THE BIRDS OF COLORADO. 



BY W. W. COOKE. 



The following paper is designed to set forth our present 

 knowledge 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 328 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 in Colorado, while 

 Colorado has less than five that are not found in Nebraska. 



The avi-fauna of Colorado is reinforcgd 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 



