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 try not to steal a glance through the lab window each time I pass. But I usually fail. You see, Marco Bisoffi, a molecular biologist and colleague of mine at Chapman, has restarted our study of telomeres* in loons as a possible tool to measure age and the effect of stress. Each week Marco churns out telomere measurements on a new set of loons, as he tries to troubleshoot the PCR** procedure. So when I walk by his lab and see him bent over his laptop, I wonder whether his promising early finding that telomeres indicate age in loons has held up.

It has. Now that Marco has run twelve males and ten females of known age, the trend is stronger than before. If you study the plot above, in fact, two patterns are evident. First, old males and females have shorter telomeres than young males and females. Second, males as a group have shorter telomeres than females. (This latter finding repeats what Jeremy Spool had found a few years ago.) There is some scatter in the data, especially among females, but both patterns show high statistical significance. Of course, we will have an even better fix on these patterns when we have run the other 83 DNA samples we have collected from adults of known age.

It is hard to exaggerate the value of these findings for loon biology and our own research in Wisconsin and Minnesota. There are countless benefits to studying loons, but one drawback has always been our inability to “age” individuals effectively. To our enormous frustration, we cannot even distinguish a 5-year-old from a 30-year-old. If this telomere pattern holds up, however, that source of vexation will be considerably diminished. In the future, we will be able to take a DNA sample from an unknown adult, measure its telomeres, and assign it to an age-class. Indeed, if the unknown bird is a male and we record both its yodel and its tendency to yodel at intruders, we shall be able to narrow its estimated age range still further — probably to within a few years.

Why does it matter that we are on the brink of being able to age adult loons accurately? First, age has a strong effect on a great range of behaviors, including aggressiveness, ability to hold a territory — which increases in young loons and then declines later in life — and even willingness to incubate eggs when black flies are abundant. Second, age impacts survival rate, especially in males. So knowing the ages of loons helps us refine our estimates of survival and improves our models of population dynamics.

Speaking of age and decline, the featured photo for this post is Linda’s Grenzer’s pic of “Clune”, the male on her lake. Despite the inevitable shortening of his telomeres, this 23-year-old still looks pretty fit in his winter attire!


*telomeres — protective DNA sequences (“end caps”) on chromosomes that permit DNA to be replicated many times but become shorter with age and stress

**polymerase chain reaction — a common molecular technique that permits efficient study of specific regions of DNA

The Loon Project is my life’s work. While I greatly enjoy teaching Chapman students, serving on committees with my colleagues, and living in southern California, a part of me resides permanently in the Northwoods with the loons.

I inherited my love of loons. Mom introduced me to them in the 1970s when we made trips to Temagami, a deep, clear, sinuous, 30-mile-long lake in central Ontario. “Listen…..do you hear the loons?” she would ask my brothers and me as we lay beneath thick woolen blankets. As a resident of far-off Houston, I recall feeling awe, and some fear, to hear the mournful wails and maniacal tremolos echo across the huge lake. I wondered what messages loons could be sending each other in the middle of the night.

So I guess I was predisposed to study loons when I re-encountered them in Michigan’s Upper Peninsula in 1992. By the time I had finished listening to Dave Evers (then director of the Whitefish Point Bird Observatory) describe battles for territorial ownership that he and his staff had witnessed during their capture and marking efforts, I was hooked. At first, I implored him to conduct further research. “Your observations suggest a cool territorial system in loons, Dave. There are important questions about behavior and ecology to address here.” But Dave’s interest was not territorial behavior. If anyone was going to follow up those exciting early findings, I, a trained behavioral ecologist, would have to do it.

I began my loon study in 1993 and ran the project on a shoestring back in the mid 1990s. Then a postdoc at Indiana University, I really had no business setting aside my work on parentage analysis by DNA fingerprinting – expertise much sought-after by universities at the time – for a logistically-challenging project that required an enormous investment of time and energy. There was no low hanging fruit here. Several years were required simply to collect enough data to publish my first paper.

It took a decade — until 2003 — to pull together a sufficient cluster of banded loons and early findings to convince reviewers at the National Science Foundation that I was doing productive, cutting-edge research. I was awarded additional funding in 2007 and 2012. But funding rates for ecological proposals are now in the 7 to 9 percent range — roughly a third of where they were 30 years ago.

