They look like...

 

Nightjars have a wingspan of 52 - 59cm. When they are flying you can mistakably identify them as a small falcon or cuckoo. They plumage is brown-greyish and is speckled with numerous white and black dots and stripes. Most of the times Nightjars lay on a dark substrate and keep very quiet. Resting on a branch or on the ground, they are really hard to destinguish from the background. When Nightjars are grooming or cleaning, they move like a branch or leaf that is moved by wind.

The remarkable thing about Nightjars is their churring, a long rattling sound with short stops. Most churrings end with an ‘errrrurrrruu’ or deep stuttering ‘fiORRRRR’. In flight they sometimes make shot 'Kweek' sounds. During displays male Nightjars clap their wings above their backs to make clapping sounds.