Lots of amphibians live exclusively around vernal pools, filled with water only in the spring. By drying out in the summer, the habitat doesn't support fish which would munch on them.
For more about frogs (or toads, which are actually a kind of frog), check out mister-toad.com
. Great name for a website.
I noticed the same clapclap noise in a nearby puddle, thinking that it is 3 weeks too early for the peepers. But equally embarrassing I did not realize the difference in sound pitch. Thanks for cleaaring this up, and thanks for the nice pictures.