The nest in the featured photo has that forlorn, unloved look that many do at the beginning. It seems, at first glance, that the female just crept up on shore and dropped an egg there — as if she had to put it somewhere. Closer scrutiny reveals that the pair had gathered a substantial bed of pine needles from the surrounding area and formed a crude bowl shape around the lone egg. Still, it does not inspire confidence that a portion of rotted oak leaf is draped over the egg. Yet the male approached me as I quickly snapped a few photos, registered the nest’s GPS coordinates, and skedaddled to see if he would incubate. He did not.

I can understand the Thunder Lake pair’s inability to adjust to changing conditions. Only an hour before seeing them, I had stumbled out of my guest room (a wonderful, secluded home by a small lake where our friends permit us to lodge in May and June). My trip to Rhinelander on the previous day had not been the relaxing, sumptuous journey I had hoped for. After a last-minute cancellation of our flight, I buddied up with two other shell-shocked passengers near the gate — one a sales director for a pharmaceutical company from Minocqua, the other a Colombian fellow from Tomahawk who sells farm machinery for a Finnish company — and we rented a car for the 3 1/2 hour drive instead of waiting overnight to be rescheduled. (In truth, Steve rented the car, since his company would cover the cost.) By the time our rental rolled into Rhinelander, it was 2 a.m. Luckily, the engine turned over in the 2007 Toyota Corolla that we leave for nine months in our storage box (the “Loonmobile”). I was able to grab three hours of shut-eye before some passing Canada Geese awakened me. Two — okay, three — donuts and a cup of coffee later, I had seen my first loon pair on Thunder.

My first lake visit in 2022.

We have learned over the years that loon pairs take a day or more to “accept” that they have laid an egg and must incubate it. On these initial days, pairs sometimes wander far from their new nest, leaving the egg dreadfully exposed. I find this curious. The egg is, of course, in danger of being found and eaten from the moment it is laid. The embryo inside it cannot begin developing rapidly until it becomes optimally warmed by the parents. Every moment spent off the eggs seems time wasted and needless risk taken. I suppose, though, that I must defer to my study animals, who have a pretty good record of turning eggs into chicks.

The Thunder pair can be forgiven more easily than most for their reluctance to incubate. The male is new to the territory and unmarked. Probably a 5- or 6-year-old, this is likely the first egg that a mate of his has ever laid. One hopes that he and his well-seasoned mate — whom we banded as a chick on Currie Lake in 2003 — can restore Thunder Lake to productivity, after a six-year chickless slump. To do so, they will have to shake off this egg-denial and get serious about breeding!

I love the painting because it is not about loons. John Seerey-Lester’s artwork*, pictured above, is about rain. The painting recalls those moments when you were out on a lake — taking in the vast expanse of its surface; gazing at an eagle circling high above; or watching a loon pair drift by with their four-day-old chick — and a rogue cloud emptied upon you. Most of us who have ventured out onto lakes can recall such an experience. In the moment, there is panic: a hastily zipped jacket, a vain attempt to find some form of shelter to thwart the impending deluge. But there is wonder and beauty in the storm itself. As Seerey-Lester’s painting shows, raindrops transform a lake’s monotonous surface into an astonishing palette of dancing splashes. Accompanied by a soothing whisper, the spectacle of a rainstorm on a north-country lake is one of nature’s wonders.

Loons cope well with rain, of course. What harm is more water, after all, when you live in water? Like most birds, loons assiduously preen their feathers, coating them with oil from a gland at the base of their tail, so water beads up on their heads and backs, but ultimately rolls harmlessly off them — like water off a loon’s back. A downpour might necessitate a few shakes of the head, inspire a few extra wing flaps, and prevent foraging for a time, owing to reduced visibility. But loons greet rainstorms with little more than a shrug.

Considering the grace with which rain appears in loons’ lakes — and, of course, its fundamental importance in supporting all life — I was unpleasantly surprised to learn this week that rainfall has likely contributed to the reproductive decline of loons in northern Wisconsin. You see, rain does not merely stimulate plant growth, raise water levels, and rinse car windows. Rainfall also washes all sorts of matter into lakes. This includes visible organic matter such as sticks, leaves, and soil but also invisible nutrients and chemicals. Many substances that reach lakes via rainstorms reside naturally in soils or on the forest floor. Others, like fertilizers and sewage, have been added by humans to the environment. Human-added materials that contribute nitrogen and phosphorous to lakes can cause populations of phytoplankton to surge, which reduces water clarity.

