If you're walking around a town or other developed area with buildings that have chimneys that are accessible from above and you hear twittering in the skies overhead you may be in the presence of one of my favorites (I have many favorites!), the chimney swift. Here is a reflection from an evening spent watching chimney swifts fly over Wellsboro, PA...
master of the skies, chimney swifts slice through the air with rapid
wing-beats followed by swift glides twittering as they go. The rapidly
moving flock comes together and then disperses; if the sky is their
canvas, its easy to think that they might be trying to cover all of it!