A drought in the western United States is thought to be primarily responsible for a short-term decrease in the North American duck population.

A preliminary report by the U.S. Fish and Wildlife Service on mid-continent breeding ducks and their habitats estimated the total duck population at 37.3 million breeding ducks in the surveyed area. That represents a 9 percent decline over last year's estimate of 41.2 million birds.

While the estimate remains 11 percent above the 1955-2007 long-term average, experts say the short-term population decrease, caused by the temporary drying of small ponds, is reason for concern.

"Habitat is the core factor driving the health of duck populations and the size of the fall flight," said Dale Humburg, chief biologist for Ducks Unlimited. "Habitat also is a key for waterfowl in migration and for hunters. This year, spring and early summer flooding in the Midwest and South, drought in the prairies, and extremely dry conditions in parts of the West Coast, could affect migration and hunting habitat."

Wet and dry cycles are common and necessary components of the health of wetlands, especially those of the prairie potholes of the northern plains. Ducks Unlimited voiced concern that the permanent elimination of habitat during dry periods could turn a cyclical downturn in the duck population into a more serious problem.

A greater long-term threat to waterfowl is the loss of grassland habitat. Ducks Unlimited reports the U.S. prairie pothole region lost more than 800,000 acres of farmland enrolled in an important erosion control program last year, and more than 3.3 million acres of native prairie land is projected to be lost during the next five years