The Nightjar family is a bird because its order is the Caprimulgiformes which is part of the Aves class, therefore being a bird. Birds are "feathered, winged, bipedal, endothermic, egg-laying, vertebrae animals". All of which, the Nightjar has, therefore being a bird.
The Nightjar has some interesting habits which are unusual such as perching along a branch, rather than across it. They are also nocturnal, although this is not unusual. Nighjar's are also in ancient foklore and were thought to suck the milk from goats. The Latin for 'goatsucker' is also, Caprimulgus. This is how Caprimulgiformes was derived.