[Mystery bird] Henslow's Sparrow, Ammodramus henslowii, photographed on the Konza Prairie Biological Station (Riley County) in Manhattan, Kansas. [I will identify this bird for you in 48 hours]
Image: Dave Rintoul, 8 July 2007 [larger view].
Please name at least one field mark that supports your identification.
I'm at the office without any book, so I'm just going to take a stab in the dark and say it's a Lincoln sparrow, because of the headstripe.
As I usually get the sparrows wrong, the nearest I can guess is a Savannah Sparrow, broad supercilium, black moustachial stripe and white outer tail.
(although it's probably a Chipping Sparrow!!)
I am really bad with sparrows, but my guess is a young Henslow's Sparrow (Ammodramus henslowii). The olivy tint around the head and reddish wings are easily seen, the the lightly streaked breast seems like what a juvenille coming into adult plummage woudl look like. Also the head seems a bit flat-ish. Plus it seems to be in a 'weedy field' habitat, which fits.
I'll add another vote for the Henslow's sparrow. "No other sparrow has an olive head that contrasts strongly with a brown back" (Golden Books guide to North American Birds)