lyway 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 fr