And this brings us back to loons. Loons rely strongly upon their vision to catch fish and other prey underwater. As our recent investigations have shown, reduced water clarity hinders loon foraging. We now know that reduced water clarity leads to poorer body condition both in breeding males with chicks and in chicks themselves. The decline in chick body condition and accompanying rise in chick mortality are essential components of the breeding decline now underway in northern Wisconsin.

Water clarity in loon breeding lakes in July declines with increased rainfall in June and July.

Why am I so determined to blame it on the rain? Because a few days ago I examined water clarity estimates from my collaborators — Kevin Rose and Max Glines of Rensselaer Polytechnic Institute — and found that water clarity in July, the best predictor of adult male and chick mass, is, in turn, strongly dependent the amount of rainfall in June and July. Just as April showers bring May flowers, June and July showers bring July algal blooms in Wisconsin lakes that make it more difficult for loons to find their prey.

Of course, rain itself is not the true villain. Rather it seems to be fertilizers, leaky septic tanks, and maybe even pet waste that human lake residents have added to the ecosystem that are contributing to the loss in lake clarity.

You might wonder if there is truly a sustained, irreversible downward trend in water clarity or whether water clarity fluctuates according to natural cycles and is merely on a downward trend at the moment. If we are simply on a temporary downward trend, then it is a decade-long trend, according to Max and Kevin’s measures of water clarity (see graph just below).

Moreover, as I reported recently (see graph below), loon males have been losing body mass for the past 30 years. So the data we currently have indicates that we are on a prolonged downward slide with respect to both water clarity and loon mass.

Male loons have been gradually losing body mass for the past 30 years; female loons do not show such a decline.

So what do we do? All hope is not lost, I think. But if our data and interpretation are correct, then we must immediately begin to monitor — and curb — chemical runoff from shorelines into lakes from sources such as fertilizers and septic systems. In the long-term, we need to understand that summer rainfall will only increase as the Earth continues to warm and cloud formation accelerates. In short, it is a bit harsh to blame the rain for loons’ current reproductive woes, but increasing rainfall in coming decades will probably push us more rapidly in the wrong direction.

*”Sudden Rain”, copyrighted by Sir John Seerey-Lester

The Upper Midwest remains — for the moment — in winter’s clutches. This fact is oddly comforting to me, stuck as I am in the pleasant but seasonless climate of southern California. In one sense, I dread the coming of spring in Wisconsin and Minnesota. Each year I find myself weeks behind schedule. While my study animals in the Upper Midwest spend April facing real problems like migrating safely, settling on their territories, contending with hopeful usurpers, and beginning their breeding effort, my Aprils are more mundane. I prepare study guides, conduct review sessions, and quiz my students about evolution and ecology in preparation for the final exam.

Despite the bland Mediterranean climate and muted seasonality of southern California, I am not entirely in the dark about the coming of spring. When I hear house finches, orange-crowned warblers, and the impossibly loud and bubbly song of the tiny house wren out my front door, I understand that loons are on the move. Last Wednesday’s birdwatching trip to the Newport Pier provided the most stark reminder yet. As I stood next to my spotting scope, scanning the ocean for pelagic rarities like jaegers and shearwaters, a familiar dark silhouette appeared several hundred yards offshore.

His crisp breeding plumage gave him away. But just to drive home the point, this adult-plumaged Common Loon uttered several awkward, truncated yodels — to the befuddlement of many Western Grebes rafting nearby. No doubt this male will improve upon his sputtering vocal performance by the time he reaches his breeding territory in British Columbia or Alaska. For the time being, his voice reminded me that I have much work to do in the coming weeks.

My tardiness reaching the Upper Midwest each year guarantees that I spend the first two weeks of every field season in a tizzy. I race frantically to a boat landing, drop my canoe into a lake, and make a beeline for the breeding pair. After ID’ing them from their leg bands, I throw the canoe back up on my roof rack, drive to the next lake, and repeat the process. It is not the way one might choose to take in the wonders of early spring!

But enough of my problems. Loons are the ones most impacted by late spring thaws. As Linda Grenzer’s above photo from northern Wisconsin shows, lakes are only beginning to open up in the Northwoods. This means that, for now, breeding loons must be content feeding and resting on rivers and dodging ice floes, like the bird below from Linda’s video:

Perhaps today’s warmth will melt enough ice for breeding pairs to begin to land and stake out their territories for the year. They must rue each day that passes before settlement. A recent statistical analysis showed that each four days that pass before loons can take possession of their territories pushes back nesting one day. On the one hand, this is good news, because it means that breeders bounce back from late ice-outs by being more thoroughly recovered from migration and ready to nest when they finally do settle. On the other hand, like me, they must be pretty anxious to get their summer’s toil under way.

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.

