RICH HILL, Mo. - After 32 years of hunting ducks here in the wetlands of west-central Missouri, Chuck Geier knows when temperatures will drop and waters will freeze. That means he also knows when the birds will fly and hunting will be best.

 Except that now much of what he knows is in question.

"It used to be by Dec. 6, this place was frozen," said Mr. Geier, 51, a national sales manager for a telecommunications company. "That's not true anymore."

From the "prairie potholes" of Canada and the Upper Midwest to the destination states of Arkansas and Louisiana, the rhythms of the cross-continental migratory bird route known as the Mississippi Flyway are changing.

Here in Missouri, where the five-year winter temperature average has been on a striking ascent, hunters say birds are arriving later and sticking around longer before bolting for warmer redoubts. Elsewhere, wetlands are not freezing over the way they once did.

As hunters point their shotguns toward the sky and fire, a question echoes in the spent powder: what, please, is up with the ducks?

"People say it's cycles, every five to seven years, but it's just been too long," said Mr. Geier of the warming trend, which he traces to the late 1990s. "It's a wake up call."

Five-year averages for "duck use" days on some conservation areas in Missouri show peaks that come a week or more later in the year than do the 30-year averages. Hunters have said in state surveys that they want later hunting seasons, reflecting the later arrival of major weather systems that move birds into the state.

The Mississippi Flyway is one of four main migratory bird routes that bisect the country, and hunters in some other regions of the country have also reported shifts in duck behavior.

"We're having milder falls, later winters," said Dave Erickson, chief of the wildlife division for the Missouri Department of Conservation. "What we don't know is if the trend that affects migration and the hunters' desire for a longer hunting season is a temporary fixture or a permanent fixture."

Sure science is elusive. Scientists and state wildlife officials say there is no clear-cut data to support the reports of changes in duck behavior, but the patterns are familiar. They note that various other animal species, from songbirds to frogs and foxes, are developing different breeding and migration patterns.

"We're seeing northern range shifts of lots of birds and butterflies," said Dr. Camille Parmesan, a professor of conservation biology at the University of Texas and a member of the United Nations panel recently awarded the Nobel Prize for its work documenting climate change.

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