Sapsuckers make two kinds of well. Deep round holes tap xylem in spring, before leaves emerge, while shallower rectangular wells cut into phloem later in the season and must be maintained by repeated visits or they seal over. The bird's tongue is brush-tipped rather than barbed, suited to lapping rather than spearing.
Because the wells stay open, other animals exploit them. Ruby-throated hummingbirds arriving north before flowers open feed heavily at sapsucker wells, and in parts of the range their breeding distribution tracks sapsucker presence. Warblers, kinglets, squirrels, porcupines and many insects use them too.
The damage to individual trees is real but usually modest; heavy repeated drilling can girdle a small stem or admit fungal infection, which is why the species is sometimes treated as a pest in orchards. The claim that sapsuckers routinely kill healthy mature trees is not supported by the evidence. How far northern hummingbird ranges truly depend on sapsuckers is still debated.