I do not think of myself as a cheerful bearer of bad news. Yet I repeatedly bring it. Each time I meet a new lake resident and secure permission to cross their land and observe their loons, I brace myself for the inevitable question: “Is it true that loons mate for life?”. I gently share the truth with them. “No, they don’t; but they really have a strong allegiance to their territories!” The idea that loons love their homes, not their breeding partners, provides scant solace to most folks.

Having spent a quarter century disappointing loon lovers in Wisconsin and Minnesota, I have been searching for scientific news to share about breeding pairs that sheds a warm, wholesome light on loon mating behavior. My quest is not inspired by guilt alone. As a scientist, it makes sense to me that adults should become acquainted with their mates and benefit from doing so. How could complex, long-lived animals like loons jointly defend a territory, build a nest, divvy up incubation duties, and raise young together — as seen in Linda’s photo above — without benefitting in some way from their association?

At last, my current study of predictors of breeding success has revealed one way in which loon pairs do benefit from a long-term association. The graph below shows two patterns. First, there is a gradual improvement in hatching success over time as a pair remains together on a territory. These numbers jump around a lot owing to limited samples of pairs that have been together ten or more years.

Second and more clear cut is the improvement in hatching success between a pair’s first year together (Year 0) and their second (Year 1). As you can see, loon pairs improve their chances of hatching eggs by almost 10% between these first two years.

Now we can only speculate about the cause of this dramatic improvement in breeding ability among young pairs. Perhaps pair members synchronize their breeding behavior better in their second year together than in their first. Maybe pair members rotate incubation duties more crisply in year two, thus seldom leaving the eggs uncovered and unguarded.

Of course, the challenge that romantic couples face of living and working harmoniously, following an awkward adjustment period, has a familiar ring to it. That challenge is depicted in “Period of Adjustment“, a 1962 comedy-drama film based on a play by Tennessee Williams. It is heart-warming to see that loons — like actors Jane Fonda, Jim Hutton, Lois Nettleton, and Tony Franciosa — bounce back strongly after a rough first year.

I have had a lot to say about male loons and their experience. Indeed, the fact that the male decides where to place the nest means that he develops a tight bond to his familiar territory and fights hard — harder than his mate — to keep it.

But females too play a vital role in breeding success (like the pictured female photographed by Linda Grenzer on Bear Lake). How might female experience affect the outcome of a nesting attempt? Now that I have begun a detailed analysis of causes of breeding success and failure, I have started to ferret out the difference that female experience makes. I am only halfway done, but I can already see that the number of years a female has spent on her territory strongly affects the date her chicks hatch. As the graph below shows, females that have just arrived on a new territory — because they have evicted the previous female owner or replaced a dead one — have an average hatching

date of 22 June. In contrast, 4-year veterans on territories hatch their eggs, on average, 5 days earlier — June 17th. Now this might not sound like much of a difference in hatching date. But when you are tasked with stuffing your voracious chicks with fish, watching them grow rapidly to adult size, and hoping they get proficient enough at foraging, flying, and avoiding trouble to eke out a successful migratory flight to Florida, you take every extra day you can get!

You might ask: “Does this delay occur because of female inexperience with breeding in general or does it come about because of lack of experience on a specific territory?” The delay seems to be associated with lack of familiarity with a specific territory, because females that pick up stakes and move to new territories show delays in hatching date just like the ones they suffered on their first territories.

The cause of the improvement in nesting schedule with experience is likely to be energetic. That is, a female that knows how and where to find food on a lake is able to recover from spring migration quickly and begin the lengthy and arduous process of raising chicks. A female that is still learning where to find food on her new lake spends extra time before she reaches a suitable body condition to commence breeding.

Don’t male loons have just as big a problem restoring their body condition and thus readying themselves for a nesting attempt? Perhaps. But males experience minimal breeding costs until they begin joint incubation duties with females. Apparently males’ energetic deficit from migration does not hinder the breeding schedule.

You probably noticed that the graph depicts a curve, not a line. Older age-classes of females — those that have spent 5 or more years on a territory — actually begin to nest later than females that have spent 4 or fewer years there. What on Earth could explain this peculiar pattern?

Since it makes little sense that a female’s experience on a territory could begin to work against her after several years, we must look beyond experience for an explanation. The later hatching dates of more experienced females probably arise from reproductive senescence — a decline in reproductive performance that occurs with advancing age. Senescence is well-known in mammals and also many birds. We should not be surprised to see such a pattern in loons.

It is exciting to discover and ponder the reproductive quirks of female loons. Like many of our findings, this one only became visible because we studied thousands of nests, by hundreds of marked loons, across decades of their breeding lives. That, of course, is our bread and butter.

