Take your laptop outside, turn up the volume and listen for the reply to the recording of the Grey Tree Frog. It is possible they may not be in your area, hence little chance of tadpoles in rain barrels. These frog sounds can be found on FrogForum as well as Sounds Of North American Frogs Soundtrack CD Album produced by Smithsonian. These recordings do get the attention of my Copes Grey I raised from tads, and allows me the opportunity to learn various other frog sounds. Highly recommend this Smithsonian CD but much of it can be played without investment.