Science is, by nature, cumulative. Theories put forward centuries, decades, or years ago form the foundation of ideas we test today. If those theories fail to explain patterns we see in nature, they are refined or discarded and replaced by new theories that themselves must be tested ceaselessly and revised or rejected.

For our part, we scientists spend years learning our field, which means achieving a deep understanding of the sweeping theories that have withstood the test of time. We also must have an intimate knowledge of recent findings of colleagues in the sub-discipline that forms the context of our own research.

The way that scientists carry out the scientific process should sound robust and logical. It is. This approach has led to steady progress in our understanding of the world and a guarantee that — although we may occasionally take a wrong turn in understanding some process or phenomenon — we shall not stray too far and for too long.

But the innate teamwork that typifies the scientific process has a major drawback. So desperate are scientists to keep up with discoveries and hypotheses of others in our own discipline that we expect to replicate their findings in our own work. Of course, such replication is vital to the scientific process; repeated similar findings confirm for scientists that we are seeing consistent patterns and have a solid understanding of nature. In our tendency to look for and find what others have found, though, we are often blind to what is novel. Indeed, if we discover some oddity, we are more likely than not to try to reconcile it with current theory by treating it as an aberration or an artifact of our procedures, rather than a truly new pattern that we do not yet understand.

So it was with spotlighting by loons. For years, I had observed the visits of territorial loons to their neighbors’ lakes. This behavior was curious, to be sure, but my training convinced me that these visits must have an explanation within the fabric already woven by other scientists. No, we would not expect territorial pairs with chicks ever to leave them at home and visit their neighbors with chicks. It made no sense. But until I took a long, hard, robust look at our data, I simply shrugged and trusted that someday we would be able to make sense of it based on what my scientific colleagues had found in other species.

On the other hand, all scientists are aware of this bit of wisdom familiar to fans of Sherlock Holmes:

When you have eliminated the impossible, whatever remains, however improbable, must be the truth. —Arthur Conan Doyle

Most ecologists encounter this situation seldom. It is, as you must imagine, a most unsettling outcome. However, in trying to test hypotheses to explain intrusions by loons with chicks into neighboring lakes, I encountered this situation exactly. Territorial loons, I thought, might be visiting their neighbor’s territories to look for food. That possibility did not stand up to scrutiny; intruders rarely forage during intrusions. Neighboring pair members, I reasoned, might intrude to learn about nearby territories, so that they would be positioned to “trade up” to a new one, given the opportunity. This possibility works for neighbors that failed to produce chicks, which sometimes trade up to the territory next door, but not for neighbors that hatched chicks themselves, which do not. Neighbors with chicks might intrude because — if they wish to draw attention away from their own chicks — they must go somewhere. This explanation fails because parents would be better off, in that case, visiting one of the many uninhabited lakes throughout the study area, where they could forage without interference from other loons and replenish their energy reserves. The fact that parents target other territories with chicks in precise, laser-like fashion rather than studiously avoiding them indicates that they are visiting specific territories with a specific goal. Thus, the improbable explanation that remains after all impossibilities fall away is spotlighting.

To conclude that loons are reciprocally spotlighting each other’s chicks is unsettling. No one has ever proposed such a convoluted mechanism of territory defense before. Our ability to develop the spotlighting hypothesis depended upon knowing loon behavior intimately. When you consider that: 1) nonbreeding floaters are obsessed with finding chicks to gauge territory quality for eviction attempts, 2) pairs with chicks are desperate to hide them from floaters, and 3) floaters are strongly attracted to other adults already intruding in a territory, it is not a great conceptual leap to suppose that adults eager to hide their own chicks would visit the neighbors to draw floaters to the neighbors’ territory and the neighbors’ chicks.

