Hot Birds abk Naubd 



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

 result of climatic conditions. For instance, in regions of heavy rainfall 

 a bird's colors are usually much darker than they are where tlie 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 anj 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 accompanied 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 naming 

 them a third name, or trinomial is employed, arid the possession of such a 

 name indicates at once that a bird is a geographic or racial representative 

 of a species, with one or more representatives of which it intergrades. 



Returning now to the Kobin. Our eastern Robins always have the outer 

 pair of tail-feathers tipped with white and, in adults, the back la blotched 

 with black; while Robins from the Rocky Mountains and westward have 

 little or no white on the outer tail-feathers, and the back is dark gray, with-' 

 out 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 specieSj but as a subspecies, and consequently, 

 apply to it a subspecific name or trinomial, plarifsticus migratoHus propin- 

 quus, {propinquus, meaning nearly related.) 



A further study of our eastern Robin shows that in the southern parts 

 of its breeding range (the Caroliiias and Georgia), it varies from the 

 northern type in being smaller in si^e and much paler and duller in color; 

 and to this second geographical variety is applied the name Planesticut 

 migratoriua ackmsterus, (ackrusterut, meaning less highly colored). 



After the recognition of western and southern races of the Robin under 

 three names (trinoinial) it would obviously be inconsistent to apply only two 



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