Variations of this kind are spoken of as geographic, racial, or subspecific and the birds exhibiting them are termed subspecies. In naming 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 representative 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 westward 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, Planesticus migratorius propinquus, (propinquus, meaning nearly related.)
A further study of our eastern Robin shows that in the southern parts of its breeding range (the Carolinas and Georgia), it varies from the northern type in being smaller in size and much paler and duller in color; and to this second geographical variety is applied the name Planesticus migratorius achrusterus, (achrusterus, meaning less highly colored).
After the recognition of western and southern races of the Robin under three names (trinomial) it would obviously be inconsistent to apply only two names (binomial) to our eastern bird, the former being no more subspecies of the latter than the latter is of the former. In other words, to continue to apply only generic and specific names to the Eastern Robin would imply that it was a full species, while the use of a trinomial for the Western or the Southern Robin shows them to be subspecies. As a matter of fact we know that there is but one species of true Robin in the United States, consequently in accordance with the logical and now generally accepted method, we apply to that species the name Planesticus migratorius, and this is equally applicable to Robins from east, south or west. When, however, we learn that the Eastern Robin is not a species but a subspecies, we repeat the specific name by which it was made known and call it Planesticus migratorius migratorius.
It may be asked, Why give names to these geographical races? Why not call Eastern, Western and Southern Robins by one name, Planesticus migratorius, without regard to their climatic variations?
In reply, two excellent reasons may be given for the recognition of subspecies by name; first, because in some cases they differ from one another far more than do many species, when it would clearly be inadvisable to apply the same name to what are obviously different creatures. For example, it has lately been discovered by Mr. E. W. Nelson that the small, black-throated, brown-breasted, Quails or Bob-whites of southern Mexico, through a long series of intermediates inhabiting the intervening region, intergrade with the large, white-throated, black-and-white breasted, Bob-white of our northern states. It would be absurd to call such wholly unlike birds by the same name, nor could we give a full specific name to the Mexican Bob-white since at no place can we draw a line definitely separating it from the northern Bob-white. Furthermore, the use of only two names would conceal the remarkable fact of the intergradation of two such strikingly different birds; a fact of the first importance to students of the evolution of species.
For much the same reason we should name those birds which show less pronounced variations, such as are exhibited by the Robin. Here we have a species in the making, and in tracing the relation between cause and effect, we learn something of the influences which create species. Thus, climate has been definitely proven so to alter a species, both in size and color that, as we have seen in the case of the Song Sparrows, marked climate changes are accompanied by correspondingly marked changes in the appearance of certain animals. In naming these animals we are, in effect, giving a 'handle to the fact' of their evolution by environment.
Since it is evident that a bird may vary much or little, according to the governing conditions and its tendency to respond to them, no fixed rule can be laid down which shall decide just what degree of difference are deserving a name. It follows, therefore, that in some cases ornithologists do not agree upon a bird's claim to subspecific rank.
In North America, however, questions of this kind are referred to a committee of seven experts of the American Ornithologists' Union, and their decision establishes a nomenclature, which is accepted as the standard by other American ornithologists and which has been adopted in this volume.
Foreign birds of wholly accidental occurrence, most of which have been found in North America but once or twice, are included in the systematic list of North American birds, but are not described or figured in the body of the book, where their presence would tend to convey an erroneous impression of their North American status. Furthermore, records of the presence of birds so rare as these can be properly based on only the capture of specimens.