It was jarring two days ago to look at the database we use to track our breeding loons. During most years, the first week in June is the peak of incubation. At this time, breeding pairs that laid eggs early are well into the 28-day period that will carry them to hatching; others have lost an early clutch and replaced it. But steady, determined incubation is the rule at this time of year.

Not so this year. As the screen grab from Evelyn and Tarryn’s data entry shows, 2019 is yet another year of severe black flies. I had a sinking feeling that black flies would be a plague when I was in the study area in May.

As our recent paper shows, cool springs are killers. In years when April and May are cooler than average, black flies live longer than average. From the standpoint of a female fly, cold weather makes flight and dispersal more difficult, so a female in a cold spring is likely to delay her quest for the blood meal she needs to nourish her eggs. Not all females postpone reproductive activity, however, so cold springs reduce synchrony between female flies. The result, from a loon’s perspective, is a longer period when flies are around to harass them on nests. In contrast, warm springs cause a well-synchronized explosion of fly biting activity. During warm years, Simulium annulus blackens the skies for a few days and exacts an awful toll on incubating pairs during that brief period. Many pairs, though, are able to weather the onslaught, maintain viable eggs (no doubt aided by warm temperatures), and incubate the eggs to hatching despite the interruption.

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Elaina’s photo of the nest on Hilts Lake is typical of what we see in a cool spring. Flies are abundant, to be sure. In the photo, you can see many in the air above the nest but also scores hanging onto the arched roof on this platform. There are enough flies around the nest to keep the pair from incubating the eggs, but few enough so that this infestation does not reflect an explosion, such as would occur in a warm year.

Screen Shot 2019-06-10 at 11.07.16 AMNow that I have got you worried — sorry — you must be wondering where 2019 ranks relative to other years. What kind of productivity can we expect? I ran a quick back-of-the-envelope analysis by comparing rate of chick production in each year with the proportion of pairs sitting on nests during the first week of June. Even with the one “outlier” (2013), there is a tight correlation. That is, we can usually tell pretty well how productive a year our loons are going to have by looking at the proportion of all pairs on eggs in early June. If 2019 falls near the line, then the 0.45 proportion of sitting pairs in early June of this year predicts 0.42 chicks per territorial pair. This is not terrible — look at 2011, for some perspective — but not what we had hoped.





It is my 60th birthday today. I ran 5.3 miles just to show that I still had it. Since I find running unpleasant, I listen to music in an effort to distract myself as I run. (This works poorly.) Amidst the classic hits on my playlist today (by Jim Croce, Don Henley, Talking Heads, David Bowie) was an outlier: “New Rules“, a 2017 hit by Dua Lipa. Wikipedia describes New Rules as a “tropical house, electronic dance music and electropop song with a drum and horn instrumentation”. While I do not understand most of that description, I understand enough to know that few men my age would find the song meaningful. When you consider that the song concerns a young woman’s efforts to break up with an unfortunate boyfriend, its profound significance to me becomes even more alarming.

I can explain. New Rules describes a very logical practice common to many of us: trying to interpret a vexing phenomenon and chart a course forward by looking, not at a single event, but a series of related events. The idea, of course, is that an isolated event might be misleading, whereas many events, studied together, offer a better picture of the world. And this is what scientists like me do. We resist the temptation to generalize to the world from a single event and instead study sets of related events and use statistical tests to discern patterns that may or may not align with our hypotheses. When I heard New Rules for the first time, I laughed out loud, because Dua Lipa sings “Now I’m standin’ back from it, I finally see the pattern” — referring to her boyfriend’s persistent misbehavior. This is just exactly what I have devoted my life to doing… a very different context.

Here is the latest puzzle in which I am trying to see the pattern: old female loons are reluctant incubators. That is, when sitting out in the open on a nest becomes unpleasant, old females are less likely to put up with the unpleasantness and continue sitting on the eggs. Instead, they tend to bail on the breeding attempt, to their great cost and that of their mate. In loons, of course, the unpleasantness derives from black flies that specialize on sucking loon blood, Simulium annulus, which are in profusion at the moment in the study area. (The photo below is from Clara Lake, where the pair is beseiged.)

