population

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What if we had an early warning system in loons that could alert us to population decline, like the proverbial canary in a coalmine?

Male loons might serve as such an early warning system. That is, careful monitoring of the health of male loons might provide a good indication of the health of the loon population as a whole. How is this possible? Because the more we study the breeding ecology of loons, the more stark differences we find between the sexes. And — more to the point — male loons have some chinks in their armor that females do not.

Most fundamentally, males are 25% larger than females. Greater size places greater energetic demands on males. Males are living “closer to the edge” than females and might often fail to acquire enough food during the season to maintain good body condition. Thus, a decrease in the quality or quantity of food — which could set in motion a population decline — should strike males first and hardest. Indeed, as the graph below shows, the average mass of male loons has declined in northern Wisconsin over the past 30 years in a way that suggests they are having more time finding food now than they used to. (Note that females have not declined in mass during the same period.) The obvious conclusion: something in Wisconsin lakes has changed in the past three decades that has impaired males’ ability to feed themselves.

Average masses of male and female loons in northern Wisconsin, 1991 to 2021. Male mass has declined significantly during this period, while female mass is unchanged.

Long before I discovered that male masses were in decline, I had begun to worry about male loons. You see, male loons live shorter lives than females. This means that there are simply fewer adult males around. In fact, the majority of non-territorial adults (“floaters”) in the loon population are females. Since males are in short supply, the loss of an adult male breeder on a lake or territory sometimes leads to that territory becoming vacant. In fact, in 23 of 24 well-documented instances where an adult breeder’s death was associated with a territory vacancy, the dead breeder was a male. Vacant territories are, of course, a harbinger of overall population decline.

Sadly, recreational fishing does not help the situation. Possibly because males’ greater size makes them a bit more desperate to feed themselves, male loons are twice as likely as females to be hooked by anglers or become entangled in fishing line. This pattern is well-documented in New England loons, but the same scenario plays out in the Upper Midwest. Specifically, of 47 known fishing entanglements among our study animals, 33 involved males, and only 14 involved females. Angling mortality, then, exacerbates what is already a female-skewed sex ratio owing to early male senescence.

It is difficult to predict the future, but I think you can see why I am concerned. Male loons appear to be in trouble. We cannot say for certain whether mass loss by male loons will cease or continue. Furthermore, we have no evidence to date that the 4% net loss in mass by males since 1991 has negatively affected their survival. So it is too early to panic about these patterns. But it is also hard not to feel like a miner glancing anxiously at his lethargic canary.

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I have said a number of times that we do not know how the Minnesota loon population is doing. That blanket statement is misleading. In fact, two well-organized efforts to gather data on Minnesota loons — both run by the DNR and staffed by armies of citizen scientists — have been under way for decades. These massive efforts have given us glimmers of information about the status of loons in the state that I will summarize here.

The Minnesota Loon Monitoring Program

Begun in 1994, the Minnesota Loon Monitoring Program relies upon volunteers to count loons within six regions in the state and produces a summary report every five years. A second project, the Volunteer LoonWatcher Survey, also run by the DNR, began in 1979. Since the MLMP aligns closely with one important goal of our Minnesota work — assessing the status of the Minnesota loon population — I will limit my comments to that survey.

Loon density (number of loons per 100 acres of lake), according to the Minnesota Loon Monitoring Program. (From 2020 report by Krista Larson; Minnesota DNR. Reproduced with permission.)

What Do the MLMP Data Show?

Of course, using volunteers — some without boats and binoculars — to measure loon numbers increases uncertainty in measurement. But despite inevitable fluctuations in measurements from individual regions and years, the MLMP survey seems to be an effective tool for estimating loon populations. So it is reasonable to look at MLMP data and expect to see meaningful patterns.

One of the first patterns you notice in the MLMP survey data is the noise within it. The true density of loons on lakes within each region of Minnesota should not vary much from one year to the next, because loons are long-lived and reproduce at a low rate. Yet the MLMP data show huge fluctuations in loon density from year to year, especially in Itasca, Otter Tail, and Becker Counties. That substantial scatter in the data is important, because it makes interpretation difficult.

