Seasonal Changes in Community Composition . In order to clarify ecological 

 differences among bird species sampled by the roadside wildlife survey routes, 

 each species was assigned a feeding/nesting guild and a resident status (table 8). 

 Examination of seasonal changes in the relative composition of the samples with 

 respect to these two parameters should well describe the ecological makeup of 

 these grassland bird communities. 



Seasonal changes in species numbers of yearlong residents (R), summer res- 

 idents (S), winter residents (W), and migrants (M) are shown in figure 37, along 

 with seasonal changes in the percentages of composition among these same cate- 

 gories of the total number of individual birds observed each month. These and 

 other data indicate that the peaks of spring and fall migration occur from mid- 

 April to mid-May and from early September to mid-October, respectively. Yearlong 

 residents predominate outside the breeding season, both in terms of number of 

 species and the percentage of individual birds. Migrants and winter residents 

 are a relatively small component of the community even during winter, spring, and 

 fall. During the months of April through August, summer residents were the predom- 

 inant species, both In terms of species numbers and percentage of individual birds. 

 During June, the heiglit of the breeding season, they accounted for 80% of all spe- 

 cies and 71% of all individuals. The number of yearlong resident species remains 

 fairly constant throughout the year, as expected; slight decreases in winter are 

 likely related to winter decreases in abundance and/or conspicuousness of these 

 species. 



Seasonal changes in trophic composition of bird community samples are shown 

 in figure 38. The four trophic categories shown correspond to the major food 

 source categories listed in table 8 as part of species guild assignments. It Is 

 evident that the number of species relying on vertebrates or on seeds and vege- 

 tation are relatively constant yearlong, and that the great Increase in species 

 number during the breeding season is due primarily to the addition of omnivorous 

 and, especially, invertebrate-feeding species. The influx of Invertebrate feeders, 

 which is especially evident in terms of percent of total Individuals, is related to 

 the drastic summer increase in prey availability. Species relying on vertebrates 

 as a major food source (all of which are raptors) represent the highest trophic 

 level, and, while comprising a relatively large percentage of the total number of 

 species (especially in winter), comprise a small but constant proportion of the 

 total number of individuals, never accounting for more than 3% of the total. The 

 percentage of contribution of these relatively large-bodied species to the total 

 sample biomass would be considerably greater; however, their high observability 

 tends to bias sample abundances upward, and it is doubtful that they comprise even 

 one percent of the total biomass of bird communities sampled. Only breeding-sea- 

 son trophic levels were used for analysis; non-breeding season dietary shifts 

 undoubtedly resulted in greater representation of herbivorous birds and lower 

 representations of insectivorous and omnivorous birds than are shown in figure 

 38 for the winter months. 



Figure 39 shows seasonal changes in representation of feeding guilds (feeding 

 strategy as defined in table 8) among the communities sampled. As would be ex- 

 pected in this predominantly grassland habitat, the ground and brush foraging 

 guild predominates at all seasons, both in terms of number of species and per- 

 centage of total individuals. The extreme rarity of trees within the areas 

 sampled drastically reduced the structural diversity of habitats available to 

 birds, and accounts for the rarity of bird species adapted to tree foliage 



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