We examined the crop contents from 129 Rock Ptarmigan (Lagopus mutus) collected between 1967 and1971 on Amchitka, Aleutian Islands, Alaska. On a yearly basis two plant species, Empetrum nigrum and Equisetum arvense, made up 87% of the diet. Only in July and August were they less than 90% of the diet, and in those months they constituted 65% of the diet. During the warmer months (May-October) when a variety of plant species were available, the Amchitka ptarmigan fed upon growth forms of plants high in nutritive value (particularly Equisetum strobili). Its main winter food was the vegetative portions of Empetrum, one of the few abundant evergreen vascular plants on the island.
Emison, William B. and White, Clayton M.
"Foods and weights of the Rock Ptarmigan on Amchitka, Aleutian Islands, Alaska,"
Great Basin Naturalist: Vol. 48
, Article 11.
Available at: http://scholarsarchive.byu.edu/gbn/vol48/iss4/11