Vertical Migration
In the effort to find winter quarters furnishing satisfactory living conditions, many North American birds fly hundreds of miles across land and sea. Others, however, are able to attain their objectives merely by moving down the sides of a mountain. In such cases a few hundred feet of altitude corresponds to hundreds of miles of latitude. Movements of this kind, known as "vertical migrations," are found worldwide wherever there are large mountain ranges. Aristotle first mentions vertical migration: "Weakly birds in winter and in frosty weather come down to the plains for warmth, and in summer migrate to the hills for coolness ..." (Dorst 1962). The number of species that can perform this type of migration pattern is obviously limited to those species adapted to breeding in alpine areas.
In the Rocky Mountain region vertical migrations are particularly notable. Chickadees, rosy finches, juncos, pine grosbeaks, Williamson's sapsuckers, and western wood pewees nest at high altitudes and move down to the lower levels to spend the winter. The dark-eyed juncos breeding in the Great Smoky Mountains make a vertical migration, but other members of the species, breeding in flatter areas, make an annual north-south migration of hundreds of miles (Van Tyne and Berger 1959). There is a distinct tendency among the young of mountain-breeding birds to work down to the lower levels as soon as the nesting season is over. The sudden increases among birds in the edges of the foothills are particularly noticeable when cold spells with snow or frost occur at the higher altitudes. In the Dead Sea area of the Middle East, some birds that breed in this extremely hot desert move up into the surrounding cooler hill during the winter (Thomson 1964).
The vertical migrations of some mountain dwelling gallinaceous birds (mountain quail and blue grouse) are quite interesting because the annual journey from breeding to wintering grounds is made on foot. Mountain quail make this downward trek quite early in the fall well before any snows can prevent them from reaching their goal. Blue grouse perform essentially the same journey in reverse. During midwinter, these birds can be found near timberline eating spruce buds protruding above the snow.
These illustrations show that the length and direction of a migration route are adapted to the needs for survival and are met in some cases by a short vertical movement or great latitudinal travels in others.