Unseen singers.

It was a bad day to be a frog yesterday. The overnight rain had raised the watertable on the flood relief basin near my home. Whilst the swollen streams and drains provided the frogs with more of an excuse to sing during the day, that singing had also attracted a flock of herons, that had descended on said frogs.
The herons were, of course, having little impact upon the intensity or the variety of the frog chorus, the variety being the most noticable thing.
There are the popping songs that sound like bubble-gum bursting or a bubble of gas breaking the surface of a viscous liquid; there's the song like a pencil being dragged along a serrated wooden surface, and there's the song that sounds like a balloon being harshly rubbed. Those were the most noteworthy, amongst the croaks, and chirrups and other bizarre calls made by the singers, unseen in the water and the grass.