The three species of ptarmigan are mostly plant-eaters, though they also consume beetles, caterpillars and other insects. Ptarmigan live in very cold habitats, and insects only figure into their diet during warm weather. Ptarmigan eat berries, buds, twigs and seeds year-round.
Ptarmigans are often very selective in the plants they eat. The rock ptarmigan, for example, mostly eats dwarf birch. During the winter, this tree makes up most of its diet, with willow making up the rest. The willow ptarmigan or red grouse, as it is known in Scotland, relies heavily on one of two specific plants, depending upon the region in which it lives. In North America, the bird eats willow almost exclusively, with the plant making up as much as 94 percent of its diet during the winter. In Europe, the bird eats heather almost exclusively, with 95 percent of its diet consisting of heather from autumn through winter.
Young ptarmigans are more omnivorous than adults, as they need the added protein from insects to grow.