1921 BIRDS OF MONTANA 13 



Transitiou. One tivc, the hox elder (Acer )u(jHii(hi) is found only in this zone, 

 in the eastern part of the state, and the j;iven ash (Fra.riniis hiiiti alaia) occnrs 

 sparingly in the same ret>ion. 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 /.ones. There 

 are easily observed two di'finite sorts of Transition, that of th.- jii'airies 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, hi tlie prairies 

 and mountain valleys tiie chief trees are the cottonwoods {Popaliis Ijalsainifirit, 

 P. angustifolia and /'. fri(Ii(jc(ir/)(i) , the last named only in mountain valleys on 

 the west side of tiic eontiuciital divide. In the foothills and lower mountains the 

 ])rincipal trees are the yellow jiine {I'iinis scupitlorKDi) and the Douglas fir 

 {rs( iidolsiifia ta.rifolia). Tlie yellow i)ine is the only one thoroughly charactei-- 

 istic, as the fir is found also iu the Canadian zone. Engelmanu spruce (I'icia 

 I )i(i(}}nanHi) occurs to a liiiiHed extent in this zone, and limber pine (Pimis flcr- 

 ilis) and tamarack (Larix ocrulrnlalis) 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 C'arter County, forming fairly dense forests of tall 

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

 8onoran areas. 



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

 part of the mountain valleys and all the lower slopes 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 [)artly gi'ass 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 -^JtOi) to fi.riOO in the northern. West of the divide, where the 

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

 ern part of the state. 



in the northweslei-u mountain region conditions occur which uuike 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 M.OOO feet, while not many miles away are nuich less 

 iiuniid ari'as. 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 Jlission Jlountains, are dense forests of Englemann spruce, where Cana- 

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

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

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

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

 I'.y Upper Sonoran species, such as the Grasshopper Sparrow, Bullock Oriole, and 

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

 sph( rie moisture conditions, rather than by changes in temperature. Therefore 



