PHYSICAL FEATURES OF BIRDS. 271 



other as the bird beats the air with its wings, or keeping 

 taut and tense as the bird soars, apparently a motionless 

 speck in the blue above our heads. The number of pri- 

 maries is usually constant, being typically ten ; but in the 

 highest birds the first primary suffers various degrees of 

 reduction, ranging through short and spurious to obsolete, 

 in which last case it has disappeared, and but nine 

 primaries remain. 



Like the hand, the wing of a bird is capable of express- 

 ing emotion. The challenge to battle, the attitude of 

 defense, the tenderness of the nest-mate, the helpless need 

 of the tiny nestling, all are expressed by various move- 

 ments of the wings. As to the feathers covering it, the 

 wing is in many birds one of the most highly varied areas 

 of the plumage, vying with the head and the tail in the 

 brilliancy of its coloring. 



The wing may also be used as a musical instrument; 

 though in this case it is instrumental music, of course. 

 The woodcock whistles chiefly through the use of its 

 curiously marrowed outer three primaries. 



Coloration 



The plumage changes with the season, with the 

 nesting period, with the age of the birds, with the climate, 

 and with a change of food. It is a well-understood fact 

 that birds are lighter in plumage in arid regions and 

 darker in humid regions. For instance, one of our 

 sparrows, the common song sparrow, having a range from 

 Arizona to Alaska, is a pale, sandy-colored bird in Arizona 

 and dark sooty-brown in Alaska. Between these ex- 

 tremes are to be found nine intergrading species, accord- 

 ing to Coues. Some animals are more affected by climate 

 than are others, and this is a rather extreme example. 



