Understanding Waterfowl: Ducks on the Move

By Mike Brasher, PhD; Bram Verheijen, PhD; Lisa Webb, PhD; Heath Hagy, PhD

The migration has shifted. Such is the claim many hunters have made to explain their observations of fewer birds at traditional locations. Others point to delayed migrations or birds wintering farther north, due to any number of possible causes. Duck hunters are an observant lot, and their questions frequently provide the impetus for scientific investigations that provide a deeper understanding of waterfowl behavior.

In recent years, numerous studies have asked whether bird distributions or migration patterns have changed, and the results are clear—they have, in various ways and for various reasons. Whether neotropical songbirds, migratory waterfowl, or resident game species, the number and distribution of birds across today’s landscapes are not the same as those our grandparents witnessed. For waterfowlers, well-known examples include shifts in the winter distribution of Canada geese from midlatitude regions to more northern locales. Additionally, white-fronted and snow geese have moved from the Gulf Coast into agricultural landscapes of the Mississippi Alluvial Valley and southern Great Plains. But for ducks, our scientific understanding is still developing.

While earlier research has shown that migrations are occurring later and that winter distributions for some duck species have shifted north, scientists continue to seek additional details and explanations. In this article, we present the results of a recent study in which we used 60 years (1960–2019) of band recovery data to quantify changes in the winter distribution of mallards, northern pintails, and blue-winged teal in the Central and Mississippi Flyways. Thanks to reports from generations of duck hunters and annual banding efforts by state and federal agencies, we obtained records from over 350,000 band recoveries for use in addressing these questions.

This study was unique in asking whether long-term changes in band recoveries, and thus winter distributions, differed among subpopulations of ducks. We defined three subpopulations according to the geographic area in which the recovered ducks had been banded: the Canadian Prairies and Western Boreal Forest, the US Prairies, and the Great Lakes and Ontario. For each subpopulation, we used a metric known as a “kernel density estimator” to delineate regions in which 50 percent and 95 percent of band recoveries were most concentrated. These areas were identified separately for October, November, December, and January to assess whether shifts were more profound in one month than in another. Of primary interest was whether band recovery distributions had changed between a historical (1960–69) and more recent time period (2010–19).

We characterized recovery distributions using multiple metrics, but in this article we discuss only changes in the centroid (a measure of average recovery location) and in the total area of the region encompassing band recoveries.

Band recovery distributions during October and November indeed changed between the 1960s and 2010s but not in a consistent pattern across species or subpopulations. In contrast, shifts in distributions were strongest and most consistent for mallards and northern pintails in December and January, moving northward anywhere from 35 to 240 miles. However, there were important differences among subpopulations and between the overall (95 percent of recoveries) and core (50 percent of recoveries) distributions.

DISTRIBUTION OF MALLARD BAND RECOVERIES IN JANUARY
(during 1960-69 and 2010-19)

For example, the centroid of the overall distribution of January recoveries for mallards banded in the Canadian Prairies and Boreal Forest shifted 240 miles from south-central Arkansas in the 1960s to northwestern Arkansas in the 2010s, reflecting increased harvest in the south-central Great Plains. In contrast, overall recoveries of mallards banded in the Great Lakes and Ontario shifted northeastward 140 miles, while recoveries for mallards banded on the US Prairies shifted due north 160 miles. The overall area accounting for band recoveries also shifted north for all mallard subpopulations, with prominent contractions in southern wintering regions such as coastal Texas and Louisiana. Interestingly, the size of these areas increased, suggesting that the distribution of mallards has become more widespread at the northern extent of their wintering range.

Changes in the core recovery distributions for mallards in January were generally less than for overall recoveries, as the centroids shifted a shorter distance and the area accounting for recoveries decreased in size for two subpopulations. We found that patterns for northern pintails were generally similar to those of mallards, with northeastward shifts in recovery distributions and overall expansions of the regions where recoveries occurred.

Using 60 years of band recoveries, this study sheds new light on changes in duck distributions in the central United States. Most importantly, this and a companion study that examined shifts in average band recovery locations for 15 species of ducks from 1960 to 2019 demonstrate that while winter distributions have changed, notable differences occur among species, months, and subpopulations. Overly general statements about changes in duck distributions do not account for these important differences.

Regarding the question of whether migrations have shifted, scientists note that migration itself is a behavior that allows birds to cope with seasonal changes in resource availability. When landscapes and environmental conditions change across longer time frames, we should expect migratory behaviors to produce similarly persistent shifts in species ranges and seasonal distributions. There is little argument about whether this is happening. Rather, research is now focusing on the factors underlying these changes and asking whether they can be addressed through management, conservation, or land-use activities. The next phase of this study is attempting to answer these questions, helping us understand bird responses to changing conditions and how future conservation efforts can address the needs of ducks and hunters across dynamic landscapes.

Visit ducks.org/migrationchanges for more information about this study and a companion paper that documented long-term changes in band recovery locations for 15 species of ducks in the Central and Mississippi Flyways.

Dr. Mike Brasher is senior waterfowl scientist for Ducks Unlimited, Inc. Dr. Bram Verheijen is a postdoctoral researcher and Dr. Lisa Webb is assistant unit leader for the US Geological Survey Missouri Cooperative Fish and Wildlife Research Unit, University of Missouri. Dr. Heath Hagy is project leader for the US Fish and Wildlife Service Habitat and Population Evaluation Team.