I think that it's all down to floating or borrowed verses. The Outlandish Knight has a similiar sort of plot, although in this case the maiden drowns the knight in self defence - he deserves it as he has already drowned six maidens previously. Again the parrot sees what has happened and the maiden bribes him:

"Well done, well done, my Pretty Polly
You have tuned your notes well to me
Now your cage shall be made of the finest beaten gold
And the doors of the best ivory"