Six days later, they're taking recognizable shape as birds and nearly filling the nest. It's quite small, interior diameter no more than 3.5 inches..
We're careful not to disturb them, but when I checked a few minutes ago their beaks were wide open, expecting food. I returned two minutes later with the camera to find they'd fallen asleep - though the one on the right is keeping an eye open.
If we ever figure out what they are, we'll report back.