At the moment, controversy is raging in the birding world, well, at least in that bit of it that affects Cambridgeshire. If you are interested in birds at all, you will probably know that we have two very similar species of summer visiting warblers, little greenish-brown jobs that glean leaves, namely, the Willow Warbler and the Chiffchaff. Very very similar. If they sing, no problem: the WW does a descending cadence based on its name "willo wileeloelooy warwbbbbb" or something like that, while the chiffchaff calls "chiff chaff chiff chaff", etc (unless he is German, in which case he say "zilpzalp).
But if they DON'T sing for you, they can be difficult to tell apart, and indeed when I was a lad, we used to call them "willowchiffs", and left it at that. Later on, I learned to be more sophisticated and referred to them as "phylloscs", from the generic name Phylloscopus (= leaf gleaner), implying that what you had glimpsed could be an even more exotic member of the genus.
Later, with a lot of effort, I - like a zillion others - began to sort them out on such features as call ( WW = dysllabic whoo'eet, CC = monosyllabic wheet), structure (extension of primaries greater in willow warbler) and leg colour (light = WW, dark = could be either). And we all went home happy.
But not any more. The whole goddam business has gone racist, at least as far as the Chiffchaff is concerned. There are Iberian races, Siberian races and, for all I know, Jewish, Welsh and Cantabrian races, distinguished by such subtle differences of plumage that even as we speak, Cambridge ornithologists are fighting duels to the death on Parker's Piece in defence of buff versus dusky and brownish-green versus greenish-greyish-brown.
By the way, I had some long-tailed tits on my suet feeders today. No risk of confusion there.
Or is there? It's such an uncertain world when you get to my age.