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 Sonoran 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 juncos, 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 
