I would guess that birch wouldn't be a great bellywood candidate for a bamboo backing, but all things are possible-I've seen several birch bows but never worked with it myself. I've heard people (myself included) tell someone that you can't make a bow from this wood or that wood or with that design, and then they turn around and make a nice bow from it anyway. I would trap the boo so that it is narrower than the birch, make it as long as possible, and go really slow on the tillering-a slight hinge will probably be likely to chrysal. I'm looking forward to seeing how it turns out. We have loads of sweet and yellow birch here, too. Is the birch from a stave or board?