196 University of California Publications in Zoology [Vol.12 



arrival of the closely related Dcndroica aestiva brewsteri at the same 

 latitude in the San Diegan district of southern California; and this 

 in spite of the lower zone of the Colorado Valley. 



The Sonora yellow warblers were abundant at all suitable places 

 along clown the river from Picacho, the stations being indicated in the 

 accompanying table of measurements. They adhered closely to the 

 willow association, the mode of their forage zone lying somewhere in 

 the crown foliage <>f the willows and cottonwoods, the height above the 

 ground of course varying with the stature of the trees. The females 

 were hard to locate, but the incessant song of the male birds rendered 

 the latter easy to detect. Observation on April 19 in a very extensive 

 willow tract in ' ' Charlie 's Valley, ' ' eight miles east of Picacho, showed 

 them to be regularly spaced out through the top foliage at an estimated 

 frequency of four per acre. Thus, at this point, a quarter of an acre, 

 with a depth of perhaps twelve feet of foliage, w-as the forage allow- 

 ance of each nesting pair of warblers. This was a greater congestion 

 of warbler population than in most places, because of the evident 

 unusual favorableness of the environs. 



At another place, five miles above Yuma, May 3, there was only 

 one singing male to two acres of willows. With a minimum of 640 

 yellow warblers to the square mile, their aggregate numbers in the 

 Colorado Valley must reach an enormous total, considering the vast 

 area of first bottom. Since there is in the Museum collection a female 

 specimen (no. 4265) of Dendroica aestiva sonorana taken by Cooper 

 May 4, 1861, at Fort Mohave, it is reasonable to suppose that this 

 form breeds north in uniform abundance along the valley to the Nevada 

 line. In spite of the numbers of the birds, we failed to run across 

 nests. This may, however, have been due to nesting being deferred 

 until later than the middle of May. 



The series obtained of this warbler (forty-seven specimens, nos. 

 13604-13650) provides so much material of a bird not heretofore 

 adequately represented in our collections, that a rather detailed exam- 

 ination of its characters seems warranted. Furthermore, so many 

 examples of one species of migratory bird from so limited an area, 

 and taken practically within a month's time at one season, would 

 seem to be a basis from which to obtain a fair idea of normal variation. 



Referring first to the accompanying table of measurements, it will 

 be noticed in the males that the average and the mid-point of the 

 range practically coincide ; that the variation in the six respects varies 



