Variation in North American Ravens

Abstract

DURING several years past the writer has given considerable time to the study of the Raven (Corvus corax) as it occurs in North America. This work was undertaken in an effort to ascertain the number of geographical races and the limits of their ranges. Summation of the facts brought to light has been delayed in the hope that additional material from pertinent localities might be secured, but, as it seems unlikely that this hope will be realized in the very near future, it may be advisable to publish a short account of what has been learned, so that it may be available to other students in this field. There are at least three factors that make the recognition of different races in C. corax difficult. First, because of sameness in color, only variability in size and proportion is useful in separating races. Second, individual variation in size in birds of the same region is so great that even a fairly large series of specimens obtained at random may be misleading. Third, we have little knowledge regarding migratory movements in the species, so that it is unsafe to assume that birds taken outside the breeding season are members of the breeding population where they are secured. It has apparently been the custom of some authors to combine measurements secured from specimens of both adult and immature ravens, a method that is sure to result in erroneous conclusions. A bird may have assumed most of its adult plumage and still not have attained full size. All the birds used in this study are believed to be adult, having been separated from immatures by the shape of the tail-feathers. This selection resulted in decreasing greatly the number of specimens available for use, as it was found that most collections contain a large proportion of immature birds. A total of 120 specimens of adult birds has been used, admittedly far too few for satisfactory solution of some of the more complex problems involved. Gratitude is expressed to the following institutions and collectors for use of their specimens: Colorado Museum of Natural History, Donald R. Dickey Collection, Hancock Foundation, Los Angeles Museum, Museum of Vertebrate Zoology, Museum of Zoology (Ann Arbor), San Diego Natural History Museum, Louis B. Bishop, Ed. N. Harrison, Stanley G. Jewett, Robert T. Moore, J. R. Pemberton, Kenneth E. Stager and John G. Tyler.