I love my work and have enjoyed learning about loon behavior, ecology, and population dynamics over the past 27 years. The project is more important now than ever before for loon conservation. With the future of loons in Wisconsin somewhat in doubt, our long-term measurement of breeding success and territory occupancy of marked birds in a large, fixed set of lakes provides us with a vital “early warning system” to detect population decline.

I am excited to invite you to support my efforts to learn about loon behavior and ecology while creating educational opportunities for undergraduates. Here is a link that will take you to our brand new “Donate” page. Thank you in advance for any amount you are able to give — and for your commitment to the loons of the Northwoods!



The Nose Lake male I observed today had a conspicuous scar on his head. This particular scar, which I dutifully sketched on my datasheet, was located on the right side of his head, behind and beneath the eye. In comparison, the head of the Nose Lake female was sleek and without blemish. Scarred males paired with pristinely-plumaged females are a common sight. In fact, the scar I recorded today was the 72nd of the study – and the 63rd seen on a male. Moreover, this scar has persisted for a month; Linda photographed this bird on May 6th, and the scar was obvious then.

Scars on heads of males, which occur when they grasp each others’ heads and necks in a territorial battle, bring to mind the short, violent lives that many males lead. Slowly and surely, we are beginning to understand why male battles are fiercer than female battles. Part of the explanation for this pattern has to do with nesting behavior. We know from analysis of nesting behavior among color-marked breeding pairs that male loons control the placement of the nest. While we do not know why males control nestsite placement, we can see that male control of nest placement cranks up the stakes for male territorial battles. Why? Because male loons learn by trial and error where to place the nest. Once a male has nested successfully on a territory, he reuses that good nest location again and again, boosting his hatching success. Therefore, once established on a territory where he has nested successfully, a male has a large stake in holding that familiar territory. If evicted from there, the male must relearn where to nest and where not to nest on a new territory, which costs him precious time and energy. In contrast, females, which do not control nest placement, can freely move from one territory to another without paying a penalty in lost familiarity and, hence, breeding success. Since one territory is, in effect, as good as another to them, females should fight less hard than males to remain on a territory – and they behave as predicted.

A second part of the explanation for violent male battles is rapid senescence. Again, while we do not yet understand why males should age so badly, compared to females, the contrast in senescence has strong implications for male behavior. A male reaches a point – in his mid-teens typically —  where he is in rapid decline. That is, he is losing body condition and is at great risk for losing his territory. With the future offering little reproductive promise, many males in their mid-teens increase their aggressiveness and territory defense so that they can squeeze another year or two of breeding out of their territory. This, of course, is the terminal investment finding that I have been blogging about for the past months. (By the way, that paper has just been published online.)

With two factors – male nestsite selection and senescence – at play, we can begin to understand why males might be so violent. The factors are additive. A fifteen year-old male on a familiar territory is both falling into decline and facing a steep loss in breeding success, if evicted. So he has two good reasons to fight like hell to hang on.



I have just completed my paper on black flies. The paper presents evidence that black flies cause nest abandonment, which was lacking in the literature before. The evidence is pretty convincing, I believe. (We shall see what my scientific colleagues think when I submit the work for publication in the next week.)

In the course of looking at black fly impacts on nesting, I stumbled into two  interesting findings. These findings were serendipitous, like much of what scientists report. That is, I was keenly focused on one topic — black flies and nest abandonment — when I made a finding related to another topic — other causes of abandonment. In fact, I analyzed statistically a whole set of factors, some seemingly unrelated to black flies, that might have predicted nest abandonment. Among these were age of the male, age of the female, duration of the pair bond between them, exposure to wind (which might have kept the flies at bay), size of breeding lake, and distance from the nest to the nearest flowing water (from which black flies emerge as adults).

I was excited, but also baffled, to discover two new predictors of nest abandonment. First, pairs on large lakes are less prone to nest abandonment than pairs on small lakes. Second, pairs containing an old female are far more likely to abandon a nest owing to black flies than are pairs containing young females.

