Winter Diets, Gut Lengths, and Interspecific Competition in Alaskan Ptarmigan

Abstract

CLOSELY related species that live in the same region frequently eat different diets, which is usually attributed to interspecific competition for food. Morphological specializations may occur as a result of such competition, and allow each species to utilize its own particular food most effectively. An example of this is the three species of ptarmigan in interior Alaska: Willow Ptarmigan (Lagopus lagopus), Rock Ptarmigan (L. mutus), and White-tailed Ptarmigan (L. leucurus). In winter the ground is usually covered by snow and all three species browse on shrubs. They often occur in the same place and sometimes in mixed flocks, though more often in flocks of one species. Flocks of each species often use the same pieces of ground at different times, but all three species do not occur on all the available ground. Willow Ptarmigan eat almost entirely the twigs and buds of willow (Salix spp.), Rock Ptarmigan largely resin and dwarf birch (Betula glandulosa and B. nana, which may be conspecific) buds and catkins, and White-tailed Ptarmigan many cones of green alder (Alnus crispa) as well as birch and willow (West and Meng 1966; Weeden 1967, 1969; Moss 1973). Associated with these differences in diet, the ptarmigan have bills and gastroliths of different sizes (XVeeden 1969). The present paper shows that they also have guts of different lengths. This is further documentation of the theory that gut length is related to diet (Leopold 1953, Rieck et al. 1971, Gardarsson 1971, Moss 1972, Pendergast and Boag 1973) and gives some insight into the way in which present food habits may have evolved.