298 Distribution of Land Birds. [zoE 



Passing now to the Sonoran Area, we find ourselves in the most 

 Strongly marked and highly differentiated region of the State. It 

 contains not only a large number of species which are strictly lim- 

 ited to it, but even a few genera, such as Columbigallina^ Micropal- 

 las and Pyrocephahis, It is here that the orioles and thrashers 

 attain their maximum abundance, and develop certain species not 

 found elsewhere in the State. 



The only region to which the Sonoran Area is closely allied is 

 the Great Basin Area, which it resembles in many important details. 

 It is a desert region with a very slight rain fall, while certain 

 sections are utterly destitute of moisture. Consequently such species 

 as inhabit it have become acclimated to a very unusual environ- 

 ment and are kept within tolerably distinct limits. As would 

 naturally be expected, most of the birds inhabiting this region are 

 pale and dull in color, due partly to the bleaching effect of the sun, 

 but more largely to natural selection, by which protective colors are 

 assumed; but we find also a {^vi conspicuous departures in birds of 

 tropical brilliancy in such species as Pyrocephalus rubinens mexi- 

 caims and Icterus cucidlatus nelsoni. The former is a resident of 

 Central and South America, and serves to illustrate the close rela- 

 tionship existing between the Sonoran and Tropical Regions. 



The distribution of Gambel's partridge ( Callipepla ga7nbeli) may 

 be taken as illustrating the limits of the Sonora Area in California; 

 and in order to show that this Area does not reach the coast in Cali- 

 fornia it may be well to consider the range of this species somewhat 

 in detail. In Cassin's work on the birds of the Western States (p. 

 47) the following is quoted from Col. McCall: "First, then, with 

 respect to its western limit: This species was discovered by Dr. 

 Gambel ' on the eastern side of the Californian range of mountains 

 in 1 841.' He did not meet with it on the western side; nor has 

 it, as far as made known, been found there since that time by 

 others/' Mr. Belding, in Land Birds of the Pacific District (p. 

 14), cites several instances of its occurrence in California. Agua 

 Caliente, San Diego Co. (F. Stephens); Mojave River and Fort 

 Yuma (Heermann) Cooper, 1885. '* Their range toward the north 

 Is not known to be above 36°. At Fort Mojave they are numerous,' ' 

 etc. On the whole, wx may say that this species has its center of 

 abundance in Calfornia along the Colorado River, whence it dimin- 

 ishes in numbers toward the north and west, reaching its northern limit 



