Research efforts have uncovered important clues in the puzzling decline of scaup populations and have raised new questions biologists must explore
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.
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.