Birds of central or southern México reach the southern part of Coahuila; the Thick-billed Parrot, Hooded Yellowthroat, and Rufous-capped Atlapetes are examples. A boreal forest on the higher slopes of the mountains of southeastern Coahuila is suitable for certain northern birds such as Goshawks, Pine Siskins, and Brown Creepers. Some species of birds ordinarily associated with western North America are present in Coahuila only in its southeastern part; striking examples of disjunction in range thus occur. Probably sometime in the past these birds were distributed throughout most of Coahuila. When this area became arid, these species disappeared from all of Coahuila except from the high mountains in the southeastern part. For example, Steller's Jay and the Scrub Jay are absent in the Sierra del Carmen of northwestern Coahuila but do occur in southeastern Coahuila.
Migrants of the eastern flyway as well as migrants associated with western North America pass through this section of Coahuila. The following breeding birds are associated with this province: Goshawk, Band-tailed Pigeon, Thick-billed Parrot, Golden-fronted Woodpecker, Ladder-backed Woodpecker (D. s. giraudi), Pine Flycatcher, Buff-breasted Flycatcher, Vermilion Flycatcher (P. r. mexicanus), Steller's Jay, Scrub Jay, Mexican Chickadee, Black-crested Titmouse (P. a. atricristatus), Cactus Wren (C. b. guttatus), Robin, Blue-gray Gnatcatcher (P. c. amoenissima), Hutton's Vireo (V. h. stephensi), Bell's Vireo (V. b. medius), Hartlaub's Warbler, Summer Tanager (P. r. cooperi), Pine Siskin, Rufous-capped Atlaptes, and Black-throated Sparrow (A. b. grisea).
The Chihuahua-Zacatecas Biotic Province.—This province in Coahuila covers the arid, interior, western desert area; it consists of rolling plains with mountains that rise islandlike above the general surface. Some of the mountains, such as in the Sierra del Carmen and the Sierra del Pino, are more than 9000 feet high. The major part of this biotic area lies within the Lower Sonoran Life-zone. Areas of the Transition and Canadian life-zones are present on some of the higher mountains; their discontinuity results in a discontinuous distribution of the conifer-dependent avifauna.
The large desert restricts the movement of birds considerably. Major results of this include isolation of certain populations and absence of others in the boreal islands. For example, Miller (1955a:157) noted that the "dispersal of conifer-belt birds to and from the Sierra del Carmen, although not as difficult as to well separated islands [such as off the coast of Baja California], is nevertheless a formidable matter to accomplish across the great deserts of Texas, Chihuahua, and Coahuila." Miller (loc. cit.) noted also that the avifauna of the Sierra del Carmen, due to its insularity, is unbalanced and stated that "as a consequence of unbalance, species that are present show ecologic extension and unusual numerical relations." At least in this type of environment, an extension or expansion of the ecologic habits of the related types takes place when some species are absent.
This isolation influences local variation among some of the birds found in Coahuila. Niches elsewhere usually occupied by certain species, absent here, are occupied by other species. These other species thus enjoy an ecologic freedom and can expand their niches in the absence of related types of similar ecologic scope. For example, Miller (1955a:158-159) reported that Hairy Woodpeckers occurred only casually in the Sierra del Carmen and that the Ladder-backed Woodpecker has spread out and seems to occupy the niche or niches usually characteristic of the Hairy Woodpecker. Changes usually thought of as of subspecific character seem to be taking place between the Ladder-backed Woodpeckers of the Sierra del Carmen and of other areas, possibly because the Ladder-backed Woodpecker in the Sierra del Carmen is extending its ecologic sphere more than in areas where the Hairy Woodpecker exists. Restriction in dispersal due to geographic isolation has probably hindered gene flow, thus allowing rapid local adaptation, recognizable in variation at the infraspecific level. Miller (loc. cit.) listed other birds that have expanded their ecologic scope; his work should be referred to for further details.
The following birds are associated with this province: Black Vulture, Scaled Quail (C. s. pallida), Turkey, Elf Owl, Green Kingfisher, Hairy Woodpecker (D. v. icastus), Ladder-backed Woodpecker (D. s. cactophilus), Wied's Crested Flycatcher, Buff-breasted Flycatcher, Vermilion Flycatcher (P. r. flammeus), Black-crested Titmouse (P. a. dysleptus), Cactus Wren (C. b. couesi), Curve-billed Thrasher (T. c. celsum), Blue-gray Gnatcatcher (P. c. amoenissima), Hutton's Vireo (V. h. carolinae), Summer Tanager (P. r. cooperi), and 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