I forget the specific bonus I get for singing butterflies... I think it's level-based, and I think it's 124xp.
So yesterday, I was at my home that was cluttered with foodstuffs anyway, and decided to explore massive eating and butterfly singing. I would say that all went pretty much to plan... it takes a lot of energy to get a butterfly to sing. You can keep doing it, but you better keep eating, too.
But the one unexpected thing is that sometimes I got double xp for a sing. (Sometimes I got 248xp.) I formed little hypotheses about why this was happening and tried to test them, but never could figure out what the trigger was for when it happened.
(And then, whoops, I died, just before shutdown. All for science.)