Interesting. Willow Grouse (Lagopus lagopus) are white in the winter over the rest of their range (Scandinavia & points east). It's only the British/Irish race Lagopus lagopus scoticus (our "red grouse") that stays brown all year round. Maybe somebody's introduced some foreign birds. I suppose it's possible they or their ancestors made their own way across from Sweden or Norway in a storm, but grouse don't generally move away from where they were born according to my books.