THE BIRDS OF COLORADO. 
BY W. W. COOKE. 
The following paper is designed to set forth our present 
linowledge of the distribution and migration of Colorado birds. 
There is also included a bibliography of the subject and an 
historical review of the progress of ornithological investigation 
in this State. 
The total number of species and varieties of birds known 
to occur in Colorado is 360, of which are known to breed. 
This is a larger number of species than has been taken in any 
state east of the Mississippi and is exceeded by only one state 
of the Union, that is by Nebraska with nearly four hundred 
species. 
The reason for this great variety of bird life is found in 
the geographical position of the State and the physical charac- 
teristics of its surface. 
From the Atlantic Ocean to western Iowa but slight 
changes occur in the avi-fauna. But with the decreased rain- 
fall and the increase in altitude from there westward, a great 
number of new forms appear. The greatest change is at the 
eastern foothills of the Rocky Mountains, which is the natural 
dividing line between the eastern and middle provinces of the 
United States. But while many of the western forms extend 
as stragglers eastward into Kansas and Nebraska and especially 
into the Black Hills of South Dakota and northwestern Ne- 
braska, a large number of eastern forms do not pass west of the 
semi-arid region of twenty inches of annual rainfall and are 
not found in Colorado. It is due to this fact that Nebraska ex- 
ceeds Colorado in the number of species taken in the State. 
All of the eastern species reach Nebraska and nearly all the 
western forms extend into -northwestern Nebraska. This is 
strikingly shown in the case of the Warblers. Nebraska has 
more than twenty Warblers that do not occur iu Colorado, while 
Colorado has less than five that are not found in Nebraska. 
The avi-fauna of Colorado is reinforced by species that 
belong more properly to the regions on all sides of it. The 
basis may be considered as the species that range over the 
whole of the United States and those that are most abundant 
