o the west of the
Sierra Madre Oriental Axis are higher, more rugged, and more dissected
than those of the Coastal Plain and are marked by scattered desert
ranges, buttes, low hills, and knobs.

Most of Coahuila is arid. Rainfall is moderate on the Coastal Plain and
is low west of the central mountains. Baker (1956:128-132) and Muller
(1947:35-38) give good summary discussions of the topography and
climate of Coahuila, and the reader is referred to these for further
details.

Biotic Communities

Baker (1956:132) stated that "the biotic communities of Coahuila might
be divided in accordance with the three physiographic areas of the
State: the Gulf Coastal Plain, the mountains, and the desert plains of
the Mesa del Norte." Goldman and Moore (1945:348-349) listed three
biotic provinces in Coahuila: the Chihuahua-Zacatecas Biotic Province,
in the western half of the State; the Tamaulipas Biotic Province, in
the northeastern part of the State; and the Sierra Madre Oriental
Biotic Province, in the southeastern part of the State. Merriam (1898)
noted that definable portions of the Lower Sonoran Life-zone, the Upper
Sonoran Life-zone, the Transition Life-zone, and the Canadian Life-zone
can be distinguished in Coahuila. In my study of the distribution of
the avifauna of Coahuila, I found that the three biotic provinces
listed by Goldman and Moore (_op. cit._) as major headings and
Merriam's life-zones as supplements are the most satisfactory
divisions.

_The Tamaulipas Biotic Province._--This province consists of lowland
plains and a few isolated ranges of low mountains. The average rainfall
is 23 inches (Baker, 1956:130), considerably more than the 10 inches
falling in the western part of the State. In the northeastern section
of the State, the moderate amount of rain, mesic vegetation, and close
proximity to the eastern migration pathway importantly influence the
types of birds found.

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 T