1921 BIRDS OF MONTANA 13 



Transition. One tree, the box elder (Acer negundo) is found only in this zone, 

 in the eastern part of the state, and the green ash (Fraxinus lanccolata) occurs 

 sparingly in the same region. But these two trees are not found in all the Sono- 

 ran areas of the state. 



The Transition zone occupies the largest area of any of the zones. There 

 are easily observed two definite sorts of Transition, that of the prairies and 

 mountain valleys, and that of the lower mountain slopes, foothills and pine hills. 

 All the faunal regions of the state contain areas of this zone. In the prairies 

 and mountain valleys the chief trees are the cottonwoods (Populus halsamifera, 

 P. angustifolia and P. trkliocarpa) , the last named only in mountain valleys on 

 the west side of the continental divide. In the foothills and lower mountains the 

 principal trees are the yellow^ pine (Pinus scopulorum) and the Douglas fir 

 {Pseudoisuga taxifolia). The yellow pine is the only one thoroughly character- 

 istic, as the fir is found also in the Canadian zone. Engelmann spruce {Picca 

 engelmanni) occurs to a limited extent in this zone, and limber pine (Pinus flex- 

 ilis) and tamarack (Larix occidentalis) are common in it in certain localities. 



The chief Transition areas in the southern prairie region are the pine hills. 

 These are flat-topped prairie buttes, whose tops are open grass land, but whose 

 sides are clothed with yellow pine, usually scattering and sparse, but occasionally, 

 as in the Long Pine Hills of Carter County, forming fairly dense forests of tall 

 trees. These pine hills mark isolated tracts of Transition surrounded by Upper 

 Sonoran areas. 



The two northern prairie regions are entirely Transition, and the greatest 

 part of the mountain valleys and all the lower slopesi and canyons also belong to 

 this zone. The prairies and valleys are mainly grass land, with sage-brush areas 

 in some parts of the state, and with cottonwood groves and willow thickets along 

 the streams. The foothills are partly grass and partly timber, with occasional 

 areas of forest of either yellow pine or fir. East of the continental divide the 

 Transition zone occurs from about 4,000 to 6,000 feet in the southern part of the 

 state, and from 3,500 to 5,500 in the northern. West of the divide, where the 

 valleys are generally lower, this zone goes down to 3,000 feet, even in the north- 

 ern part of the state. 



In the northwestern mountain region conditions occur which make it diffi- 

 cult to define the limits of this zone. The humidity of slopes facing west is much 

 greater than those facing east. This causes conditions much like the Canadian 

 zone at elevations as low as 3,000 feet, while not many miles away are much less 

 humid areas, where the elevation is no lower, but which I strongly suspect are 

 Upper Sonoran. Thus, on the east shore of Flathead Lake, directly at the base 

 of the Mission Mountains, are dense forests of Englemann spruce, where Cana- 

 dian birds such as the Golden-crowned Kinglet and Winter Wren breed. On all 

 of the mountain slopes surrounding these areas are Transition forests of yellow 

 pine, fir and tamarack, extending at least 1000 feet higher. Both above Flat- 

 head Lake and below its outlet, are areas of the same elevation as these, inhabited 

 by L^pper Sonoran species, such as the Grasshopper Sparrow, Bullock Oriole, and 

 Arkansas Kingbird. These phenomena are brought about either by soil or atmo- 

 spheric moisture conditions, rather than by changes in temperature. Therefore 