Second, despite the noise, it is clear that the density of loons varies greatly between regions. The DNR’s analysis shows only one loon per 100 acres of lake in Kandiyohi County (southwestern part of the state) but three or more loons per 100 acres in Itasca County (northern part). The four other surveyed regions — Becker, Otter Tail, Aitken/Crow Wing, and Cook/Lake — have loon densities that fall between these two extremes. Differences in density across the state are significant, because they help us identify regions of particular importance to a species. With apologies to loon lovers in southern Minnesota, if loons are three times as dense in Itasca County as in Kandiyohi, then Itasca is a much higher conservation priority. (This sort of “triage” perspective is the bread and butter of conservation biology.)

Third, population trends — the aspects of the survey in which we are most interested — are dimly visible within the data, despite year-to-year scatter. According to the DNR analysis, two regions — Cook/Lake and Itasca — have seen small declines in loon density since 1994; one — Otter Tail — has seen a small increase; and the other three regions have experienced no significant change. These conclusions highlight one difficulty we face in assessing the MLMP results. If there are two bits of bad news and one bit of good, what do we conclude about Minnesota’s loon population as a whole?

According to the DNR summary, “MLMP results suggest that Minnesota’s loon population remains stable with an average of 2 loons per 100 acres of lake across all six Index Areas.” It would be pleasing to conclude, as this statement does, that: 1) there is one statewide population pattern and 2) that this overarching pattern could be encapsulated so simply. But the DNR’s summary seems to gloss over some worrisome trends in the data.

What Do the Data Show for the Past Ten Years?

In light of the brevity of the DNR’s summary, it seems worthwhile to take a deeper dive into the MLMP. One oddity of the 2020 DNR report is that it uses 1994 to anchor the trend line. Why 1994? Simply because this was the inaugural year of data collection. But we are most interested in the trend over the past decade, because that gives us a better sense of what is happening now. Over the past ten years, Crow Wing/Aitken and Itasca regions both appear to have suffered sharp declines in loon density; Cook/Lake has declined slightly; and Becker and Otter Tail regions have been more or less stable. Only Kandiyohi County provides good news, as it appears to have greatly increased in loon density in the past decade. But since loon density in Kandiyohi remains far below that in Crow Wing/Aitken and Itasca, the good news from Kandiyohi does not even begin to offset the disappointing findings from Crow Wing/Aitken and Itasca.

What is the Real Take-Home about Minnesota Loons?

We cannot reach any firm conclusion about the status of Minnesota loons based on the Minnesota Loon Monitoring Report. There is simply too much scatter in the data for that. However, careful inspection of recent findings reveals worrisome downward trends in two vital loon population hubs. I take these troubling signs seriously. With support from the National Loon Center in Crosslake, I am accelerating my effort (begun in 2021) to establish a large marked study population in one of these two hubs — Crow Wing County. In the next few years, we will produce estimates of adult survival, reproductive success, and other demographic parameters to construct a new population model for the region. Our fine-grained analysis will indicate whether the downward trend suggested by the MLMP data is real and sustained or whether those of us who wish to conserve loons in Minnesota can breathe a great, collective sigh of relief.

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As we motored around the Whitefish Chain in late May, the thought crossed my mind. As I looked over Katy and Jordana’s field notes from Minnesota in June, it occurred to me again. And by the time we banded five tightly-packed loon pairs on Ossawinnamakee Lake in a single night in July, I had become utterly convinced: there are more loons per lake, on average, in our new Minnesota study area than in our long-time study area in northern Wisconsin.

Of course, it is one thing to have a gut feeling that a natural pattern is out there and quite another to demonstrate that the pattern is real. Indeed, having enough self-discipline to wait and test a hypothesis instead of blurting it out and selling it as fact is what separates science from…..well, something less than science.

So I examined our data from both study areas. If there are more breeding loons per acre of lake in Minnesota, the difference should be evident from a statistical analysis. I looked at all lakes between 165 and 740 acres in both study areas for which we have reliable data, divided lake area by number of breeding pairs, and ran a test. The result: In our Wisconsin study area, a loon pair’s average territory size is 282 acres, while in Minnesota an average pair occupies a territory of only 180 acres. This is a highly “significant” statistical difference, which means that the huge disparity seems to represent a real pattern, not just a chance result.

