USE OF THE MAP 



The four Life Zones iudicated on the accompanying 

 map are those mentioned in the data given under the 

 headings Geographical Distribution and Breeding Range. 

 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 Yellowpines, 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, piUon pines, and sagebrush. 

 Last of all, 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 

 between breeding seasons, as the birds of the Eastern 

 United States migrate north and south. This changing 

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

 vertical migration. 



