From 1963 through 1965 we studied the summer feeding areas of six pairs of Yellow-bellied Sapsuckers (Sphyrapicus varius) in northern Lower Michigan. The sapsuckers fed on phloem sap obtained in different ways and from different types of trees, varying with the season. Animal visitors–insects, birds, and mammals–were consistently associated with sapsucker feeding trees, attracted to the flowing sap and/or insect life at the trees. Butterflies, moths, flies, hornets, and bees were the most numerous insect groups. Birds totaling 20 species appeared at the feeding trees. The Ruby-throated Hummingbird was the most numerous of all birds, including the sapsucker itself. Segregation of the sexes of hummingbirds to specific feeding areas was observed. A female hummingbird built a nest within fifty feet of a feeding tree, and its activities centered about both the tree and nest. Bats, northern flying squirrels, and red squirrels were always associated with sapsucker trees. In studying the round-the-clock activites and interrelationships of all the animals visiting the sapsucker trees, we found that a prominent social hierarchy developed among the more frequent visitors and that the summer feeding trees of Yellow-bellied Sapsuckers have a signficant effect on the local ecosystem.