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I observed the reluctance of an old female to sit on eggs on Oneida-West four days ago. The very tame male that just took over the territory this year  — a 9 year-old hatched on McCormick who settled first on Oneida-East, and then shifted to the territory on the opposite end of the lake — had a nape that was seriously chewed up. The feathers on his nape were misaligned, showing that the skin beneath them was swollen from countless bites of black flies. Disrupted nape feathers are a telltale sign that the loon has been incubating for a long period in the presence of many black flies. But the female of the pair, a veteran of 23 years breeding on Oneida Lake and mother of at least 18 fledged chicks, had a pristine nape. While her appearance was more pleasing to the eye than the male’s, it revealed her dark secret: she had been neglecting her incubation duties after laying an egg in the nest a few days before.

One pattern is clear here. Statistical analysis of known-age breeding pairs has shown a very strong tendency for pairs containing old females to abandon their eggs when black flies are severe. In contrast, old males, young males, and young females incubate clutches enthusiastically when black flies abound. Why on earth would old females have evolved to drop the ball on incubation as they do?

As a biologist, I have learned that most behavior is adaptive — it tends to increase the breeding output of the individual displaying the behavior — so I am inclined to interpret what might be called irresponsibility on the part of old females as calculated to help them in the long run. Perhaps old females have somewhat weaker immune systems than young females, so that exposing themselves to countless bites and the harmful protozoans that flies transmit might weaken them, making them vulnerable to territory eviction by a healthier female competitor. By refusing to incubate and weaken themselves, old females are costing themselves a single chance to rear young but might be protecting their ownership of the territory, which increases the young they raise down the road.

My explanation for old females’ behavior is just one possibility. It is quite likely to be incorrect. Like Dua Lipa, I will have to stand back a bit more — and collect more data on old females — to see the true pattern.

What is better than finding out that your just-published article has been featured by an online science media outlet? Finding out from your dean! An hour ago this happened to me as I strolled out of our new science building.

We were excited to learn this article has excited some attention. It was a bit of a sleeper. Published in a good — but not spectacular — journal, our investigation of the flies’ impacts and loons’ logical responses to them caught the eye of the journal’s media department. I will not bore you by rehashing our findings, which I have discussed before. By the way, a related media blurb included Linda Grenzer’s cool photo of the male on her lake sticking on the nest in 2017 despite flies biting him mercilessly. (Another of Linda’s related photos appears above.)

Fortunately, this year has been a mild one for black flies. So while pondering the harsh negative impacts that black flies often have on loon nesting behavior and breeding success, we can all relish their absence.


Last week was a tense one. Dozens of pairs had laid one or two eggs, but black flies descended on them, making incubation impossible for most. Of the fifteen or so pairs with nests last week, only two incubated at all, and one of these pairs sat only during the first twenty minutes of our observations. (The other was Linda Grenzer’s notorious overachievers, Clune and Honey.) All other pairs spent their time in the general vicinity of the nest — but diving frantically, shaking and tossing their heads, even snapping their bills fruitlessly at the relentless biting insects.

Already frazzled by the grading of 131 all-essay final exams, calculation of final grades, and strident pleas of disappointed students — I fielded another while writing this post — I feared another dreadful year of nest abandonments, like 2017.

What a difference a week makes! While not altogether gone, the flies are dwindling rapidly. Breeders that could only view their eggs from afar 7 days ago are back on them. I caught the male in the photo — from Two Sisters Lake-Far East Territory — about to hop cheerfully onto his nest, after an incubation switch. He and his unbanded mate still have 27 days of incubation remaining. But, like all of our focal pairs, they have overcome the first major hurdle.

Ending a short run of bad luck, we just had our paper accepted that describes impacts of black fly infestations on loon nesting behavior. As I have explained in many previous posts, Simulium annulus wreaks havoc with loons’ reproductive efforts. The biological relationship between the fly and the bird is of substantial scientific interest, and we are pleased to have finally brought our low-level data collection on this relationship to fruition.

On the other hand, our celebration of this achievement has been cut short by the cold weather still gripping northern Wisconsin. Why? Because one of our findings was that unseasonably cool springs often bring extended periods of fly abundance. So we face the prospect that the breeding season of 2018 will illustrate the threats to loon breeding we just described so vividly in our article.

There is also reason for hope. As the above figure shows, early ice-outs resulting from warm spring weather ensure that flies will be only a minor nuisance to loons. Late ice-outs pose a problem, but the results vary from a severe rate of nest abandonment (as in 2014, the worst year ever for fly-caused abandonments) to modest impacts. Let’s all hope that 2018 is one of those years when the correlation between cool spring temperatures and severe fly infestations breaks down.

