Diversity and Abundance of Spring Migratory Birds Using Habitat Islands on the Great Plains

Abstract

Relationships of area with numbers of species and individuals of spring migrants were examined for 69 shelterbelts (forest islands) in eastern South Dakota. Total abundance and number of species were as highly correlated with area during spring migration as during the breeding season. The relationships of area with total abundance and number of species were highly similar between 2 yr of study. Area was more important in determining abundance and number of species than either diversity of plant species or isolation of the islands. The dispersion of migrants among islands, indicated by the relationships of area with total abundance and species numbers, could have been a result of passive dispersal, selection for larger area or behavioral interactions. Passive dispersal was unlikely because the isolation of an island had no significant influence on abundance or diversity. The diversity and abundance of migrants were modified by habitat conditions (i.e., vegetation diversity), indicating that migrants select the forest islands they inhabit. Migrants may select large areas but should increase with area at an increasing rate, but this did not occur. Dispersion among islands may be the best way for migrants to replenish their energy reserves when food is scarce. Two facts suggest that migrants may interact to disperse themselves relative to food. Application of a model for interacting species provided increasingly better fits to ecological groups that increasingly confined their foraging within shelterbelts. The density of birds was greater in smaller islands than in larger islands owing to species that did not feed solely within the islands. The density of birds that did feed primarily within the islands remained constant with changing area.