Recently park staff have been asked why there seem to be fewer Abert's squirrels (Sciurus aberti) in Rocky Mountain National Park than there were in the past. In answering this question it is important to remember that these shy, often difficult to observe squirrels have very large natural fluctuations in their populations due to food availability. Recent droughts in the area of the park may have resulted in less food available for these specialized feeders. Also, Abert's squirrels have very large home ranges and only occur in mature ponderosa pine (Pinus ponderosa) stands, so increases in human encroachment can cause decreases in Abert's squirrel numbers. Finally, anecdotal evidence suggests that Abert's squirrels are less able to recognize cars as predators than other squirrel species, and are often killed on roads.
Abert's squirrels are one and a half to two pound grayish, reddish, or black squirrels with white undersides. They are most easily distinguished from other squirrels by their prominent ear tufts that are more prominent in the winter and may almost disappear in the summer. Their head and body is about 12 inches long, and their tail is about nine inches. They may live seven to eight years in the wild, although there is little real data on life span.
Ponderosa pine trees provide not only a home but also most of the Abert's squirrel's diet in Rocky Mountain National Park. During the summer they eat the needles, seeds, buds, and male and female cones from these trees. Because they are the only squirrel that does not make food caches, they must be active all year around. During the coldest parts of the winter they will leave their nests for short periods of time to nibble through the ponderosa pine's bark to get to the inner sap conducting layer. When possible, they also eat mushrooms and other fungi, and occasionally they will eat mistletoe, insects, carrion, antlers and bones. They either nest in witches' brooms (misshapen balls of twigs caused by dwarf mistletoe) or make ball-shaped nests, one to three feet in diameter, out of twigs lined with moss, bark, leaves, fur, lichens, and similar materials about 20 to 40 above the ground. Abert's squirrels breed in April or May, carry their young for about 40 days, and usually give birth to three or four babies. Except when breeding or caring for young, they are solitary.
While population numbers may fluctuate from year to year, these fascinating and charming squirrels are in no danger of disappearing from Rocky Mountain National Park.