An article in today's Wall Street Journal about the US Geological Survey's annual frog and toad census features interviews with Museum herpetologist Dr. Alvin Braswell and biologists from NCSU and the NC Wildlife Resources Commission:

Is That the Croak of the Pickerel? Frog Census Is a Tad Uncertain [2]
Each year, volunteers head out and listen to frogs, trying to estimate the abundance of the creatures by identifying each species' unique call. Now scientists have devised an experiment called 'Ribbit Radio' which suggests the official frog-call count isn't all it's croaked up to be.