Abstract
The ruby-throated hummingbird has the most extensive breeding distribution of any hummingbird species in North America. In its most northern distribution in Canada, its spring arrival often occurs before herbaceous flowers are in bloom, and in many parts of its range it breeds in habitats where floral nectar is scarce or unavailable. Evidence is presented to support the hypothesis that the dates of spring arrival and northern breeding distribution of the ruby throat, and probably the rufous hummingbird, are determined by a commensal relationship with the yellow-bellied sapsucker, in which the principal energy source for the hummingbirds is sucrose in sap from trees drilled by the sapsuckers.
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