For pretty-much-ubiquitous birds, I still had a hard time telling House Finches and Purple Finches apart, unless they were both in view at the same time. My old field guide (Golden, circa 1966) emphasized the color difference, and provided pictures where the color was basically the same. *sigh* And then last year, I decided I deserved a book that at least had the correct current names (no more Oldsquaw!). Lo and behold, arrows indicating the streaky sides and dark back and head of the male House! the almost unpatterned face of his mate! the strong facial marking on the female Purple! the more uniform color and light belly of the male!
It’s a poor worker who blames her tools, but what a difference the right tools make.