Some seed eaters have pouches in their throats that hold food while they are gathering the seeds. Insect-eaters eat suet from bird feeders. A bird's feathers provide alot of insulation. Birds fluff up their feathers and trap air between the feathers. The ptarmigan has feathers on its feet to help keep warm.
Birds huddle together for warmth.
The grouse will hide in snowdrifts to keep warm. Shivering helps to keep some birds warm because it increases body heat. Snowy Owls come from the north in search of food.