In Illinois, clues in decline of scaup on rivers

Champaign, Ill. — What is happening with the lesser scaup?

A study of avian malarial infections in female lesser scaup during spring migration may offer new insights into the puzzle of why the continental population of this diving duck has been declining so dramatically.

Previous research has found that female ducks that are in poor condition during migration are less likely than healthy ducks to produce a healthy, large brood once they settle into their summer breeding sites.

In an attempt to gain greater insight into the mechanisms responsible for the declining populations, researchers from the Illinois Natural History Survey collected 130 female lesser scaup at 24 stopover sites along the Illinois and upper Mississippi rivers in 2014 and 2015.

They measured body fat levels, which are thought to be the most important predictor of subsequent reproductive potential, parasitic infection status, body mass, wing size, and other features.

The researchers found that infected ducks were in poorer condition than uninfected birds, but faced a cause-and-effect challenge: Did the infection cause ducks to be in poor condition, or did ducks in poor condition have a greater likelihood of becoming infected with parasites?

“The cool thing is that we have strong circumstantial evidence supporting the idea that body condition is driving the changes in the prevalence of infections,” said Loren Merrill, an INHS scientist and now with the U.S. Army Corps.

In 2014, nearly 28 percent of ducks tested positive for infection, but in 2015, that prevalence increased to 47 percent. Also, body fat levels decreased substantially from one year to the next, but at nearly the same rate for both infected and uninfected ducks.

The similar rate of body fat decline for infected and uninfected birds suggests that the increased prevalence of infection in 2015 may have been related to the greater proportion of poor-condition birds being more susceptible to infection, or to a relapse of latent infections.

“Incidences of infection rates may help us understand the causes for the declines that the lesser scaup is experiencing,” Merrill said. “In general, studies on migrating birds are focused on food quality in stopover and nesting sites. However, there may be more subtle effects that are partly responsible for population changes over time.”

Climate change, in particular, is an important factor to consider. Mosquitoes, which are the parasite-carrying vectors, are predicted to move farther north of their typical range because of a warming climate. As a result, the lesser scaup infection rates may increase as ducks come into contact with mosquitoes for a longer period of the spring migration.

The continental lesser scaup population peaked in the early 1970s and has declined ever since.

“The population has remained well below the long-term goal of the North American Waterfowl Management Plan,” said Jeff Levengood, a researcher at INHS. “This research was part of a larger, multifaceted study to examine the health of lesser scaup migrating through Illinois on their way to the breeding grounds to the north. These studies can help us identify causes of reduced condition of female lesser scaup at a critical time and guide management actions to alleviate the impact these stressors.”

The study is set to be published in the Canadian Journal of Zoology.

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