Understanding Waterfowl: The Magic of Migration

When and where waterfowl migrate is determined largely by day length, weather, and food availability

© GARYKRAMER.NET

—By Tom Moorman, Ph.D.

Migration embodies much of what fascinates us about waterfowl. It compels us to wake up at 4 a.m., slog through the marsh, and set decoys before dawn—all in hopes of intercepting a few birds before the majority continue to points unknown. The essence of Ducks Unlimited's mission is to conserve habitat to ensure that future generations can also marvel at the sights and sounds of migrating waterfowl.

Waterfowl are very adaptable. They have evolved with an ability to fly long distances in search of the most favorable habitats and environmental conditions, which can vary year to year based on weather. Migration may seem like an arduous journey, but this behavior enables waterfowl to find food-rich habitats on wintering and migration areas when landscapes to the north are frozen and covered with snow. This allows most of the birds to survive and breed in subsequent years.

Day length, or photoperiod, is the main factor that drives migration. However, changes in day length only prepare birds physiologically for migration. Weather events are largely what stimulate them to actually migrate.

Most hunters recognize that weather affects migration, and many of our most memorable hunts occur on "flight days," when large numbers of birds are moving south on the heels of a powerful cold front. However, waterfowl are flexible in how they respond to weather conditions, which can affect the timing and distance of their movements. This flexibility can affect the number of birds that we see while hunting. You can blame the weatherman when mild weather causes waterfowl to remain to the north, but remember to thank him when winter arrives early and forces birds to move south far and fast. Whether it works in our favor or not, we cannot control the weather, and the birds are simply responding in a way in which they are hardwired—to migrate only as far as necessary to find food and open water. 

Differences in migration timing and distance among waterfowl species are related to the birds' feeding ecology. For example, blue-winged teal, which feed on seeds and invertebrates in shallow wetlands, reliably depart the breeding grounds in August and September because their preferred feeding areas are the first to freeze in fall. Gadwalls, which feed almost exclusively on aquatic plants in slightly larger and deeper wetlands, usually depart breeding areas in October and typically do not wait for winter to arrive before heading south. Like bluewings, they must fuel up, store fat, and depart the breeding grounds before their preferred foods are trapped under ice.

Meanwhile, most diving ducks and mallards may remain up north well into November, until Old Man Winter finally gives them a hard nudge. Canvasbacks, redheads, scaup, and ring-necked ducks typically feed on aquatic plants and invertebrates in larger, deeper marshes that are the last to freeze, allowing them to linger and store fat until bitter-cold temperatures force them to migrate to warmer latitudes.

Mallards sometimes don't arrive on wintering areas until late December, and then in reduced numbers in some years. Mallards, northern pintails, American green-winged teal, and American wigeon consume waste grain in both dry and flooded fields, in addition to feeding on seeds and aquatic plants in wetlands. For these birds, especially mallards, cold weather often isn't enough to send them packing to their winter haunts; significant snowfall must also cover their food supplies in harvested grain fields and wetlands. Mallards in good condition, with fat making up nearly 25 percent of their body mass, can withstand periods of up to seven days without food, allowing them to wait out short periods of harsh weather. If a winter thaw occurs, they may even move north again. This "strategy" enables the birds to minimize the risks associated with migration and allows them to return to the breeding grounds as soon as possible in spring. 

Weather also influences migration and winter habitats. Droughts or floods during the growing season can reduce summer plant growth and thereby impact fall and winter food abundance. Fall and winter droughts and floods can also impact food availability. A lack of water, or water that is too deep, can limit access to food resources, while widespread shallow flooding can create extensive new feeding habitats, causing waterfowl to disperse widely across the landscape. 

Waterfowlers hope for winter storms that dump snow and freeze wetlands to the north, pushing waves of waterfowl down the flyways. They also wish for enough rainfall to flood traditional hunting areas. But always remember the adage: "Be careful what you wish for."

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Photo © GARYKRAMER.NET

 

Last year, waterfowl appeared to be scarce across much of the eastern half of the continent. Early winter storms in October and early November had many hunters cheering the weather forecasts, but in late November temperatures warmed substantially as far north as prairie Canada, where snow and ice were nearly nonexistent well into January. According to the National Centers for Environmental Information, the period from August 2018 to January 2019 was among the warmest on record for the United States and the wettest in more than a century in eastern states. The combination of warm temperatures and heavy precipitation undoubtedly exerted a strong influence on the migration by creating abundant feeding habitat, which significantly reduced the southward movements of waterfowl. That was a perfect recipe for a challenging duck season for hunters.

Given the impact of the weather on waterfowl migration, what might the future hold for waterfowlers? In short, they should expect significant variation in the timing of waterfowl migrations, which will likely coincide with increasingly variable and warmer weather conditions. Waterfowl managers will need to understand these long-term trends and consider changes in season dates and wetland management to provide habitat for ducks and hunters during peak waterfowl abundance in their respective areas. 

DU scientists are working to understand potential changes in the winter distribution of waterfowl. And, importantly, we are contemplating ways to communicate any changes that are detected to hunters. We cannot control the weather, but we can adjust our conservation programs to ensure that waterfowl have habitat options despite highly variable weather conditions. We can also help hunters understand how waterfowl are likely to respond to warmer winters in coming years. 

The next time you hear wings ripping the air as a flock of dabblers or divers drops altitude to investigate your decoys, remember you are witnessing a finely honed behavioral adaptation that has allowed the birds to thrive for millennia. Hopefully, you will make a good shot, and as your dog retrieves your prized bird, know that your support for Ducks Unlimited will help perpetuate the experience for generations to come. 


Dr. Tom Moorman is Ducks Unlimited's chief scientist.