Winter is a very hard time for the herbivores of Yellowstone. The grasses the grazing animals depend upon are dry, dormant and less nutritious than during the rest of the year. The other problem is that they may be buried under many inches, if not feet, of snow. So food is less available, and the weather is very cold,requiring the animals that stay active in the winter to turn their fat reserves into heat to stay warm.
This bison calf has a snowy nose because it has been pushing snow out of the way to find food. Bison do migrate to lower elevations in the winter, and also leave the park. This causes problems for them as they are hunted, and rounded up and often sent to slaughter since local ranchers are concerned they may transmit a disease called brucellosis to cattle. The irony of the situation is that the cattle gave the disease to the bison in the first place.
Bison, however, are well adapted for the cold Yellowstone winters. During the winter their coat of coarse hair can be as much as two inches thick. This provides excellent insulation against the cold.
The snow does not accumulate in such deep layers around the hot springs and other thermal features so the bison there do not have to move as much snow around as the ones who winter well away from the hot springs. There are dangers, however, of life around the hot springs. Bison are huge, heavy animals. They can weigh 2,000 pounds and sometimes as they walk around the thermal features they break through the thin crust of soil and fall into the hot springs and die.
I photographed this bighorn sheep ram in one of the lower elevation areas of the park. Migration to lower elevations where there is less snow is a solution to the problem of winter adopted by many of the herbivores in the park.
I was very surprised to see this squirrel active early in January. Most of the squirrels hibernate during the winter, but this one was not at all sleepy it seems. It scampered across the trail in front of me, ran up into this tree, paused for a few seconds and then was on to another tree.
These cow elk form large herds with their calves and younger elk. Here they are watching the landscape from a high point to better detect their main predator, wolves.
As winter takes its toll on elk, bison, deer and other animals in Yellowstone animals such as this coyote will benefit from feeding on the carcasses of winter killed animals. They also feed on mice and voles, animals that stay active under an insulating blanket of snow.
I was surprised by the number of birds I saw during my winter trip. Many of the ducks, and this beautiful trumpeter swan, forage in the Firehole River. Water flowing from geysers, and hot springs into the river keeps it flowing year round which provides habitat for a host of birds.