94 University of California Publications in Zoology [Vol. 21 



to a greater or less degree by most of the animal species of the same 

 general region, and students of animal variation have come to regard 

 this tendency to dark colors as a result of the excessive humidity of 

 the surroundings. General correlation of the two is obvious. How- 

 ever, the whole of the region involved is not uniform in degree of 

 humidity; neither are the several races of Passcrella uniform in 

 coloration. There is in the several subspecies, as we have seen, 

 progressive darkening from a relatively grayish bird (unalaschcensis) 

 to one of extremely dark brown coloration (fuliginosa) . The darken- 

 ing in color, however, does not coincide with increase of humidity in 

 the territory occupied; at least — and this is the important fact — in 

 that occupied by these birds during the breeding season. 



It is a general assumption that the conditions prevailing in the 

 summer home, not in the winter habitat, stamp the character of a 

 variable and migratory species, such as is here considered, but the 

 observed facts concerning these races of Passerella will bear a diiferent 

 interpretaton. Sinuosa, one of the three paler colored subspecies, 

 inhabits a region that is, in part at least, of as great humidity as the 

 habitat of townsendi, a darker colored form. Fuliginosa, nesting 

 under much less humid conditions than townsendi, is of still darker 

 coloration. Thus the appearance of these birds seems in flat contra- 

 diction to the correlation we have traced between humidity and color. 



If, however, we do not insist upon conditions in the summer home 

 as the only ones of importance the apparent discrepancies may be 

 reconciled. In the three northern subspecies {unalaschcensis, insularis 

 and sinuosa), the paler colored forms, the summer habitat is in part 

 a region of exceedingly heavy rainfall, and in general one where 

 heavy fogs and cloudy weather prevail during much of the time. It 

 is a region of great humidity. The subspecies concerned, however, 

 migrate far to the southward, and spend the winter amid relatively 

 arid surroundings. Of the summer habitat of the three southern 

 subspecies {annectens, townsendi, and fuliginosa) , the darker colored 

 forms, the most humid region includes most of the range of townsetidi. 

 Fuliginosa, still darker colored, spends the summer in much more dry 

 and sunny surroundings. These three subspecies do not travel so far 

 southward in winter as do the northern forms, and they all {annectens, 

 townsen-di and fuliginosa) winter within the hiunid coast belt. They 

 spend the winter months in a region where this period of the year 

 constitutes the "rainy season," where the precipitation is much 

 greater, and cloudy and foggy weather much more prevalent, tlian 



