By Richard M. Kaminski, Ph.D.
Dr. Frank C. Bellrose, renowned scholar of waterfowl migration, wrote in his classic book, Ducks, Geese and Swans of North America, "Most of the 775 species of birds in North America migrate, but because waterfowl are highly visible in migration, they epitomize this phenomenon to most people. And to most hunters, waterfowl migration is an eagerly awaited event because of the drama they witness from a blind and the satisfaction it brings to them."
My passion for waterfowl "hatched" near my hometown, Manitowoc, Wisconsin, on Lake Michigan, where I witnessed countless migrating ducks, geese, and swans from beach and boat blinds. As Mary Hopkins sang, "Those were the days, my friend. We thought they'd never end."
Indeed, those days of spectacular skeins of migrating waterfowl have not ended, though some redistributions of the birds have occurred along the flyways. Breeding waterfowl populations have been up and down, generally every other decade since I began waterfowling in the 1950s with Pa's pump gun and paper reloads. These population dynamics are normal. Waterfowl populations rebound with improved habitat conditions as most North American waterfowl species have proven to us repeatedly, and most recently during the 1990s. So long as changing seasons and waterfowl habitats exist along the flyways, waterfowl migrations also will persist. However, waterfowl abundance and their habitats will not be constant; we should expect changes in both time and space.
Although some facets of waterfowl migration remain puzzling, research has fused certain pieces of the puzzle. Because we are currently experiencing or anxiously awaiting fall migration, I'll summarize some of our knowledge on why, when, and where waterfowl migrate during fall in North America.
Why do waterfowl migrate? According to Dr. Bellrose, "the biological reason for migration is survival." Additionally, survival is a prerequisite for reproduction. These two life-history components are all that matter to waterfowl and all wildlife. Hence, through eons and natural selection, waterfowl have evolved behavior to avoid intolerable conditions (e.g., cold and ice, food inaccessibility, disturbance, predation, etc.). They move from one location to find suitable habitats and resources elsewhere, ultimately to survive and produce offspring. Thus, most species of North American waterfowl endure the physical perils and energetic costs of migration each fall by leaving their breeding grounds and flying hundreds to thousands of miles to wintering grounds to survive and subsequently reproduce in spring.
When do waterfowl migrate? The timing of annual migrations is correlated with a complex of physical factors and environmental cues. Our scientific understanding suggests that photoperiod (i.e., proportion of daylight to darkness) is an important stimulus for migration. Even without scientific evidence, this seems reasonable as seasonal variation in daylight and darkness has been essentially constant since the formation of planet Earth. Increasing day length in spring stimulates neural centers in birds' brains and endocrine systems, causing hunger, foraging, and significant gains in fat to fuel migration. This springtime stimulus triggers a "biological clock" in waterfowl and other birds, priming them to progress through reproduction, summer molt, and autumn premigratory body conditioning.
Premigratory body conditioning and zugunruhe, a German word referring to nighttime restlessness and associated activities (e.g., intense feeding, exercising flight muscles, vocalizations, etc.) predispose birds to migration. Indeed, waterfowl get restless and begin migrating in fall even before cold weather and reduced food-and-water availability force them southward.
Some waterfowl migrate in fall before major cold crunches. Well-known examples include blue-winged teal, northern pintails, northern shovelers, gadwalls, wood ducks, canvasbacks, and ruddy ducks. However, cold, windy, and snowy fronts that lock up food and water stimulate major movements of cold-hardy waterfowl, such as mallards, American black ducks, large-bodied races of Canada geese, and certain sea ducks (e.g., goldeneyes, buffleheads, and long-tailed ducks).
Dr. Sidney Gauthreaux, a Clemson University professor and 40-year scientist of bird migration, explains, "When extreme cold temperatures are moving in, waterfowl can sense that ponds are going to freeze, and they're ‘gone with the wind' to reach more favorable conditions. When major cold fronts pass, we see them, on radar, moving from adversity." You may recall a mass migration of waterfowl in November 1995. It was reported that 90 million waterfowl may have engaged in that "grand passage"; so dense were the flocks of birds that radarscopes at major Midwest airports couldn't distinguish birds from airplanes.
Where do waterfowl migrate? Robert Helm, a veteran Louisiana waterfowl biologist, correctly stated recently, "Birds go where they find food, water, and safety. They stay as long as these habitat qualities satisfy their needs."
I frequently tell my students that waterfowl are "sampling specialists" because of the dynamic environments they inhabit; they seek to find suitable habitats that reward them with resources linked to their survival and reproduction. The ultimate reward is "fitness"—a term meaning genetic representation of individuals in subsequent generations. Remember, that's all that really matters to waterfowl and all wildlife. Thus, waterfowl migrate to habitats where past and present "sampling" revealed good prospects for fitness.
But, what if "sampling" reveals suitable habitats and resources distant from traditional wintering areas, such as waterfowl may have experienced during recent warm falls and winters? Then, waterfowl may remain north of traditional terminuses until harsh environmental conditions force them southward. Why? Arguably, because migration is physically costly, and the real rewards are survival and fitness—hence, some waterfowl remain as close to the breeding grounds as their bodies and resources permit. Some have hypothesized that mallards and Canada geese are lingering at northern latitudes, especially when food, water, and safety exist. Might we see increased winter staging (or stalling) around 35 degrees latitude and temperature at least for these species? I can't answer this question without scientific evidence, which is currently lacking, but we are conducting research to address this and other questions related to wintering duck distributions.
To sustain migrations of waterfowl in North America, changing seasons and suitable habitats must persist throughout their annual cycle and range. Indeed, migration is an adaptive strategy hinged on finding suitable habitats. Although timing and destinations of migrants will remain somewhat puzzling and unpredictable, we should feel fulfilled helping provide suitable habitats and knowing our conservation efforts will be rewarded by sights and sounds of waterfowl migrating overhead and the periodic free-fall of ducks and geese into our decoys.
Dr. Kaminski is a Professor of Waterfowl Ecology and Management, Department of Wildlife & Fisheries, Mississippi State University, and an avid waterfowl hunter.