Many thanks for all the comments!

A couple of additional points:

We do feed exclusively black sunflower seeds. I think this deters the
house sparrows, for some reason - maybe they're too lazy. Dunno.

I also agree that the squabblers - I mean, house finches - seem to
like to feed en masse. I wonder if this is to provide better
anti-predator cues? We do have a semi-resident sharpie that harvests
finches from the neighborhood, so that could well be an issue.

This morning we had an interesting feeding behavior: after I filled
the feeder (they empty it at least once a day; little piggies!), I
immediately got a couple of chickadees and a RB nuthatch, then a quick
flock of house finches - then THEY all flew off and a lovely
golden-crowned sparrow came in and was VERY aggressive toward the
finches, but not toward the chickadees. They amiably shared the feeder
for a half hour or so. It was interesting, hearing all the finches in
the trees and adjacent honeysuckle thicket complaining bitterly the
whole time.

I think I will take the advice of adding a tube feeder with only a few
perches available and see what happens (then I'll have to worry about
the squirrels, but that's another issue...).

Cheers,

Josh
josh at pogo.cqs.washington.edu