EXPLANATION OF DISTRIBUTION MAPS. 
The accompanying maps are intended in the present connection only as aids 
in defining the ranges of our birds. Yet a few remarks as to the determination 
of the regions indicated may not be out of place. 
It has been found that the most important factor governing the distribution 
of plants and animals is temperature; and that the next most important factor, on 
land, is humidity. Regions of uniform conditions in these reSpects each possess 
characteristic assemblages of plants and animals, and many of these are so sensi- 
tive that they are not found beyond the realm of such conditions. Plants, espec- 
ially trees and shrubs, are most satisfactory indices of life areas because the indi- 
viduals are fixed throughout their lives in one place. But some birds even though 
capable of extended locomotion seem to be quite as sensitive, while many other 
birds of more hardy constitutions are also restricted to narrow limits because they 
are dependent on certain endemic plants for food. 
life Zones are belts of uniform temperature (of the summer season, or season 
of growth in plants and reproduction in animals). faunal areas are regions of 
uniform humidity of the atmosphere (also of the summer season). Lines separat- 
ing two Zones are therefore theoretically isotherms, or lines connecting points of 
equal temperature; while lines separating Faunal Areas are isohumes, or lines con- 
necting points of equal humidity. As the direction and extent of Zones are 
obviously dependent in the main on latitude and altitude, and the extent of 
Faunal Areas depend on adjacency of bodies of water, the coast-line running 
nearly north and south, it follows that Faunal Areas are subdivisions of Life 
Zones. It must be kept in mind that there is seldom an abrupt demarcation be- 
tween adjacent Zones on Faunal Areas. The lines approximately indicate the 
middle of the region of mergence between two such areas. 
The areas differentially colored on the maps have been outlined as accurately 
as the information at hand permitted; still the boundary lines must be considered 
mainly provisional until the State is carefully surveyed zoogeographically. 
The Life Zones of California have been distinguished as follows: 
alpine-arctic 
Boreal -< Hudsonian 
(Canadian 
{ Transition 
AUSTRAL { lower Sonoran 
On the maps for reasons of convenience all of Boreal (including Alpine-Arc- 
tic, Hudsonian and Canadian) is colored green; Transition is colored blue; Upper 
Sonoran, yellow; and Lower Sonoran, red. 
The Faunal and Sub-faunal Areas here recognized may be classified as 
follows: 
