In Coahuila, the Coastal Plain and the Rio Grande Plain lie in the path of the northernmost trade winds; they account for the more humid eastern slopes of the mountains of the northeastern part of the State (Muller, 1947:38). Nevertheless, the northeastern section of the State is semi-arid and can be placed in the Lower Sonoran Life-zone. The vegetation consists mainly of thorny shrubs and small trees with a liberal admixture of yuccas, agaves, and cacti, and closely resembles that of southern Texas, northern Nuevo Leon, and northern Tamaulipas (Goldman and Moore, 1945:354). Migrant birds from the eastern flyway and less commonly migrants from western North America pass through northeastern Coahuila. The following breeding birds seem to be associated with this province: Harris' Hawk, Bobwhite (_C. v. texanus_), Scaled Quail (_C. s. castanogastris_), Yellow-billed Cuckoo, Groove-billed Ani, Green Kingfisher, Golden-fronted Woodpecker, Hairy Woodpecker (_D. v. intermedius_), Ladder-backed Woodpecker (_D. s. symplectus_), Vermilion Flycatcher (_P. r. mexicanus_), Cave Swallow, Gray-breasted Martin, Black-crested Titmouse (_P. a. atricristatus_), Carolina Wren, Long-billed Thrasher, Curve-billed Thrasher (_T. c. oberholseri_), Blue-gray Gnatcatcher (_P. c. caerulea_), Hutton's Vireo (_V. h. carolinae_), Bell's Vireo (_V. b. medius_), Yellow-throated Vireo, Red-eyed Vireo, Summer Tanager (_P. r. rubra_), Olive Sparrow, Cassin's Sparrow, and Black-throated Sparrow (_A. b. bilineata_). _The Sierra Madre Oriental Biotic Province._--Southeastern Coahuila is in this province that includes mountains in southern Nuevo Leon, southwestern Tamaulipas, and eastern San Luis Potosi. Areas classifiable as Canadian, Transition, Upper Sonoran, and Lower Sonoran in life-zone are found in this province. This region of Coahuila receives the highest rainfall; this is evidenced by the luxuriant growth of boreal plants living in the higher places there (Baker, 1956:131). Spruce, pine, and aspen occur at higher elevations and oaks, thorny shrubs, and grasslands are present lower down. Birds of central or southern Mexico 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.