Other December nature events
• Tundra swans returned to Cottonwood Marsh and Sombrero Reservoir at the end of October. They move around a lot, so also check other open bodies of water, such as Valmont and Baseline reservoirs. Other recent rarities in the county include varied thrush, black-throated blue warbler, black-throated gray warbler, brown thrasher and common redpolls.
• Curt Brown reported flowering Easter daisies on Nov. 15 and offers a six-pack of beer to anyone who finds earlier blooming native wildflowers in Boulder Open Space and Mountain Parks.
• Sulphur butterflies were still being observed in mid-November. Keep watching for unseasonable events.
• Up to 100 meteors per hour radiate from a spot near Castor during the Geminid meteor shower that lasts for about two weeks, peaking Dec. 14.
• The Boulder Audubon Christmas Bird Count occurs Dec. 17. Check boulderaudubon.org for details.
The clang of horn against horn resounds in December. While other animals sleep or just try to stay alive, Rocky Mountain bighorn sheep are giving the lie to their peaceful image. It's the rut! Ewes are in heat, and rams are dueling with clashes that can be heard a mile away.
In "Rocky Mountain Mammals," David Armstrong writes: "During the breeding season the usually disorderly and nervous behavior of bighorns gives way to utter chaos." Rams back up. They charge. They ram each other. And they do it again and again with some duels lasting for hours. The massive, curled horns that can weigh up to 30 pounds absorb most of the shock, and their brain is cushioned by large sinuses in the skull, so the warriors rarely die.
The winner of the battle wins the harem and the right to mate with the ewes. Young are born April through July, often on mountain ledges safe from predators. When only a day old, lambs can rock climb almost as well as their mothers.
In summer, the sheep move to higher elevations and roam in small bands, grazing on grasses, sedges and forbs with the older rams remaining separate from the ewes and lambs. In November and December, they descend again to lower elevations.
Both sexes are characterized by horns, though ewes sport smaller spikes. Unlike most ungulates, bighorns don't shed their horns after the rut. Instead, the horns continue growing, laying down annual rings. Armstrong writes that a ram with a three-quarter curl is about 10 years old.
Bighorns wear the color of earth, as opposed to mountain goats, their spiffy, white cousins. Mostly dun-colored or brown, the white rumps of bighorns flash like lights when the animal runs.
Other than humans and the occasional mountain lion or avalanche, disease is the greatest threat to bighorns. Lungworm, which often leads to pneumonia, and other parasitic diseases plague many Colorado herds. Bighorns have no immunity to diseases spread by domestic sheep and do not fare well when territories overlap.
The Rocky Mountain bighorn, Ovis Canadensis, is found only in the Rocky Mountains, though it formerly migrated down to the prairies in winter. By 1950, the state population had dropped to 2,200 animals, the lowest number ever recorded here, and the Colorado Division of Wildlife started reintroductions.
Now an estimated 7,040 Rocky Mountain bighorns, designated the official state animal in 1961, are thriving in 79 herds. Desert bighorns, Ovis Canadensis nelsoni — a smaller, more endangered cousin — have also made a comeback in the western part of the state. Though they don't occur in Boulder County, you may spot them around Colorado National Monument.
Boulder County's largest bighorn herd roams the remote North St. Vrain River drainage above Buttonrock Reservoir, where they were re-introduced after being extirpated from the county early in the 20th century. A few sometimes wander into Boulder Canyon, and they have even been seen on Flagstaff Mountain. Other places to see these iconic creatures are on Mount Evans, on the slopes near Georgetown and in Rocky Mountain National Park.
Ruth Carol Cushman and Stephen Jones are authors of "Wild Boulder County" and "The North American Prairie."