154 UNIVERSITY OF COLORADO STUDIES 



representatives of certain western species gradually become fewer, and 

 finally disappear, while some eastern forms, seldom seen near the moun- 

 tains, become quite common. The northern violet-green swallow, 

 although quite common in 'the foothill canyons, was seen only within a 

 few miles of Boulder. Its near relatives, the cliff and barn swallows, 

 with a much broader range, were seen almost everywhere on the 

 trip. The cliff swallow was particularly noticeable. In the Chalk 

 Bluffs region many clusters of the mud" nests of this species were 

 found.' 



A bird seldom seen on the plains, but more or less common in the 

 foothills, is the canyon" wren. This species was observed only at a 

 point four miles west of Trilby, where the spurred towhee was also seen 

 for the last time. Two other wrens were frequently met with, the 

 western house wren in the fringing forests of the St. Vrain and the 

 Little Thompson and the rock wren which extends even to the more 

 barren regions farther east and north. A rock wren's nest, carefully 

 excavated, showed an excellent arrangement for concealing the young 

 birds. The tunnel had the form of a capital Y, the nest, constructed of 

 fine plant fibers, being located at the forking point. The floor of this 

 tunnel and especially of the two branches, was lined with a layer of fine 

 fibers resembling chopped tow. Two of the young fluttered from the 

 nest, when it appeared wholly deserted. A careful examination was 

 now made of the two side passages. Here no less than five young birds 

 were carefully hidden away in the fibrous lining of the nest, which they 

 resembled closely in color and general appearance. 



Of the eastern birds that became abundant a distance out from the 

 foothills the crow was perhaps the most conspicuous. This bird was 

 seen along the rivers near New Windsor and also farther east, but was 

 not observed on the high plains. On the other hand, the mocking bird 

 was seen in the Pawnee Buttes region as well as along streams. 



Most of the brilliantly colored birds, such as the Bullock oriole, 

 evening grosbeak, lazuli bunting, yellow-throat and chat, were not seen 

 away from the well-watered and timbered country. In drier places, as 

 on the high plains, these species are replaced by birds having a dull 



- See Fig. i, Plate II. 



