I think of Nightjars as being birds of southern England but they are actually quite widely found in the UK including parts of Wales, northern England and southern Scotland. But I first heard them on the Somerset Levels in my youth and my strongest memories of them are on heathlands in Dorset, Hampshire, Norfolk and Suffolk.
Their song is a prolonged churrrrring. There’s nothing quite like it – it sounds almost mechanical. It is most often heard as light ebbs away and is delivered by a sitting bird – if it turns its head then the volume will increase or decrease depending on where you are in relation to the bird.
There’s not a lot of variation, but here’s a bird from the Netherlands;
… and one from Poland;
…and another from the UK:
Churrrrrring. Easy to identify but not so easy to be in the right place to hear it.