"Many species of birds that depend on grassland or savanna habitats have shown substantial overall population declines in North America. In order to understand the underlying causes for these declines, we examined the habitat requirements of grassland birds in six regions of the continent. Open habitats were originally maintained by ecological drivers (continual and pervasive ecological processes) such as drought, grazing, and/or fire in tallgrass prairie, mixed grass prairie, shortgrass prairie, desert grassland, and longleaf pine savanna. In contrast, grasslands were created by occasional disturbances (such as fires or beaver activity) in much of northeastern North America. The relative importance of particular drivers or disturbances differed in different regions. Keystone species of mammals (grazers such as prairie-dogs and bison in western prairies and dam-building beavers in eastern deciduous forests) played a crucial, and frequently unappreciated, role in maintaining many grassland ystems. Although fire as important in preventing invasion of woody plants in the tallgrass and moist mixed prairies, grazing played a more important role in maintaining the typical grassland vegetation of shortgrass prairies and desert grasslands. Heavy grazing by prairie-dogs or bison created a low S2grazing lawnS3 that is the preferred habitat for many grassland bird species that are restricted to the Great Plains and desert grasslands. Ultimately, many species of grassland birds are vulnerable because people destroyed their breeding, migratory, and wintering habitat either directly by converting it to farmland and building lots, or indirectly by modifying grazing patterns, suppressing fires, or interfering with other ecological processes that 3 originally sustained open grassland. Understanding the ecological processes that originally maintained grassland systems is critically important for efforts to improve, restore, or create habitat for grassland birds and other grassland organisms. Consequently, a high priority should be preservation of large areas of natural or semi-natural grassland where these processes can be studied. However, some grassland birds now primarily depend on artificial habitats that are managed to maximize production of livestock, timber, or other products. With a sound understanding of the habitat requirements of grassland birds and the processes that originally molded their habitats, it should be possible to support their populations on S2working landS3 such as cattle ranches, farms, and pine plantations. Proper management of private land will be critical for preserving adequate breeding, migratory, and winter habitat for grassland and savanna species."