PKEFATORY NOTE IX 



compact, being well marked by a large proportion of liighly 

 characteristic, if not wholly peculiar, species. The resulting 

 aspect of the bird-fauna is far more strongly pronounced than 

 is ordinarily found to be the case with areas of corresponding- 

 dimensions. As might be expected from aridity of such extent 

 and to such degree as is witnessed in the Colorado Valley, the 

 prime mark of the birds of the region is that iDallor of colora- 

 tion which is now well known to result from the combined effects 

 of heat and cfryness. It is the extreme of a condition very sen- 

 sibly offered by the birds of the Great Plains at large. In some 

 cases we here find that the modification of a common stock has 

 produced forms sufficiently distinct from their respective allies 

 to meet the requirements of "species"; while in many more 

 instances strongly marked geographical races are developed by 

 the same natural causes, operating less intensely, less continu- 

 ously, or upon less susceptible material. It is unnecessary in 

 this i)lace to cite examples, as such cases are already well known 

 to ornithologists. It may be added, as a curious fact in the 

 matter of the modifications here witnessed, that the tail is length- 

 ened in many cases of birds which otherwise differ fr^om their 

 respective allies mainly by the bleached coloration just noticed. 



A few words upon the progress of our knowledge of the birds 

 of the region under consideration wiU not be out of place here. 

 It is only within the last twenty-five years that we have acquired 

 any considerable information respecting the ornithology of the 

 Colorado Basin. Shortly after NuttaU and Townsend largely 

 increased our knowledge of Western birds from localities much 

 further north, Dr. William Gambel gave us welcome advices 

 in various pai^ers published by the Philadelphia Academy from 

 1843 to 1849 ; and this naturalist may be considered as a pioneer 

 in this field. He was succeeded by Dr. S. W. Woodhouse, who 

 accompanied an expedition to the Zufii and Colorado Eivers, 

 and prepared a valuable paper published in 1853 in Sitgreaves's 

 Report. Mr. Cassin's well-known "Illustrations", completed 

 in 1856, contain colored figures of many interesting species, 

 and include the timely field-notes of Col. G. A. McCall, Dr. A. 

 L. Heermann, and other naturalists who had made personal 

 observations iu the field. A stride forward was taken when the 

 ReiJorts of the Pacific Railroad and Mexican Boundary appeared ; 

 the technicalities of the subject being admirably worked out by 

 Professor Baird in these volumes, while the same publications 

 include the field-notes of the naturalists attached to the several 



