^°'/^^^] Grinnell, Chestnut-backed Chickadee. 377 



locality. Two species of approximately the same food habits are 

 not likely to remain long evenly balanced in numbers in the 

 same region. One will crowd out the other ; the one longest 

 exposed to local conditions, and hence best fitted, though ever so 

 slightly, will survive, to the exclusion of any less favored would-be 

 invader. However, should some new contingency arise, placing 

 the native species at a disadvantage, such as the introduction of 

 new plants, then there might be a fair chance for a neighboring 

 species to gain a foothold, even ultimately crowding out the native 

 form. For example, several pairs of the Santa Cruz Chickadee 

 have taken up their permanent abode in the coniferous portion of 

 the Arboretum at Stanford University, while the Plain Titmouse 

 prevails in the live oaks of the surrounding valley. 



In accordance with the above outlined theories of distribution 

 it is easy to account for isolated breeding colonies, such as that 

 of Parus rufesceiis rufescens in northern Idaho (Fort Sherman and 

 Coeur d'Alene Mountains). Fall stragglers, wandering unusually 

 far and finding themselves suddenly amid familiar conditions, 

 would tarry there to breed, and with the continuance of a favor- 

 able state of affairs, and with no serious competition, might soon 

 result in a well-established colony, itself a center of distribution. 

 The record of rufescefis from Mt. Shasta (July 14) seems to have 

 been based on a lone straggler, for the species has not been found 

 there since. (For references and localities see beyond.) 



As has become a generally accepted idea, the young plumages 

 of birds, if different at all from those of the adults, present a gen- 

 eralized type of coloration ; or, to express it in another way, the 

 young more nearly resemble recent ancestral conditions. The 

 familiar examples of the spotted, thrush-like plumage of the young 

 robin and the streaked, sparrow-like plumage of young towhees 

 and juncos are cases in point. Accepting this phylogenetic 

 significance of ontogeny, we find the chickadees giving some 

 interesting illustrations. 



Although the adult of barlowi has the sides pure smoke-gray, 

 the Juvenal plumage possesses pale rusty sides. This points 

 towards a rusty sided ancestor like neglectus. This also agrees 

 perfectly with the distributional evidence of origin. The adult of 

 neglectus has pale rusty sides ; the young also has rusty sides, but 



