A pine siskin would have the following, which the bird in your first two photos lacks:
- a prominent wing-bar
- a very thin, pointed, dark bill
Juncos have a thicker, pale bill, which your first photo, especially, shows really well. Also, in the first photo you can see the adult plumage beginning to come in: the dark head, reddish-brown back, and paler underparts. It's not as diagnostic as the bill and the emerging adult plumage, but your bird also seems to be behaving more like a junco (foraging on the ground/deck) rather than a siskin (which you'd expect to see acting more like a goldfinch, flitting around your thistle feeder).
Some googling for images turns up the following, which show the range that the juveniles go through as they transition to their adult plumage:
And from another expert:
Here's a page from Sibley online that shows a juvenile from a different junco population, but all the juvies look much the same and here you can see the stripes that may have thrown you off:
This is a good online guide in general: http://sibley.enature.com/