Sapsuckers drill a myriad of holes in trees. Generally shallow, and they cause the sap to run. Hence, the name. The sap also attracts bugs; which the sapsuckers and other birds also consume.

Not too sure if the sapsucker activity is all that damaging to the tree. We have a number of trees at our place that are rather riddled, and still seem to grow well.

The deeper holes that cause access to the heartwood are another story entirely. These can be caused by broken limbs and the like.