Doc Tom
The reference is in the Peter Kennedy book mentioned above . I haven't got the book with me at the moment , but will post what it says tomorrow . There are no known records of the original Cornish words .

Tug the Cox ,

Yes it is a metaphor , but the choice of the nightingale for the symbol is interesting if the bird itself was unknown in Cornwall .

Did the songwriter(s) and the people who sang The Nightingale in Cornwall know the sweet singing bird itself or only the metaphor ?

I don't know if it's true that this song is a "Cornish variant of the song". The song was widely known in Cornwall , but not elsewhere - as far as I am aware - until the latter years of the nineteenth century .
Barry ,
I'd love to hear the cowboy version .Does it yodel?