■2 68 Grinnell, Ckestnut-backed Chickadee. \^n\^ 



head in hudsonicus is broccoli brown, while in rtifescens it is dark 

 hair brown. The back of hudsotiiciis is pale grayish olive brown, 

 while in riifescens it is chestnut brown. The sides and flanks of 

 hudsonicus are rather pale hazel brown, while in rufescefts they are 

 deep hazel brown approaching chestnut. Otherwise the two spe- 

 cies look practically alike. 



These differences are just those we find so commonly in two 

 conspecific representatives, one occupying an arid habitat, the 

 other a comparatively more humid one. Indeed we can find 

 exactly parallel cases in certain other bird races occupying the 

 same two regions as the chickadees in question, but which as yet 

 are not disconnected by intermediates, and in which the degree of 

 difference is not so great. (For e.x?Lm^\e, Melospiza Ii?icolni liti- 

 colni and Alelospiza lincolni striata, and Regulus calendula calendula 

 and Regulus calendula grinnelli.) It is the same story, of intensi- 

 fication of browns and decrease in size under the conditions of a 

 moist climate. 



As to the greater relative decrease in length of tail in rufescens, 

 it may be suggested that it is an observed rule among the Paridae 

 (and in some other birds of similar habits, though not without 

 exception) that those species which habitually forage highest 

 above the ground in the foliage of tall trees possess the relatively 

 shortest tails, while conversely those which haunt low thick trees 

 or underbrush exhibit the greatest caudal development. (For 

 example, Psaltriparus and Charnaa.) These conditions doubtless 

 bear some definite relation to mode of flight. The shorter the 

 flights the slower they are, and therefore the greater must be the 

 tail surface distally in furnishing sufficient opposition to the air to 

 direct or arrest flight. At any rate, rufescens haunts much higher 

 and more open trees than hudsonicus. 



It seems to me reasonable to suppose that Parus hudsonicus 

 approaches closely the common ancestral form. Its wide range, 

 which, if we take the Old World Parus cinctus of such close resem- 

 blance as conspecific, is almost holarctic, favors this idea. At 

 some early period there may have been no representative of Parus 

 in the Northwest Coast belt. By a process of invasion of indi- 

 viduals of the hypothetical stock form (which we may call Parus 

 pre-hudsotiicus) from the adjacent region, and their subsequent 



II 



