BIRDS OF BOULDER COUNTY, COLORADO I 79 



The nomenclature used is that of the third edition of the American 

 Ornithological Union Check-List and the 16th "Supplement" published 

 in The Auk for 1912. 



Geography of Boulder County 



Boulder County extends approximately twenty-four miles north 

 and south and thirty-two miles east and west. The Continental 

 Divide forms the western boundary and rises to elevations of from 

 12,000 feet to over 14,000 feet. The eastern third of the county is a 

 part of the plains region of Colorado and has an elevation of from 

 5,000 to 6,000 feet. Differences in elevation between the eastern and 

 western boundaries of the county are accompanied by a great variety 

 of climates. Hence, for purposes of bird study a division of the 

 county into zones is necessary. The plains and the region above 

 timberline form fairly distinct zones. The intervening mountainous 

 country is forested, the character of the tree growth changing with 

 elevation. Three forest types are usually distinguished, the Western 

 Yellow Pine or Rock Pine, 1 the Lodgepole Pine, 2 and the Engelmann 

 Spruce. 3 The data available for a classification of bird-life according 

 to altitude are not, however, sufficiently definite to make advisable 

 the use of more than two zones in the forested region. The open and 

 dry Yellow Pine region of the foothills and lower mountains has been 

 taken as one zone. The dense and somewhat more moist forests 

 of Lodgepole Pine, and those of Engelmann Spruce and Alpine Fir, 4 

 have been grouped as another unit and called the Mountain Zone. 



The Plains Zone lies approximately between 5,000 and 6,000 feet 

 in elevation. It was formerly a semi-arid, treeless region with the 

 exception of the cottonwood and willow bordered creeks. Much of it 

 has now been transformed by irrigation into cultivated fields, though 

 tracts of considerable size remain in their original dry condition. 

 There are many ponds; a few are natural, though most of them are 

 the result of irrigation. Some cat-tail marshes occur, although none 

 of large extent. Shade trees in the towns and about the farms furnish 



1 Pinus scopulorum. j Picea engelmanni. 



1 Pinus murrayana. * Abies lasiocarpa. 



