With the warming temps and spring rains, comes a chorus of frog song in Concord's marshes and vernal pools. These early crooners are hard to spot, so learning their calls breeds our deeper familiarity. Here is a great website out of the U. of Michigan with images and varied calls for all of the common frogs and toads of the Northeast, http://animaldiversity.ummz.umich.edu/site/topics/frogCalls.html. I've also added the link to this website under "Resource Links" in the right-hand column of this webpage.
On the April monthly walk at Great Meadows we heard pickerel frogs, green frogs, and spring peepers.