Introduction

Adult male Rock Ptarmigan, courtship plumage; n. Canada (Nunavut), May

Figure 1. Year-round range of the Rock Ptarmigan in North America.

Its trim form and its more direct, buoyant flight make the Rock Ptarmigan to the Willow Ptarmigan what the Teal is to the Mallard. One of the most beautiful sights of this north country is a flock of fast-flying ‘rockers’ skimming around the snow-packed dome of some hill above the tree-line. The sky is intensely blue, and below, the forest tapers out into the distance to mark the course of the valley. A whir of wings rises above the wailing of the wind, and suddenly they are gone (Clement, in Todd 1963).

Rock Ptarmigan are the quintessential arctic bird, living only on inhospitably cold and windswept tundra regions around the northern hemisphere. This is the only bird species in which all populations spend their entire life cycle on the tundra, retreating only from northernmost regions during the long period of winter darkness. Probably as a result of their limited contact with humans, Rock Ptarmigan are often exceptionally tame and easy to observe at close range.

Nevertheless, their behavior has received little serious study so far, particularly in North America. Their plumages, on the other hand, have been studied extensively over the past century, revealing an unusual sequence of 3 body molts from spring to autumn, the first of these resulting in a rapid and dramatic change in appearance from immaculate white to dark brown in both sexes. Females, however, complete this molt before males and the result is one of the most striking sexual differences in conspicuousness in birds. Thus, territorial males are often visible from 1–2 km away with the unaided eye, whereas females sitting on their completely exposed nests are so well camouflaged that they are often difficult to find from less than 2 m away—arctic foxes have been observed walking right past them.

As in other dimorphic species, the opposing effects of sexual selection and predation presumably influence the expression of conspicuousness and camouflage in Rock Ptarmigan and may help to explain both geographical and seasonal variation in the plumages and molts of this species.