Here is an excerpt from allaboutbirds.org on the topic under discussion.
Field Sparrows in the Great Plains tend to be larger, paler, and grayer than their brighter, browner counterparts in the East, although there is considerable overlap in both plumage and size. On Great Plains individuals, the only warm body coloration is restricted to the crown (but with a gray central crown stripe) and a bar at the shoulder. The back is brown with gray streaks.
On Tuesday 26 September 2017, 1:00:36 PM GMT-5, Carol Joan Patterson <0000003a0ccbe138-dmarc-request...> wrote:
I too have had an interesting experience with Field Sparrows, but much closer to home: Donald's Goshen fields. When I first was there, the Field Sparrows had a downward trill, but years later this sparrow type was replaced with sparrows having an upward trill. So we had a different Field Sparrow population.
From: Joseph Neal <joeneal...>
Sent: Tuesday, September 26, 2017 10:33 AM
Subject: A GRAYISH SPARROW
Greater Prairie-Chicken would certainly be a high priority for many Arkansans visiting The Nature Conservancy’s Joseph H. Williams Tallgrass Prairie Preserve in the Flint Hills of northeastern Oklahoma. A few in our Bison, Birds, Botany, and Butterfly (and Bugs) group saw chickens during our field trips September 20-24.
No chickens for me, but I was first in line when we flushed a small bird from the grass along a trail near the headquarters area. Fortunately, it was out of the grass and onto perch in open, gnarly branches of a Blackjack Oak.
Most of us saw it. It even remained long enough for photographs. Streaky, sparrow-sized, with obviously long tail and a modest but noticeable eye ring. My call: juvenile Field Sparrow, but it seemed big and gray.
Thinking the bird might be something different, I sent my photos to biologist Dan Reinking of George Miksch Sutton Avian Research Center in Bartlesville. He agreed on Field Sparrow, but pointed out my problem might involve differences among subspecies. The lighter-colored, smaller sparrows mainly in the eastern US including Arkansas are assigned to Spizella pusilla pusilla. Grayer, larger sparrows in the western parts of the Field Sparrow range are assigned to S. p. arenacea. In other words, what we flushed and watched for a while was probably this western form.
As Doug James observed many years ago, birds are distributed on the landscape according to their ecological requirements. Tallgrass Prairie Preserve includes significant areas of Post Oak-Blackjack Oak woodlands called Cross Timbers, the broad band of open hardwood forest separating our eastern forests from Great Plains grasslands. My suspicion is that evolution has been at work in the Cross Timbers, favoring bigger grayer Field Sparrows.
Sorting out this kind of stuff makes of every field trip a re-voyaging of Darwin’s ship, Beagle, that carried him to the Galapagos and eventually to the theory of evolution. Writing of the finches he found, "… one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends." Or for our voyageacross the Tallgrass archipelago, September 20-24, 2017, substitute Field Sparrow.