These are marsh frogs and they are not British. They were introduced into Britain some time back in the 1930s and since then they’ve bred and spread. Hearing them is one thing, seeing them is quite another. Like a lot of elusive wildlife, just getting the slightest glimpse is amazingly satisfying. Bill Oddie can just see little eyes poking out. He's not quite sure what provokes them to call because they were going one minute and then suddenly it’s all quiet. Bill impersonates some frog calls, to see what happens. The frogs seem scared, but he gets an audience of cows. Another marsh frog calls. Sounds like a big one. They’re much much bigger than than common frogs and it's an extraordinary sound, but if that lot was singing in your garden pond at night, it’d drive you totally nuts.