190 University of California Publications in Zoology [Vol.12 



in the adjacent mesquite belt. Each pair of vireos was closely delimited 

 in its forage beat by that of its neighbor. Each pair in its own area 

 actively resented encroachment by others of its own species. The 

 vireos worked a rather low zone of foliage, from the ground up to a 

 height of six or eight feet, just about the same, again, as in the case 

 of the Lucy warbler. It is of interest to note that the Sonora yellow 

 warblers, which arrived much later, after the willows had leaved out, 

 were spaced much closer, but foraged through a greater depth of 

 verdure, from the summits of the willows to within five or six feet 

 of the ground. The vireo's domain was but slightly impinged upon. 



At Ehrenberg. Arizona, a newly started nest was found. March 29. 

 It was at this date about half completed, and was attached to the 

 forking stalk of a guatemote five feet above the ground. It would 

 appear that many nests meet with disaster from their being built. 

 as they so often are. in openings between thickets. These openings 

 serve as passage ways for browsing cattle, which as they crowd through, 

 force the supporting branches aside and demolish the nests. Evidence 

 of a number of instances of this type of catastrophe came to notice. 

 A partly completed nest found on the California side near Pilot 

 Knob, May 12, the owners being seen in the vicinity, would, together 

 with the previous instance, indicate a length of breeding season of 

 at least six weeks. Repeated trials, following such common accidents 

 as the above, may account for this. 



A nest found April 24 on the Arizona side, five miles above Laguna. 

 was located three and one-half feet above the ground on a horizontal 

 willow branch, beneath and darkly shaded by several small willow 

 trees growing close together at the margin of an overflow slough. The 

 nest preserved (no. 759). is of normal construction for vireos of this 

 genus. The chief constituents are weathered mesquite and willow bark 

 strips and spiderweb and cocoons ; the inner lining is of fine round 

 grass stems and shreds of dry grass blades. The eggs, four in number, 

 were advanced about one-third in the process of incubation. The 

 egg-shells are pure white, dotted very sparsely about the large ends 

 with bay and hazel. 



The series of twenty-three adult specimens taken (nos. 13510- 

 13532 s ) establishes clearly the identity of the Colorado Valley birds 

 with Vireo belli arizonae of south central Arizona. The range of this 

 geographic race is thus carried over into the confines of California 

 along the southeastern frontier of the state. 



