UPDATE: 23 February 2011 - All opinions garnered on this mystery sparrow have supported its identity as Grasshopper Sparrow.

This small sparrow (smaller than nearby American Tree Sparrow) was running through grasses (mouse-like) and generally behaving as an Ammodramus sparrow (silent; short-flights; avoiding perching above the ground) on 13 November 2010 at Fossil Creek Reservoir in Larimer County, Colorado (USA).

Click on an image to enlarge.

Image 81 shows streaking on underparts.
Image 83 shows flat head, large eye, crown stripes.
Image 108 shows malar strip and streaking on breast, suggestive of Baird's Sparrow.