Receding glaciers sculpted the shallow wetlands of the Prairie Pothole Region in central North America, and ten thousand years ago melting glaciers created millions of shallow depressions called potholes that became North America's most important waterfowl breeding habitat. This area supports insect and plant life important to waterfowl, where ducks and geese feed on insects hatching in the wetlands, nest in prairie grasses, rear their young in permanent and temporary wetlands, and remain until the ducklings are big enough to fly south in the fall. The Prairie Pothole Region lies at the heart of the Central Flyway, one of North America's four major waterfowl migration corridors, but since the 1900s, forty to seventy percent of its wetlands have been drained for agriculture and other development, making conservation of what remains more critical. The U.S. Fish and Wildlife Service protects habitat by purchasing waterfowl production areas, wetland easements, and grassland easements to conserve migratory bird habitat and preserve duck and goose breeding grounds. The prairies of the Great Plains are also disappearing, though they provide food, nest sites, and shelter for a unique mix of songbirds, game birds, raptors, and other grassland birds. Before the 1800s, prairie grasslands covered nearly one-third of the United States, but since then nearly half has been converted to farmland and other development, contributing to sharp declines in grassland bird populations; in the past half-century, populations of nearly nine out of ten prairie bird species have steadily dropped, and today grassland birds are the most imperiled group of birds in North America. Protecting large tracts of grassland habitat, including places such as Maga Ta-Hohpi Waterfowl Production Area, is identified as one key to protecting these birds. In the Great Plains, low rainfall and frequent fires historically created ideal conditions for grasslands, with tallgrass prairie in the wetter east, shortgrass prairie in the drier west, and mixed-grass prairie in between. Grassland birds depend on these habitats year-round, and conservation efforts include purchasing waterfowl production areas, establishing conservation easements with private landowners, and working with partners. Insects in wetlands and grasslands provide essential protein for birds in spring and summer, but pesticides and insecticides reduce the number and diversity of insects available for birds to eat. Because most grassland birds nest on the ground, their eggs, incubating adults, and fledglings are especially vulnerable to predators such as raccoons and skunks, and waterfowl production areas, grassland easements, predator removal, and delayed haying help improve nesting success.