USE OF THE MAP 



The four Life Zones indicated on the accompanying 

 map are those mentioned in the data given under the 

 headings Geographical Distribution and Breeding Eauge. 

 They represent climatic conditions of temperature in the 

 regions indicated. The "Boreal" extends from the tree- 

 less, snowclad summits, far above the timber-line down 

 through the coniferous forests. Next in coldness is the 

 "Transition," which begins at the Yellowpiues, overlap- 

 ping the Boreal a little, and containing some species of 

 oaks, buckbrush, manzanita, and some sagebrush. Lying 

 between the Transition and the almost tropical heat of 

 the "Lower Sonoran " is the "Upper Sonoran," where 

 we find the juniper, oaks, pinon pines, and sagebrush. 

 Last of aO, the "Lower Sonoran" is the warmest. In it 

 lie the hot valleys and desert regions of California, and 

 here flourish the live oaks and mesquites. Many Cali- 

 fornia birds migrate from one to another of these zones 

 during the breeding season, as the birds of the Eastern 

 United States migrate north and south. This changing 

 from lower to higher altitude, or the reverse, is termed 

 vertical migration. 



