Chimney swifts

An alley behind our previous apartment offers a great place to watch a thousand or more chimney swifts barreling in herds into a tall brick chimney attached to an older apartment building.  They gather around dusk and circle a few hundred times in swirling clouds of birdity, seeming to wait for the light to be just so before entering.  Or maybe they wear watches so they can turn in at a respectable hour.  How they all nest and/or roost in there is a major mystery.  The event seems to be most observable later in summer, before the colder weather comes, and the fledglings are flipping out with the whole flying thing.  I just hope no one lights up the building’s boiler before the birds move south to friendlier climes for the winter.  This terrible photo is no help at all in conveying the coolness of the occurrence.