ahuila), 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
Mexico reach southeastern and southwestern Coahuila. Representatives
of the Gulf Coastal Plain in Tamaulipas and Nuevo Leon 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 Mexico, 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 east