SparrowB WESTERN BIRDS 



"Observe, also, that the Mexican Song Sparrow at the 

 southern extremity of the range of the species is the 

 smallest of the race, 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 Spar- 

 row attains a length of nearly nine inches. 



"If we compare only the palest Song Sparrow with only 

 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 

 intergradation 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 as inde- 

 pendent growth as a species if it were separated from the 

 parent stem. Doubtless some day the separation will 

 come, when we shall have several species of Song Spar- 

 rows, each with its group of races, but at present we 

 have only one species, divided into some twenty-three 

 subspecies, 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 adap- 

 tability and ready adjustment to differences to food and 

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

 general law of increase in size northward, are exception- 

 ally marked, and not equaled by those of any other 

 North American bird — a further indication of an in- 

 herent plasticity. 



"The species is comparatively non-migratory. Several 

 races, notably in California, are permanently resident, 

 and a number of contiguous and restricted areas may 



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