the chimney swift is a gregarious species, and is seldom seen alone. it generally hunts in groups of two or three, migrates in loose flocks of 6–20, and (once the breeding season is over) sleeps in huge communal roosts of hundreds or thousands of birds. like all swifts, it is a superb aerialist, and only rarely seen at rest. it drinks on the wing, skimming the surface of the water with its beak. it also bathes on the wing, gliding above the surface of a body of water, briefly smacking its breast into the water, then flying off again, shaking its feathers as it goes. it has been recorded by pilots flying more than a mile above the surface of the earth, including one seen at . it is incapable of perching upright like most birds do; instead, it clings to vertical surfaces. if it is disturbed while at rest, the chimney swift will clap its wings loudly once or twice against its body; it does this either in place, or while dropping down several feet to a lower location. this behavior can result in a loud "thundering" sound if large roosts of the birds are disturbed. the sound is thought to be the bird's way of scaring away potential predators.