By Mike Brasher, Ph.D.
For much of North America, the winter of 2013-2014 will not soon be forgotten, as it brought record snowfall and the coldest temperatures in at least 20 years to many locations. These extreme winter conditions were caused by deep troughs in the jet stream, which enabled Arctic air to plunge southward into regions east of the Rockies. Granted, bone-chilling temperatures occur routinely across Canada and the northern tier of the United States, and it's not uncommon for large portions of the continent to experience below-average temperatures for brief periods each year. But last winter was still harsher than most in many areas.
The invading Arctic air first arrived in December and persisted well into March. In Duluth, Minnesota, for example, the average temperature from December through February was 4°F. In Watertown, New York, the average high during March was 31°F. And in Winnipeg, Manitoba, the temperature rose above freezing on only eight days between November 1 and March 31.
While ice cover on the Great Lakes is normal in winter, this past year it was nothing short of spectacular. At its peak on March 6, more than 92 percent of the surface of the Great Lakes was locked in ice, the second largest ice coverage in 40 years of record keeping. Even more impressive, one-third of the surface of the Great Lakes was still covered with ice during the last week of April, and the official ice-free date wasn't declared until June 7, the latest on record.
When duck hunters hear of extreme winter weather, they instinctively focus on how it may impact their hunting success. In northern states, such forecasts can inspire dread of a rapid freeze-up that will bring an abrupt end to the waterfowl season, while in southern states the prospect of an Arctic cold front or winter storm in the Midwest can create eager anticipation for major movements of birds to their wintering grounds. The severity of winter weather is known to affect the timing and intensity of waterfowl migration, but occasionally conditions become so extreme that the birds find themselves in a life or death struggle against the elements. The winter of 2013-2014 provides a recent example, as late February and March brought numerous reports of starving and dying ducks, primarily on the Great Lakes and in northeastern states.
Waterfowl possess remarkable adaptations to survive in cold weather, including dense layers of insulating feathers, counter- current blood flow to reduce heat loss through their feet and legs, behavioral modifications to reduce exposure to the elements, the ability to carry large fat reserves, and perhaps the greatest adaptation of all-migration. Considering these adaptations, it's natural to wonder why waterfowl would ever find themselves at risk of death due to winter weather.
The greatest challenges faced by waterfowl during severe winters are elevated energy demands required to maintain their internal body temperature and restricted feeding opportunities, resulting from ice and snow cover on foraging habitats. During severe cold snaps, waterfowl often simply hunker down to conserve energy until the weather moderates and foraging habitats thaw. During these periods, the birds must burn fat reserves to keep warm, which ultimately causes loss of body mass. As long as they are brief, these bouts of cold weather pose little threat to waterfowl, because the birds can rapidly replenish fat reserves when warmer weather returns. However, if extremely cold temperatures persist or worsen, the birds' body mass will continue to decline, ultimately reaching a point at which fat reserves may be insufficient to sustain the birds. At this stage, waterfowl must either migrate or face starvation and eventual death.
While instances of death due to starvation are seemingly rare in waterfowl, some species appear to be at greater risk, such as those with more restrictive diets or whose preferred foods and habitats are in northern latitudes. These species include various diving and sea ducks that forage on mollusks, fish, and crustaceans in large, deep bodies of water. The primary species impacted by bitter cold temperatures during the winter of 2013-2014 were red-breasted and common mergansers, white-winged scoters, common goldeneyes, and scaup. Most observations of starving and dying ducks occurred on the Great Lakes, which during normal years remain sufficiently ice-free to give birds access to underwater food sources throughout winter. But this past year, many waterfowl that attempted to overwinter on the Great Lakes found themselves locked out of essentially all foraging areas by late February due to the unrelenting winter, resulting in the loss of many birds to starvation.
This is not the first time waterfowl populations have experienced extremely cold winters, and it certainly will not be the last. It is striking how similar the observations from 2013-2014 are to those recorded during previous severe winters, including those of 1935-1936, 1976-1977, 1978-1979, 1983-1984, and 1993-1994, to name a few. Although these events seldom have significant impacts on overall waterfowl populations, exceptions do exist, such as the winter of 1976-1977, when more than 60 percent of the Atlantic brant population was thought to have perished while bays and estuaries along the East Coast were locked in ice.
Fortunately, as long as waterfowl have sufficient habitat in traditional wintering areas across North America, the risks posed to the birds by any single winter weather event are small. Indeed, this reality highlights the importance of robust conservation efforts on the wintering grounds, which help ensure that waterfowl survive the winter and return north the following spring in good breeding condition.
Based in Lafayette, Louisiana, Dr. Mike Brasher is biological team leader of the Gulf Coast Joint Venture.