Milder cold fronts can cause behavior changes in waterfowl on the wintering areas, even if they don't stimulate mass movements. "Temperature affects food choices," Humburg says. "When the temperature falls, ducks shift from seeds and moist-soil foods to ‘hot foods' such as corn. But if the weather moderates after a front, the birds may shift right back. I've had some great hunting in the late season over natural, moist-soil wetlands. Any time you have a temperature change there may be a diet shift."
Although waterfowl may fly south at any point during a cold front, they often use the northerly winds behind a cold front to migrate more efficiently. These arctic winds create the chilling side effects associated with cold fronts, but they also provide tailwinds for migrating birds.
"Generally, most waterfowl arrive just behind a front," Gauthreaux says. "Powerful fronts are huge wedges of cold air, and they tend to travel fast. Waterfowl are fast fliers so it is possible for them to jump ahead of a front, but the conditions aren't really favorable for them to do that because they would be flying into the wind. We see much larger movements behind a front because the flying conditions are more favorable."
Warm fronts are warm air masses that are replacing cooler air masses, but their effect is often more gradual than that of a cold front. Expect to see low-hanging clouds, precipitation, falling pressure, and often fog as a warm front approaches, followed by rising temperatures and stable pressure. Warm-ups and the weather phenomena associated with them can have a surprising impact—and not always a negative one—on your hunting.
Many hunters see a warm forecast and assume the hunting will be slow. But if your area has been frozen for a week, a warm-up can be just the ticket to bring the ducks back.
"If lots of birds that are scattered over a wide geographical area are suddenly pushed south because of hard weather, you can get a sharp population increase in concentrated areas farther south," Gauthreaux says. "As soon as conditions moderate, many of them may go back north. In a way, they act like the mercury in a thermometer, moving down (south) when it is cold and up (north) when it is warm."
Humburg says the effects of a warm-up can be dramatically different depending on the time of year. "Things just may go flat during a long warm period in November," he says. "But you can definitely see some reverse migration later in the year, especially from mid-January on. Many different biological things are going on at that time of year that weren't taking place in November. By that time of year, 90 percent of mallards are paired. This causes changes in habitat preferences and behavior. Pairs become more isolated, and all birds are putting on fat reserves for their return migration."
Rain is often associated with warming periods in the winter, and it can have both immediate and long-term effects on hunting. "As it's falling, heavy precipitation will put birds down," Gauthreaux says. "From an aerodynamic standpoint, raindrops disrupt wind flow over the wings, causing birds to expend too much energy when flying."
But the situation can change quickly once the rain passes through. Increased runoff from large amounts of rain can flood fields and bottomland, providing new food sources for waterfowl. At the same time, the feeding areas that birds had been using may become too deep—puddle ducks can't reach food in much more than 18 inches of water.