Although I recorded this on 6 December, it seems appropriate to share it while listening to raindrops on a wet, soon-to-be-winter's night.
Do you recognize this species?
6december by nhbh
The recording features two different calls of Pacific Chorus Frogs (also called Pacific Treefrogs). [It's one recording that repeats.]
Most of what you're hearing is an advertisement call. It's a two-part call, sometimes written as rib-it, rib-it (or krek-ek, krek-ek — you can choose what suits your ears best!).
The second call is more subtle. If you listen very carefully at ~2 seconds and again at ~10 seconds you can hear the trilled encounter call. According to californiaherps.com, this call is given by males to establish spacing when a chorus begins or later if another male approaches (or calls) too closely.
I didn't get any pictures that night, but here are a few of a Pacific Chorus Frog on Bodega Head in May 2012.
Are you hearing Pacific Chorus Frogs at wetlands near you?
P.S. For more information and pictures of this species, check out the post from 17 March 2012.