Absolutely Lesser Goldfinch. By the way, I don't know how much time you spent looking at warbler pics, but you would have saved yourself that effort by noting the bill shape: warblers have much thinner bills. Bill shape and size is often a good clue to point you in the right direction; bright colour patterns you'll likely remember without trying, but the bill really needs to be looked at (unless we're talking about a toucan or spoonbill etc). Just a friendly hint. -O-

Sorry, the image I tried to upload was too big, I changed it to 72 dpi and now it's too small. Our friend already emailed us back that it's a song sparrow.

immature or female Painted Bunting ... not a match because it lacks the distinct dark head. I saw it again today, and the most likely bird is a Lesser Goldfinch. http://www.allaboutbirds.org/guide/Lesser_Goldfinch/id

I agree with BN. It is an adult Sharp-shinned Hawk. Although my reasoning slightly differs: the "hooded" appearance of this bird is obvious from this angle (how the dark head continues over the nape of the neck). This is a terrific way to tell the similar Cooper's and Sharp-shinned apart (but only as adults!). A Cooper's has a "capped" look (dark crown of the head and a lighter gray nape of the neck). The shape is also indicative of a Sharp-shinned (squat appearance with a small head and fat chest, lack of a neck, large eyes and small bill). The auriculars (cheeks) actually aren't a reliable way to tell Sharp-shinned from Cooper's, but it is a semi-useful way to tell male from female in both Sharp-shinned and Cooper's. I don't like to use it unless it is really obvious. Kind of iffy. In summary, this is a Sharp-shinned Hawk. It is hunting birds.