I ran to the house and grabbed the scope and Coolpix P5100 and came back out to the driveway. From 15 feet away I started digiscoping the birds as they calmly ignored my presence.
After a few pics I set the camera to capture a video. I watched as the young bird appeared wobbly on its perch. I wondered whether the bird was ill, but felt that it would be puffed up if it were. I think that it was still trying to get its sea legs.
Examination of the young bird revealed heavy streaking on the chest and bright, buffy wing bars on the greater coverts, (2). Its beak appeared a bit shorter than the adult bird's, but it could just be angle of view. Unfortunately the field guides provide no help in identifying young siskins, and a search of the internet produced only references to the brown/buffy color and short, pointy rectrixes relative to adult birds.
I've had Pine Siskins all spring. At least one bird has been seen daily since January, and occasionally two. They usually show up first thing in the morning (before the Goldfinches) or late in the evening. I've always suspected a nest was nearby, and hopefully this is now proof!