What does it mean? Having done the easy part — finding a difference — we are now faced with the thorny task of explaining it. Innumerable hypotheses leap to mind. (1) The slightly different climate of central Minnesota might support a denser breeding population than the northern Wisconsin climate. (2) Lake chemistry might be more favorable in Minnesota and thus explain the difference in density. (3) Minnesotans are somewhat more apt to put out artificial nesting platforms for loons (21 of 105 territories; 20%) than are Wisconsinites (23 of 216; 11%), which might support more loon pairs. (4) Predators might be more abundant in Wisconsin and/or food scarcer. (5) Minnesota lakes might be more convoluted in shape and thus contain more natural boundaries that allow coexistence of more loon pairs on the same area of water. (However, a quick glance at lake shapes suggests the opposite — that our Minnesota lakes are more round.) (6) Human harassment of loons might be more intense in Wisconsin. (I have not noticed any such pattern, however.) In short, we have lots of questions and no answers, at present!

Now, it is important to take a step back. We have under active investigation only about 10% of all Wisconsin loons. In this beginning phase of Minnesota research, we have only 1.4% of all Minnesota loons in our study. (The nice loon photo by Katy Dahl above shows only one of about 14,000 loons in the North Star State.) So our two study areas — especially the one in Minnesota — capture only a snapshot of a small subsample of each population. The overall statewide loon densities might be quite different. Still, the two study populations are similar in numerous ways, including latitude, degree of human lake usage, and deep affection of almost all lake residents for the species. The ability of Minnesota loons to live “shoulder-to-shoulder” tells us something profound, I think, if we can only ferret it out.

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In a recent post, I described how the popularity of loons and the willingness of many folks to pontificate about them without solid data or thoughtful scientific analysis makes loons unique. I tried to point out that this practice can be harmful, if we make misleading statements about loon conservation at a time when some loon populations are declining.

Now, let me give an example. Millions of state and federal dollars have been spent in recent decades in attempting to measure the effect of methylmercury (the toxic form of mercury) on wild animals, especially birds. Without a doubt, more funds have been spent analyzing mercury impacts on loons than on any other aspect of loon biology. What have we learned from this body of work? The major take-homes are that: 1) mercury certainly can affect behavior and survival of adult loons and chicks if it occurs in a high enough concentration in their tissues, 2) high mercury levels tend to occur mainly in loons living on small, acidic lakes, which have negative effects on loons that have nothing to do with mercury, and 3) harmful concentrations of mercury do not occur in most geographic areas within the breeding range. In short, despite an abundance of research and the expenditure of millions of research dollars across three decades, we have no direct evidence that mercury negatively impacts loon populations. In fact, the consensus among loon scientists is that mercury probably has little or no negative impact on most populations.

The situation is dramatically different with lead. Careful analysis of loon carcasses in New England has shown us that lead is quite deadly and affects a great many loons. (The featured photo above shows a deadly lead sinker in the stomach of a loon that died a few days ago in Wisconsin. Photo by Wild Instincts.) In a 2017 study, Grade et al. determined that a whopping 48.6% of the loons they examined had been killed by lead sinkers and lures. The authors estimated that this mortality rate had reduced the New Hampshire loon population by 43%.

The contrast between mercury and lead is stark. Mercury might affect loon survival and breeding success slightly in a few isolated populations. Lead has been shown to cause half of all loon deaths in one state and to make an enormous dent in the loon population of that state.

The contrast between these two toxins goes further. Mercury exposure is pushed to high levels mainly through burning of fossil fuels like coal and oil, which contain mercury. So reducing loons’ exposure to mercury requires a long-term effort to reduce burning of fossil fuels over a large geographic area. In contrast, loons are exposed to lead through our use of lead sinkers and fishing lures. The remedy for lead-related loon deaths is simply to implement use of lead-free fishing tackle in lakes where loons breed. (Steel, tin, and tungsten are common alternatives). Indeed, lead bans are now in place across New England.

Here is the problem. Despite the lack of evidence that mercury affects loons in nature, mercury has become the “go-to” environmental toxin mentioned by many loon researchers. Mercury has become such a prevalent scapegoat in grants, reports, and even published papers that many of us are not keeping its limited impact on loons in proper perspective. A clear-headed, candid, objective review by a loon researcher with a strong background in mercury toxicity would do wonders for loon conservation. At a time when studies have just reported long-term declines in two loon populations, those of us trying to conserve loons would do well to focus our attention on the real enemy.