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 lot of science is scut work. While I do ponder my findings, develop new hypotheses, and publish papers that might (slightly) change the way that others view the natural world, these activities are, in fact, only the most glamorous ones in the scientific profession. I spend far more time worrying that I am mistaken about a result. You see, scientists are nit-pickers who know from personal experience the difficulty of proving something is true. Scientists are skeptical of their own findings as well as those of their colleagues. We are always on the lookout for false assumptions, biased data samples, misleading correlations, and experimental results that are artifacts (false outcomes) of our methods. Thus, we spend a good deal of time poking and prodding our data, turning it this way and that, and making certain it is bullet-proof, before we try to publish or present it to colleagues. Scientists run many tedious and seemingly repetitive statistical tests aimed at testing a single hypothesis and ruling out alternative explanations for patterns we find in our data. If a flaw in our reasoning, an untested assumption, or a problem in experimental design weakens or invalidates our findings, we want to discover it ourselves in the solitude of our office — not have a listener at a talk or a reviewer of a grant proposal do so.

Scientists have a much higher threshold for accepting statements as fact than does the public at large. Indeed, flawed and misleading conclusions — which would bring harsh criticism to a scientist uttering them — are rampant in public discourse. The biased sample problem leads to many misleading conclusions. Following a political debate, TV commentators always tell us who won, based upon a sample of viewers. Unless one is very careful to control the political makeup of an audience, however, the outcome of such a poll is certain to be biased. Naturally, Fox News and CNN have audiences that differ greatly in their political leanings; in addition, people who watch debates on television or in person represent a biased sample of voters, not the population at large. Finally, viewers are more likely to see the candidate they favor as the winner of a debate, so favoritism towards one candidate will make that candidate more likely to be seen as the winner, even if their performance was worse that their opponent’s. That is, if 65% of all voters favor Ms. Sims over Mr. Peach ahead of the debate, and 55% of all voters say afterwards that Ms. Sims won the debate, Mr. Peach almost certainly performed better, because he beat his poll numbers.

I face the biased sample problem constantly in my analysis of loon behavior. For example, we have observed that loons shifting from a first to a second breeding territory tend to move a very short distance, often settling to breed on a new lake right next to their old one. It is tempting to surmise that loons that move between territories cover only a short distance in order to take advantage of their knowledge of the local area and ease their transition to the new breeding space. This sounds plausible but ignores the fact that shifters are not a random cross-section of the population. Instead, these loons are almost all old individuals with low fighting ability that have been evicted from first territories. Moreover, the new territories they shift into are not average breeding territories but new, untested ones with limited nesting habitat that seldom yield offspring. So old, worn out loons do not seem to be carefully choosing to settle in a new breeding space that they know well; rather, they are desperately setting up a new territory near their original one — and in a place that no other loon wishes to use — because it is not worthwhile trying to compete for a proven territory anywhere else.

mentions correlation

At the moment, I have turned a critical eye towards black flies and nest abandonment. I have “known” for decades that black flies cause high rates of nest abandonment in certain years, as they did in 2017. But it is one thing to know something is true, and quite another to convince other scientists of what you know. So I have gone back to field records from 1994 to 2017 and tallied occasions when field observers reported severe infestations of black flies on loons or around their nests during the early nesting period. Then I looked at the correlation between reports of severe black flies and rates of nest abandonment across years. The result, as shown in the figure above, is unsurprising. In years when black flies were reported to be abundant, nest abandonments were very common.  (By the way, that data point in the upper right corner is 2014.)

While I was certainly not on pins and needles during this latest analysis of black flies, it is a crucial piece of the puzzle. Lacking any direct data showing that black flies caused loons to abandon nests, my best evidence to support this conclusion was that cool springs lead to a high rate of nest abandonment. The strong correlation pictured above now implies a direct causal connection between flies and abandonments.

I breathed a sigh of relief at this finding. I am not sure how I would have responded if I had found that years of severe black flies were NOT correlated with rates of nest abandonment. Yet I cannot rest. I can imagine a scientific reviewer complaining that the correlation might have resulted from observer bias. For example, once an observer starts to notice that black fly population is high early in the year and possibly related to nest abandonment, he or she might be more likely to report severe black fly infestations on subsequent days. Such behavior by field observers might explain the correlation I found, at least in part.

