Each year as the summer months come to an end and autumn sweeps across Canada, young loons stretch their wings and begin their first winter migration.

But according to the results of a recent Bird Studies Canada project, the iconic common loon is facing an uncertain future.

The Canadian Lakes Loon Survey, released in July, found that, nationally, the bird’s reproductive success declined between 1992 and 2010. The survey suggests that if the current rate of decline continues, there could be a steady decrease in loon numbers within 20 years.

That’s serious news for the health of Canada’s lakes, as the loon’s high position in the food chain makes them a strong indicator of lake health.

Douglas Tozer, the survey’s lead author, says a major cause of the loons’ reproductive problems is elevated levels of mercury and acid precipitation in lakes, caused primarily by atmospheric pollution.

Elevated levels of mercury have serious effects on the health of loons, making them slower and adversely affecting their behaviour. The study says the birds spend less time collecting food, and chicks become less adept at avoiding predators because of compromised immune systems. The survey also found that lakes with high levels of acidity — and therefore fewer fish to eat — negatively affect loon reproduction.

The survey, which began 32 years ago as a citizen scientist-based study of Ontario lakes, covers some 4,500 lakes across the country. It also found that reproductive success was higher in the West than in the East, was higher on larger lakes than on smaller lakes and increased as acidity decreased.

Tozer says that trends in reproduction can serve as early warning signs for potentially more serious problems later. “This is one of those unique situations where we’re getting an early warning. Usually we don’t find out until most of the birds are gone.”

To read the survey, visit birdscanada.org/volunteer/clls/resources/CLLSsummary.pdf