Since I am on a roll, I will add that the form of eavesdropping that loon pairs appear to do on each other’s yodels to keep track of each other’s breeding success is rather novel. That is, behavioral ecologists have long known the animals listen to each other and intercept each other’s messages in order to boost their own reproductive opportunities. Nightingales, for example, use their neighbors’ songs to determine where intruders are, so that they can defend their territories more effectively. But the kind of eavesdropping that I propose in loons — whereby loons use intercepted yodels to learn about neighbors’ chicks, spotlight neighbors’ chicks, and thus cause nonbreeders to evict neighbors — is far more sinister. The fact that eavesdroppers are causing harm to the loons whose yodels they intercept sets the loon system apart from other forms of eavesdropping that have been described in animals.

2009 video from Spider Lk, Oneida County.

I know. Web sites, books, “loon experts” — and our own hearts — tell us that loons are fundamentally good. Even when a pair with chicks hides them along a shoreline while confronting a raft of intruders, all of the attendees of the social gathering seem so respectful to each other, so congenial. But I am going to ask you to take the same sort of journey that I did as a scientist recently. Despite having been steeped for decades in narrow, Pollyannaish loon lore that holds that all loons are friends and are looking out for each other and the population as a whole, open your mind to the possibility that loons, like humans, do not always see eye to eye. Maybe, like me, you will find that this brings you closer to them.

I often thank my lucky stars that I am a field biologist. Being outdoors, especially in Wisconsin, is a huge perk of my profession. There is something thrilling about being in a situation where nothing is planned and anything can happen. Yet as glorious as it is to be outdoors, field work is perilous. Although I might tell the National Science Foundation, or Chapman, or the U. S. Geological Survey (who provide leg bands) that I have much of loon behavior figured out — that I have systematically tested all hypotheses and eliminated all plausible alternatives — I never feel confident when I am watching loons. They invariably surprise you by doing something that defies explanation. “What does that adult have to gain from wailing so often and so loudly when there is no other loon nearby to hear it?” or “Why is that loon wasting time alarm calling at the harmless muskrat?”. Rarely does a day in the field go by when I do not scratch my head at least once at inexplicable loon behavior.

Paradoxically, the best place to be when you are trying to figure out why animals behave as they do might be in your office, crunching the numbers, without any animals in sight. Free of distractions and laser-focused on the data, sometimes you discover a pattern that gives you a clear answer to a central question.

That happened to me yesterday. I was puzzling over a weird finding. In the midst of analyzing patterns in territorial intrusion, I was surprised to learn that territorial loons intrude more often into neighboring territories with chicks than do young, non-territorial floaters that are looking to settle on a territory. How on Earth could this be so? Floaters are young adults that are on the prowl. They search widely for territories with chicks, use those chicks as a badge indicating a good territory, and then return to try and evict a pair member in order to seize the territory for themselves. So it is floaters, not territory holders, that should be obsessed with finding, visiting, and competing for territories with chicks. Territory holders should have as their priority simply holding onto the territory they already own.

I must point out here that intrusions into territories with chicks, regardless of which loons make them, are generally a bad idea. As many of you have seen, territorial loons do not appreciate landings or close approach by intruders when they have chicks to protect and are much more apt to attack intruders at such times. This fact only thickens the plot. Now we must try to understand why a territory owner — a loon with something valuable to lose — would take a chance at being injured by visiting a nearby territory with a super-aggressive owner!

Lacking any other obvious path forward, I dove even more deeply into the curious tendency of territorial loons to seek out neighbors with chicks. Late in the breeding season, territory owners can be partitioned into two groups: those with chicks and those without. So I could look to see if, as one might predict, territory owners that had failed to raise chicks — and who therefore might be looking to trade up to a better territory — were those most likely to intrude. But quite the opposite was true. Territory owners rearing chicks of their own were much more likely to intrude at neighboring lakes with chicks than were territory owners that had no chicks.

