What Factors Might Affect Duck Population?
Weather and landscape conditions, food availability, water depth and more can all affect the number of ducks living in a given area at any time
"Where are the ducks?" It is perhaps the most frequently uttered question in duck blinds across North America. Answering this question requires an understanding of the various factors that influence waterfowl movements and habitat use during fall and winter. Migratory behavior is deeply ingrained in most waterfowl species, and although seasonal changes in the amount of daylight stimulate this innate urge, the precise timing of the birds' migrations can vary considerably among species, individual birds, and locations.
Large movements of waterfowl regularly occur in response to weather conditions such as powerful cold fronts, subfreezing temperatures, heavy snow, and flooding rains. Over longer periods of time, changes in agricultural practices, habitat loss, declining water quality, and milder weather patterns can impact waterfowl distribution along the flyways. Moreover, as experienced waterfowlers know, local wetland conditions and hunting pressure can have a tremendous impact on the number of birds that we see from our blinds.
In recent years, scientists have made remarkable strides in better understanding the many factors that influence waterfowl migrations and habitat use across this continent. For example, waterfowl biologists at Mississippi State University and the Missouri Department of Conservation recently developed a system linking the timing of major waterfowl migration events to weather conditions. Using a numeric weather severity index, the researchers were able to forecast duck migrations based on the extent and duration of subfreezing temperatures and snow cover in northern and central states. Species-specific models confirmed that cold-hardy birds such as mallards and American black ducks often endure extended periods of harsh winter weather before moving south. In contrast, American wigeon, American green-winged teal, and northern shovelers were found to be among the least cold-tolerant ducks, which migrate well in advance of hardier species. These models are also being used to study the potential effects of increasingly mild winters on future migration patterns, and serve as the basis for weekly duck migration forecasts, which are posted online during fall and winter by researchers at the State University of New York College of Environmental Science and Forestry (schummerlab.weebly.com).
Waterfowl distribution and movements can also be influenced by landscape-level changes in habitat availability caused by heavy rainfall and associated river flooding.
A classic study of northern pintails equipped with radio transmitters in southwest Louisiana discovered that many of the birds left the Gulf Coast and flew northeast to meet major rain events in the Mississippi Alluvial Valley, presumably to take advantage of newly flooded feeding habitats. A mid-1980s study of mallards in Mississippi and Arkansas found a similar pattern, with birds moving up to 30 miles in response to heavy rains and overbank river flooding.
Other changes in the distribution of migrating and wintering waterfowl can occur over many years or decades. On Chesapeake Bay, declining water quality and the loss of wild celery and other submersed aquatic vegetation caused long-term declines in the abundance of canvasbacks and many other waterfowl species. In the Central Valley of California, the distribution of white-fronted geese and dabbling ducks shifted regionally over a 10- to 20-year span in response to changes in rice production and wetland restoration efforts. And along the Texas Gulf Coast, milder winters, declining rice agriculture, shrinking coastal wetlands, and urban expansion have caused a noticeable decrease in wintering dabbling duck numbers in recent decades.
Although weather and landscape conditions can impact waterfowl distribution on continental and regional scales, waterfowl numbers in specific areas are often influenced by more localized conditions. Researchers at the University of Missouri, in partnership with Ducks Unlimited and state wildlife agencies, tracked female mallards with GPS devices to learn how their winter movements were affected by various landscape features. In this study, mallards were found in greatest abundance in areas that had a variety of wetland types in close proximity to high-energy agricultural crops. These findings were consistent with the results of a separate investigation in Mississippi and long-standing recommendations to manage for a diversity of waterfowl habitats on migration and wintering areas.
On an even smaller scale, waterfowl use of individual wetlands can be heavily influenced by variables such as food availability, water depth, and vegetation structure. Energetics drive waterfowl behavior during winter, so wetlands that contain abundant food resources are typically of greater benefit and receive greater use by the birds. However, food production alone isn't enough to guarantee that large numbers of waterfowl will flock to a particular wetland. Extensive research has shown that waterfowl foods must be accessible at the appropriate depths – shallow for dabbling ducks and deeper for divers – to be exploited by the birds. The structure and variety of wetland vegetation is also important. In many cases, wetlands with a 50:50 ratio of emergent plants to open water provide the optimal mix of food and cover for waterfowl.
Of course, waterfowl use of even the highest-quality habitats can be greatly reduced by excessive disturbance, whether from hunting, boating, vehicle traffic, or other human activities. Most waterfowlers who hunt on public land have witnessed the adverse effects that excessive pressure can have on the number of ducks they see as well as the birds' willingness to decoy. While disturbance typically has its greatest impact on individual wetlands, a study by the Illinois Natural History Survey found evidence that total duck use was lower in areas where waterfowl had fewer places to feed and rest without being disturbed. Similarly, a radio telemetry study in the Mississippi Flyway discovered that female mallards tended to select wetlands in areas with higher levels of sanctuary.
As these and numerous other scientific investigations reveal, a wide range of factors – acting on both large and small scales – can influence how many ducks we see during our hunting trips. And no matter how good the habitat is where we hunt, sometimes the birds simply find other areas they like better. Fortunately, exciting new technologies are being applied by researchers to help us understand these complexities in ways that were previously not possible. While we may never be able to fully explain when and where waterfowl go, these new revelations will help us demystify the movements and habitat preferences of these remarkable migratory birds.