Along the Silver Thread Scenic Byway in the heart of the San Juan Mountains, wildlife can be seen in streams, rivers, open meadows, forest edges, cliffs, canyons, valleys, and high mountain passes. Elk are abundant from late October through early spring and are best seen in meadows or along the highway just before sunset or after sunrise. Bald and golden eagles appear in cottonwood trees, on cliffs, and soaring on air currents; bald eagles build large stick nests in cottonwood trees and usually stay close to the river to fish, while golden eagles are present in winter and can be seen soaring in the canyon or along the Rio Grande. Moose were historically said to have lived in the San Juan Mountains, though no records verify this, and between 1990 and 1993 one hundred moose were transplanted by the Colorado Division of Wildlife from northern Colorado and Utah to an area near Spring Creek Pass because its willow thickets provide food, shelter, cover, and escape from predators. Yellow-bellied marmots, the largest member of the squirrel family in North America, have dark brown fur with a yellowish-brown underside and are also called whistle pigs because their whistle-like signal can be heard as far as one mile; they can be seen on high mountain passes, in open meadows, or near rocky outcrops. In 1999, lynx were reintroduced into the San Juan Mountains to reestablish this Colorado endangered species; they are recognized by two-inch ear tufts and flared ruffs of hair on each side of the lower jaw, prefer high-elevation mixed forests with thick underbrush used by snowshoe hare, and have dispersed widely into many habitats. Rocky Mountain bighorn sheep graze on plants in meadows, open woodlands, and above timberline and seek steep rocky cliffs for protection; historically they migrated from mountain range to mountain range, but today they remain year-round because roads and subdivisions have broken up their migration routes. Mule deer, recognized by their large mule-like ears, are most active in the morning and evening as they feed in open meadows and scrub brushland and often cross the highway unexpectedly. During summer, Western and Mountain Bluebirds are common in open valleys, nesting in holes in trees and wooden fence posts, and civic groups have constructed and installed thousands of bluebird boxes along fences to attract them. In the San Luis Valley each spring and fall, Sandhill Cranes and the endangered Whooping Crane can be spotted during migration between Idaho and New Mexico; Sandhill Cranes are four feet tall with six-foot wingspans and grayish-brown coloring, while adult Whooping Cranes are slightly larger, white with black wing tips, and have wingspans up to seven feet.