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One of the challenges of studying loons is that they are so well-loved. Many people have observed them, written down notes about them, and — here’s the problem — shared their speculations about all aspects of loon biology with others. Those of us who study loons are in a position of power, because the public looks to us for information. It takes some humility, when a journalist asks us a question about loons, to admit that we do not know the answer. Admitting ignorance is embarrassing. It disappoints the questioner. It makes us feel inadequate and uninformed. Yet admitting ignorance is vital. Our willingness to say we understand fully some aspect of loon biology that we do not — on websites, at conferences and in print — poses great problems for loon science and conservation.

Wait. Published material on loons should help move our understanding forward, right? Yes and no. Well-researched, robust science on loons improves our understanding; anecdotal, speculative work based on small samples of loons, inexpertly analyzed, and passed off as fact does not. In the field of loon behavior and ecology, a huge “grey literature” exists, which consists of popular loon articles, books, websites, unpublished Master’s theses, and low-brow pseudo-science that eked its way into the lower echelons of science journals. When such sketchy information makes up the majority of the material publicly available about loons, there is a real danger that speculation and pseudo-science might drown out real science.

There is good news, however. Our understanding of loon biology is better now than ever before. Across the continent, loon researchers have started to mark individual loons, examine many aspects of their ecology and behavior, use powerful statistical tests, and publish their findings regarding loons in peer-reviewed scientific journals. This last step is critical, because peer review means that three or more scientists are criticizing a paper submitted for publication anonymously and candidly before publication. In most cases, scientists who review loon research do not themselves study loons, so they can bring an important bit of objectivity to the process and read what a loon scientist writes without preconceived notions about the species. Reviewers who are not loon researchers, in effect, are helping pull the study of loons into the mainstream of scientific research. If scientific studies on loons are treated with the same level of rigor as those on fruit flies, downy woodpeckers, wolves, elephant seals, and angelfish, loon science will eventually become as robust and reliable as science carried out on other species.

Such rigor in loon study is long overdue. Now that we see multiple populations of loons declining in number or reproductively, we must do better. We need to advance from “there are still loons on my lake, so the population is stable” to careful, longitudinal quantification of adult survival, juvenile survival, breeding success, and other demographic parameters that can contribute to a valid statistical population model.

Why does it matter? Because when we fall into that very human trap of expounding upon a topic without a foundation of scientific fact, people sometimes listen and use our pronouncements in ways that we did not foresee. Case in point: the Minnesota loon population. Any population ecologist who looks at the data — well, lack of data, in this case — will tell you the following. We truly have no idea whether the population of loons in Minnesota is rising, falling, or remaining steady. We simply have not marked adults, carefully recorded their rate of return to their territories, measured the number of chicks they have produced, marked those chicks, measured the rate at which those chicks return, and plugged all of these data into a statistical model. Without such a thoughtful, complete analysis of survival and reproductive success, any statements about the Minnesota loon population are simply speculation — speculation that could be seized by others to undermine conservation efforts. Indeed, one difficulty faced by the “Get the Lead Out Minnesota” campaign (which anyone who loves loons and wildlife should support strongly) is that there are many baseless statements to the effect that the Minnesota loon population is stable in the media and the grey literature.

So, a plea. Let’s emulate population ecologists in describing our knowledge of loon populations in Minnesota and elsewhere. Let’s apply rigorous techniques and wait until the research has run its course to reach any conclusions. In the meantime, let’s have the courage to utter those most honest but difficult few words: “We don’t know”.

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I am fearful of new challenges. In 1993, when I began to study the behavioral ecology of loons on a cluster of 12 Wisconsin lakes, doubt gnawed at me. How can one carry out meaningful, publishable, scientific research, if one’s study animals are large, aquatic, diving birds that range over huge territories, dive constantly, and must be followed in boats? Would my work be severely limited in scope, like that of researchers on porpoises, whales, and sea turtles? I had no training in field techniques for study of aquatic animals, only my childhood experiences paddling canoes over vast stretches of Lake Temagami in central Ontario. But my fascination with loons — which also grew during summers on Temagami — and my sense that canoes could be an effective means of tracking them without altering their behavior pushed me forward. And so, for reasons that I do not understand, I began to treat seemingly insurmountable problems with funding, logistics, and personnel as mere nuisances. And I ignored warning signals that any reasonable young scientist would have heeded. I began to study loons.

