In Colorado, the alpine timberline lies at about 11,500 feet, somewhat higher on south-facing slopes and a bit lower on shaded, northern flanks of the mountains. This biologic barrier, characterized by krummholz (stunted, wind-shaped conifers) and pockets of bristlecone pine, marks the upper edge of the Subalpine Zone; above timberline is the alpine tundra, a sun-drenched ecosystem of lichen-splattered rocks, glacial lakes, tussock grass, cushion plants and an amazing diversity of wildflowers.
Ravens are the most conspicuous birds of the alpine tundra, joined by American pipits, horned larks, mountain bluebirds and those ubiquitous American robins during the warmer months. White-tailed ptarmigan, though difficult to spot, are also fairly common and brown-capped rosy finches congregate near the rock-rimmed lakes. Avian hunters include golden eagles, prairie falcons and northern goshawks and the open woodlands of timberline attract white-crowned sparrows, Wilson's warblers, Clark's nutcrackers, pine grosbeaks, Cassin's finches and olive-sided flycatchers. Mammals of the alpine tundra include elk (primarily bachelor herds), mule deer, bighorn sheep, yellow-bellied marmots, pikas, short-tailed weasels, golden-mantled ground squirrels, chipmunks, coyotes and the occasional mountain lion; mountain goats have been introduced in some areas and are expanding their range.
When the snows of autumn arrive in the Alpine Zone, most birds depart for southern climes or migrate to lower elevations and most mammals den up, hibernate or descend to the milder life zones of the Rocky Mountain corridor. By winter, only the ptarmigan, ravens and mountain goats are evident amidst the frigid snowscape of the alpine tundra.
