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).