"Flying cigars."
I encountered a flock of about 50 of them yesterday afternoon, twittering as they flew. Probably trying to fatten up before their trip to western South America. They can't perch, so you will only see them in the air.
My personal story about Chimney Swifts was in church, a decade ago or so, when one came down the old church chimney. Yes, during Pentacost service! The service was interrupted while I gently snatched the bird while he was trying to get through a closed window and carried him outside.
They are Interesting birds