Woodpeckers do their
damage by pecking into the bark of trees looking for
boring insects beneath the bark. They can hear the
critters moving around but usually cannot find them
with the first hole. So, they make one hole and then
move a little horizontally and make another one
until they are successful. That is why you will see
a serious of horizontal holes in the bark of trees
that are being bothered by woodpeckers.
This is one of those situations where the woodpecker
damage is actually a secondary sign of a more
borers. The bird is only
interested in the tree if it has these insects
working inside it. So, the real problem is the borer
which is cutting tunnels in the wood of the tree.