As it happened, I discovered this last vexing pattern late in the day and could not dwell upon it. At 2:32 a.m. — during that inevitable hourlong period of sleeplessness that comes each night — I figured it out. While successful rearing of chicks is the ultimate goal of an adult loon’s life, chicks pose a great hazard too. To a floater, a territory owner’s chicks signify a high-quality territory, and so chicks raised in one year guarantee the owner will spend the next year fending off eviction attempts from floaters. It follows that owners should take any and all steps they can to keep floaters from learning about their chicks. Simply decamping and leaving your chicks alone during early morning is a good strategy, because floaters learn about chicks chiefly after spotting their conspicuous parents on the water and landing near them. If you are not on your territory, then no floater is likely to find your hidden chicks. But being away from your own territory and also intruding into your neighbor’s territory is doubly beneficial for a loon with chicks, because your presence will draw other adults to the neighbor’s territory (and away from your own territory nearby) and increase the likelihood that your neighbor’s chicks will be the ones that are spotted. That is to say, neighboring pairs with chicks seem to be locked in a desperate, reciprocal effort to expose each other’s chicks to floaters in order to protect their own territory ownership.

As I write this, I am listening to the hideous whine of a circular saw next door. Our own neighbors have contracted with the loudest and most inefficient construction crew west of the Mississippi to renovate their home. I find the noise, the clutter, the truck traffic, and the ceaseless cursing and shouting tiresome, to say the least. But I am fairly confident that our neighbors are not conspiring with outside forces to get us evicted. So I guess we have it pretty good.

We all focus most of our attention on breeding pairs and their chicks. Why not? Breeders are the loons we get to know — day after day, year after year — as they struggle to hold their territories, choose nest sites safe from raccoons, incubate their eggs for 4 long weeks, and then protect helpless offspring from all manner of environmental threats. We admire the toil and turmoil they face each year and are downcast when they lose their breeding position, nest, or chick. And we grieve when they die. I am probably unusual in this regard, but I am also inspired by observing the challenges that breeding pairs routinely confront and overcome.

In our obsession with breeding individuals, though, we forget about the many loons living on the margins. These are “floaters” — mostly 2 to 7-year olds who spend the entire summer without a fixed home. Floaters are the individuals that forage alone on small lakes or skulk along the outskirts of defended territories, occasionally socializing with or accosting territory holders. Unlike territorial loons, they drift about.

Although their lives might seem simpler and less stressful than those of territorial loons, floaters — even young ones — face challenges of their own. Our work has shown that 2- to 4-year olds are much lighter than 5- to 7-year olds, are more submissive to territory holders during territorial intrusions, and almost never initiate battles for territorial ownership. Yet these youngsters do intrude into breeding territories. We presume, therefore, that even as they mature, young floaters collect information about owners. Our data show that young floaters intrude strategically into territories within a focused area (usually about 10km in diameter, see figure below from our recent paper) so as to meet and interact with owners of their own sex that they might

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evict down the road.

Having reached optimal adult condition, floaters of 5+ years of age begin to size up owners with greater urgency, choosing to battle those that appear weak or are noticeably weaker than they were on a previous visit. Male floaters probably also listen to the quality of a male owner’s yodel, because certain acoustic elements of the yodel convey information about the yodeller’s body size and condition. As we have seen repeatedly, intrusions by floaters of 5+ years of age are not welcomed by territory holders — and can be dangerous for both parties.

Considering that 2 to 4 year-olds are still reaching optimal body condition and 5 to 7 year olds are putting themselves at risk by actively seeking territories, it is surprising that floaters of both age groups survive at a rate just as high (about 90% annually) as established pair members (see figure below). Apparently the risks of probing and competing for a territory among young loons are roughly equivalent to those that come with


territory defense and chick-rearing among older individuals. (Note from the figure that only older age-classes, and only males, show lower survival.)

We are used to the invisibility of floaters. That is, we see and study them as intruders into defended territories, but we seldom consider where they come from or how they live. So I always get a jolt when a dead floater turns up, like the tame 5 year-old male in Linda’s photo from the Lake Nokomis area. These rare unpleasant finds are a good reminder that gaining a territory is a long and difficult slog.