Diversity and Density of Shelterbelt Bird Communities

The number of bird species and the density of each species were monitored in 69 shelterbelts in eastern South Dakota during spring migration and breeding seasons in 1976 and 1977. A total of 44 different species of birds were found during breeding and 68 species during spring migration. Approximately 60 to 80% of the species in a shelterbelt eat insects as part of all of their diet. Most of these bird species are territorial. Usually only 1-2 pairs of a territorial species will reside in any one shelterbelt. This low density is caused by the limited habitat area that shelterbelts provide. Shelterbelts are, essentially, forest islands surrounded by cultivated and natural grasses. Area of the shelterbelt accounted for approximately 60% of the variation in the number of species and density of the bird communities in both season as a result of the “island effect”. The limited food space provided by these forest islands makes ecological isolation among coexisting species necessary for birds to replenish energy stores lost due to migrational flights. The importance of shelterbelt area on species numbers during the breeding season can be partly attributed to the minimum area requirement of territorial pairs during breeding. Some species will not reside in shelterbelts below a minimum size due to the large territory size these birds require. However, minimum area does not explain the upper limit placed on the number of species that will coexist in shelterbelts. Diffuse utilization of the limited food supplies was postulated as setting the upper limit. Bird species that coexisted tended to exhibit different foraging strategies, thus reducing overlap in use of food resources. Implied increases in territory size with increases in the number of coexisting species were found. Theories on species-area models were re-evaluated in terms of competitive saturation. Ability of the species source pool to supply enough competitively different species to saturate the available food space for the smallest islands was postulated as the reason for the high species-area slope found. All species-area relationships were evaluated in terms of one general curve and were considered a sub-section of that curve. Placement on the curve, and consequently, the slope of the species-area relationship, was related to immigration and extinction rates, based on the effective source pool size. The effective source pool size was related to the actual source pool size, the distance of the archipelago from the source pool, and the overall vagility of the species comprising the source pool. The idea of diffuse competition influencing the territory size of bird species was further investigated by mapping territories of yellowthroats, house wrens, and brown thrashers in 2 large shelterbelts. Results indicated that territory size of these species was larger in belts with greater number of coexisting species than in the smallest shelterbelt size colonized by one pair of each species. The variation in community diversity and density unexplained by area was attributed to environmental factors and sampling error. The effect of area was removed. The transformed data were analyzed to provide management alternatives using multiple regression to delineate the environmental factors influencing community diversity and density during both migratory and breeding seasons. A shelterbelt that is subjected to light grazing to eliminate a heavy shrub understory and enhance development of a lust; herbaceous lay considered optimal for both diversity and density. Dense rows of shrubs along the borders of the belts also contributed to an increase in the bi9rd population. Heavy grazing or mowing of the belt after the belt was well established reduced bird diversity. Utilization of tree species that provide open foliage conditions, such as Siberian elm, led to increased bird diversity and density. Multiple regression analyses of the environmental factors influencing 15 of the bird species commonly inhabiting shelterbelts were performed. IN general, the 14 species preferred a shelterbelt configuration similar to that described for the diversity and density measures. In addition, specific preferences of each species suggested ways of modifying the bird community composition. Removal of eastern red cedar may lead to a reduction in noxious species. Planting of green ash, due to it infection by heartrot, and retaining snags enhances the presence of hole nesting species such as house wrens and woodpeckers. Other species preferences are discussed