The dividing line between the ranges of the black-tailed and mule deer in general is the summit of the Cascade Mountains. In summer the two races come together and doubtless mingle in the Cascades but as fall approaches they migrate. The mule deer moves eastward to the yellow-pine areas on the lower slopes while the blacktails descend westward to the denser Douglas fir and hemlock forests on the western flanks of the Cascades. In the breeding season the two races are separated. However, as shown by [Cowan] (1936A: 219), in the Pemberton Valley, British Columbia, the two forms occur together during the breeding season and intergradation occurs there. Intergradation occurs also in the Lake Wenatchee area of Chelan County. Observation of hunters' specimens showed some undoubted intergrades among the more abundant, typical mule deer.
Both the mule deer and blacktail have a characteristic bounding gait, unlike the smooth run of the white-tailed deer. The tail is usually held down, rarely erect. When not frightened they move with dainty steps, making little noise.
Fig. 137. Black-tailed deer (Odocoileus hemionus columbianus), buck resting at midday, Van Trump Park, Mt. Rainier, Washington, August, 1931. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 260.)
The black-tailed deer in certain areas is more nocturnal than diurnal. They feed principally in the early morning and evening. In the Olympic Mountains, on the North Fork of the Quinault River, in July, 1937, blacktails were watched feeding and playing until midnight. Also in December, 1939, deer, near the forks of the Skykomish River, were moving about, browsing as late as 11 p. m. while a light snow was falling. In the San Juan Islands, in the summer of 1939, deer were observed feeding at all hours of the day.
The blacktail lives in some of the most dense jungle areas of western Washington. On Whidby Island and some of the San Juan Islands the brush and vines grow so densely that a man can scarcely penetrate them. Were it not for the deer trails, formed by generations of deer, our small mammal investigations would have been far more difficult. The blacktail lives also in the dense forests where fir, hemlock and cedars grow to heights of more than 150 feet. Here lack of light allows only ferns and moss to form an understory vegetation. These forests often clothe the steep glacial hills and the trails of the deer on such hills show them to be adept climbers. Deer trails generally avoid fallen trees and other obstacles. When startled, however, a stump, fence or log is easily leaped by a blacktail. Often they are in small bands of 6 to 10 individuals, but almost as often are solitary, or in pairs.
The habitat of the mule deer is generally more open than that of the blacktail. In the summer, it may occupy rough and rugged country; in the higher Cascades individuals were found in the rocky and brushy country and in open glades and meadows. Farther east they were in the open yellow-pine forests where extensive grassy slopes, free of trees, existed. In some parts of northeastern Washington they lived in the larch and lodgepole pine forests almost dense enough to be "blacktail country." In the Okanogan Valley and on the northwestern corner of the Columbia Plateau mule deer lived in open prairie country where a few cottonwoods and willows were the only trees.
The mule deer seem more social than the blacktail. In the late summer they gather in bands of 10 to 20 or more. In winter, under pressure of hunger, they gather in herds and raid haystacks and pastures. Farmers in the Methow Valley, Okanogan County, report herds of 200 to 400 mule deer about a single haystack.