Tiger swallowtail, Papilio canadensis
Ventral view. The butterfly displays an interesting behaviour that I hadn't noticed before: on this breezy day it's using its hindwings to clasp the stem of the Clintonia borealis plant from which it's feeding.
I had identified this individual as the Eastern Tiger Swallowtail, Papilio glaucus, but now I'm leaning toward calling it the Canadian Tiger Swallowtail, Papilio canadensis. The underwside of the forewing has nearly a continuous yellow submarginal band ratierh than the distinct rounded spots of P. glaucus. Birch trees (Betula spp.) are abundant in the area.