From Christopher Rustay: At approximately 1 pm on 20 Dec 2015, I was walking through vegetation about 1.5 feet high on the upstream portion of the Bluewater Lake bed. I heard several high seep calls that I took to be from a Savannah Sparrow. The bird moved a few feet within the vegetation, and then flew about 25 feet. I was able to get it in my binoculars just as it landed, where I could see it was streaked and fairly evenly patterned above, with a primary mixture of browns and white/off-white. I slowly approached and it moved part way up in a small clump of vegetation, and then flushed (calling again) to my left, flying 50 feet or more. I was not able to refind it.
I determined this was a Savannah Sparrow based on the calls, general shape, and a brief view where I was able to get only a general sense of its plumage patterns. It was a rather small, medium-to-short-tailed sparrow with a marked chest (seen poorly). Just before it took off for the second time I had a decent profile view from less than ten feet away but without binoculars (it flew again before I was able to raise my binoculars). Most evident was the "Savannah Sparrow profile," with a slightly raised crest (alert) before it took to the air and gave me a naked-eye view of its profile in flight, flicking its wings and moving across the current of 20 mph wind before landing again.
Unfortunately I didn't have the opportunity to get an extended clear view of a perched bird, so I had to make the identification based on a process of elimination and extensive past (and recent) experience with the species' general shape, impression, and calls. I am confident this was a Savannah Sparrow because the combination of sound and detected features ruled out other possibilities. Its relatively compact and medium-to-short-tailed appearance ruled out a Spizella, such as a Brewer's Sparrow, which would have appeared longer-tailed, slighter of build, and with a smaller bill. Nor did it exhibit the behavior and shape of any Ammodramus or of a Sagebrush Sparrow; the latter showing gray in the head and also a longer tail. Vesper Sparrow would appear larger and longer-tailed than this bird, and in the admittedly brief view I had, I think I would have detected the prominent eye-ring and facial pattern which gives a Vesper Sparrow a distinctive appearance (but then I wasn't able to clearly detect a prominent supercilium either, which a Savannah Sparrow is expected to show, but the view was quick). I did not see any white in the outer tail when the bird was in flight and slightly spread its tail when it landed. Lincoln's Sparrow, when poorly seen, can have a similar combination of a medium-to-short tail, bill not notably large or small, a streaky pattern, and a usually quite evident raised crest when alert. The high seep notes were not consistent with calls of Lincoln's, and I didn't get an impression of the warm wash a Lincoln's would have on the upper breast and malar region, nor did it pump its tail in flight.
Calls, especially high call-notes, are notoriously difficult to describe. Other species do make high seep notes, though not all discussed above do. I hear Savannah Sparrow calls thousands of times each fall through spring in the Sacramento Valley and elsewhere in California, and what I heard here sounded like a Savannah: quick, high calls lacking buzzy overtones or significant change in pitch (perhaps ascending in pitch, but I experience the sound as a single note). My recollection is that most Spizellas have slightly higher-sounding calls and Sagebrush Sparrows have a sharper call.
As someone from outside the area, I'm not sure how unusual a Savannah Sparrow is for this location and time of year, so I don't know how high a bar I need to clear to establish this record. Above is what I saw and heard and the process by which I ruled out other candidates.