Geographic Orientation of American Tetraonidae

Differentiation of both species and genera within the grouse family (Tetraonidae) has been pronounced in North America. Each of its species has become adapted to specific types of habitat. These vary greatly, from arctic tundra to northern desert scrub and humid forest of both deciduous and coniferous types. All the species are racially variable in some degree, from the sage grouse (Centrocercus urophasianus), with 2 races, to the ruffed grouse (Bonasa umbellus), with 13. The races tend to be correlated with the ecological climax area in which they live. There are a few cases in which this correlation is not obvious and racial variation seems to be entirely the result of geographical isolation. The present status of grouse depends chiefly on the extent to which modification of required habitat has taken place; the greatest changes have occurred in the grasslands and the least in the arctic-alpine areas. American grouse occupy a wide range of habitats, mostly of a subclimax or successional stage type, in the arctic and temperate portions of the continent north of Mexico, from the rocky barrens of the Far North and high mountaintops of the West to the prairies of the Gulf Coast. These occupied habitats are representative of most of the major ecological areas of different climax types found within this great expanse of country, although major gaps occur in the evergreen and tropical types of the Southeast, and the southern desert scrub, chaparral, and woodland types of the Southwest (Fig. 1). In this extensive and ecologically varied area, grouse have differentiated into nine species, all but two of which are confined to North America. According to the classification of Peters (1934:24-42), which differs from the present arrangement only in recognizing two species of Canachites instead of one, there are nine species in seven genera of grouse confined to North America, as compared with eight species in five genera confined to the rest of the world excluding North America. Only two species, the willow and rock ptarmigans (Lagopus lagopus and L. mutus), are present in both the New and Old Worlds. Thus it would appear that evolution within this family of birds has been very active in the relatively limited geographical area of North America. Each species of grouse appears to have become adapted to a particular life form of vegetation. There is some overlap, but, by and large, each North American species has its own individual preference for a specific type of habitat. In some places, the preferred forms of vegetation are climax types for the regions where they occur; in other places, they are stages in succession to the climax types. However, even though the species as a whole may occupy habitats of equivalent life form (deciduous, coniferous, etc.) throughout its