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 eastern United States, 6 per cent breed in the eastern
rather than the western United States, 30 per cent breed in both the
eastern and western United States, 20 per cent are restricted to the
Republic of Mexico, and the southern parts of Arizona, New Mexico, and
Texas, and 1 per cent (Aztec Thrush and Rufous-capped Atlapetes) is
endemic to the Republic of Mexico.
It is instructive to consider also the origin of avifaunal elements at
the level of Family. According to Mayr (1946:11) most North American
families and subfamilies clearly originated in the Old World, in South
America, or from a North American element that developed in the partial
isolation of North America in the Tertiary. Three other elements, the
Panboreal, the Pan-American, and the Pantropical are represented by
some North American families and subfamilies. Because of the obscurity
of the place of origin of certain groups, an additional unanalyzed
element must be recognized.
The Caprimulgidae and Picidae probably originated in North America
(Mayr, 1946:26). Although the Psittacidae are Pantropical in
distribution, indications are that they probably originated in the Old
World (Mayr, 1946:17). The Phasianidae, Turdidae (_Myadestes-Hylocichla_
group), and Sylviidae (Polioptilinae) seem to have originated in the
Old World (Mayr, 1946:27). However, Mayr considered these groups to
have had a secondary center of proliferation in North America, and I
thus consider these groups to have a North American origin. Mayr
(1946:27) considered the Trochilidae, Tyrannidae, and Icteridae
Pan-American in distribution; however, he suggested that they probably
originated in South America, and I here treat them as South American in
origin. No representatives of the Pan-American element that probably
originated in North America have been recorded from Coahuila nor have
members of the Panboreal element (Mayr, 1946:11) been recorded in the
State. According to my analysis, representatives of families of birds
known to breed in Coahuila and those that probably breed there thus
seem to have been derived historically from the following sources:
Old World 24.7%
North America 37.0%
South America 24.0%
Unanalyzed 14.3%
Mayr (1946:28-29) gave examples of analysis by geographic origin of the
breeding species of several districts of North America. For instance,
at Yakutat Bay in southeastern Alaska the South American element of
breeding passerine species was 3 per cent, the North American element
39 per cent, and the Old World element 58 per cent whereas at Sonora,
Mexico, the South American element of breeding passerine species was 27
per cent, the North American element 52 per cent, and the Old World
element 21 per cent. The breeding avifauna of Coahuila is thus in line
with Mayr's analysis, resembling that of Sonora to a considerable
degree at the taxonomic level of Family.