Dunno. Maybe the mouse just said "To hell with it! I'll use a bit of string!" And I have never been able to think of a plausible reason why the sea should want a feather either.

The book it came from had a page of acknowlegements giving the sources of the other songs , but there is no origin given for this one. The notes say the "lep" is an old form of "leapt" thus hinting that the song *was* taken from elsewhere.