92 COLOR AND CONCEALMENT 



If we compared only the palest Song Sparrow with the darkest, we 

 might well believe, so unlike are they, that each form represents a 

 distinct species; but when we include in our comparison representatives 

 of all the twenty-three races of Song Sparrows we find complete inter- 

 gradation in color and in size. Nowhere can one draw the line. As 

 the climatic conditions under which the birds live change, the birds 

 keep pace. Cause and effect go hand in hand. Here we have a species 

 in flower, as it were, a single Song Sparrow stalk with its twenty-three 

 blossoms, any one of which might make an independent growth as a 

 species it if were separated from the parent stem. Doubtless some day 

 the separation will come, when we shall have several species of Song 

 Sparrow, each with its group of races, but at present we have only 

 one species, divided into some twenty-three sub-species, or species in 

 process of formation. 



A variety of reasons may be advanced to account for the pronounced 

 geographical variations shown by the Song Sparrow. Its wide range 

 indicates physical adaptability and ready adjustment to differences in 

 food and habitat. Its variations in size, while they conform to the 

 general law of increase in size northward, are exceptionally marked, 

 and are not equalled by those of any other North American bird a 

 further indication of an inherent plasticity. 



The species is comparatively non-migratory. Several races, notably 

 in California, are permanently resident in contiguous and restricted 

 areas. Such strictly non-migratory species are continuously subjected 

 to the influences of their environment, which are heightened by perma- 

 nent isolation. But even the most migratory forms come early and stay 

 late, and are thus in the breeding area for a much greater part of the 

 year than, for example, many Warblers which come in May and leave 

 in August. 



But, suggest as we may the various factors which appear to be active 

 in producing such geographic variations as the Song Sparrows exhibit, 

 they are not potent with all birds, even when other things are equal, 

 and it seems probable that some species are in an active state of de- 

 velopment and readily respond to the influences of their surroundings, 

 while others are fixed and make no such response. The latter represent 

 older types of birds, which are, so to speak, near to or a part of the 

 trunk of the bird's family tree, while the former class includes the birds 

 at the terminal branches of this tree. 



Color and Concealment. It is clear that the life of the individual 

 bird, and hence the continued existence of its species, depends pri- 

 marily on its success in securing food and in escaping from its foes. The 

 colors of predaceous species must, therefore, tend to conceal them from 

 their prey (aggressive or deceptive colors), while the colors of those 

 that are preyed on are designed to- conceal them from their enemies 

 (protective colors) . 



When, however, a bird's food may be secured without the exercise 

 of caution, when it apparently is not preyed upon, or when it is protected 



