I'm reviving this thread because I heard this song recently with some different words from those in my head. Those came from when I first heard it, from the Corrie Folk Trio (as they were at the time) on the Hoot'nanny show about 1963. In those days I taped a lot of radio and TV programmes, and that tape is one that I recently digitised, so I have checked their version.

Besides some minor differences from the words up-thread and in the DT, the difference that struck me, which has prompted me to find this thread and dig out my recording, is at the end of verse 2. Instead of repeating "For there's none ..." as in the first verse, they sang
"And on my heart my singin' bird would sing itself to rest".

My initial thought was that continuing the rhyme from "nest" and "breast" would be original, and the repetition of "For there's none ..." a corruption. Or could it be the other way round, the CFT having decided to "improve" the song?

They also had a third verse (which I had entirely forgotten until I listened to the recording just now) quite different from those up-thread.

In grief I wander east and west
O'er mountain, vale and shore.
And yet I know I will never find
The lass that I love so well*
But I will bravely face the world
To hear you sing once more.

* "The lass that I do adore" would seem better, maintaining the rhyme.

If he's determined to hear the bird again and yet expects never to find the lass, that implies that the bird is not a metaphor for the lass. That would be consistent with the verse about climbing the tree and bringing the bird "To the arms I love the best".

Does it all make sense? Not entirely!

And does it take us any closer to the origin? Nope!