So it was in Minnesota. Although one might surmise that beginning a field project on loons in one state would be much like doing so in another, this is not so. True: loons are loons. We see many of the same behaviors, hear the same basic calls, and witness the same sorts of human-loon interactions in Minnesota that we have seen over the past 29 years in Wisconsin. But all else is new. Starting a major field study in the Crosslake area has reminded us that we have an army of friends, lake residents, and supporters in Wisconsin. These folks have housed us, fed us, carried us around in their boats at times, and — most important — provided us with a trove of information on our study animals to supplement our field data.

And our Minnesota study lakes are far larger than those in Wisconsin. Only a masochist would attempt to study loons on the massive Whitefish Chain — where about half of our Minnesota study animals reside — by canoe. So a growing list of Minnesota friends and supporters have provided us with boats — thanks, John, Mike, Mary, Keith, and Dawn! — that permit us to cover the big water. (By the way, several others have made our work possible by providing housing — thanks, Melanie, Charlie, Mary, Jim and Jon!) In fact, we have learned that we can move about far more easily on huge lakes than on the tiny lakes where most of our Wisconsin loons live. Moreover, we can hold our position in the water more effectively and work in greater comfort on the Chain, providing winds are calm.

However, loon capture is another matter. Having caught rather few loons on huge lakes in Wisconsin, I was concerned that my team would waste many hours each night scanning the dark water before our spotlight came to rest upon a tiny light smudge that would become, on approach, a loon parent and a chick that we could capture. In truth, we do spend somewhat more time searching for Minnesota loon pairs that we are accustomed to. Furthermore, locating loon families acoustically is more difficult in Minnesota, because Minnesota loons seem less vocal at night than their small-lake brethren in Wisconsin. But once located, loons in Crow Wing County have proved easier to capture. So my irrational fear that loon capture would be slower and more difficult in our new western study area was unfounded.

What progress have we made in Minnesota so far? Despite the ill-timed failure of an outboard motor that forced us to cut short our night and limp back to our boat landing using only a single canoe paddle and three tote box lids, we have marked 37

adults and chicks in four nights. We banded sixteen loons on Ossawinamakee alone last night. In a few hours’ time, five anonymous territorial loon pairs on Ossie have become a valuable set of individuals whose behaviors, life histories, and survival rates we can track to enrich our understanding of loon breeding behavior and population dynamics. Moreover, our experience in Wisconsin tells us that the brief capture and marking process leaves little or no imprint on loon behavior. Loons caught and marked one night act the next day as if the event never happened. They display the same casual indifference towards us and other humans that they showed on the day before.

On the other hand, we ourselves are greatly changed after we capture and mark loons. Marked loons are individuals to whom we are committed forever afterwards. Yes, we get scientific data from them. But marking creates a lifetime bond between observer and loon. We know these birds. We cheer as chicks we marked return as adults to the study area and claim territories. We mourn when marked parents lose a chick or abandon a nest. And we move heaven and earth to guard these individuals and come to their rescue, if they need it. It has proved impossible to maintain pure scientific indifference to our study animals.

In short, Minnesota loons are excellent study subjects. They ignore our visits to their territories and forgive us immediately after capture and marking. My initial fears and doubts about marking and observing Minnesota loons have subsided. We can now see that we will learn an immense amount about territorial behavior, breeding ecology, and population dynamics of Minnesota loons — if we are willing to shoulder the burden of an intensive field project in a new state on these most engaging birds.

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Our work in Minnesota is just beginning. Although we have great support from the National Loon Center and seem to be getting lots of folks from across the state interested in our work, there is much left to do. At present, we are scouting lakes in the Crosslake area and hoping to find chicks so that we can band the chicks and their parents. Only through this arduous process can we establish a large study population, measure survival rates, and construct a population model for Minnesota loons. So far we have only fifteen banded birds in our entire Minnesota study area.

Make that sixteen. On Wednesday, Katy and Jordana found a new nesting pair on Lower Whitefish. This duo — the nesting loons, not Jordana and Katy — has the distinction of using the oldest, most sunken, washed-out nesting platform that we have yet seen in Minnesota. (Most nesting platforms that we have found in our new study area are quite bouyant and well maintained.) The platform location is also exposed to a substantial wind fetch, not to mention endless wakes from passing boats. And yet, drawn to this location by the predator-resistant nesting opportunity, a pair of loons has made this site their home.

