90 COLOR AND CLIMATE 



II. FUNCTIONAL 



Pursuit I ^ y m a-le 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 I Vocal, by male and rarely female. 



{ Mechanical, by male and sometimes female. 



Special 



Dances, mock fights, aerial evolutions, construction of 

 bowers, decoration of play-grounds, attitudinizing, strut- 

 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 8ong Sparrow 

 (Diagram A. 23; Plate VII) attains a length of nearly nine inches. 



