When you see one, you'll know it

A maxim adopted by birders and the American Golden Plover here is a good example. I've (rather sadly) spent a fair time looking through Golden Plover flocks trying to make individuals into other species. "That eye-stripe is a big brighter", "It's beak is a bit longer", "What longs wings it's got" (pantomime response - "All the better to fly across the Atlantic") or "That bird is a bit greyer" are examples. But each time, deep down, you know it is just a Golden Pl;over until you see the real McCoy like this one.
Eagles and Buzzards are another classic example. If you think it is either a Buzzard or an Eagle then it is a Buzzard.