Molting Adaptations of Rock Ptarmigan on Amchitka Island, Alaska

Abstract

-The molting pattern of Rock Ptarmigan (Lagopus mutus gabrielsoni) from Amchitka Island differs from that of other described races in the feathers molted, temporal and sexual sequence of molt, and extent of molt. Partially dark feathers may completely cover the head of females as a part of the winter plumage. Many new brown feathers or retained old ones are also found on the upper parts primarily of females but of both sexes in winter. To date, only the mid-summer and male autumn plumage, on which the description of this race was based, has been documented. The Rock Ptarmigan (Lagopus mutus) has distinct races occurring on such islands as Newfoundland, Greenland, Iceland, Scotland, the Aleutians and Japan. Of the 13 Palearctic races recognized by Vaurie (1965), only one (L. m. millaisi) from Scotland is said to retain some brown feathers in winter (Vaurie 1965, Cramp and Simmons 1980). None of the 10 Nearctic races discussed in Ridgway and Friedmann (1946) is said to do this, although adequate descriptions of the winter plumage for the races chamberlaini (includes sanfordi, fide Gabrielson and Lincoln [1953]) and gabrielsoni were not available. Gabrielson and Lincoln (1959) accepted eight races for Alaska (one was newly described after Ridgway and Friedmann) for three of which, all Aleutian Island forms, winter plumages have not been described. In the original description of L. m. gabrielsoni from Amchitka, Aleutian Islands, Murie (1944) had only summer and autumn specimens but suggested that the winter plumage was almost certainly completely white. At least two insular races of Willow Ptarmigan (Lagopus lagopus scoticus, L. 1. variegatus) and the southernmost population of the race L. 1. maior are known to retain brown feathers in winter (Vaurie 1965, Cramp and Simmons 1980). In addition, stocks of L. mutus welchi from Newfoundland have been transplanted to warmer climates (Salomonsen 1939), where they have been found to retain brown feathers in winter; in some populations, where winter conditions and snow prevail through the summer, white dorsal feathers are retained as a part of the summer plumage (Salomonsen 1939). It seems adaptive to retain those complements of feathers that will provide most security from predation in regions of variable snow cover or unique environmental conditions. Populations that retain such mixtures of feathers represent a departure from the type or normal molt pattern and sequen e described by Salomonsen (1939). His type in the annual molt-activity of various feathers falls into five categories, some of which pe tain to both sexes equally while others pert in to only one. These categories may be generally summarized as: 1. One molt (basic in autumn) per year (includes most large feathers and some under body feath rs). 2. Two molts (basic plus alternate in winter) per year (mainly body feathers in males only). 3. Two molts (basic plus supplemental in summe ) per year (lesser wing coverts and under body feathers). 4. Two molts (alternate plus supplemental) per year (tertiaries in males, wing coverts in females). 5. Three molts (basic, alternate and supplemental) per year (upper parts, head, a few wing feathers and scapulars). In addition to chronicling the sequence of molt in Rock Ptarmigan, Salomonsen (1939) sp culated about the ecological functions of the molt, as well as the physiological and environmental variables affecting molting times, extent of molt, and coloration of plumage. Watson (1973) and Hewson (1973) presented data that raised doubt about some of Salomo sen's earlier hypotheses. Our findings also differ from the sequences that Salomonsen described. While studying the avifauna of Amchitka Isl nd, Alaska, we noticed that the winter plumage of the Rock Ptarmigan there, the southernmost Nearctic population except for Newfoundland, included many colored or partially colored feathers. We found that the par-