•264 Grinnell, Chestnut-backed Chickadee. Ilulv 



THE ORIGIN AND DISTRIBUTION OF THE CHEST- 

 NUT-BACKED CHICKADEE. 



BY JOSEPH GRINNELL. 



The Chestnut-backed Chickadee {Farus rjifescens) is a boreal 

 species of peculiarly limited distribution. It is almost exclusively 

 confined to the humid Pacific Coast region of North America, 

 within which it is the most abundant, and in many places the only, 

 member of the genus Farus present. We find it characteristically 

 at home within the densest coniferous forests, or along their edges, 

 where there is much shade and an even temperature. 



The range of the Chestnut-backed Chickadee is nearly two 

 thousand miles long north and south, extending from a little north 

 of Sitka, Alaska, to some forty miles below Monterey, California. 

 (See Map I.) But its width is very narrow, only within the confines 

 of Oregon and Washington exceeding one hundred miles and else- 

 where usually much less, save for one or two isolated interior colo- 

 nies to be mentioned later. 



The influences determining this queer-shaped distribution area 

 may be safely assumed to be atmospheric humidity, with asso- 

 ciated floral conditions. For this habitat coincides quite accu- 

 rately with the narrow coastal belt of excessive cloudy weather and 

 rainfall. 



The specific character distinguishing Farus rufescens from all 

 other American chickadees is the color of the back, which is an 

 intense rusty brown approaching chestnut. It is of common note 

 that the most evident efifects of similar climatic conditions on 

 other animals is a corresponding intensification of browns, espe- 

 cially dorsally. We may therefore consider the Chestnut-backed 

 Chickadee, as indicated by its chief specific character, to be a prod- 

 uct exclusively of the peculiar isohumic area to which we find it 

 confined. 



Farus rufescens^ from Sitka to Monterey, has a chestnut-colored 

 back. And from Sitka to Point Arena, between which we find the 

 extremest humidity, another conspicuous character is uniform, — 

 the color of the sides, which are also deep rusty brown. But from 

 Point Arena south to San Francisco Bay (Marin District), these 



