Several groupings of species showing similar habitat requirements are 

 clearly evident in this ordination, and have been enclosed by circles for 

 clarity of explanation. In the upper left are the characteristic grassland 

 birds, the horned lark, lark bunting, and chastnut-collared longspur, which 

 nearly always occur together in flat-to-roll ing, open grasslands. The vesper 

 sparrow, Brewer's sparrow, and rock wren form a closely-associated group char- 

 acteristic of th.e heavily dissected big sagebrush badlands and scablands . A 

 fairly large group of species characteristic of tall shrub and tree-shrub hab- 

 itats is evident, and includes the loggerhead shrike, commom crow, brown thrasher, 

 common yellowthroat, common flicker, mourning dove, yellow warbler, ring-necked 

 pheasant, clay-colored sparrow, eastern kingbird, rufous-sided towhee, and black- 

 billed magpie. Another group inhabiting somewtiat drier, more broken shrublands 

 includes the Brewer's blackbird, lark sparrow, and field sparrow. Seasonal 

 changes in habitat selection by certain species (pronghorn antelope, mule deer, 

 white-tailed deer, western meadowlarks, and horned lark) may also be seen in 

 this figure. For example, the western meadowlark showed an apparent shift 

 from moderately dissected topography in the breeding season (April-June) to 

 flatter, more open topography in the post-breeding season (July-October). 



SMALL MAMMAL COMMUfUTY PARAMETERS 



Community Composition 



Small mammal capture data are summarized in table 36. In 7200 snap-trap 

 nights, 348 individuals of seven rodent species were captured, and in 9200 live- 

 trap nights 143 individuals representing four species were captured. Neither 

 of the voles nor the least chipmunk was captured in live-traps. 



The deer mouse was the dominant species in both overall snap-trap (92% of 

 all individuals captured) and live trap data (50% of all individuals captured). 

 In all but two of the habitats snap-trapped, the deer mouse was dominant (table 

 38). In both riparian western bulbush (sites #15 and #20) and silver sagebrush/ 

 western wheatgrass-blue grama (site #33) habitats the deer mouse was an equal 

 co-dominant with the western harvest mouse. Lewis et al. (1978), working near 

 Colstrip, Montana, also noted a preponderance of deer mice (76-80% of captures). 



The deer mouse occupied nearly all habitats sampled and in fact showed the 

 broadest niche of all small mammal species studied. It is probable that the deer 

 mouse is the most abundant mammal in the mine study area and all of McCone County. 



In live-trap results the western harvest mouse was a close co-dominant of 

 the deer mouse, comprising 47% of all individuals and 38% of all captures as 

 opposed to 50% and 61% respectively, for the deer mouse (table 37). The majority 

 of western harvest mouse captures occurred on live-trap grids during the last 

 three months of the study, coinciding with the increased proportion of breeding 

 females beginning in July. During September and October, which were the last 

 two months of sampling, on the sagebrush control grid ^hich had the highest mean 

 of monthly western harvest mouse captures)the western harvest mouse was captured 

 twice as frequently as the deer mouse. This suggests a shared numerical domin- 

 ance relationship between the two species in this habitat type, which fluctuates 

 with the seasonal cycle of rodent abundance; the deer mouse dominates during 

 periods of low density, yielding to the western harvest mouse for an undetermined 

 duration following breeding season and peak density. 



173 



