In the west, the Willow vs. Alder identification puzzle involves the Cordillerian vs. Pacific-Slope empids. All four of these species look identical and each has a subtly different call. We always laugh at the distribution map in Sibley for the Pacific-Slope Flycatcher. It shows them distributed up and down the west coast from Baja California to southern Alaska with a couple of those little green dots that Sibley uses to show accidentals in places like Pennsylvania, upstate New York, Louisiana, etc. Now that's a gutsy call!
Accordingly, whenever we can't identify a local empid, we immediately assume that it's a vagrant Pacific-Slope that's wandered into the northeast. - ¿Quien sabe?
Larry and Mona