Clark's Grebes have paler backs than Western Grebes, and, especially, lighter colored flanks. In this view, the eye border and bill color clearly identify this bird as a Clark's. One can then observe flank color with confidence.

In using flank color as the lone I.D. criteria (such as on a sleeping bird where the head and bill are hidden), it's important to be able to see the pale flanks on both sides of the bird. The reason is that both species often roll their bodies to one side, exhibiting much more white than on a bird of either species sitting flat on the water. This posture can confuse even expert birders into over-estimating the number of Clark's Grebes present in a mixed flock. So, while white flanks can be diagnostic, they need to be used carefully and only from angles showing both sides of the bird simultaneously.