The Great Scaup Mystery

© Brent Gale

By Jean-Michel DeVink, Ph.D., and Stuart Slattery, Ph.D.

Most duck hunters don't need a waterfowl biologist to tell them that there have been fewer bluebills heading south each fall. The breeding population of scaup has declined steadily since 1980, dropping from about 6.3 million birds in the 1970s to fewer than 3.5 million in 2007. Much of this decline occurred within Canada's western boreal forest, where most scaup breed, and to a lesser degree on the Canadian prairies. Although the continental population is almost 40 percent below the North American Waterfowl Management Plan goal, scaup are certainly in no danger of extinction. Nonetheless, despite a growing body of research, we are still not sure what can be done to rebuild scaup populations and whether their decline indicates larger environmental problems.

During the 1980s and early 1990s, many of the continent's waterfowl populations declined, partly because of drought on the prairies. But when most other species began to rebound, scaup continued to decline. Their inability to bounce back caused a stir within the waterfowl community. So, in 1998, DU Canada cosponsored a workshop with the U.S. Geological Survey at the Northern Prairie Wildlife Research Center to assess our knowledge of scaup. Forty-five participants representing state, federal, academic, and nongovernmental organizations, including many of DU's biologists, attended the workshop. After a series of presentations, followed by much discussion, participants agreed that, based on our limited evidence, the decline appeared to result from a reduction in either female survival or production of young.

Attendees speculated that increased contaminant exposure and/or decreased food availability along with other unknown habitat changes on wintering, staging, and breeding grounds could be primary causes of the scaup decline. These factors were thought to be limiting the females' ability to store nutrients for migration and egg laying, thus influencing reproductive efforts later in spring. Assessing how changes in wintering or migration habitat might be affecting reproduction would prove difficult, because researchers had little information from primary breeding areas in the boreal forest.

This list of potential causes helped frame scaup research for the next seven years, and in 2006, more than 60 biologists and managers from across the continent reconvened in Bismarck to reassess old hypotheses, discuss new ones based on recent information, and plan next steps.

Population Change

Changes in duck populations can result from changes in one or more components of the annual breeding cycle, such as adult survival, nest success, and clutch size. Two recent studies found that factors affecting female survival and productivity are most likely to have influenced scaup population trends.

As with most ducks, scaup nest success varies greatly by location and from one year to another. A study in the Manitoba parklands found that scaup nest success appeared to be lower than that of ring-necked ducks, possibly because scaup nest more in the uplands than over water. But in the western boreal forest of Alaska, no difference was found between nest success of scaup and other ducks, and in some years, scaup nest success was fairly high. In other years and in some locations, nest success in the boreal forest was considerably lower. The reasons for such highly variable nest success in wilderness areas are unknown, and unfortunately, we lack historic data on nest success by which we might determine if there has been a long-term decrease in this vital rate. And we know even less about other vital rates contributing to scaup productivity, especially in the western boreal forest.

Similarly, a decline in female survival could reduce populations. Several die-offs related to parasite infections have occurred, but the relative importance of these events is difficult to assess. We are uncertain if such die-offs have been increasing or have always occurred, but recent research found that the levels and species of parasites present in wintering scaup were similar to historical values.

Little is known about the causes and rates of scaup mortality during and outside the breeding season, but scaup harvest undoubtedly contributes to annual adult mortality. Since 1986, bag limits on scaup have been reduced several times in an attempt to help the population recover. However, biologists are uncertain whether harvest restrictions have had or will have any affect on the size of scaup breeding populations. Everyone agrees that more research is urgently needed to better understand this issue, including direct estimates of harvest rates and adult survival.


Some biologists have speculated that environmental contaminants may be a factor in the scaup population decline. However, researchers needed to determine which contaminants the birds were encountering and whether those pollutants were affecting the ability of adult females to breed or survive. And if so, were sufficient numbers of birds being affected to result in continental declines?

Research conducted before the first workshop indicated that although some scaup in the Great Lakes and on both coasts accumulated high levels of a few contaminants, the greatest concern was selenium. Selenium is a trace element required in small quantities for good health, but too much of it could deform or kill developing ducklings or cause health problems in adults. Concern about the potential effects of selenium heightened when researchers discovered that zebra mussels accumulate selenium in high concentrations. Studies showed that zebra mussels were fast becoming a preferred food of scaup, particularly during winter and migration in the Great Lakes.

Meanwhile, breeding ground studies conducted in the western boreal forest and parklands after the first workshop found that selenium concentrations in scaup eggs were below critical levels and that most eggs under observation hatched. Moreover, females on the breeding grounds also had much lower concentrations of selenium in their bodies than those sampled from wintering and staging areas. One study even found that females with more selenium were in better body condition, on average, than those with lower levels.

