How Birps ARE 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 migratoria propingua, (propingua, meaning sue 
related). 
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