There are two subspecies of White-crowned Sparrow that regularly occur in Utah. Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha; also called "Interior West White-crowned Sparrows") breed here, as you might expect, mostly in the mountains. Gambel's White-crowned Sparrows (Z. l. gambelii) winter here, having migrated down from their breeding range in the western taiga of northern Canada. Around the same time Gambel's are arriving from the north, the Mountains are heading south, leaving the state to winter in southern Arizona and northern Mexico. There are a few eBird records of Mountain subspecies wintering in Utah, but I've never actually seen one documented here in Utah: I suspect it is very rare if it occurs at all in Utah in winter, but please comment below if you've documented Z. l. oriantha in Utah in winter.
Telling these two subspecies apart is relatively easy. Gambel's have a yellowish bill and pale lores; Mountains have a pinkish bill and dark lores. (The lores are the area between the eye and the bill. Look to see whether the dark line that both subspecies have behind the eye continues in front of the eye to connect with the stripe on the side of the crown.) Bill color is a little less reliable in young birds, but even the young ones usually have the appropriate color lores.
It is always possible that one of the other three subspecies of White-crowned Sparrows could show up in Utah, but this would be very unlikely. Start by practicing telling these two subspecies apart, and you'll be more prepared for a vagrant of another subspecies. For more on distinguishing subspecies of White-crowned Sparrows apart, see this and this post from David Sibley; for the VERY interested, see this extensive treatment, also by Sibley.
|Adult Mountain White-crowned Sparrow, Z. l. oriantha, Logan, Cache County, Utah. 11 May 2011.|
|Adult Mountain White-crowned Sparrow, Z. l. oriantha, Antelope Island, Davis County, Utah. 7 May 2011.|
|Adult Gambel's White-crowned Sparrow, Z. l. gambelii, Washington County, Utah. 19 Mar 2011.|
|Immature Gambel's White-crowned Sparrow, Z. l. gambelii, Cache County, Utah. 20 Feb 2011.|
|Immature Gambel's White-crowned Sparrow, Z. l. gambelii. Gunlock Reservoir, Washington County, Utah. 16 Mar 2007.|
(All photos copyright Ryan O'Donnell)