At lunch time, I noticed that the birds (a couple dozen sparrows and two pairs of cardinals) kept flying away from the hopper feeder and then coming back. As I looked around for the source of their skittishness, I spotted a hawk -- not in his usual tree (the bee-tree walnut about 20 feet from the house) but another on the east edge of the yard.
It was my_partner_doug who thought to get the camera and take a few snapshots through the window. So this isn't a great picture, but you can see that it's a hawk. \o/ Good hunting, skybrother.