The region covers the Dakotas, Iowa and Minnesota, plus bits of several other states. It also includes portions of Alberta, Saskatchewan, Ontario and Manitoba in Canada. But it's being diminished in size, due mainly to corn growing.
Tax subsidies promoting corn-based ethanol are hastening the area's destruction. Source: a recent study released by the National Wildlife Federation, with data gathered by researchers at the University of Michigan.
More than 3.2 million acres of the Prairie Pothole region in the U.S. have been plowed under in order to make room to grow more corn. Much of that corn is grown to meet America's questionable need for ethanol, an ingredient in fuel that allegedly creates cleaner combustion and lowers emissions when blended into real gasoline.
The USDA provides corn-growing incentives, such as tax breaks, crop insurance and low-interest loans which virtually guarantee a grower's profit regardless of the crop yield. If the demand for ethanol remains steady, an additional 10.6 million acres of corn could be planted in the next year. Grower's will reap a profit without taking any risk, and bird populations will keep declining.
NOTE: I've used the term "grower" instead of "farmer" because most entities that now grow corn are corporations.