I first heard their rapid, chittering calls as they zoomed their aerobics above me yesterday, May 14. They are "late" this year: the last two years I noticed them first on May 11. (Perhaps they were here a day or two ago, but were subdued by all the overcast and rain.) Always they seem to be in flight, chasing insects on the wing--I never see them perch. They are a cheerful ornament to spring, here.
Chimney swifts are long-distance migrants, coming all the way from South America. They are so called because these days they nest almost exclusively in chimneys--which is harder and harder to do since many homeowners today cap them. (Long ago, they nested in cavities in trees, but natural cavities that meet their needs are rare today.)
Learn more about them from the Cornell Lab of Ornithology, my chief go-to site on birds and bird song.
Rob Curtis, WordsonBirds.blogspot.com