Vesper Sparrow, Pooecetes gramineus

Note the three the well-known field marks for this species: the white eye ring, the small rufous shoulder patch (a lesser covert, often difficult to see in the field), and the white outer tail feather. The absence of a strong supercilium (eyebrow) helps distinguish Vesper Sparrow from the similar looking Savannah Sparrow.

I'm guessing this one is a juvenile, as the adult markings are present but appear not to be fully developed.