
Interestingly the notes in a Nightjars 'jar' or 'churr' are emitted at a rate of 30 or 40 per second and ringers can also testify to an interesting 'hiss' that is given by birds in the hand. This could be used to 'see off' any would be attackers by mimicking a snake.
Rebecca Nesbit has posted a clip on Youtube (below) which shows Mike Gould going through the process of Nightjar ringing.