58 CALIFORNIA ACADEMY OF SCIENCES [Peoc. 4th Ser. 



VARIATION 



There are but two distinguishable stages of plumage in 

 this species (besides the natal down) — the juvenal and those 

 of the first and subsequent winters. The juvenal differs from 

 the later stages mainly in being mottled or spotted over much 

 of the area that is subsequently uniformly colored. After loss 

 of the juvenal plumage, in the first August and September, 

 there are no further changes in appearance, immatures of the 

 first winter being indistinguishable from adults ; after the first 

 year there is but the one molt annually, at the end of the sum- 

 mer. There are thus no seasonal changes in appearance other 

 than those produced by the mechanical wear and tear upon the 

 plumage, amounting to fading of colors proportionate to the 

 nature and extent of exposure, and obliteration of finer mark- 

 ings as the feather tips are worn away. There are no appreci- 

 able sexual distinctions of color or markings, merely slight 

 average differences in measurements, males averaging a trifie 

 larger than females. 



It is seen that the wrens of this genus have a wide zonal 

 range, and that where a change in life-zones acts as a check, 

 it is, in this group as in most others, usually an absolute barrier 

 to the species rather than an accompaniment of subspecific va- 

 riation. It is also evident that environmental requirements 

 ai-e not rigid, the species being readily adaptive within rather 

 wide limits. While the species as a whole is wide ranging, 

 however, there is a strong tendency toward separation into 

 different forms or races distinguished by peculiarities of color 

 and proportions. Study of this variation demonstrates close ac- 

 cordance of appearance with locality. In this group of birds each 

 faunal area represented tends to a remarkable degree to pro- 

 duce its own peculiar type. So that it is seen that Thryomanes, 

 wide ranging and with the faculty of thriving under varied 

 climate and surroundings, exhibits to an extraordinary extent 

 — perhaps as part of this same adaptability — variation of ap- 

 pearance in accordance with every change in environment. 



It must be understood, of course, that the differences 

 alluded to are not always conspicuous. There are frequently 

 variations in proportions requiring careful measuring for de- 

 tection, or slight differences in color which in scattered speci- 



