MIGRATION AND ORIGINS 273 



em wintering population probably traverses its entire breadth east- 

 ward in spring. Many maritime birds of the western coasts in winter 

 come to nest and migrate farther northward in the passes through the 

 Brooks Kange. 



Many of the land birds of the Pacific-coast States (table 11a) have 

 been reported in migration northward between the Coast Range and 

 the Rocky Mountains in northern British Columbia (Swarth, 1936). 

 I suspect that those which are headed for Alaska pass through south- 

 western Yukon into the upper Tanana and Yukon Valleys. Those of 

 them which reach Old Crow may be suspected of avoiding the cross- 

 ing of the Ogilvie Range to travel along the Yukon until they can 

 enter Porcupine Valley. In this way they would be following the 

 eastward routes of inland travel observed among loons, geese, ducks, 

 and gulls in their visible flights past Old Crow. 



The species which migrate northward between the Mackenzie and 

 Rocky Mountains (table lib) are seen in considerable numbers during 

 migration in southern Yukon. It is likely that they pass over the 

 valley of the upper Liard River into the headwater valleys of the 

 Yukon and proceed from there westward into Alaska. With ex- 

 ception of the sandpipers, 13 of these species are very common nest- 

 ing birds at Old Crow. If we consider the Ogilvie Mountains likely 

 to bar them from a direct approach to the Porcupine, they could pass 

 along the Yukon to the Porcupine and enter its valley from the west, 

 as do the maritime wintering birds. There is an objection to con- 

 sidering that these birds would enter Porcupine Valley from the 

 west because some of them have been observed in the Yukon Valley in 

 Alaska a week or more earlier than they appear only 140 miles 

 northeastward at Old Crow, But not enough dates of northward 

 progress are yet available from the right locations for us to use the 

 calendar to plot the probable routes of these migratory flights. Some 

 eastern birds may cross the mountains in northern British Columbia 

 to reach their nesting grounds around Cook Inlet, but the majority 

 of these populations nest north of the Alaska Range and seem likely 

 to enter Alaska along the Yukon. 



The five species which migrate through the United States east of 

 the Mississippi (table lie) pass northward in the Mackenzie Valley, 

 probably holding to its warmer western watershed. In its northern 

 part they bear west to enter Yukon. Since golden plover (Pluvialis) 

 are rare at Old Crow they probably traverse northern British Co- 

 lumbia and southern Yukon to occupy their extensive Alaskan nest- 

 ing areas. The gray-cheeked thrush {Hylocichla minima)^ blackpoU 

 warbler {Dendroica striata), and fox sparrow {Passerella) migrate 

 extensively through southern Yukon with other birds from east of 

 the mountains. 



