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 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.
Migrants of the eastern flyway as well as migrants associated with
western North America pass through this section of Coahuila. The
following breeding birds are associated with this province: Goshawk,
Band-tailed Pigeon, Thick-billed Parrot, Golden-fronted Woodpecker,
Ladder-backed Woodpecker (_D. s. giraudi_), Pine Flycatcher,
Buff-breasted Flycatcher, Vermilion Flycatcher (_P. r. mexicanus_),
Steller's Jay, Scrub Jay, Mexican Chickadee, Black-crested Titmouse
(_P. a. atricristatus_), Cactus Wren (_C. b. guttatus_), Robin,
Blue-gray Gnatcatcher (_P. c. amoenissima_), Hutton's Vireo (_V. h.
stephensi_), Bell's Vireo (_V. b. medius_), Hartlaub's Warbler, Summer
Tanager (_P. r. cooperi_), Pine Siskin, Rufous-capped Atlaptes, and
Black-throated Sparrow (_A. b. grisea_).
_The Chihuahua-Zacatecas Biotic Province._--This province in Coahuila
covers the arid, interior, western desert area; it consists of rolling
plains with mountains that rise islandlike above the general surface.
Some of the mountains, such as in the Sierra del Carmen and the Sierra
del Pino, are more than 9000 feet high. The major part of this biotic
area lies within the Lower Sonoran Life-zone. Areas of the Transition
and Canadian life-zones are present on some of the higher mountains;
their discontinuity results in a discontinuous distribution of the
conifer-dependent avifauna.
The large desert restricts the movement of birds considerably. Major
results of this include isolation of certain populations and absence of
others in the boreal islands. For example, Miller (1955a:157) noted
that the "dispersal of conifer-belt birds to and from the Sierra del
Carmen, although not as difficult as to well separated islands [such as
off the coast of Baja California], is nevertheless a formidable matter
to accomplish across the great deserts of Texas, Chihuahua, and
Coahuila." Miller (_loc. cit._) noted also that the avifauna of the
Sierra del Carmen, due to its insularity, is unbalanced and stated that
"as a consequence of unbalance, species that are present show ecologic
extension and unusual numerical relations." At least in this type of
environment, an extension or expansion of the ecologic habits of the
related types takes place when some species are absent.
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).
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.