A purely hypothetical question!
But lets say you're grafting away in your patch one day and you're lucky enough to see a white-tailed eagle fly by; could you count it?
After all they're all pretty much straight from cages having been flown over from some foreign land and deliberately released. A bit like a budgie really. Just because the person releasing it is wearing an RSPB badge doesn't make it any more wild than a muscovy duck.
So I say you can't count them, at least not until I get one flying past my house one day or I get a muscovy duck, both will do...