The shift is evident in Texas farming statistics. In 1950, the state raised 57 million bushels of corn; in 2000, it raised 236 million bushels. Much of the increase came from the irrigated fields of the Panhandle. Waste grain was a mixed blessing for wintering ducks and geese -- on one hand, it provided a huge new supply of carbohydrate-rich food; on the other, it was low in protein and other nutrients.

Irrigation itself also had mixed effects on waterfowl. Irrigation water helped fill playas when snow and rain didn't, but row-crop farmers filled or drained many playas and deepened others to act as tailwater pits. The deeper ponds held water longer than the old playas, but they had less shallow water and produced less natural food and cover as a result. And plowing around the margins of playas reduces the amount of food and cover, just as it does in the prairie potholes.

The new landscape was a complicated tangle of shifting climate, farming practices and ecological responses, but for 30 years or so, it was a dependable oasis in the Central Flyway, and the numbers of waterfowl wintering there continued to grow. The mid-winter inventory in 1981-82 found 187,000 mallards in northwest Texas alone. During one particularly favorable winter in the 1970s and early 1980s, more than 800,000 pintails passed the winter in the Panhandle. Snow geese and Canadas recognized the reliable source of food and water as well.

Unfortunately, the aquifer couldn't sustain the growing demands of irrigation. By the late 1950s, water levels in some parts of the Ogallala were dropping as much as five feet a year.

Faced with the rapid decline in available water, farmers have taken steps to use it more carefully. Still, the water level in the Ogallala on the southern plains continues to drop. Studies done in the last five years suggest that the top of the water table may drop 150 feet or more by 2050 -- in some places that will mean no more groundwater.

This loss of groundwater will have several effects on the southern plains and their waterfowl. First, many of the springs that feed rivers in the region will become less dependable or may dry up entirely. Reservoirs will shrink or dry up, especially during droughts, depriving ducks and geese of the roosts they've learned to use over the last 50 years. This has already happened on the Buffalo Lake National Wildlife Refuge south of Amarillo and the Optima National Wildlife Refuge in the Oklahoma Panhandle. Less standing water will mean fewer wintering waterfowl.

Farmers who are anxious to reduce their demands on the aquifer will choose crops that require less water. As the amount of waste grain dwindles, more birds will move elsewhere.

More careful use of irrigation water means less run-off. Playa basins that have been filled as an unintended consequence of farming will once again fill or dry up as the vagaries of rain and snowfall dictate. These kinds of fluctuations have already emerged in the Texas Panhandle. Bill Johnson, waterfowl biologist with the Texas Department of Wildlife and Parks, has been following duck numbers in the Panhandle. In 2001, only a few playas had any water at all, and more than 90 percent of them froze during the winter. That year, less than a thousand pintails and about 10,000 mallards showed up on the mid-winter survey. In 2005, more than half the playas had water, and most of them stayed open through the winter -- the mid-winter count found 225,000 pintails and 160,000 mallards.

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