It's that time of year - you know, the time when spring loudly announces its arrival in some parts of the world. I grew up in the midwest United States, on a farm near a marsh, where lots of these little cuties live:

Yes, that's a frog know as the Spring Peeper and when there are hundreds of them going at their evening calls, the sound can be deafening. This site has some great recordings of what the various spring / summer frogs sound like . When I hear these frog calls, I know the seasons have changed!
What are some sounds that always tell you time is moving forward? What are the sounds, and what seasons do they go with?