Steely-vented Hummingbirds (Amazilia saucerrottei) are one of the most common hummers in Costa Rica. You'll hear this sound in a lot of your travels there.
Lesser Nighthawks (Chordeiles acutipennis) sit on the ground of the chaparral hilltops and trill this toad-like trill on the summer nights around central Arizona. This one was at Peck's Lake [Arizona] May 20th, 1999. The sound is very hard to localize, since it is so low, quiet and "generic" (high-pitched sounds are much easier to find), and they are generally closer to you than they seem to be.
This species gets together in a kind of lek - several calling at once, and occasionally flying up in a fluttering courtship display while singing this strange song. 06
