Funnily enough this came up in conversation at Christmas. According to a friend of mine who seems to know everything, or at least has lots of opinions, a 'pear tree' was a large tear-shaped (so possibly pear-shaped)birdcage. Don't know how true it is but it makes more sense than getting too accademic about the origins of the song. Didn't ask about the other eleven days though, they are probably much simpler than they sound as well.