Nutrition of Willow Ptarmigan in Northern Alaska

Abstract

THE arctic and subarctic regions of Alaska and northern Canada are extensive areas occupied by large populations of relatively few species of animals. Many of these are browsers, such as moose (Alces alces), porcupine (Erethizon dorsatum), snowshoe hare (Lepus americanus), tundra hare (L. othus), and two species of Ptarmigan, Rock (Lagopus mutus), and Willow (L. lagopus). These species obtain most of their nutrition from the stems, bark, twigs, buds, and leaves of brushy plants and trees. Beyond the limit of trees, however, only a few genera of bushy plants are available, the most abundant being Salix (willow), Alnus (alder), and Betula (birch). Salix appears to be the most important genus and sustains Willow Ptarmigan and moose throughout the winter in the arctic slope area of northern Alaska. Willow Ptarmigan have perhaps a more specialized and uniform diet than any other northern animal since during many winter months up to 94 per cent of their diet consists of buds and twigs of willow, and 80 per cent of this may be of a single species. The adaptation of the ptarmigan's own digestive enzymes and the ability of its intestinal and cecal microflora to provide nutritive and energy requirements to the bird from a single generic plant source is unusual and interesting. This paper reports on the results of one of a series of studies which has been undertaken to understand these adaptations and to document seasonal and latitudinal variations in the diet of these birds. The population we studied breeds in the valley of the Colville River, north of the Brooks Range in northern Alaska and migrates south through passes in the mountains to winter throughout the river valleys of the range as far south as the Koyukuk River, a northern tributary of the Yukon. This population has been under observation since 1948 by Laurence Irving and Simon Paneak who have recorded changes in seasonal abundance and movements in several papers (Irving and Paneak, 1954; Irving, 1960; Irving et al., 1965). The population segregates into age and sex groups in its migration south in the fall and these groups become distributed along a north-south gradient through the Brooks Range (Irving et. al., 1965). Samples from ptarmigan flocks were collected as often as possible from Umiat (690 24! N lat., 152? 07' W long.), Anaktuvuk Pass (680 09' N, 151? 46' W), Crevice Creek (670 22' N, 152? 04' W), and Bettles Field (660 55' N, 1510 28' W) (Figure 1). Details on sex composition of the