If you look below youtube videos, there is a brief description of the video (double downward pointing arrowheads to the right will expand the description if it is longer than the default space). The description says it is adapted from a song by Al Stewart (known for "Year of the cat").
A quick Google finds the Al Stewart web site and another search gets The Lyrics.
That said. I don't know the song, or how it was performed by Al Stewart. No doubt, there will be some Mudcatter who is a fan, who will have interesting anecdotes to add.
Quack!
GtD.