What a ragbag of inheritance our language has.
I've been pondering why the hair round the feet of horses and ponies is called "feather", in the singular, no matter how legs or how many horses we are talking about.
What other nouns behave like this? Wool, at clipping-time, is a mass noun; the fleeces (plural) as a whole are wool, not wools, though a grader at the mill would define different "wools" by their staple length and fineness. Fluff. We wouldn't call the fibre gathered by the vacuum cleaner "fluffs" even if it had come off several cats. A high quality duvet is filled with down, not downs, despite the filling having come from more than one bird. It's something to do with volume, mass or quantity. Some uncountable quality makes these things mass nouns.
Rice, gold, butter. Milk, honey, marmalade. Sugar, grass, sand. Hay, straw, bedding. Cutlery, furniture. Concrete. All these are mass nouns. Not pebbles, rocks, or apples.
So feather is a mass noun when it relates to horses, but not when it relates to birds! Feathers with an "s" are something entirely different, and structurally different from hair (which, incidentally, on humans, in English, is also a mass noun – we never say, "I love your hairs.")
Why? I dunno. It just is.