Why do some grouse have feathers on their legs and feet?

Grouse are ground-dwelling birds that often feature feathered legs and feet as a protective adaptation. While most birds have feet covered in scales, these specialized feathers provide insulation and help the birds survive in cold environments.123

For example, members of the genus Lagopus, known as ptarmigan, inhabit cold regions and mountain-top environments. These birds are distinct among the grouse family because their toes are covered with stiff feathers both above and below. This adaptation allows them to thrive in harsh, wintry conditions, where they browse for food and burrow into the snow to sleep.1