The comment about the unicorn wasn't made in deadly earnest either, you know!
As it happens, I've just been talking with someone who said the only time he'd ever heard a nightingale was in London, so there you go. He is now on the trail of the Cornish language expert and of the "Eos Whek", so we can all sleep soundly in our beds tonight. He also says that anything written in "modern" Cornish is fake rubbish and shouldn't be be relied on to prove anything at all, so you can all put away your "Holiday Cornish in Two Weeks" phrasebooks, they're rubbish.