If birds gather on any spot, it is unoccupied. Clamor by night betokens nervousness.
Birds will not gather where there are people (unless the potential for food is greater than the perceived hazard). Birds gathered alone is hence a natural sign of an unoccupied space.
Birds seldom fly at night. They often roost together for safety, and any alarm is quickly taken up by others to enable the flock to escape. An enemy creeping through a roosting area may create such arousal. In this way, roosting birds are good night guards.