I confess that I remain stymied by most sparrows. So, I'm doing some serious time at my local community gardens simply looking at sparrows. Especially Chipping Sparrows, not only because they are charming but because large flocks of them often hold a Clay-Colored. So I read with interest a recent exchange of ID tips on Clay-Colored sparrows on the MassBird list-serve. One authority weighing in was David Sibley, and he has kindly summarized his remarks (including some not in his guide) at his blog. Another commentator, Phil Brown, has updated his blog with some side-by-side diagnostic shots to illustrate Sibley's tips.
Now that Sibley mentions it, I noticed a fine partial eye-ring on a cooperative Chipping. I'd never seen that before. Time to get back out and look again.