90 COLOR AND CLIMATE 
II. FuncrionaL 
Pursuit By male when similar to or brighter than female. 
‘** ) By female when brighter than male. 
Display ...... By male of accessory plumes and other appendages. 
Battle ...... By male using spurs, wings, bill, etc. 
Music { Voeal, by male and rarely female. 
‘'*** | Mechanical, by male and sometimes female. 
Dances, mock fights, aérial evolutions, construction of 
bowers, decoration of play-grounds, attitudinizing, strut- 
Special ting, etc. 
habits ... 
a. By male before the female. 
b. Among the males alone. 
Color and Climate—The immediate effect of climate on a bird’s 
plumage is to increase or decrease its general tone of color; thus, those 
representatives of a species living in arid regions are paler than repre- 
sentatives of the same species living in humid regions. The degree of 
difference is closely related to the annual rainfall, as it indicates rela- 
tive humidity; and where there are no abrupt changes in climate, these 
climatic variations change as gradually as the conditions which cause 
them. At first so slight that only the expert systematic ornithologist, 
with access to large series of specimens, can detect them, they become, 
in some instances, so pronounced that not only the general tone of 
color but pattern itself is affected. It is on such variations that most 
subspecies or geographical races of birds are based. (See Allen, ’77.) 
Among North American birds, they are best illustrated by the oft- 
cited case of the Song Sparrow. (See Diagram A and Plate VII.) Twenty- 
three races of this exceedingly plastic species are recognized. They are 
distributed from the Valley of Mexico northward throughout the 
United States and a large part of Canada to the Aleutian Islands. 
Note, however, that only two of them are found east of the Rocky 
Mountains, where climatic conditions are comparatively uniform; 
while California alone has ten resident races, an indication of its great 
diversity of climate. 
The Desert Song Sparrow (Diagram A. 8; Plate VII) the palest 
race, inhabits the Colorado desert region where the average rainfall is 
about six inches; the Sooty Song Sparrow (Diagram A. 19; Plate VII) 
the darkest race, is found on the northwest Pacific Coast where the 
annual rainfall averages over ninety inches. 
Again, observe that the Mexican Song Sparrow (Diagram A. 1) at 
the southern extremity of the range of the species is one of the smallest 
races, measuring some six inches in length, and that there is a gradual 
increase in size northward until the maximum is reached at the northern 
extremity of the range of the species, where the Aleutian Song Sparrow 
(Diagram A. 23; Plate VII) attains a length of nearly nine inches, 