One of the quirks of the loon breeding system is that males choose the nest location. I have mentioned this fact many times in posts. I still scratch my head over the pattern, which makes no sense on many levels. Why would a breeding system evolve in which one sex or the other has sole discretion for deciding where the nest goes? In such a case, death or eviction of the choosing partner is devastating for its mate.

Consider this scenario. A male and female settle on a new territory on a protected cove in a large lake. At first they struggle to hatch young, but they begin to do so after a few years and enter a period of steady chick production. After 12 years, the male dies. Since he is the repository of information about safe and dangerous breeding sites in the territory, the veteran male’s death leaves his mate in the lurch. She must wait patiently while a new male takes over as breeder and stumbles badly for a few years by placing nests in the path of hungry raccoons. Ultimately, the new male learns by trial and error where eggs will be safe from predators, begins to reuse those favorable locations, and becomes an accomplished breeder. But, having endured several years of her mate’s reproductive ineptitude, the female has lost precious breeding time.

This scenario plays out incessantly in common loons. We field observers experience some of the frustration that veteran females must feel as we watch the poor nesting choices of their novice male partners. Until a few days ago, however, we did not appreciate the stark contrast between male and female breeding experience.

As the graphs above show, males do not simply improve in hatching success in their first year or two on a territory; they improve over a period of at least 20 years! And the cumulative impact of this steady improvement is massive. A male’s odds of a successful hatch are 35% better in year 20 than in year 1.

On the other hand, females do not experience better hatching success in year 20 on a territory than in year 1. In fact, there seems to be the faintest whiff of improvement for females over the first five years on a territory — and a tailing off around 20 years on — but these apparent patterns are not borne out by statistical tests.

By the way, these findings do not mean that a veteran female might not happen to enjoy higher hatching success as her tenure on a territory increases. What it means — and this can be confusing to think about — is that any boost in hatching success an individual female enjoys results from the growing nesting experience of her mate, not herself.

I am still scratching my head over this dramatic difference in breeding behavior between male and female loons. It still baffles me on many levels. But the dramatic, long-term gain in nesting ability of males does help me understand the viciousness of male territorial behavior. A male that has reaped enormous benefits while spending two long decades to learn the ins and outs of his breeding territory should fight desperately to keep it.

Sometimes during a night of capture, when we have finished color-banding a loon and are releasing it back into its territory with its family members, I say to the bird, half jokingly, “Welcome to the Loon Project”. But I mean it. Once we place colored leg bands on a loon, we start to feel a kinship with that loon and take an active interest in its well-being.

The bond we feel with each banded loon grows as team members report its trials and tribulations across many years of its life. “Red over blue-stripe on Lumen is soooo tame!”. “Omigosh, that female on Lumen was super aggressive when two intruders landed in her territory this morning”. “Red over blue-stripe really scared a kayaker that came too close to its chicks today”. “Red over blue-stripe fed its chicks 58 times during the hour I was observing the family; those chicks begged relentlessly.” “Red over blue-stripe just skulked around the southern end of the lake this morning while her mate foraged with a new unbanded female. She looked so bummed out.” “There is a new breeding female on Birch today; she is red over blue-stripe!”

Just as we mourn when a male or female is evicted from its territory by a young adult, we cheer when it bounces back and claims a new territory nearby with a new mate. If one of “our” loons should be injured by a lure or fishing line, we spring into action to save it.

Knowing and caring about our study animals makes it more enjoyable and rewarding to observe them. But the warmth and connection we feel towards our loons is really just a pleasant byproduct of a coldly pragmatic research philosophy: mark every loon you can, and track marked individuals obsessively throughout their lives.

Why are we so fixated on marking loons and studying marked individuals? Because marking and reobservation allows us to turn anecdote into science. If one watches five unmarked adult loons circling and diving together in early July on Brandy Lake, and two of the five birds yodel at each other, one might conclude that two members of the group must be males that became aggressive for some reason. If, on the other hand, the five loons are color-banded, we can begin to make inferences about behavior. We might observe that the group consists of two territorial pair members from Brandy and three intruders: a 3-year-old male floater reared on Johnson Lake, a 7-year-old male floater raised on Bullhead, and an 11-year-old female breeder from neighboring Arrowhead Lake. We might further note that the two yodelers are the 9-year-old territorial male and the Bullhead floater. And finally, we might observe that the 3-year-old and neighboring female fled from the group of 5 following the yodeling incident and flew off shortly afterwards, while the 7-year-old male engaged in many simultaneous dives with the male breeder and stayed 36 more minutes before departing from the lake.*

Of course, one visit to a breeding territory does not by itself lead to any useful scientific conclusions, even when loons are marked. But when this day’s observations are combined with those by scores of other field observers on hundreds of marked loons and thousands of early mornings, statistical patterns begin to emerge. Indeed, in a paper we just published, we document how floaters (nonbreeders too young to claim a territory) behave differently as they age, how territory owners tailor their aggressive behavior to floaters of different ages, and how loon parents optimize defense of chicks differently as they grow. So the accumulation of observations on marked, well-known loons made possible several steps forward in our understanding of territorial behavior.

