392 BULLETIN 191, UNITED STATES NATIONAL MUSEUM 



tion between the other two races, one north and one south of it. Mr. 

 Ridgway (1904) describes it as "similar to P. r. rujescens but with much 

 less of chestnut on sides and flanks, which exteriorly are pale gray, the 

 chestnut also paler and duller." Dr. Joseph Grinnell (1904), in dis- 

 cussing this group, remarks that "this southward paling of the lateral 

 feather tracts seems to be parallel to the relative decrease in the humidity 

 of the regions occupied." And this paling is carried still farther, with 

 the practical disappearance of the chestnut sides in the next race to 

 the southward, harlowi. The Nicasio chickadee seems to occur in its 

 purity only in Marin County; it intergrades with P. r. rujescens in 

 Sonoma County to the northward; and San Francisco Bay and the 

 Golden Gate seem to form an effectual barrier between it and P. r. 

 barloivi to the southward. 



In support of his theory that the two species Parus hudsonicus and 

 P. rtijescens were derived from common ancestry. Dr. Grinnell (1904) 

 calls attention to the fac.t that "the young of barlozvi has the sides paler 

 rusty than neglectus, neglectus slightly paler than rujescens, but rujescens 

 has the sides slightly more rusty than hudsonicus, a sequence which 

 accords well with the present theories of origin." His hypothesis is 

 based, of course, on the generally accepted theory that immature indi- 

 viduals more closely resemble the ancestral form than the adults do. 



We have no reason to think that the habits of the Nicasio chickadee 

 are very different from those of the chestnut-backed chickadees else- 

 where. The eggs are characteristic of the species, as described under 

 the preceding form. The measurements of 39 eggs average 15.7 by 

 12.3 millimeters ; the eggs showing the four extremes measure 17.6 by 

 12.2, 16.5 by 13.0, 14.3 by 12.0, and 15.2 by 11.9 millimeters. 



PARUS RUFESCENS BARLOWI GrlnneU 

 BARLOW'S CHICKADEE 



Plate 56 



HABITS 



Farther south along the coastal area of middle California, from San 

 Francisco Bay to a little south of Monterey Bay, we have this paler, 

 less rufous-sided race of the chestnut-bac.ked chickadees. In his 

 original description of this race, which he named in honor of Chester 

 Barlow, Dr. Grinnell (1900b) characterizes it as "similar to P. rujescens 

 neglectus, but the sides pure smoked gray without a trace of rusty." 



The San Francisco Bay region seems to form the northern barrier 

 to the distribution of this race farther northward, and climatic c.ondi- 

 tions apparently prevent its extension farther south, a certain degree 



