2 74 Grinnell, Chestnut-backed Chickadee. \^^\^ 



best physical condition, and just before the stress of winter food 

 shortage. Even in the most sedentary of birds, in which no other 

 trace of a migratory instinct is discernible, this fall season of 

 unrest is plainly in evidence. I may suggest not unreasonably 

 that autumnal migration may have had its origin in such a trait as 

 this, the return movement in the spring becoming a necessary 

 sequence. (See Loomis, Proc. Cal. Acad. Sc, 3rd Series, Zool- 

 ogy, II, Dec, 1900, 352.) It is a matter of abundant observation 

 that autumn is the season when we find the most unlooked-for 

 stragglers far out of their normal range, and when sober, stay-at- 

 home birds, like Pipilo crissalis and the chickadees, wander far 

 from the native haunts where they so closely confine themselves 

 the rest of the year. It is also the experience of collectors that 

 the greatest number of these stragglers are birds-of-the-year, 

 which thus, obeying the ' mad impulse,' are led away from their 

 birthplace into new country, where they may take up their per- 

 manent abode, and be less likely to compete with their parents 

 or others of their kind. Then, too, crossbreeding of distantly 

 related individuals is more likely. The records of the Santa Cruz 

 Chickadee outside of its regular breeding range are all of August 

 to October dates (Haywards, Gilroy, San Jose, etc.). 



Thus, as above indicated, by the occupancy of new territory the 

 number of individuals which can be supported will correspond- 

 ingly grow. Hence a vigorous colony will spread out along lines 

 of least resistance, being hindered by slight faunal changes, but 

 completely checked only by topographic or abrupt climatic barriers. 

 Pariis hudsofiiciis and its near relative Parus rufesceris are boreal 

 species, the former inhabiting the Hudsonian Zone and the latter 

 a certain portion of the Canadian. It seems reasonable to suppose 

 that rufescens differentiated in the northern part of the humid coast 

 belt, which has been called the Sitkan District. This is a faunal 

 subdivision of the Canadian Zone, and its northern part approxi- 

 mates more closely Hudsonian conditions than southerly. Grant- 

 ing that the early center of differentiation and distribution of Parus 

 pre-hudsoniciis rufescens was in the northern part of the Sitkan 

 District, then the route of emigration would be confined to the 

 narrow southward extension of that faunal area. The habitat of 

 Parus rufescens thus gradually acquired the long north and south 



