[Illustration: In November bighorn sheep rams end their summer-long isolation from the ewes, move down from the higher slopes, and begin a bloodless but taxing ritual of strength and endurance to determine the harem master. The sharp reports of clashing horns may carry for kilometers, and the contests continue for weeks until the dominant ram emerges. (Note the Many Glacier hotel complex in the valley below.)] [Illustration: Hummingbirds, like shrews and other small-bodied, warm-blooded animals, exist at the theoretical thresh-hold of life. Because of their small size, body volume is not large enough in relation to surface area to prevent a rapid loss of body heat. To compensate for this, metabolic rates must be high; food is rapidly processed and used up. Thus, since fat reserves are not practical on such small animals, they must eat at frequent intervals. Two species of hummingbirds—the rufous and the calliope—are found in Glacier. Pictured is a female rufous (which weighs about the same as a dime) landing on its lichen decorated nest to feed its two young on a protein-rich mixture of nectar and small insects.] [Illustration: The insect-eating yellowthroat prefers moist habitats. Unlike many of its treetop-dwelling relatives, this tiny (10-11 cm.) warbler is usually seen near or on the ground.] [Illustration: Bands of bighorn ewes and lambs do not summer as high as the rams and are often encountered in the scrub-forest zone. Note the gnarled limber pine in the foreground of this photograph taken on the south face of Altyn Peak.] Reaching the mountain wall, the goats scramble upward to a ledge, sending scree streams pouring from several clefts. Encountering a narrow, steep snowbank, they do not hesitate but continue across the slope. Above the rock fingers of this peak the gathering clouds grow black. A sudden crack of thunder hurries me down the trail. Although geologically young, the Rocky Mountains in Glacier are composed of soft sedimentary rocks that are easily assailed by the many agents of weathering and erosion. If not rejuvenated by continual uplift, these magnificent peaks will glimmer but briefly in the long memory of the planet. Already the sharp countenance of this land is being softened by the ongoing forces of erosion. Chief among these is water, which attacks the mountains everywhere. In addition, frost action continually exploits rock fractures, breaking down blocks of rock into talus and scree. Avalanche and rockfall sweep down the slopes. Layers of softer rock erode quickly, undercutting more resistant rock and creating overhangs which gravity, in time, will collapse. The lashing rain catches me on this sun-and-storm-contested pass. Ice, gravity, wind, and especially water—all attack a land that dares the clouds. The Rising of the Sun and the Running of the Deer: A Glacier Year