Aristotle recorded the belief and Pliny the Elder repeated it, adding that the milked animal went blind. The birds do gather around grazing livestock at dusk, but for the moths, beetles and flies that the animals stir up from vegetation. A nightjar's bill is small and weak, and its huge gape is for catching insects in flight.
The churring song is the other feature that draws attention. A singing male produces a sustained mechanical trill from a perch, sometimes for several minutes without an obvious pause, changing pitch as he turns his head, which makes the source hard to locate. The song is produced vocally; the sharp wing-clap in display flight is not.
Camouflage is the species' main defence. Roosting birds sit along a branch rather than across it, and the plumage matches bark and leaf litter closely enough that a bird can be approached to within a metre. How well nightjars locate prey in near darkness, and how much they rely on silhouetting insects against the sky, is not fully settled.