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

At the beginning of the 21st century, most ducks hunters didn't need a waterfowl biologist to tell them that there were fewer bluebills heading south each fall. The breeding population of scaup declined steadily from the 1980s to the early 2000s, dropping from about 6.3 million birds in the 1970s to fewer than 3.5 million in 2007. Since then, the population had a modest recovery, reaching approximately 5 million in 2012, but has again declined to 3.6 million in 2022. Much of the previous 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 approximately 21 percent below the North American Waterfowl Management Plan (NAWMP) objective of the long-term average (LTA; 1955-2014) today, scaup are certainly in no danger of extinction. Thanks to the growing body of research, we now have a clearer picture of the contributing factors to this marked decline, but there is still much uncertainty to be researched. 

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 decades, and we have since expanded our understanding of how these various hypotheses have contributed to changes in the scaup populations throughout North America. 

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 studies found that factors affecting female survival and productivity are most likely to have influenced scaup population trends, and a study in 2009 suggested that breeding probability of lesser scaup was lower than expected. 

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 difficult to quantify, but it is believed the main contributing factor to nest failure is predation. Unfortunately, we lack historical 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 research found that the levels and species of parasites present in wintering scaup were similar to historical values.

Much remains unknown about the causes and rates of scaup mortality during and outside the breeding season, but recent studies have found no evidence of scaup harvest being additive. This explains why there was not the desired population response when bag limits on scaup were reduced several times since 1986 in an attempt to help the population recover. Recent complex population models suggest that juvenile female survival plays an important role in scaup population dynamics, but declines between 1983 and 2006 were driven primarily by a long-term reduction in fecundity.

Contaminants

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. Overall, research into contaminants as a major cause of scaup decline has revealed no support for this hypothesis. 

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 habitats. 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.

Declines in habitat quality (oftentimes leading to a reduction in body condition) on stopover areas can influence survival, recruitment, timing of breeding, and brood attentiveness. A study in 2014 provided evidence that body condition is indeed an important factor in determining reproductive success of breeding lesser scaup. These results underscored the important role of spring staging wetlands in building nutrient reserves and in influencing reproductive success. 

There is also debate about whether birds affected by poor-quality staging grounds are able to compensate by acquiring nutrients from higher quality habitats or whether they just do not breed. For instance, scaup may have shifted to alternative migration routes and staging areas in response to changes in habitat conditions. 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. 

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 30 years. 

Habitat changes in the heartland of the scaup breeding range have been observed. In some cases, wetlands have been lost or degraded because of development activities (e.g., mining, oil and gas, agriculture), but in other cases wetlands may no longer be suitable for nesting scaup. For example, climate change has led to wetlands warming up earlier in the spring and getting hotter in summer. Research suggests this change has caused shifts in the types of numbers of aquatic invertebrates in these wetlands, including a reduction in those that female scaup and ducklings prefer to eat, such as amphipods. The decline in food quantity and quality could cause ducklings to grow slower and 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. 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.

It may be too late to fully understand what precipitated the decline of scaup starting some 40 years ago. However, the waterfowl community has banded together to try and better understand the status of this species and what are the current population-limiting factors on the landscape. While some things are still unclear, our understanding of the effect of habitat changes on lesser scaup have increased greatly in the past 20 years. 

Today, scaup populations remain below the NAWMP LTA objective, and the recent "peak"of 5 million in 2012 is also almost 20 percent below the NAWMP aspirational objective (80th percentile of the LTA) where populations should reach during population peaks. This further cements the importance of collaborative efforts to study and manage species impacted by our changing world. With time, wisdom, and persistence, we will improve our understanding of scaup populations, and with targeted management actions, we may return to the days when scaup are more abundant. 

Dr. Jean-Michel DeVink is Ducks Unlimited Canada's National Manager of Conservation Planning based out Saskatoon, Saskatchewan.

Dr. Stuart Slattery is the National Manager of Ducks Unlimited Canada's Institute for Wetland and Waterfowl Research in Stonewall, Manitoba.

Ashley Tunstall is a Conservation Intern at Ducks Unlimited HQ in Memphis, TN.