I'm still not sure exactly which sub-species of flicker this was. per this site.

"Northern Flickers are large, brown woodpeckers with a gentle expression and handsome black-scalloped plumage. On walks, don't be surprised if you scare one up from the ground. It's not where you'd expect to find a woodpecker, but flickers eat mainly ants and beetles, digging for them with their unusual, slightly curved bill. When they fly you'll see a flash of color in the wings – yellow if you're in the East, red if you're in the West – and a bright white flash on the rump."

That flash of color in the wings was what first got my attention, as the bird flew into the tree above me and settled onto a branch. It was quite striking (mostly yellowish but with a bit of red, and the darker spots).

Apparently, the yellow shafted and red shafted flickers often interbreed, and sometimes both may with the gilded flicker.   That may explain why the coloring on the bird I saw was not exactly like the coloring of the photos I've found.

But I suspect the bird I saw may have been a female red shafted flicker.
red shafted flickers diagram.
She would not have had the bright red streak on her cheeks that the males have.