Z. l. leucophrys
Z. l. gambelii
Look carefully the next time you see a White-crowned Sparrow, Zonotrichia leucophrys. Four races (= subspecies) are currently recognized by the American Ornithologists' Union. Two can be found in Minnesota--see photos above. They can be told apart by the lore color (between their eyes and bills) and perhaps by bill color. These races are shown in Sibley, but the illustrations are not labeled scientifically. The National Geographic guide (third edition) does a better job
The range information quoted below is from Chilton, G., M. C. Baker, C. D. Barrentine and M. A. Cunningham. 1995. White-crowned Sparrow (Zonotrichia leucophrys), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu.bnaproxy.birds.cornell.edu/bna/species/183
Birds with black lores, Z. l. leucophrys, breed "in north tier east of Hudson Bay, in Rocky Mountain ranges from s.-cen. Canada south through n. New Mexico, in various Intermountain Ranges south to cen. Nevada and s. Utah, and in Cascade-Sierra Nevada axis south to s. California; winters in lowlands across southern tier of United States, generally rarer westward." Their bills are pink. (The photo above was taken near Dundas, MN.)
Birds with gray lores, Z. l. gambelii, breed "across northern tier from Alaska to Hudson Bay; winters south through cen. Mexico, generally rarer eastward." Their bills are orange or pinkish orange. In the Hudson Bay region, birds intermediate between these two races can be found (although I have never seen an intermediate bird). (The photo above was taken in Aberdeen, SD.)
The other two races of White-crowned Sparrow are found along the Pacific coast from British Columbia south--Z. l. pugetensis is the most widespread (the slightly larger Z. l. nuttalli is found south of Los Angeles). Both of these coastal races are unlikely to be found inland, even in winter. I took the photo below at Morro Bay, California. Note the brownish back and underparts, dusky white head stripes, and yellow bill.
Z. l. pugetensis