Jordana shared the news of this new nesting pair with me in the most thrilling possible way. She told me there was a nest and sent me a video of one of the pair members swimming underwater and behaving protectively towards it. I immediately ran the video and caught an unmistakable sparkle on the bird’s left leg. (Note the bright white spot on the left leg of the loon in the featured photo and the video below.) The sparkle told me that the bird had a silver band on its leg that was catching the sunlight; the fact that the left leg was banded told me that this loon was an “ABJ” (adult banded as a juvenile) and not an “ABA” (adult banded as an adult). Hence, this loon, when we capture it and confirm its age and natal origin from the number on its USGS metal band, will be the first known-age individual in our Minnesota study area.

So this is not just any pair of loons. No, this Minnesota loon pair represents a first for the new Minnesota study area, because one of the two pair members is an individual whose age can be known precisely. Kevin Kenow of the USGS captured and marked this silver-banded loon as a chick four to six years ago on a different part of the Whitefish Chain. At present we must say “four to six years ago” because Kevin placed only a single band on the left leg of this bird, and he banded six other loon chicks similarly. Until we capture it, we will not know which of those seven chicks grew into this breeder. However, we can be virtually certain that the banded loon is a male, because most females settle to breed many miles from their natal lake.

Why am I so excited to see this young adult settle in our study area? Because one very important demographic parameter we seek to measure in Minnesota loons is the survival rate of juveniles and young adults. This, you may recall, is the cohort of the Wisconsin loon population that has suffered a high rate of mortality in recent years and thrown that population into decline. While we will be able to estimate the survival rate of breeding Minnesota loons — a crucial parameter in its own right — after another year or two of work, it will take five years or so before we have seen enough settlements of ABJs like this male to produce a good estimate of young adult survival in Minnesota. But the sight of this first young settler shows that we are on the road.

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Although I am stuck in California teaching for another few weeks, others have been hitting the lakes. Based on thirty or so lakes from which I have had reports — mostly Linda’s work, but also a few lake residents — we have an early read on the return rate of adult loons from last year.

Let me explain. April and May are exciting months for the Loon Project, because we hustle from lake to lake to see which of our banded adults have returned and which have not. In a typical year, the vast majority of our study animals have managed to survive the winter, navigate fall and spring migrations successfully, and take possession of the territories they occupied the previous year. The figure hovers around 80 to 90%. A high rate of annual survival is vital to our population. The low reproductive rate of loons is sufficient to sustain the population only because most adults survive each year.

During years when I am able to steal away from my pedagogical commitments, I find these “censusing” visits oddly thrilling. On my first lake visit of the year, I fancy that the male and female both pause for a moment, wheel in my direction, and think, “Where’s he been?” This might not be pure imagination; after all, I have been observing most of these individuals for a decade or more.

Last year seemed an exception to the typical high rate of adult return. Fully a quarter of the adults that we left behind safe and sound in the early fall of 2019 failed to come back in the spring of 2020. While some of these adults had merely been evicted from their territories, most were dead. Coupled with our recent finding of population decline in northern Wisconsin, the low 2020 return rate weighed on my mind last spring. On the other hand, return rates bounce around. So I tried to avoid jumping to the conclusion that adult survival was going downhill.

Recent reports from the lakes this year have placed concerns about adult survival front and center again. Each of Linda’s almost-daily census visits seems to bring fresh news about a missing adult or breeding pair. Early on, Linda reported that a new male had replaced the long-time resident male on Manson. Okay, that happens, I thought. Nothing to worry about. The Deer Lake female, freshly marked in 2020, also turned up missing. A Halfmoon Lake visit brought no better news; the 2020 female from there was gone as well. Hildebrandt and Julia, always occupied by pairs and frequent chick producers, were vacant. Linda’s trip to Nokomis Lake was most devastating of all. Towards the eastern end of the lake, both members of a long-term pair with a consistent record for rearing chicks were AWOL. And Linda turned up only one unmarked loon from the entire 2200-acre Nokomis flowage, which usually supports three breeding pairs.

