How are arctic birds different than those in warmer climates?

A bird that I hope to see in Churchill, Manitoba, Canada is the Willow Ptarmigan. In the summer these birds have dark brown or black striped feathers, called plumage. They change their color by growing white feathers for the winter months to camouflage themselves by blending into their surroundings. This bird is different from birds in warmer temperatures. Just click on the link below to see the difference. You will see how the Ptarmigan has feathers on its feet.

http://www.hickerphoto.com/ptarmigan-8033-pictures.htm

My challenge to you is: Why do the Willow Ptarmigans grow feathers on their feet?