1922] Swarth: Birds and Mammals of. the Stikine Region 147 



of the river valleys traversing the barrier (as the Stikine) became 

 such as to permit the existence of animal and plant life, and various 

 species extended their ranges up or down such channels of communica- 

 tion, as circumstances permitted. One factor that may be of present 

 importance in limiting the use of the Stikine Valley as such a channel, 

 so far as birds are concerned, is the late advent of summer conditions 

 on the lower Stikine compared with the regions on either side, a 

 seasonal tardiness that unquestionably prevents the nesting of many 

 species in this intermediate strip at the time of their arrival from the 

 south. (In this connection see Dawson, 1889, p. 596 ; Emmons, 1911, 

 pp. 9-10.) 



The general hypothesis outlined seems to accord with conditions 

 as we now see them. Among birds it may be pointed out that it is 

 consistent with the manner of occurrence of forms like Hylocichla 

 ustulata usiulata and H. u. swadnsoni, closely related subspecies of one 

 species but behaving at this point like two distinct species. It is con- 

 sistent with the extension inland a certain distance of forms like 

 Sphyrapicus varius ruber and Passerella iliaca fuUginosa, and toward 

 the coast of Piranga ludoviciana (see Swarth, 1911, p. 95) and Empi- 

 donax trailli aln^rum, and it is consistent with the restriction on one 

 side or the other of such distinct and sharply delimited forms as 

 Bombycilla garrula pallidiceps and Cyanocitta stelleri stelleri. 



In this connection, certain facts regarding the present-day migra- 

 tion of birds in the region should be considered. If there were any 

 general travel between the interior and the coast such river valleys 

 as the Stikine would be the highways most generally followed. Our 

 work in the region shows pretty clearly that such is not the case. It 

 seems evident that the birds of the interior travel north and south 

 almost entirely east of the coast ranges. Some of them, it is well 

 known, even pass east of the Eocky Mountains as they get farther 

 south. Birds of the coastal region remain west of the mountains. 



"We found many characteristic inland species of birds, some breed- 

 ing, others, perhaps, merely migrants, as far down stream as Great 

 Glacier, some thirty miles from the coast. The species seen there 

 include Penthestes atricapillus septentrionalis, Setophaga ruticilla, 

 Hylocichla ustulata swainsoni, and Vireosylva gilva swaAn^oni, none 

 of which has been taken on the coast. Dendroica aestiva was an abun- 

 dant migrant, though almost entirely absent at the mouth of the 

 river. The Stikine in its passage through the Coast Kange travels 

 almost due south, and it is natural that birds from the headwaters 

 should migrate in numbers along this valley. It is not so clear why 



