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jennier@daknet.com (Jennier) sent this comment:

Common Nighthawk (Chordeiles minor chapmani)

Common Nighthawks in Florida have many vocalizations and two sounds made by wing feathers. One of the feather sounds is familiar and is produced at the bottom of a dive. This is characteristic of the male. The female may do a "wing flutter" on a flat, circling flight after a disturbance. This sound is different from the dive boom sound, but is loud enough to be noticed at about 50 feet.