They do seem successful at colonising new territory. I can remember seeing my first one here in England when I was a kid in the mid sixties. I couldn't find them in my bird book and wondered what it was at first. By the early 70's they were everywhere having spread over from continental Europe. They soon went on the shooting lists as a pest species. I have them in the garden and nesting regularly but of late the numbers seem to have stabilised and they are not as common as only a few years ago. They seem to found strong pair bonds and if you shoot one its mate will come looking for it. I dislike shooting them unless I can get both. Lagopus.....