He did not know it, but Al from West Horsehead produced the straw that broke the camel’s back. His report from this morning that the 8-year-old West Horsehead male had been replaced by a 6-year-old male from neighboring East Horsehead hit me especially hard. The sample had become large enough that I could not longer deny the pattern. Looking at the number of returns right now, we have found only 21 of 31 adult loons that should be on territory. Now, we will track down some of these missing birds. A few will be alive and breeding on a seldom-visited lake near their old territory. And that will give us a momentary lift. But an adult return percentage in the low-70s, as we are seeing for the second straight year, will not sustain our breeding population for long.

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By training I am a behavioral ecologist. That means that my background and experience help me understand what behavioral answers have evolved in response to the ecological problems animals face, like avoiding predators and finding a mate. So I am especially interested to learn why loons that are rearing chicks abandon them for periods of an hour or more to visit the neighbors, and why female territory holders are able to surrender their territory to a superior opponent and live for another day while male territory holders in the same predicament seem unable to sense the danger and often die in territorial battles. But such questions pale when compared to a single, burning question we have faced for the past year on the Loon Project: “What is causing the northern Wisconsin loon population to decline?”. That question has become a nagging source of unease that prevents me from feeling fully comfortable anywhere and at any time.

There are many possible reasons for the decline: the exploding eagle population, decreased fish numbers, human impacts like increased boating or angling. And, of course, climate change, which impacts temperature, rainfall, and extreme weather events, is the elephant in the room. Learning about and systematically eliminating each potential cause of the decline will require me to find and collaborate with other scientists who know about fish, eagles, human impacts, and climate. In other words, cracking this nut will force me far outside my comfort zone.

We have glimmers. My collaboration with Sarah Saunders showed us that increased rainfall, increased human settlement, and the North Atlantic Oscillation – a broad-scale climatic event that influences weather in the northern Hemisphere – are all linked to both lower breeding success and lower adult survival of our loons.

A month or so ago, Linda and her husband, Kevin, speculated that increased boat traffic on large lakes might be the cause of the reproductive decline of loons in Wisconsin. They reasoned that more big boats might churn up the water, reduce water clarity, and make it harder for loons to find their prey under water. Such a scenario might make chicks grow more slowly now than 25 years ago and cause higher chick mortality.

Water clarity has always been a prime suspect among factors likely to influence loon survival and breeding success. As visual predators, loons must be affected by water clarity. Right? Yet we have no evidence to date that clarity affects loons. Brian Hoover’s recent paper, for example, showed that juvenile loons try to forage on lakes similar to their natal one in pH, but not in clarity. Our analysis from several years ago showed that young loons tend to settle on breeding lakes similar to their natal one in overall size and pH – but, again, water clarity is not a factor. Moreover, a glance at our study lakes shows that loons survive well and produce chicks on lakes that range from crystal clear (20 feet of visibility or more) to very murky (4 feet or less of visibility). If loons live and breed successfully on lakes that vary so greatly in clarity, perhaps clarity simply does not matter at all.

Nudged by Linda and Kevin to look once more at water clarity, I finally had some success. When two new collaborators at Rensselaer Polytechnic Institute provided me with thirty years of water clarity data based on satellite overpasses from my Wisconsin study lakes – and I plugged those data into my statistical models – suddenly clarity mattered. To be specific, mean water clarity during July was a significant predictor of chick mass. Clear water produced fatter loon chicks! Furthermore, chick survival decreased significantly in cloudy lake conditions.

Wait……what does this pattern mean? If you are like me, you think of water clarity as being constant or static for a lake. That is, you consider Two Sisters Lake as a very clear lake and Pickerel Lake as a murky lake. And you are correct. But those lakes – all lakes – fluctuate in clarity seasonally, annually, and even over days or weeks. Runoff events caused by rainstorms reduce clarity, for example, because silt and other materials are carried by streams into lakes. So you can have a bad few weeks or month for clarity on a lake that is generally quite clear. And a very clear lake can gradually become less clear over the years. The new satellite data are showing us that such short-term fluctuations in water clarity are associated with lower chick mass. It is a conceptual leap, but the obvious interpretation here is that short-term losses in water clarity impair foraging by loons and reduce the amount of food they are able to provide for their chicks.

It is early days. My collaborators are refining their estimates of water clarity from the satellites for northern Wisconsin and promise improvements by October. Meanwhile, I am left to ponder two things. First, water clarity in northern Wisconsin has declined over the past ten years, as the featured graph shows. Second, if recent declines in lake clarity really do hurt loons’ ability to catch prey for their chicks, what can a single loon researcher do about it?