Hi CM- I have no experience with western towhees, let alone young ones. I've looked at Sibley and don't think your bird is a towhee. Both the Spotted and Green-tailed (yours looks like it has a green tail) have strong malar stripes (a light stripe each side and separate from the throat patch). Your bird does not show this at all. Your bird shows one weak wing bar whereas, the Spotted has two strong ones and the Green-tailed has none.
For me the bill is too big for a towhee and looks more blackbird-like. I am thinking young Brown-headed Cowbird.
Juvenile songbirds can be brutal since they often have a very short-lived plumage that most birders don't get to see. And I'm not even sure this bird is a juvenile.
This is a tough angle but I think this may be an Indigo Bunting. Fall birds can show some streaking (not usually not this extensive), the bill looks about right, the whitish throat seems about right, and it has a weak wingbar with brownish flight feather edges. If you have a side view shot, that might be helpful.
John, I like Indigo Buntings better too. They're so cute! Oh, wait. What was the topic?
I think the coverts look OK. I've looked over some photos on the web and it seems to be present in at least winter adult males and probably some females as well. Here's the plate from the National Geographic guide that shows pretty strong coloration on the covert edges too.