I would love to know what makes the frogs call. For the last week or so the pond spotted marsh frog (sounds like only one at the moment) has been quiet. Last night I filled up the pond - was getting a bit low with the 30C days we've been having lately - and tonight the spotted marsh frog is calling again. The difference in water height isn't huge - about 10cm. In the pond that means the difference between having water just covering the dam weed, and having a depth of water on top of that. Perhaps the depth of water on top is what's needed to make the frog feel comfortable to call.