A grouse of subarctic and subalpine North America. In many regions its plumage changes with the seasons, and feathered feet help it cope with snow and cold.
In many regions it is white in winter and mottled red-brown in summer. Some subspecies in western Norway and the Kazakh steppe remain reddish brown all year.
Willow Ptarmigan burrow into snow to shelter from cold and wind. A snow chamber provides a winter resting place where plumage and surrounding snow help separate the bird from the outside air.
A 1980 paper reported experiments with six birds at three northern Finnish sites during 1975–1977. The birds lived in outdoor pens and could enter snow. Researchers measured temperature, humidity and carbon dioxide in artificially constructed snow burrows.
Burrow temperature usually stabilised within two or three hours, but could continue rising for eight hours when outside air was below minus 35 degrees Celsius. Warming depended on initial snow temperature and density. Snow thus formed chambers retaining heat from the bird.
IUCN Red List (BirdLife listing) · Least Concern · Global
BirdLife’s IUCN category table lists Willow Ptarmigan as globally Least Concern. The Finnish experiments show that snow conditions affect the warming of resting chambers. Places suitable for snow roosting are relevant winter-habitat features alongside food resources.