228 Distribution of Land Birds. [zoE 



the San Joaquin and Tulare Lake region is comparatively destitute 

 of trees. Still, the oak remains always the characteristic form 

 wherever any trees grow. The country bounded on the east by the 

 Colorado River, on the west by the Sierra Nevada Mountains (and 

 farther south by the Coast Range), and extending a little north of 

 latitude thirty-six degrees, is an arid region where the cactus forms 

 the prevailing vegetation. This desert region constitutes the Son^ 

 oran Area. The only part of the State not yet considered is the 

 southern coast district, or what I have called the Californlan Tran- 

 sition Region. This country Is mountainous, but the mountains are 

 for the most part not high. The vegetation is of three distinct kinds 



sagebrush or Artemisiay oaks and conifers. There is no part of 

 the State in which this Artemisia is wholly absent, but it is in this 

 region that it attains its maximum abundance. 



Having now glanced at the main topographical and floral features 

 of California, let us briefly consider the theories governing the dis- 

 tribution of birds, and see if the faunal areas, as enumerated above, 

 are in accordance with our theories. In the first place, it must be 

 distinctly borne in mind that in speaking of the geographical dis- 

 tribution of birds we refer only to their breeding range. Birds have 

 such wonderful powers of locomotion that it is impossible, in most 

 cases, to draw any sharp lines limiting their distribution even then, 

 but regions may be indicated in which many forms have their center 

 of abundance during the breeding season, and outside of which they 

 are more or less rare. Such regions are called faunal areas. But 

 what are the causes which make a bird abundant in one region 

 and rare or wanting in an adjoining district? Dr. Merriam says:^ 

 "Omitting reference to the effects of physical barriers, which explain 

 the differences in the life of disconnected continents, it may be 

 stated that temperature and humidity are the most important causes 

 governing distribution, and that temperature is more potent than 

 humidity." As applied to life in general this is undoubtedly the 

 true theory as is also his statement **that the temperature during the 

 season of reproductive activity determines the distribution of life,'* 



F 



but when we consider the case of birds, we find that the environ- 

 ment is perhaps the most powerful factor in determining and defin- 

 ing life areas. To be sure, it is due almost entirely to temperature 



•Fauna, No. 3, pp. 26-27, 



