The most significant change for North American birders in the 2013 AOU Checklist supplement is the split of Sage Sparrow into two species: Sagebrush Sparrow and Bell’s Sparrow. In this arrangement Bell’s Sparrow includes the distinctively dark coastal California subspecies belli, as well as the much less distinctive interior California subspecies canescens. Sagebrush Sparrow is monotypic (no named subspecies) and breeds throughout the Great Basin region.
The new challenge is to distinguish Sagebrush Sparrow from the interior subspecies of Bell’s Sparrow, and Peter Pyle has put together a preliminary guide to these species. The simplest summary is that Sagebrush Sparrow has stronger streaks on the back and a weaker lateral throat stripe, but the differences are small and affected by wear. You can download the pdf here:
Feel free to leave comments here, including links to other resources that might help sort out this ID problem.
A blog post by Lauren Harter from Feb 2013 briefly discusses the status in the Lower Colorado River Valley, where both species winter, with a photo of an apparent canescens Bell’s Sparrow.
(A summary of all the new AOU changes is at the ABA blog)