TO makes a few obvious points. I am Cornish. I have known this song a long time, since I was child. I have lived in many places. It is only recently that I have discovered what a nightingale looks like. Having not studied these things, I have never had any knowledge of whether nightingales existed where I lived, neither had I any idea what their song sounded like. In view of this, and presumably many other people's similar background, I doubt if the absence of nightingales in Cornwall can have anything much to with the origins of the song, either in or out of Cornwall.
I also know that a lot of my fellow compatriots(or whatever term you care to use) have spent a lot of time and ingenuity creating a body of "Cornish language folksongs", or convincing themeselves that such a heritage exists. Well, I would be the first to be absolutely delighted if we could rediscover such a priceless collective possession. But, alas, I have come to the conclusion, like most people, that it doesn't exist. Cornish language versions of the Nightingale, or anything else, are modern inventions.