BIRDS OF BOULDER COUNTY, COLORADO 227 



the difficulty of securing direct evidence in problems of this nature 

 none of the theories have been satisfactorily established. 



It has been suggested that birds nesting in the higher zones migrate 

 north along the plains and then follow the mountain valleys up to their 

 breeding areas. Denis Gale believed that the Rocky Mountain 

 Hermit Thrush reached its home by a mountain route and that it did 

 not occur on the plains. While thrushes are certainly not common as 

 migrants near Boulder, they occur on the plains and have been recorded 

 as common at times at the Barr Lakes east of the county. In the 

 migration of the Mountain Bluebird large flocks occur both on the 

 plains and in the forested regions and some of the dates for arrival in 

 the upper part of the Yellow Pine zone have been as early as any for 

 the plains. The spring of 191 2 was very late in the Mountain zone, 

 yet Robins, Bluebirds and a Ruby-crowned Kinglet were noted long 

 before the snow had bared the ground. The presence of birds going 

 to or coming from more northern breeding areas further confuses 

 the data. It seems likely, however, that such a vertical migration will 

 be found true of the ground-nesting birds, some of which arrive on the 

 plains before the snow has left their summer habitat. 



In the first part of September, flocks of Robins are common in the 

 forested region, and in the experience of the writer their direction of 

 flight was very consistently toward the east, whether in the Yellow 

 Pine or in the high regions of the spruce forests. While evidence of 

 this kind is only suggestive, the trend of the Continental Divide along 

 the western border of Boulder County in relation to Middle Park and 

 northwestern Colorado also suggests a possible migration route across 

 the range into Boulder County. A map of Colorado shows the Divide 

 to be at its most eastern position at this point, while the location of 

 the mountain ranges in the central and southern part of the state are 

 such as to make the east slope the more direct route south. It would 

 be interesting to determine whether flocks of Robins actually cross 

 the range in the autumn flight. Such evidence would be good proof 

 of a migration route and would account for such records as the finding 

 of a dead Sora on Arapahoe Glacier. Gale believed that a Northern 

 Shrike which he saw in the mountains had crossed the range, follow- 

 ing up the migrating flocks of Juncos. 



