Yellow-bellied Sapsuckers [Sphyrapicus varius] visit our yard twice a year like clockwork, once in the spring and once in the fall. They are always a treat to watch, and always frequent our birch tree. Their foraging technique is what gives them their interesting name.
The Yellow-bellied Sapsucker makes two kinds of holes in trees to harvest sap. Round holes extend deep in the tree and are not enlarged. The sapsucker inserts its bill into the hole to probe for sap. Rectangular holes are shallower, and must be maintained continually for the sap to flow. The sapsucker licks the sap from these holes, and eats the cambium of the tree too. New holes usually are made in a line with old holes, or in a new line above the old. – Cornell Lab of Ornithology’s All About Birds