Are +you sure they are woodpecker holes? Was it dead standing wood? Woodpeckers will not try to make their nest unless the tree is already dead or diseased. Softer wood.
Sapsuckers, a species of peckerwood, as the name implies feed on the sap and probably to a larger degree the insects drawn to the sap of the wounds they inflict on living trees. They peck living trees, often in a series of rows as in Weylin's stave, returning regularly to feed. They maintain these sap pits over generations, so the visible damage on the back of the bow can be traced decades back through the wood, but as Weylin suggested, the tree is continually healing the wound, as the sapsucker is continually renewing the wound.
Weylin, I would scrape down a bit until the craters are about gone. Dave Lawson made a nice hunting weight flatbow with similar sapsucker damage AND lots of rot. To my knowledge it is still shooting.