Further research clarified this discrepancy between higher contamination in the south and no obvious effects on breeding areas. In a captive study, breeding females quickly eliminated much of the selenium in their bodies once they were no longer fed contaminated food. Because the breeding grounds are generally thought not to be contaminated with selenium, birds with high levels in their body on staging areas likely will reach normal levels by the time they lay eggs. Therefore, it seems unlikely that selenium is causing problems for breeding scaup. But there is still uncertainty about whether it is causing some birds not to reach the breeding grounds, or not to breed when they get there, particularly birds that winter in contaminated areas. New research is also examining other potentially harmful contaminants.

Habitat Conditions and Reduced Food Availability

Spring Staging

Historically, large numbers of scaup used wetlands in the upper Midwest for staging while on their way to breeding grounds farther north, and some even used these wetlands as breeding habitat. Researchers speculated that large-scale habitat loss and degradation of many remaining wetlands in the region were adversely affecting female body condition and thereby contributing to scaup declines. Indeed, subsequent research showed that female scaup passing through this region were in poorer body condition than birds several decades before, a difference linked to reduced quality of food available in the remaining wetlands.

These results underscored the important role of spring staging wetlands in building nutrient reserves. However, we don't really know how scaup migration patterns responded to these habitat changes or whether poor body condition was or is experienced by sufficient numbers of scaup to have continental impacts on the population. Many birds may have shifted to alternate migration routes   and staging areas. Currently, researchers using satellite-tracking devices are studying the movements of scaup through the Mississippi Flyway to better understand migration patterns. You can follow   these transmitter-fitted birds by visiting

There is also debate about whether birds affected by poor-quality habitats are able to compensate by acquiring nutrients from habitats farther north, or whether they just do not breed. Limited studies from the boreal breeding grounds found that birds arriving at several northern locations were in relatively good condition and that there were no apparent differences from historic estimates of body mass, timing of nest initiation, or clutch size. These results suggest that compensation is possible or that too few birds are affected to detect a difference. However, the influence of reduced body condition on a female's ability to breed, or even migrate north, is unknown.

Breeding Grounds

While the focus was initially on wintering and staging areas, researchers also examined the boreal forest and discovered an alarming trend. Wetlands in the northern boreal forest of both Alaska and Siberia were disappearing—quite literally being drained away in some cases. These wetland losses have been linked to climate change, which is thought to be melting the permafrost seal. This seal holds water in many wetlands and lakes, and without it, water drains into the soil over time. Indeed, at several important scaup breeding areas in Alaska, 25 percent of wetlands have disappeared since 1950, mostly in the last 20 years. This habitat change has not been investigated in the heartland of scaup breeding in Canada, but could be occurring there as well.

Some wetlands aren't disappearing because of climate change but are warming up earlier in the spring and getting hotter in summer. Research suggests this change has caused shifts in the types and numbers of aquatic invertebrates in these wetlands, including a reduction in those that female scaup and ducklings prefer to eat. The potential implications are that less food and a poorer quality diet could cause ducklings to grow slower and could delay wing feather development of post-breeding females and young, reducing survival on the southward migration.

So where does all this research leave us? Certainly, we have advanced our knowledge of scaup, but we still have many more questions. It is unlikely that contaminants are affecting scaup to the degree once speculated, although the topic needs further investigation, particularly as it relates to winter survival of birds in contaminated habitats like the Great Lakes. Habitat change on spring staging areas in the upper Midwest probably affects a portion of the scaup migrating north to breed each year. But the severity of those effects and the number of birds affected by these changes remain elusive. Studies from the boreal forest have also uncovered changes in habitat and food availability. These discoveries could have impacts on both female survival and productivity, but direct links between these changes and scaup survival or reproduction still need to be clearly identified.

Attendees at the 2006 workshop identified seven priority issues ranging from continued work in the boreal forest to evaluating scaup harvest management. Participants also recognized that while significant progress had been made, much work lies ahead, and the best way to maintain momentum was to create a new Scaup Action Team under the North American Waterfowl Management Plan. The mission of the Scaup Action Team is to ensure that new knowledge about scaup helps inform management, that scaup research is coordinated, and that greater attention is paid to supporting scaup research, management, and conservation actions.

It may be too late to fully understand what caused the more than 20-year decline of scaup, so our emphasis has now shifted to understanding why they are not recovering, which may or may not be due to the same reasons. However, the waterfowl community is banding together for progress. And with time, wisdom, and persistence, our concerted efforts could return us to the days when scaup were more abundant.

Dr. Jean-Michel DeVink is an environmental scientist with Jacques Whitford, a private consulting firm in Fredericton, New Brunswick.

Dr. Stuart Slattery is a research scientist with DU's Institute for Wetland and Waterfowl Research in Stonewall, Manitoba.