The Sierra Madrean Element of the Avifauna of the Cape District, Baja California

Abstract

The Cape district, or terminal third of the peninsula of Baja California, has long been recognized as an area especially rich in endemic species and subspecies of birds. The district, as defined Nelson (1921:plate 31), includes that portion of Baja California lying south of a line, arching sharply to the north, Santa Rosalia on the Gulf coast at latitude 270 20' N to Santo Domingo Point on the Pacific coast at latitude 260 19' N. Grinnell (1928:7) listed 46 species and subspecies of birds endemic to this region. Of these, 43 were considered him to have arrived from the north, ... over continuous land, rather than across the Gulf to the eastward. The invasions of the Cape presumably occurred by way of its basal territory, California, Arizona, and Sonora. The hypothesis of northern (meaning not trans-Gulf) origin of most of the vertebrates endemic to the Cape district has also been expressed Nelson (1921) and Johnston (1924). In a later statement, Grinnell (1928:12) concludes that of all the areas of differentiation in Baja California, can be said that the Cape district appears to have acted most potently with respect to bird life, . (italics mine). The idea is implied Grinnell that there has been a marked differentiation in the Cape district of pioneer ancestral stocks, and that because of this differentiation, the avifauna of that region now includes an unusually large number of endemic forms. However, Oberholser (1919:211) pointed out that the distinctive Brown Towhee of the Cape district (Pipilo fuscus albigula), a form connected a series of intergrading populations with the Brown Towhees of the Pacific coast of California, actually resembles P. f. mesoleucus of Arizona, New Mexico, and northwestern M6xico more than it does the coastal races to the north, although mesoleucus on the one hand, and the Brown Towhees of California and Baja California on the other, are geographically isolated. Davis (1951:98) suggested that, rather than being a strongly differentiated endemic of the Cape district, albigula was actually a non-differentiate, a form which, far being very distinct the ancestral stock which pioneered in the Cape district, might actually be rather similar to it. According to this viewpoint, it is the Brown Towhees of the Pacific coast that have diverged farthest the original stock, leaving the towhee of the Cape district a relatively little differentiated relict contrasting sharply with them. It is the intent of this paper to consider the variational trends exhibited the endemic species and subspecies of birds that occur, at least in part, in the mountains of the Cape district, and to test the strong differentiate versus weak differentiate hypothesis for this group. There is no fossil evidence Baja California to indicate the history of the flora of that region. However, the mountains of the Cape district support a vegetation that contains a Sierra Madrean woodland element evidently derived the Madro-Tertiary flora. As Axelrod (1958) points out, the occurrence in Miocene floras of southern California of fossil equivalents of modern woodland species of the Cape highlands, together with fossil equivalents of living species typifying the Californian flora, and fossil equivalents of recent species now occurring in the Sierra Madrean woodland in the interior southwestern United States and adjacent M6xico, indicates that the Lagunan woodland of the Cape district is a segregate of the Madro-Tertiary flora. The presence in the Cape highlands of modern plant species of definite Madro-Tertiary affinities suggests that, after reaching the Pacific coast, the Madro-Tertiary flora, moving northwestward northern M6xico in the Miocene, spread down the entire peninsula of Baja California