How Birds ark Named, 



from a recognized type. But in comparatively recent years, as speci- 

 mens have been gathered from throughout the county inhabited by a 

 species, comparison frequently shows that specimens from one part of 

 its range differ from those taken in another part of its range. At in- 

 tervening localities, however, intermediate specimens will be found 

 connecting the extremes. 



Generally, these geographical variations, as they are called, are the 

 result of climatic conditions. For instance, in regions of heavy rain- 

 fall a bird's colors are usually much darker than they are where the 

 rainfall is light. Song Sparrows, for example, are palest in the desert 

 region of Arizona, where the annual rainfall may not reach eight inches, 

 and darkest on the coast of British Columbia and Alaska, where the 

 annual rainfall may be over one hundred inches. In going from one 

 region, however, to the other the gradual changes in climate are ac- 

 companied by gradual changes in the colors of the Song Sparrows, and 

 the wide differences between Arizona and Alaska Song Sparrows are 

 therefore bridged by a series of intermediates. 



Variations of this kind are spoken of as geographic, racial, or sub- 

 specific and the birds exhibiting them are termed subspecies. In nam- 

 ing them a third name, or trinomial, is employed, and the possession of 

 such a name indicates at once that a bird is a geographic or racial rep- 

 resentative of a species, with one or more representatives of which it 

 intergrades. 



Returning now to the Robin. Our eastern Robins always have the 

 outer pair of tail-feathers tipped with white and, in adults, the back is 

 blotched with black; while Robins from the Rocky Mountains and west- 

 ward have little or no white on the outer tail-feathers, and the back is 

 dark gray, without black blotches. These extremes are connected by 

 intermediate specimens sharing the characters of both eastern 

 and western birds. We do not, therefore, treat the latter as a species, 

 but as a subspecies, and consequently, apply to it a subspecific name 

 or trinomial, Merula mlgratoria propinqua, {propinquity meaning nearly 

 related). 



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