Although it has been over a decade, I still remember that morning vividly. I was observing the banded male and unbanded female on Brown Lake as they foraged on the wide portion of the lake’s eastern side. As is the case with most of our study animals, the loons were quite tame. They reacted indifferently to my red canoe as I tracked their progress slowly down the lake.
The loon pair’s relaxed foraging seemed odd during what had been a most tumultuous year on Brown. Though the female had resided on the lake since April, three different males had vied for and held the position of male breeder for portions of the season. Ultimately, “Mint-burgundy over Silver, Green over Blue-stripe” (Mb/S,G/Bs for short) drove off his competitors and became the male breeder. Evicted from Two Sisters-West in 2008, Mb/S,G/Bs had drifted about for two years before finally seizing control on Brown. Sadly, his victory in late June 2011 came too late for successful nesting to occur. So on the day of my visit, August 3rd, Mb/S,G/Bs and his mate were merely killing time before molting and readying themselves for the southward migration.
As I watched the laid back pair forage, an intruder appeared overhead. The pair watched the intruder as it slowed, descended, and parted the water’s surface to land twenty meters away. The intruder — a female hatched and reared 15 km north on Moon Lake, near St. Germain, three years earlier — was clearly uneasy. She bowed her head, dipped her bill in the water repeatedly as she drew near the pair, and initiated many brief dives as she circled them. For their part, the male and female breeder seemed to be going through the motions. They circled slowly with the intruder and peered at her when she dove but seldom dove themselves. The video below from South Two Lake depicts a similar scenario.
Afterwards I reflected upon the encounter. More clearly than ever before it seemed to me that I was watching a jittery youngster confronting two old, confident territorial loons. I am not sure why it had taken me eighteen years to do so, but I felt that I suddenly understood something very fundamental about loon territorial behavior. Loon pairs watch the behavior of an approaching intruder closely, quickly size it up — estimating the level of threat it poses to their territorial ownership — and then behave accordingly. As a result of this particular lake visit to Brown, my research team began to recognize and record “initiates dive” behavior (i.e. being the first loon in a group to make a short dive) and also “declines dive” behavior (refusing to dive when another loon nearby has done so). These advances led to new data collection and new insights into age-related territorial behavior.
Intruders, we have learned recently, provide ample signals of their age, fighting ability, and level of interest in battling for territory ownership. As the above figure shows, one of the clearest hallmarks of youth among intruders is the “initiates dive“ behavior. Young, timid intruders with no intention of vying for territory ownership are nervous Nellies, like this three year-old female was, and carry out many initiates dives. At the same time, these youngsters almost never show the “simultaneous dive“ behavior (which signals a willingness to escalate conflict), nor do they yodel or show aggression of any kind. Without question, there are dozens of other small signals that territorial pairs pick up from intruders to assess their age and degree of threat they pose.
And territory owners act upon the information they glean from intruders. That is, they treat a harmless visitor within indifference; they behave aggressively toward a dangerous intruder. The keen ability of territory owners to distinguish between intruders helps us understand how they can survive hundreds of intrusions each year without becoming exhausted. They save energy where they can and only get worked up and aggressive when they must.
These conclusions might sound obvious and intuitive. They are. And yet it took some 20 years and dozens of statistical tests to recognize and analyze the age-related patterns in behavior that allowed us to infer how both intruders and pair members betray their motives and strategies during such encounters. Fortunately, our perseverance has been rewarded. A few days ago our paper on interactions between intruders and territory owners was accepted for publication in a good behavioral journal. It should appear in print early next year. Thus, we are incrementally closer to understanding the entire territorial system of common loons.
If you would like to support our work in understanding territorial behavior, measuring population parameters, and conserving loons in the upper Midwest, consider a donation to the Loon Project HERE. At the moment, we are hoping to buy two canoes and a small motorboat, which would allow us to continue our long-term Wisconsin research while adding new lake coverage in our new Minnesota study area in 2022. Thanks for any support you can give us!