Late ice-offs can negatively impact common loon reproduction

Common Loon family (Gavia immer) swimming in the blue hour of the morning on Wilson Lake, Que, Canada Photo by Jim Cumming/Shutterstock

When spring rolls around in Ontario, you can bet boaters are eagerly counting down the days until lake ice melts away and open water heralds the return of boating season. If you’ve ever found yourself despairing over a late ice-off delaying your boat launch, know that the cottage lake’s unofficial mascot, the common loon, doesn’t fare well with late springs either. 

A collaboration between Birds Canada and Acadia University researchers suggests that late ice-off dates are negatively associated with common loon reproduction. The research includes 16 years worth of loon surveys, conducted off and on between 1982 and 2019. The research showed that years with later ice-off dates were associated with fewer loon pairs, breeding attempts per pair, and large young per pair. The research, which was conducted as part of a long-running loon study in the Sudbury area, shows how climate fluctuations impact the survival of Canada’s wildlife. 

Common loons are particular about their lakes. They have strong fidelities to the lakes they use for breeding and rearing young, which means couples will return to the same lake year after year. Loons are also long-lived; they start breeding around age six but can live up to thirty years.

Loons will return to their breeding lakes just after ice-off, says Kristin Bianchini, Waterbirds and Wetland Scientist with Birds Canada and lead author of the study. They’ll spend up to six weeks on their lake socializing and establishing their breeding territory. That can involve developing a new pair bond, re-establishing a prior one, and defending their territory from other loons looking to move in. It’s only after this six-week period that the birds will build a nest and lay eggs.

Ice-covered lakes are a no-go for the common loon. Loons need a good amount of open water to generate the speed for lift-off, so landing on an icy lake might mean the loon won’t be able to take-off again. To avoid this, Bianchini says that common loon pairs will perform reconnaissance flights on their breeding lakes. The birds will fly up to their chosen lake and check out if there’s ice cover. If so, they’ll retreat south and try again later.

It’s unknown how late ice cover negatively impacts common loon breeding, but the researchers have some suggestions. One consideration is that loons that arrive on lakes earlier may have a better chance of establishing and defending nesting territory against other loon couples. 

The timing of ice melt is also known to impact fish populations, the common loon’s food source, but the researchers acknowledge that the interactions between piecing out how fish communities respond to environmental changes like ice cover is complex. 

There’s another possibility for what might be throwing loons off during years with late ice-melt, and it’s a part of nature that many a cottager fears: black flies. Specifically, black flies that are adapted to feed specifically on common loons, called the loon black fly. These tiny insects live longer and are more abundant in Aprils with cool temperatures. Imagine the loon parents, says Banchini, trying to incubate and protect their eggs while being swarmed by black flies. “It’s pretty bad for them in some years, and it will cause them to abandon their nests. Just like people, they can’t take the black flies anymore.”

While the research found that colder springs with late ice-offs negatively impacted loon breeding pairs, it’s important to note that spring ice-offs in Ontario are generally trending towards earlier dates. Banchini says that warmer temperatures on average throughout the year will probably negatively impact loons as well. Climate change is likely have complex effects on loons, impacting everything from the availability of breeding territory to the amount of mercury in lake water. “More research needs to be done on the relationships between climate change, ice-off, and other threats to loons like mercury,” says Banchini.

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