sibley's guide says about nighthawks:
"Nighthawks are often seen in daylight, especially evening, flying over woods, fields, or towns catching insects. Their slender shape and erratic, bouding flight is distinctive."
the common nighthawk's summer territory includes most of N. America, except for the west coast, alaska and northern canada (more or less).
"Territorial male gives rasping, nasal, descending buzz "BEEErzh"; given in flight with several quick "stutter flaps." Occasionally gives rapid, clucking, "quit-quit-quit-quit" in chase; not as staccato as Anitllean [nighthawk]. At nest site female gives low clucks and purring or hissing growls. Display dive of male produces humming, whooshing "Hoooov" at bottom of dive."
On the Lesser Nighthawk, Sibley's said:
"Song up to ten seconds long: a low, whistled trill on one pitch like tremelo of Eastern Screech-Owl but longer, notes more distinct. Also a nasal laughing or bleating "mememeng." Lacksdiving display; silent in flight."
summer territory of the Lesser Nighthawk: Mexico; southern portions of Texas, New Mexico, Arizona, Nevada, California; extreme SW Utah. Rare occurances in 'dots' kind of near where the Texas panhandle forms a right angle with N. Mexico, and closer to Houston along the coast, as well as in other states (but I don't care about that right now). It looks like the Lesser Nighthawk's territpry might reach as far north as San Antonio- its hard to tell with the map in the book. I would say it probably doesn't go as far north as Austin though.
here is a hint: "lesser nighthawk molts on breeding grounds Jun-Sep; other nighthawks molt fligth feathers on wintering grounds Nov-Mar." I'll look for a pile of feather
the bird or birds I heard had more of a song (it was 'pretty') - i wouldn't have called it a buzzing or clucking or hissing growls. perhaps a purring, but that'd be a stretch.
sibley's doesn't mention whether nighthawks sing nocturnally, but I would assume that from it's NAME, it might be possible... off to the internet...