I must disagree with all of you. Hickory and boo are some tough backings. perfectly Suited for the most compression strong woods in the world.
but birch is ridiciously compression weak!!! the boo will crunch the birch in no-time. Already had this on maple-ipé trilams wich had too thin ipé... and both maple and ipé are way sronger in compression than birch.
if you're thinking about backing your bow with hickory/boo, be my guest
but don't say I didn't warn you! your's will stick together but those crunched bellies sure look ugly.
rawhide would be the best choice by FAR!
Nick