These two Grebes are look-alikes. Well...mostly. As an easterner (within the United States :-), I was at first a little bamboozled by these two species. But just a tad of research and studying clarified the field mark that needed my attention: on the bird's face, where was the line between the black and the white? If that line was above the bird's eye, then call it a Clark's. Otherwise, call it a Western. Apparently you can also use bill color to help make the identification (according to my Peterson, a Western has a greenish yellow bill with a dark ridge, and a Clark's has a orange-yellow bill), but that field mark was a tad too subtle for me, especially under different light conditions and (usually) over a long distance.
Here are my two photos. The first is a Western and the second is a Clark's. The location of the eye relative to the line between the white and black feathers is very easy to see. Even better, I can see the difference in bill color in these photographs as well.
