Both Claire and I are in New England.

This post comment was why I thought the photo was of an eastern sparrow. Still, I live in California, and had this Harris's sparrow visited my yard. So they can wander; but Montana would be within normal range.

I don't believe it is a Lincoln's sparrow. Their side/breast stripes are very narrow/thin, the chin is whitish and photo's white cheek marking tends to be buff in color on a Lincoln's.