Climate change threatens loon population, new study shows

Famous for their nocturnal calls, loons are aquatic birds often described as icons of the Northwoods. But new research shows climate change impairs their ability to feed their young.

A climate-induced decrease in water clarity could be a cause for the loon population decline in Wisconsin and more broadly across the northern United States, according to a new report from the academic journal Ecology.

Researchers used satellite imagery to study water clarity in 127 lakes across northern Wisconsin from 1995 to 2021. Increased rainfall in July each year reduced water clarity in loon territories. Over that time period, water clarity fell by about 16%, and chicks lost about 10% of their weight. Adult loons have also dropped weight each year since 1995.

The findings suggest that a decline in water clarity hurts loon reproductive success. Since loons are visual predators, they depend on high water clarity to hunt fish underwater and feed their chicks. With a reduced diet, chicks have lower survival rates.

Walter Piper, the lead author of the study and a biology professor at Chapman University, said that loon chick mass is an indication of how healthy, well-fed and likely chicks are to survive.

"Over the last 25 years, what we noticed is that you get a lot more unhealthy chicks that are below the weight that they should be for their age," Piper said. "More of them are dying."

The Northland College’s Wisconsin Loon Population Survey, conducted every five years, shows that despite successful recovery efforts, 2020 marked the beginning of a new population downturn. The adult population fell by 5% from 2015.

That decline signaled what Piper called the beginning of a new downturn associated with water clarity decline. He said there is a five to 10-year lag between the time when breeding problems hit a population and when those problems start to be felt in the adult territorial population.

"It’s becoming clear that climate change is a real danger to loons, when we hadn’t really known that before," Piper said.

Loons typically nest between May and June. Piper said the summer months, especially July, are critical to a successful reproductive season because chicks are completely dependent on their parents and cannot fly yet.

The researchers presume it’s likely that the decline in water clarity can be attributed to heavy rainfall flushing in bits of material from plants and animals, including fertilizer or pet waste.

There are other climate threats to the loon population. Experts like John Cooley, a senior biologist with the Loon Preservation Committee in New Hampshire, have seen the parasite avian malaria emerge as a source of mortality for loons in the last decade.

An ongoing recovery is underway in New Hampshire through restoration efforts, but Cooley said the birds remain vulnerable to climate change because warmer temperatures compromise nest success and more heavy rainfall events cause nest flooding.

"We’re seeing mortality in the wintertime on the freshwater lakes when loons stay too long because the ice hasn’t formed the way it used to," Cooley said. "We get a warm winter, the loons stick around, and then they’re trapped when the ice does eventually form."

Loons are at the top of the aquatic food chain, making them an important species to understanding water quality. They serve as a sentinel for identifying contaminants in freshwaters.

Like Cooley, Piper worries the population will shift out of the U.S. to more suitable climates in the north.

"It would be devastating to folks in the northern U.S. if loons disappeared," Piper said. "This is a charismatic species of enormous importance."

With more frequent rainfall, the black fly season has lengthened in some parts of the country. Black flies, which are biting insects, swarm loons as they try to incubate their eggs and drive them off their nest, Piper said.

"It’s kind of a one-two punch of climate change and overall that has just meant that they are not fledging as many chicks, not producing as many chicks that reach adulthood as they used to," Piper said.

The researchers plan to expand their study to other regions where loons breed to better understand threats to their population. That includes Minnesota, where loons are the state bird.

Cooley of the Loon Preservation Committee said there are steps that should be taken now to protect the birds and lakes.

"It’s not too late. Even these familiar iconic species like loons that we all love, we are starting to see real impacts in this natural system," Cooley said.