The frogs in our bog started singing two nights ago. The first night, it was a dozen or so loud individuals. Last night, the full chorus joined in. We're talking thousands, maybe tens of thousands of frogs, all going bananas simultaneously. It can be deafening if you're out in the yard when they start it up, but from inside it's a kind of pleasant white noise.
Updated 2/5/15 - now with audio disguised as video!
I decided to note when the froggies started singing their song this year, to eventually, maybe, try to determine whether or not there is any correlation between last frost dates, average daily temperature, etc., and when the frisky frogs fire up their pipes. ;)