Foods and weights of the Rock Ptarmigan on Amchitka, Aleutian Islands, Alaska

We examined the crop contents from 129 Rock Ptarmigan (Lagoptts mutus) collected between 1967 and 1971 on Amchitka, Aleutian Islands, Alaska. On a yearly basis two plant species, Empetrum nigrum and Equisetum arvense, made up 87% ofthe diet. Only in July and August were they less than 90% ofthe 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 oi Empetrum, one of the few abundant evergreen vascular plants on the island. The Rock Ptarmigan (Lagopus mutus), is one of the most abundant and widely distributed terrestrial vertebrate herbivores residing permanently in arctic and subarctic regions. Its circumpolar distribution covers most of the northern regions of Europe, Asia, and North America and extends southward as far as northern Japan and the Pyrenees Mountains of northern Spain. Throughout this extensive range, the Rock Ptarmigan is primarily a browser upon leaves, twigs, and buds of various plants (cf. Cramp and Simmons 1980). In many areas it is sympatric, at least for part of the year, with the Willow Ptarmigan {Lagopus lagopus) and may also share available food resources with other, primarily mammalian, browsing animals. However, neither Willow Ptarmigan nor terrestrial herbivorous mammals occur in the western Aleutian Islands. In contrast to many other areas where Rock Ptarmigan occur, Amchitka Island in the western Aleutians (Fig. 1) consists of a small land area, in which the climate is maritime with little winter snow accumulation in the lower elevations and a somewhat depauperate flora (Hulten 1960). Many foods reportedly eaten by Rock Ptarmigan in other areas (Gelting 1937, Watson 1964, Weeden 1969, Gardarsson and Moss 1970, Cramp and Simmons 1980) are rare or absent. In particular, birch (Betula) and alder (Alnus), which make up 90% (dry weight) of the winter diet of central Alaskan birds (Weeden 1969), are not on Amchitka; and willows (Salix), which are sometimes eaten by Rock Ptarmigan in other areas, are rare on the island. Other unusual features of Rock Ptarmigan on Amchitka include its moult (Jacobsen et al. 1983) and its breeding phenology (White et al. 1977). This paper describes the food and shows weights of the Rock Ptarmigan on Amchitka Island. Materials and Methods Retween August 1967 and October 1971, crop contents were removed from 129 Rock Ptarmigan collected on Amchitka (Table 1). In most cases a specimen was processed on the day of collection as follows: at the time of collection the date, location, time, general weather condition, general ground condition (dry, wet, snow covered, etc.), and flock size were recorded; the specimen was then taken to the field laboratory where the crop and its contents were removed and placed in a bottle containing 8% formalin. The bird was then weighed and gonad measurements made. About 75% ofthe collected birds were aged by the method ofWeeden and Watson (1967). The preserved crop contents were sorted into component food species, thoroughly dried at 80 C, and weighed to the nearest 0.001 gram. Samples of vegetation from one bird were used to determine nitrogen and phosphorus content. These samples were removed from the crop, sorted, and air-dried in the field laboratory before shipment (no Chesapeake Bay Center for Environmental Studies, Smithsonian Institution, Edgewater, Maryland 21037. Present address: Arthur Rylah Institute for Environmental Research, 123 Brown Street, Heidelberg, Victoria 3084, Australia. Department of Zoology, Brigham Young University, Provo, Utah 84602.