I'm sure he was counting on his great camouflage. Sharp and well exposed, a great natural history image. I may take some off the right to get him off centered.
Steve, You've captured his camo well by including the mottled background. I like the look you're getting. I could see you loosing a sliver off the bottom and right side. Memorable image!
For some reason it made me smile ,probably because of the cocked head and look.
Like this atypical picture and the rock adds to it because of similarity to the plumage.
Nice sharpness and an interesting perspective, Steve. I haven't seen white-tailed ptarmigan like this one, but I have had a fair amount of experience with willow ptarmigan and they typically are quite tame and make good photo subjects. Some alternate crops have been suggested above that you might experiment with.