Now, I like to think that I know everything about loons. When I am visiting a study lake and someone asks an easy one like, “Do loons mate for life?”, I puff myself up, lower my voice an octave, affect a mild British accent, and pontificate on the serially monogamous breeding system of Gavia immer. But I was wholly wrong-footed by these two new findings. I had been so laser-focused on black flies as the prime movers in nest abandonment that I had included age and lake size in the analysis almost as an afterthought. I had not even considered what it would mean to learn that age and lake size were significant predictors.

The statistical significance of lake size as a predictor of abandonment forced me to confront a complex variable. If numbers of black flies are correlated with nest abandonment (as they are), then it requires no great conceptual leap to infer that black fly harassment is causing loons to abandon their nests. But the fact that lake size predicts abandonment opens up a much broader range of explanations, because lake size is correlated with degree of human recreation, pH, wind exposure, wave action, available food, and numerous other factors. Having picked through the possibilities, an energetic explanation seems most likely to explain the lake size pattern. That is, large lakes provide more food than small lakes, so loon pairs on large lakes should be in better health and condition than those on small lakes. Well-fed, healthy adults with strong immune systems should be better able to cope with the blood loss and exposure to blood-borne pathogens (like Leucocytozoon protozoans, which cause a malaria-like disease in birds) than under-nourished individuals with weaker immune systems.

What about the higher abandonment rate of pairs that contain an old female? Here again, energetics might be the key. Old females senesce — they experience much lower survival and slightly higher vulnerability to eviction than young females. So it stands to reason that old females are in poorer body condition and are more likely to abandon nests when attacked viciously by black flies. Reproductive decline among old females is widespread in animals, and the tendency of old female loons to abandon nests more readily seems consistent with that pattern.

But what about males? As I have emphasized in recent blog posts, males senesce even more dramatically than females do. How is it possible that old males can continue to incubate eggs when being bitten mercilessly by black flies when old females cannot? Terminal investment appears to be the answer. Terminal investment — efforts to increase breeding output as death approaches — occurs only among male loons, even though both sexes senesce. As the months have passed, we have learned that male loons not only become hyper-aggressive when they reach old age (15 years) in an apparent attempt to hold their territory for another year or two of breeding, they also seem to show a more subtle willingness to try harder to hatch eggs and rear young to fledging. The new finding showing that old males do not abandon nests as readily as old females when beset by black flies is thus part of a growing pattern.

My tentative explanations for the impacts of lake size and sex on nest abandonment are not the end of the story, of course. Rather, they raise more vexing questions. Why on earth would a loon settle to breed on a small lake, when small lakes doom loons to poorer body condition, a higher rate of abandonment, and the likelihood of losing one or both chicks in the event they can hatch the eggs? And even if the higher rate abandonment of nests by old females fits a growing pattern, why do males and females differ so much in their life-history strategy? We do not know….and this is why I love my work!

A few years ago, my daughter and I were talking about her high school homework. I cannot recall precisely what class we were discussing, but a moment occurred when I became concerned that she was unprepared for an upcoming test. Anxiety hung in the air for a second before she reassured me. “Dad”, she said with a twinge of impatience, “I am a nerd”. It was her way of telling me that she was studious, exacting, and did not need to be told to get her work done.

As you all know by now, I too am a nerd. I wear that label — like my daughter does — as a badge of honor. The nerds I know are thoughtful, bookish folks who enjoy making fine distinctions and extracting subtle patterns from scientific data. Nerdiness of this kind is essential to a scientist, of course. Publishing our work and being taken seriously by our colleagues requires that we navigate a mine field of biased samples, uncontrolled variables, and specious correlations to arrive at valid conclusions to our questions.

Nerds are different. Most people, while chatting with a stranger in a supermarket line, can get away with saying, “Aaron Rodgers killed the Packers last year with his erratic passing in big games”. A nerd, however, would want to look at the data. S/he would examine Rodger’s passing statistics against teams with winning records and division rivals to see if, truly, he played worse in those games than in less important contests. While nerds can be annoying nit-pickers in society — the kind of people you want to avoid sitting next to at a party — we are quite valuable as scientists. We have the patience and passion to discover true causes of patterns in nature.

