along its branch.

Nightjars take great risks with their nests because they are always placed on open ground but well camouflaged, and there the hen lays her two eggs. Male and female help with the incubation, the hen taking her share only during the day. She relies on her plumage for safety and when the chicks hatch, the male and female will feed them. Eventually, the female will leave to start rearing another brood while the male continues the rearing of the first ones.

It is the nightjar’s strange song that provides its name and in some areas it was known as the churn owl. Across the country it has other names including eve-jar, eve-churr, fern owl, dorr-hawk, dog hawk, jar owl, moth-eater, night-char, night-crow and wheel bird.

Due to its night-time travels it was often associated with death and in the north-east it was called the lych-fowl or corpse fowl. It was thought unlucky to hear its song.