Marked loon populations have value over and above the strides they help us make in understanding loon behavior. Since loon numbers have clearly declined in Wisconsin in recent years and apparently also among the less-well-known loons of Minnesota, our study animals in both states suddenly have special significance. In the coming years, we hope to use our study populations in Minnesota and Wisconsin to ascertain the causes of the declines and work with others who love loons to turn things around.

*Linda’s cool photo above is of Nelson Gould, a Chapman student, who worked with us for three years.

I have pointed out many times how science proceeds not in simple linear fashion but haphazardly. A finding may appear rock solid but subsequent findings modify our understanding, even forcing us to discard earlier conclusions in some cases. Viewed from space — and over large stretches of time — we might seem to advance steadily in understanding a phenomenon. From ground level, progress in comprehending a topic is herky-jerky; we make clear progress for awhile, then reach a dead end, back up, and find a new path forward.

So it has been with loon conservation. Thirty years ago, we feared that methylmercury was a major health hazard for breeding loons and their offspring. At the time, our fears seemed well grounded. Now, having spent millions of dollars measuring mercury levels in loons and looking for its impacts, we must reassess. We now see that, at worst, mercury has the potential to harm loons in the eastern 1/3 of the breeding range — and even there only in acidic lakes. Consequently, where loon populations appear to be declining, we can cross mercury off the list of threats and turn our attention to other potential causes. By the way, it was no mean feat to cross mercury off the list of dangers to loons. Thoughtful, rigorous work by dozens of biologists across North America has made this conclusion possible.

Our study of a much narrower topic — the loon yodel — also seems to be moving forward by fits and starts. (Linda’s gorgeous photo above shows a male yodelling in typical crouched position.) A really cool paper by Jay Mager — a collaborator with the Loon Project for several years — showed fifteen years ago that the frequency of the yodel was strongly dependent upon body mass. Large males, Jay found, had low-pitched yodels. This was fascinating to us, because it meant that male loons were revealing their size to territorial competitors! Now it is well and good to advertise your size if you are a big loon. In that case, the fact that you are yodeling tells opponents that you are motivated to defend your territory, and the low frequency of your yodel informs them that you can back up that determination with beef! But consider the mixed message that a small male sends by yodeling: “I am small but feisty”. Hmmmm. Not such a deterrent to a territorial challenger, it would seem. We are still coming to grips with this so-called “honest-signalling” of body size and trying to learn how small yodelers might benefit from yodeling.

Among males whose ages are known exactly (blue) and males whose ages are estimated (red), older birds have higher-pitched yodels.

But wait! Lately we have had cause to wonder whether body size really does have a strong impact on yodel pitch. Now that a good many males that we marked as chicks have settled to breed, we know the ages of many of our breeding males precisely. Brian Hoover, a postdoc here at Chapman, took these new age data — data unavailable to Jay Mager — and looked to see whether age is correlated with yodel frequency. As you can see from the figure, age is strongly correlated with yodel frequency: old males have high-pitched yodels. Reassuringly, the better our age information is, the stronger the correlation. That is, the blue points show a tighter age/frequency relationship than the red points.

Where does this leave us? I just explained that small males have high-pitched yodels. Now I am telling you that old males have high-pitched yodels. Can it be both? It is possible that both mass and age influence yodel frequency and could exert additive effects. (Imagine the extreme falsetto of a male that was both old and small!) However, Brian examined mass in his more complete analysis and found no evidence that it affects yodel frequency after all. In other words, we might have found a mass/frequency pattern back in 2006 simply because we did not have age data that would have shown us what was really an age/frequency pattern. Statisticians are quite familiar with this frustrating phenomenon; conclusions can change dramatically when a variable that was left out of a previous analysis (age, in this case) is added.

The finding that it might be age, not mass, that affects yodel pitch would be more palatable if high-pitched yodels by old males made more sense than high-pitched yodels by small males. But, intuitively, the new finding makes less sense! This is the delightful pickle we now find ourselves in. “Delightful” because the age pattern is robust and clear, as the graph shows. A “pickle” because we have no idea, at present, why or how old males have high-pitched yodels.

Well…so it goes in science!