xxxvi INTRODUCTION 



Birds are naturally less restricted to zones than mammals and 

 plants, and in the field the question of correlating them to zones is 

 rendered difficult by the modifying conditions which complicate the 

 zones themselves. Local conditions are constantly being met which 

 produce a change of temperature within a zone, resulting in the 

 intrusion of a tongue of a higher or lower zone. Forest fires 

 make an artificial change in zones, a Canadian fir forest sometimes 

 being replaced by Transition zone chaparral. Natural modifying 

 conditions are many, and not always so patent. Slope exposure is 

 the most important. If a ridge runs north and south, its southwest- 

 ern slope, which receives the hot afternoon sun, will have, we may 

 say, a Transition zone flora and fauna, while its cold northeast slope 

 will have a Canadian zone flora and fauna. A cold mountain 

 stream, on the other hand, will bring down the flora and fauna of 

 one or two higher zones ; and Canadian and even Hudsonian plants 

 and trees bordering such a stream may thrive on its banks in the 

 Transition zone. 



So many other modifying conditions are found that the determi- 

 nation of zones is a complex matter, and must be based largely on 

 the study of trees and shrubs, as they are the most stable part of the 

 life of a region. In relating the flora to the fauna the greatest care 

 should be taken with the bird life, as a bird can at will change his 

 zone by a few hours' travel. Zonal notes should always be accom- 

 panied by dates, as breeding zones alone are of much significance, 

 birds wandering widely after the breeding season. The Lewis 

 woodpecker is a striking example of this, for, while breeding in 

 Transition zone, after the breeding season it wanders up into Cana- 

 dian and down even into Lower Souoran zone in its search for 

 mast. Most mountain birds that do not migrate to the south 

 change their zones in this way, Canadian zone birds being found in 

 Upper and Lower Sonoran zones in fall and winter. 



MIGRATION. 



Many birds wander widely east and west after the breeding sea- 

 son, and some even go north for a short distance. With many 

 mountain birds the wandering movements after the breeding season 

 amount to a vertical migration. Birds, like the grouse and quail 

 and certain species of juucos, that make only a vertical migration 

 merely come down from the snow-covered mountains into the warm 

 valleys. A number of hummingbirds perform vertical migrations 



