Pattern and Process in Grassland Bird Communities

Variations in abundance and distribution of bird species and avifaunal and community organization at regional, local, and within—plot levels were studied during 1969 and 1970 at a shortgrass prairie in Colorado and during 1970 at six additional International Biological Program (IBP) grassland sites. Patterns were generally not distinct at the regional level, although low rainfall sites tended to support fewer individuals and less biomass than more mesic sites. The dominant bird species were widely distributed, but 70% of all species recorded were present at only one of the seven sites. Local plot—to—plot differences, associated with grazing intensity, were considerably more important than the regional differences. Vegetational and avifaunal relationships of plots were determined by similarity—cluster analysis and by examination of vegetation structure. Variations in the number of breeding bird species, bird—species diversity, or equitability, were unrelated to the gradient of plots from tallgrass through shortgrass to desert, but standing crop biomass generally decreased along this gradient. The plots differed in the proportions of small, medium, and large—sized species. Shortgrass sites were generally dominated by omnivorous species; plots with a greater vegetation structure supported more carnivorous forms. Only carnivorous species occurred at the desert site. The effect of grazing on bird populations varied among the treatment plots at the various sites. In the Colorado shortgrass prairie, grazing season had a greater effect on community organization than grazing intensity. Patterns of variation were much more pronounced when single species rather than breeding faunas were considered. Grasshoppers, Lepidoptera larvae, beetles (especially curculionids, carabids, cerambycids, and scarabaeids), ants, and various seeds were the most important prey of the dominant bird species. Small sparrows and larks had generally similar diets. The diets of the larger meadowlarks and shorebirds differed from those of the former group and also differed from each other. The proportion of seeds and arthropods in the diets of Lark Buntings and Horned Larks varied considerably with time. Diurnal raptors, studied at only one site, were widely dispersed, occurring at densities and standing crops substantially less than those of small passerine populations. Owls preyed chiefly upon small mammals and insects, but the proportions of these food items in the diet differed significantly among the four species studied. The estimated energy intake of the breeding bird populations from April through August ranged from 1.01 to 2.33 kcal/m 2 ; thus the energy flux through avian consumers in grasslands is apparently very small. In general, seeds decreased in importance as energy sources and arthropods increased in importance along a gradient from shortgrass through mixed grass to tallgrass plots. Secondary production estimated for six grazing—treatment plots ranged from 3.9 to 6.9 x 10 — 3 g/m 2 . The role of birds as consumers in the dynamics of grassland ecosystems is considered. It is suggested that birds may act as controllers of other elements of system function or may not be closely evolved into the functional framework of the ecosystem at all, existing on "excesses" in production.