Introduction xill 
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 Birp 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. :— 
