Black-throated Sparrow (A. b. opuntia). Several kinds of birds, such as the Band-tailed Pigeon, occur in the "pine islands" in this province rather than on the desert floor.

There remain several kinds of birds that are not especially associated with any one or two of the above-named provinces. These birds are widely distributed and vary geographically without corresponding to the Biotic Provinces. Examples of these species are: Black Phoebe (S. n. semiatra in northern Coahuila; S. n. nigricans in southern Coahuila), Violet-green Swallow (T. t. lepida in northwestern Coahuila; T. t. thalassina in southeastern Coahuila), Black-eared Bushtit (P. m. lloydi in northern Coahuila; P. m. iulus in southeastern Coahuila), White-breasted Nuthatch (S. c. nelsoni in northern Coahuila; S. c. mexicana in southern Coahuila), Brown-throated Wren (T. b. cahooni in northern Coahuila; T. b. compositus in southern Coahuila), Crissal Thrasher (T. d. dorsale in northern Coahuila; T. d. dumosum in southern Coahuila), and Rufous-crowned Sparrow (A. r. tenuirostris in northern Coahuila; A. r. boucardi in southern Coahuila).

Some representatives of the avifauna of the central and southern sections of the Central Plateau reach southwestern Coahuila. The subspecies squamata of the Scaled Quail and eurhyncha of the Blue Grosbeak are examples. Each in Coahuila seems to be at the northern limit of its range.

In summary, there are three associations of vegetation in Coahuila and each has characteristic birds. Gross climate and topography, through their influence on vegetation, are the prime factors in the distribution and kinds of birds in the State. Some birds of central and southern México reach southeastern and southwestern Coahuila. Representatives of the Gulf Coastal Plain in Tamaulipas and Nuevo León as well as migrants of the eastern flyway occur in northeastern Coahuila. Most of the species that occur in Coahuila seem to be associated with western North America. The aridity of western Coahuila restricts, to a large extent, the diversity of the breeding populations of its avifauna. Xeric conditions surrounding some of the higher mountains are barriers to movement of some species.

ORIGIN OF BREEDING BIRDS OF COAHUILA

Probably beginning in the late Pliocene and ending in the Ice Age (Griscom, 1950:379) the refrigeration of climate in the Northern Hemisphere initiated a period of southward withdrawal of birds from the northern part of North America. Some members of the avifauna of Coahuila probably reached the State in this time. When the continental deserts were formed, or reformed, many tropical and subtropical Middle American species were forced to leave Coahuila. Species associated with arid conditions found their way there. Many representatives of the Old World element also seem to have found their way to the State during the refrigeration of climate in the Northern Hemisphere. The separation of North and South America in the greater part of the Tertiary (Mayr, 1946:9) that deterred mammals from intercontinental colonization seemingly did not hinder birds. Some South American species moved northward into México, all the way north to Coahuila.

The avifauna of Coahuila today is a mixture of the several mentioned elements. Of the breeding populations, 43 per cent breed in the western rather than the eastern United States, 6 per cent breed in the eastern rather than the western United States, 30 per cent breed in both the eastern and western United States, 20 per cent are restricted to the Republic of México, and the southern parts of Arizona, New Mexico, and Texas, and 1 per cent (Aztec Thrush and Rufous-capped Atlapetes) is endemic to the Republic of México.

It is instructive to consider also the origin of avifaunal elements at the level of Family. According to Mayr (1946:11) most North American families and subfamilies clearly originated in the Old World, in South America, or from a North American element that developed in the partial isolation of North America in the Tertiary. Three other elements, the Panboreal, the Pan-American, and the Pantropical are represented by some North American families and subfamilies. Because of the obscurity of the place of origin of certain groups, an additional unanalyzed element must be recognized.

The Caprimulgidae and Picidae probably originated in North America (Mayr, 1946:26). Although the Psittacidae are Pantropical in distribution, indications are that they probably originated in the Old World (Mayr, 1946:17). The Phasianidae, Turdidae (Myadestes-Hylocichla group), and Sylviidae (Polioptilinae) seem to have originated in the Old World (Mayr, 1946:27). However, Mayr considered these groups to have had a secondary center of proliferation in North America, and I thus consider these groups to have a North American origin. Mayr (1946:27) considered the Trochilidae, Tyrannidae, and Icteridae Pan-American in distribution; however, he suggested that they probably originated in South America, and I here treat them as South American in origin. No representatives of the Pan-American element that probably originated in North America have been recorded from Coahuila nor have members of the Panboreal element (Mayr, 1946:11) been recorded in the State. According to my analysis, representatives of families of birds known to breed in Coahuila and those that probably breed there thus seem to have been derived historically from the following sources:

Mayr (1946:28-29) gave examples of analysis by geographic origin of the breeding species of several districts of North America. For instance, at Yakutat Bay in southeastern Alaska the South American element of breeding passerine species was 3 per cent, the North American element 39 per cent, and the Old World element 58 per cent whereas at Sonora, México, the South American element of breeding passerine species was 27 per cent, the North American element 52 per cent, and the Old World element 21 per cent. The breeding avifauna of Coahuila is thus in line with Mayr's analysis, resembling that of Sonora to a considerable degree at the taxonomic level of Family.

ACCOUNTS OF SPECIES

**Podiceps caspicus (Hablizl).—On March 31, 1952, Olmstead saw "many Eared Grebes" on a pond 10 mi. E Hacienda La Mariposa. This is the first record of the Eared Grebe in Coahuila.

[Pelecanus erythrorhynchos Gmelin.—The White Pelican is uncommon, if not rare; Friedmann, Griscom, and Moore (1950:21) list it.]

Anhinga anhinga (Linnaeus).—On March 31, 1952, Olmstead noted an Anhinga perched on a submerged fence post in a lake 10 mi. E Hacienda La Mariposa. This is the first record of the Anhinga in Coahuila.

**Ardea herodias Linnaeus.—Two subspecies of the Great Blue Heron, treganzai and wardi, have been recorded from Coahuila. Friedmann, Griscom, and Moore (1950:27) listed A. h. treganzai from the State; presumably this subspecies occurs widely in low density. They (loc. cit.) remarked also that a record of A. h. wardi from Coahuila "cannot be allocated subspecifically."

Dickerman saw two Great Blue Herons in a marshy area at San Marcos (=20 mi. S Cuatro Ciénegas) on May 4, 1954. Van Tyne and Sutton (1937:12) noted the Great Blue Heron "near Boquillas [Texas], along the Río Grande, on May 10 and 15...."

**Butorides virescens (Linnaeus).—Olmstead saw a Green Heron at Boquillas, 700 feet, on March 10, 1952. Findley reported seeing Green Herons 2 mi. W Jiménez, 850 feet, on June 19, 1952, and 2 mi. S and 3 mi. E San Juan de Sabinas on June 22, 1952.