140 University of California Publications in Zoology [Vol.12 



sented, in spite of the fact that of the remainder of the series it is 

 difficult to allocate certain examples with one or the other category. 

 This has finally been done, however, as shown in the tables. The first 

 character employed for separation, and the only one holding through- 

 out the present series, is the style of black marking on the medial 

 scapulars. In the resident form, for which I am using the name 

 nitidus, these markings are narrow and more V-shaped or hastate; 

 in the presumably migratory form, which I am calling nuttalli, these 

 markings are broad, sagittate to rhomboid. I am inclined, however, 

 to believe that this character, like the others pointed out beyond, will 

 be found to fail in some cases; for a skin at hand from the Huachuca 

 Mountains, Arizona (no. 10324), is otherwise good nitidus as here 

 understood, but the scapular markings are broadly rhomboid. In the 

 Colorado series, believed to represent two forms, any of the remaining 

 characters are not separately diagnostic in every case. But the series 

 is found divisible with fair precision as indicated in the tables, by 

 an aggregate of characters. 



Besides the scapular marks above specified, the black barring across 

 the primaries averages narrower in nitidus than in nuttalli, and their 

 transverse trend is zigzag in the former, more squarely defined in 

 the latter. The transverse dark barring on the posterior lower sur- 

 face averages narrower again in nitidus, coarser in nuttalli. The 

 ground-color, whitish or hoary, is not only more extended in nitidus 

 but of a clearer white tone. In general size nitidus is distinctly the 

 smaller, and it appears to have a more rounded wing, indicative of 

 less extended migratory flight. 



While in each of these characters variation leads from one extreme 

 to the other, there are clearly two modes. The theory suggests itself 

 as accounting for this, that there are two unstable phases in the 

 poor-will, irrespective of locality, sex or age, a condition which has 

 been verified in the case of certain owls. But the facts as here pre- 

 sented militate against either that idea or the contention that the 

 paleness develops with individual senility. All the above characters 

 are paralleled in such birds as are represented by races occupying the 

 deserts on the one hand and the more northern and less arid regions 

 on the other. Some of the nuttalli examples are precisely like poor-wills 

 in the Museum collected from Humboldt County, Nevada. No skins 

 examined from anywhere north of the Colorado Valley show char- 

 acters of nitidus. 



