Right, for anyone who's still interested in a pedantic way:

The Cornish language guy had the following to say:

1. "There are no Cornish folks songs". I would say he's using both "Cornish" and "folk songs" in a strict way, but I think what he means is that any songs that exist now in Cornish are modern reconstructions, and it is very unlikely that any words or even tunes of true Cornish origin remain. [The 19th century singers of this song claimed that it was originally sung in Cornish, but as the language hadn't been spoken for over a century by that time, it is unlikely that they could know this, and it was probably fanciful wishful thinking on their part.]

2. There have never been nightingales in Cornwall.

3. "Eos" does mean "Nightingale" but he doesn't think it had an equivalent in ancient Cornish. He thinks it more likely to have been created in the 20th century revival of Cornish, and coined by some scholar from the Welsh (which is also Eos).

And (not him but me speaking now, answering a couple of posters above) yes, of course the nightingale is a symbol for rural rumpy-pumpy, in the same way as a fiddle, or a gun, or a bird-in-the-bush, or various other creatures and things can be. That doesn't make it any less interesting to try to trace the origins of the "nightingale" as a motif in Cornish and English song.

Oh, and one final point, for anyone still awake: only the male nightingale sings, so "as she sings in the valley below" is ornithologically questionable.

I think that's about all I have to say, and probably more than most people want to know.

Marje