Introduction xiii 



of varying depth, while there are several ranges, such as 

 the Elk and the Gunnison mountains, rising above the 

 plateaus. 



Colorado has the highest average elevation of all the 

 States of the Union — 6,800 feet — while there are a 

 number of peaks of between 14,000 and 14,500 feet. 



Colorado lies within the arid region ; the rainfall east 

 of the mountains seldom amounts to 20 inches, and is 

 often less ; in the mountains it often exceeds 30. The 

 bulk of the rainfall is in the summer months, and the 

 winter is generally dry. 



Analysis of the Bikd Fauna. 



The number of Colorado birds included in the present 

 work is 392. Of these 225 may be considered regular 

 breeders within the State. I have divided these into 

 three catagories, viz. : (a) Those resident throughout 

 the year, 67 in number ; (b) The strict migrants which, 

 so far as we know, never winter within the limits of the 

 State, 118 in number ; (c) Those birds which breed 

 within the State and in winter are undoubtedly less 

 numerous, so that they may be considered partly migrant 

 and partly resident, 40 in number. 



The non-breeding birds number 167. Of these the 

 largest category by far are the Casual or rare species, 

 which have only been recorded on very few occasions ; 

 these number 106. The other two groups, which are 

 not always easy to separate satisfactorily, are the winter 

 residents, numbering 28, and the birds which pass through 

 the State on the Spring and Fall migration ; these are 

 33 in number. 



The following lists show the birds in these various 

 categories. 



(a) Resident through the year — 67, or about 17 per 

 cent. : — 



