Sapsuckers eat sap, so they tap out shallow rectangular "wells", in neat little rows, in trees in order to make sap flow. These wells must be continually maintained to make the sap flow. A fact that a number of other species have noticed, such as bats and hummingbirds. In some parts of Canada Ruby-throated Hummingbirds time their spring arrival to match that of Yellow-bellied Sapsuckers!