Lagopus lagopus
Slightly bigger and more heavy build than Ptarmigan, with clearly heavier bill. Summer plumage warm brown above with white belly, showing a warmer brown than Ptarmigan. Winter plumage all white in both sexes, except for black tail. Males lacking black lores. Wings always white in all plumages in nominate ssp. lagopus, and brown in ssp. scotica.
Very characteristic calls: "Kuwa -Kuwa -Kuwa" (go back!) Also: "kauw kauwkkkrrrkekekekekeke" with a short pause after first "kauw", then a decelerating cackle, often followed by the "go back" call.
Wikipedia: map (se also Xeno-canto below)
Birdlife ecology
