There are no endemic species of birds in the Great Basin. Nevertheless, a distinctive Great Basin avifauna exists which contains components of the Mojave Desert, Rocky Mountain, and Great Plains avifaunas as well as species obligate to sagebrush and the pinyon-juniper forest. Seemingly there has been little spread of California and Sierra Nevada species eastward, but a westward extension from the Rocky Mountains of several species is indicated. While several Rocky Mountain species reach their western limits on the eastern edge of the Great Basin, others have extended into the eastern portion. Two Great Plains representatives are late arrivals, namely the Baltimore Oriole and Indigo Bunting, with evidence of introgression now occurring with related western species. A similar but longstanding situation exits for the flickers. A zone of hybridization occurs in northern Utah between two species of junco. A rather abrupt junction zone between the Great Basin and Mojave Desert avifaunas exists in southern Nevada and extreme southwestern Utah. Several species representing the Mohave Desert avifauna have extended their ranges in recent years into southern Utah. Geographically variable birds show diverse patterns of distribution along with much clinal variation and intergradation. A center of differentiation for four species occurs in western Utah in the eastern portion of the Great Basin while two more occur in the western portion of the Basin. The Wasatch Front is a dividing area between western and eastern races in several species. Extreme southwestern Utah constitutes a transition area where several species are represented by different races or intergradational populations. A study of the avifaunas of 14 boreal "islands" in isolated mountain ranges in western and southeastern Utah in comparison with the Rocky Mountain "continent" in central and northern Utah shows a close correlation between number of species present and habitat diversity. In addition, a low correlation exists between the number of species that are permanent residents on isolated mountains and the distance of those mountains from the "continent."
Behle, William H.
"Avian biogeography of the Great Basin and Intermountain Region,"
Great Basin Naturalist Memoirs: Vol. 2
, Article 5.
Available at: http://scholarsarchive.byu.edu/gbnm/vol2/iss1/5