Mystery bird solved! a juvenile Dark-Eyed Juncoe (not a Pine Siskin)

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:

• http://www.pbase.com/tgrey/image/65827008
• http://www.geocities.com/tgrey41/DarkeyedJuncosL.jpg
• http://www.geocities.com/tgrey41/DarkeyedJuncoJuv4.jpg
• http://www.geocities.com/tgrey41/DarkeyedJuncoJuv5.jpg
• http://www.geocities.com/tgrey41/DarkeyedJuncoJuv7.jpg
• http://www.discoverlife.org/IM/I_LHT/0017/320/Junco_hyemalis,_dark-eyed_junco,I_LHT1788.jpg

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:

http://sibley.enature.com/species.asp?SpeciesID=2505&Image_ID=2505_19.jpg&RecNum=1&Srch=yes&searchStr=junco&image1.x=0&image1.y=0&xLoc=&yLoc=

This is a good online guide in general: http://sibley.enature.com/