The diversity curves, however, differ from the species number and species rich- 

 ness plots in several respects. First, a sharp decrease in diversity is evident 

 for most routes between February and March. Second, diversities obtained for the 

 early post-breeding season (August) runs are comparable to those of breeding sea- 

 son (May-July) runs, although species number is much lower in August. Third, 

 diversity increases for most runs between September and October, while species 

 number decreases. These differences may be elucidated by examination of the 

 evenness component of diversity (figure 44), which provides a measure of the 

 equitability of distribution of individuals among species in the samples. 



It is apparent from figure 44 that evenness is fairly constant and similar 

 for May through August runs of all routes, and that variability is highest--and 

 also the lowest evenness values are found--in spring, fall, and winter. A drop 

 in evenness is evident for March and September runs of most routes; this is re- 

 flected by a corresponding decrease in diversity for these months. This low 

 equitability is due primarily to large migrating flocks of a few species (par- 

 ticularly horned larks) appearing in March and September at a time when sample 

 abundances of most other species (both summer, winter, and yearlong residents) 

 are low (see also figure 12). It is noteworthy that, while species number drops 

 sharply between July and August runs, evenness values rise. In fact, a general 

 increase in evenness is evident from May through August. This is probably re- 

 lated to a decline in singing activity--and hence detectibility--of horned larks, 

 as well as an increase in sample abundance of moderately common species, through- 

 out the same period. While most species which viere present in July were probably 

 also present in August, singing activity and detectibility of many species drop- 

 ped sharply between July and August. In fact, species such as the grasshopper, 

 sparrow and the rufous-sided towhee were primarily detected by sound rather than 

 by sight in July. Thus, fewer species were encountered in August, although the 

 number of species present and the relative abundances of most species--and hence 

 the evenness of the community--was likely very similar to the July data. High 

 evenness values for the Missouri River route are related to the relatively low 

 numbers of horned larks encountered on this route (figure 12); the horned lark 

 was the most abundant species on most runs of the other routes. Note that the 

 pattern of single-and two-species dominance (figures 45 and 46) is wery similar 

 to that of evenness. Samples showing a high degree of dominance by one or two 

 species (and hence a low equitability) show correspondingly low evenness values. 



All five routes run in this study fall within the Missouri-plateau-Unglaciated 

 stratum described by Peterson (1975) in his summary of the Breeding Bird Survey 

 (BBS) in the United States. The data collected during this study followed the 

 same method as the BBS data discussed by Peterson; hence, direct comparisons 

 are possible. 



June species numbers for the five routes ranged from 31 on the Flowing Well 

 route to 47 on the Missouri River route. The average for the five routes was 

 39.2. This figure is somewhat lower than the average value of 41 species per 

 route reported by Peterson for the 33 routes run in this stratum in 1973; however, 

 it is higner than average value of 37.8 reported for 1968-1973 runs of routes in 

 this stratum. These values, in turn, are somewhat lower th,T:i the 6-year contin- 

 ental average of 45.6 species per route reported by Peterson after analyzing 1670 

 routes. The average total cumulative number of species reported per year for all 

 1968-1973 runs of all routes in this stratum is 141.8, which is considerably 

 higher than the 105 summer species recorded in the study area. This may be 



163 



