I like to think it's a Vaux's Swift because there are so many of the little buggers in Portland, a particular unused chimney in the NW part of town being home to some thousand-or-so Vaux's Swifts each year. Plus, I like to imagine a hoodie-clad orni-stenciler running around Portland in the dark of night—perhaps a new face of the Audubon Society?
Here's some more detail of the puffin-ish stencil at bottom: it's a three-color job, and after seeing this one I started noticing other multi-color stencil art around town. A car. A potted plant. Though I didn't get pictures of those.