Grasslands in the Northern Prairie region support a diverse assemblage of migrant birds and facilitate fattening during migratory stopover

Abstract

Grasslands have been reduced as a result of conversion to agriculture and other anthropogenic uses. Consequently, grassland migrant birds may have limited options for migration stopover. In addition, the simple presence of birds in grassland fragments during migration periods does not imply that the habitat is providing the food resources necessary for birds to replenish fat stores and continue migration. We examined (1) the diversity of birds using grasslands during migration stopover in southeastern South Dakota and (2) whether a reconstructed prairie can provide the resources necessary for successful stopover. To accomplish these goals, we conducted line transect surveys on plots dominated by either cool- (primarily non-native) or warm-season (primarily native) grasses and captured birds on a reconstructed tallgrass prairie to determine plasma levels of triglycerides, an indicator of fat deposition, and β-hydroxybutyrate, an indicator of fat depletion. Grassland, woodland/shrub, and habitat generalist bird species were all present on study grasslands during spring and fall migration seasons. Overall, species-richness tended to be greater at warm season dominated sites during spring migration, but richness differences between sites varied by year during fall migration. Overall bird densities, however, were roughly similar between the two habitat types at both seasons, although some differences for individual bird species occurred. Plasma triglyceride and β-hydroxybutyrate levels of migrant birds using the reconstructed tallgrass prairie were consistent with those from birds in known high-quality stopover habitats. These findings suggest that grasslands that have a diversity of plant species and structural heterogeneity may be important for stopover sites for a variety of migratory birds using grasslands as stopover sites, including declining grassland species. These data further suggest that the creation and management of restored and reconstructed prairies throughout the migratory routes for grassland birds may provide quality stopover for these species.