Where's My Sibley's?
The hardest warbler identification for me is the differences in these two warblers; the Northern Waterthrush and the Louisiana Waterthrush. Both species come through Texas in the spring and present an id crisis for me. I have to grab my Sibley Bird Guide and make sure which is which. After looking in the book, I'm really not sure. Here are the characteristics of both:
"A quick judgment of the ground color of the breast and eyebrow stripe will separate most waterthrushes: bright white on Louisiana, yellowish on Northern. A yellowish waterthrush is definitely a Northern, while a whitish bird could be either species. Next look at the shape of the eyebrow stripe, which broadens to the rear on Louisiana, and since it is also bright white on Louisiana this stripe can be very conspicuous (vs tapered behind the eye and usually yellowish on Northern).
The issue for identification is that a significant minority of Northerns appear bright white on those parts, and on those birds you must look for other details to confirm the identification. The converse (Louisiana appearing yellowish) never happens. Louisiana usually shows buff on the flanks, which can be fairly prominent. If that is the only part seen it could lead to an impression of yellowish underparts, but the breast, throat, and eyebrow are always clean white."