The haunting cry of a loon pierces the silence on a glassy northern Wisconsin lake. Of the four distinct calls the bird is known for, the wail — a long, one-to-three-note call — is the most iconic, recognizable to anyone who’s spent time “up north.” The birds use the wail to beckon one another. But there is another, wilder call, the “yodel,” that sends a very different message: This is my territory.

Each year, more than 4,000 loons return to the forested areas of Wisconsin after the ice melts in late April. Breeding pairs establish a nest — typically only one per small lake— in May and hatch one or two eggs a month later. Throughout this period, loon couples often face off against “floater” loons that try to take over their nests, especially those where breeding has been successful in recent years. The stakes are high: Loons prefer to return to the same lake every year to raise their young, and some can live more than twenty years.

Jeremy Spool, holding a baby turtleIntegrative Biology Ph.D. candidate Jeremy Spool (pictured at left) spent a recent spring in Oneida County working with Walter Piper, the director of the Loon Project, who has been studying the territoriality of the common loon for more than two decades in northern Wisconsin.

“I wanted to know whether loons that successfully raised chicks on a lake would value their lake territory more than loons that failed to breed, and therefore fight harder to defend their territories,” Spool says. “It made sense that birds would choose to invest more heavily in territories that successfully produce young.”

Using a loon decoy and playbacks of aggressive male yodels to test his theory, Spool made a surprising discovery: “Loons that had recent breeding success displayed less territorial behavior toward our decoy. Loons that had recently failed to breed displayed more territorial behavior.”

Our results provide the first strong evidence of a link between breeding history and differences in territorial defense. This had never been shown before.

While the Loon Project team is still investigating exactly why — with a working theory that perhaps loons with recent breeding success adopt a different strategy for dealing with the higher rate of intruders — they already know the research is valuable. 

“Our results provide the first strong evidence of a link between breeding history and differences in territorial defense,” Spool says. “This had never been shown before. In general, building knowledge about how breeding success impacts animal behavior is a useful piece of the puzzle for understanding why some animal populations decline while others succeed and grow.”

Add climate change, shoreline development and industrial run-off to the equation, and the loons’ territorial behavior, which can result in fights to the death, takes on added significance to any assessment of population viability. Spool’s finding adds to the Loon Project’s wealth of behavioral research on the birds, which has been cited by the Wisconsin Department of Natural Resources in discussions about zoning and provides valuable data for wildlife biologists around the world. 

And Spool believes the work, which was possible due to contributions from the John and Virginia Emlen Fund and the Lowell E. and Ruth Chase Noland Memorial Fund, pays tribute to the significance the loon holds as a symbol of Wisconsin wilderness.

“There is something about the loons that resonates with people of all ages and backgrounds, and every insight we gain about the birds makes the connection stronger,” he says. “The work engages people in the scientific process, and in the conservation of our world that allows us to coexist with such fascinating creatures. Dr. Piper is constantly engaging the public about his work, and I’m happy that I have been able to add another piece to his ever-expanding story.”