BIRDS OF BOULDER COUNTY, COLORADO l8l 



considerable cover for a number of species of birds that formerly 

 must have been limited to the creek bottoms. Boulder County con- 

 tains no scrub-oak or pinyon pine, which form such characteristic 

 belts along the foothills farther south, and in the western part of the 

 state. The region corresponds to the Upper Sonoran 1 life-zone of the 

 United States Biological Survey. 



The Yellow Pine Zone, or "Foothill Zone" of botanists, 2 extends 

 from the plains to the elevation at which the open parklike growth 

 of the western yellow pine gives place to the closely growing trees of 

 lodgepole pine. Many of the more moist north slopes of this area are 

 covered with a fairly dense growth of Douglas Fir. 3 The narrow 

 creek bottoms in the canyons, which cut the zone into many ridges, 

 support a scattering growth of deciduous trees and shrubs, chiefly 

 willows, cottonwoods, aspen and box-elder with thickets of alder, 

 thorn-apple, wild plum, choke-cherry and mountain maple. The 

 smaller runs which drain the ridges into the principal creeks and to- 

 ward the plains, though dry much of the year, also support a thicket- 

 like growth of shrubs. They furnish an important cover for birds 

 in both winter and summer, as well as a considerable supply of small 

 fruits — -chiefly choke-cherry and thorn-apple — for the fall migrants. 

 On the lower border of the yellow pine zone the foothills are abrupt 

 so that there is a fairly distinct line of division from the plains zone. 

 The upper limit of the zone, however, is neither very definite nor 

 regular, as the lodgepole pine extends farther to the east and reaches 

 considerably lower elevations on the north than on the south slopes 

 of ridges. On the tops of ridges 8,500 feet may be taken as approxi- 

 mately the dividing line. The yellow pine district corresponds to the 

 Transition life-zone of the United States Biological Survey. 



The Mountain Zone as here understood includes the remainder of 

 the forested region and corresponds to the "Montane" and "Sub- 

 alpine" of botanists. It extends up to an elevation of 11,000 or 

 11,500 feet, timberline varying with topography and exposure. 



• Perhaps, following Merriam's terms used elsewhere, this would best be called "dilute Upper Sonoran." 

 3 Cf. Ramaley, "Plant Zones in the Rocky Mountains of Colorado," Science, N.S., Vol. XXVI, pp. 642- 

 643, 1907. 



3 Pseudotsuga mucronata; known also as Douglas Spruce. 