I was able to bring this patience to bear on a recent question about loon behavior. In my ongoing investigation of senescence in male loons, I faced a puzzle. The territorial yodel of males serves two purposes. It is, most obviously, a territorial call that males emit when a competitor is flying overhead or sitting nearby in the water. At such times, the yodel announces the willingness of a male to fight for territory ownership. But the yodel also serves to protect the young (see Linda’s photo, above); that is, male parents often yodel to prevent landings of flying intruders, which sometimes attack and kill chicks. Why does it matter that the yodel serves two purposes? Because I am trying to make a nerdy distinction: Do old male loons yodel more than young males because they are defending their territories, or do old-timers yodel more simply to defend their chicks?

This distinction is important. If you have been following this blog, you know that old male loons make a terminal investment in reproduction. The most obvious evidence of terminal investment by old males is their tendency to yodel more often than young males. But since yodels occur both in territory defense and in chick defense, it was not immediately obvious whether old males were yodeling their heads off at intruders simply to protect their chicks or to maintain ownership of their territories. Fortunately, we have enough yodel data from periods with and without chicks to see if the increased yodeling of old males occurs at both times. It does! Hence it seems that old males are employing the yodel call to defend their territories as well as to defend their offspring.

To a nerd, the ramifications of this finding are profound. An old male who yodels simply to protect his chicks is investing extra energy to rear his offspring to adulthood at the possible expense of his own survival. This is rather a short-term strategy, as it is aimed at rearing young to 11 weeks of age, after which young are out of the woods, and the investment has been successful. An old male without chicks that yodels, however, is taking a long-term view. Chickless males are months away from producing young. Their yodeling is aimed at guaranteeing territory ownership for many future months, even years. Although terminal investment in offspring is rare in animals, terminal investment in territory ownership is virtually unknown. So the stepped-up yodeling by old, chickless male loons is an exciting finding. As you might imagine, this result has set off quite a nerd celebration!

LMG2693 Loon Between the Ice

I always dread ice-out. While I am excited to know that I am scant weeks away from seeing the loons, ice-out now tends to occur during the high-stress period of the Chapman spring semester — and I am seldom ready for the event. As Linda’s photo from the Wisconsin River shows, loons are coming back, accumulating on the river, waiting for their breeding lake to open up enough to permit landing there. Meanwhile, my Animal Behavior students are sweating their behavioral experiments and write-ups.

Let’s focus on the positive of early ice-out for data collection. Rain and warming temperatures mean that ice-out is only days away. Soon we will begin to log the identities of returning veterans anxious to attempt another year of breeding. Since the lakes will be habitable two or more weeks before the historic mean date of ice-out, while many returning birds are still en route from the wintering grounds, we have an opportunity. In theory, adult loons that are in good physical condition generally should be those that can complete the breeding molt early and also migrate early. So the loons that show up first on breeding lakes should be those in good condition. Recent findings have shown us that this group comprises males and females about 8 to 15 years old — the prime of life for a loon. Thus, we predict that the early arrivals are in this age-class. The laggards should be breeders that are either very young — 5 to 7 years old — or very old — 20 years and older. We might expect the territories of such individuals to remain vacant for a week or more after the ice has come off of them.

If age does turnout to be a good predictor of date of territory occupation, then late return from migration could be another source of trouble for an old established breeder. That is, an old territorial bird whose body condition has begun to decline might not only need to worry about being evicted by a young, fit nonbreeder in the midst of breeding; danger might also come from the tardy return of the old bird to its territory in the spring, which could open the door for a youngster to seize the territory, pair with the old bird’s mate, and hold off the former owner when it returns.

I have painted a dire picture. We will have to use the increasingly early ice-outs like this year’s to measure date of return accurately and see if early ice-outs truly destabilize territory ownership. At the moment, I will tantalize you by reporting that breeding success across the population is higher when ice-out comes late. It is speculative at this point, but this pattern might indicate that early ice-outs lead to ousting of old, experienced breeders from their territories, which in turn suppresses chick production of the entire population. If so, I have one more reason to rue early ice-outs!