In short, I am still uneasy about my analysis of black fly impacts on loon nesting. I am looking for additional statistical analyses that could help convince a skeptical audience of the link between flies and abandonments. That is what life is like for a scientist.


A few months ago I wrote a post about the impact of black flies on nesting of loons. Some might recall that, after abandoning their first nesting attempt, pairs sometimes reuse the nest, leaving the two original eggs in place. This situation produces supernumerary eggs: two addled ones, two still alive. Despite some odd-looking clutches, though, the impact on reproduction seems minimal. That is, the presence of extra eggs in a new nest does not appear to impair incubation of live eggs. Chicks still hatch normally.

In fact, I had all but forgotten about black flies by the time it came to the loon capture season this year. You see, capture is an inherently cheery process. First of all, capture is only possible on lakes with chicks, so we only visit such lakes. Second we work at night and become so absorbed in the demands of creeping up on protective adults and their awkward, fuzzy offspring that the travails of the population at large do not enter our sleep-deprived brains. Between the adrenaline rush following a challenging capture and the warmth of feeling that accompanies the release of parents and their adorable young, nothing else matters.

One issue nagged me even during capture this year though. The great majority of chick broods were singleton chicks (like the one on Muskellunge Lake in Linda’s photo). So few two-chick broods did we encounter that each one seemed an oddity — an almost inconceivable reproductive bounty. 2017 was a surprise, because, based on many previous years of capture, I had come to expect roughly equal numbers of two-chick and one-chick broods.

In the days following my nocturnal boating adventures, I mulled over the abundance of singletons in 2017. It was then when black flies entered my mind. Was it possible that black flies had disrupted incubation to such a degree that many pairs had lost one of their two embryos early and hatched only one chick? This might happen if fly-bitten pairs spent enough time off of their nests that one, but not both, of their eggs became inviable. If so, years with many nest abandonments owing to black flies should also be those with many singleton chicks. In fact, this is the case, as the figure below shows.

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Thus, it seems that black flies inflict a double whammy: they cause widespread abandonment of nests, and nests not abandoned suffer from reduced hatching rate. To make matters worse, cold spring weather, which prolongs the lives of black flies, also causes hypothermia of loon embryos, endangering their survival.

Now I have somewhat simplified the factors that cause singleton chicks in loons. I certainly have to explore additional factors, looking, for example, to see if loons are more prone to laying one-egg clutches during severe black fly outbreaks (although a quick check of the data revealed no such pattern). But it seems that we have yet one more reason to hope for rapid and sustained spring warmup in the Northwoods.

I have an almost-annual tradition. Each year at this time, I watch helplessly as black flies harass incubating loons, drive them off of their nests, and force them to dive to gain momentary respite. In years of mild infestation, I breathe a sigh of relief to see that the nest abandonment rate is low. In severe years, I sit down at my computer and try to determine what factors might cause the little demons to hammer loons so hard. Today, I found this. Rate of nest abandonment is strongly correlated with April temperatures. Specifically, cool Aprils seem to cause more nest abandonments. The pattern is strong.

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As usual, I am late to the party. Black flies are of considerable interest because of their negative impacts on humans and domestic animals, so we know a good deal about their biting habits. An hour ago, I excitedly e-mailed one of the world’s experts on black flies to report that cool weather seems to lengthen the period when these pests bother loons. “I am onto something”, I thought. “I must get the word out.” He politely informed me that cool water temperatures slow the emergence of the flies, and cool air temperatures stretch out their lives. He then pointed me to thirteen scientific papers from the past half-century that reached the same conclusion!

The momentary humiliation I experienced was a small price to pay for the knowledge gained. I am now poring over this literature to learn what I can. I have found that most studies of black fly biting patterns are quick-hitters — snapshots from black flies caught in traps over a year or two. Since we possess over two decades-worth of data, our nest abandonment finding expands the information pool considerably. In addition, we have not merely measured how many flies are around or biting, but their apparent impact on the breeding productivity of a well-loved bird. So my excitement about our result is only slightly diminished.

Setting aside the scientific significance of our finding, what does it mean for the Wisconsin loon population? We cannot control outside temperatures, so there seems little hope that the finding will help us mitigate the impact of black flies on loon nests. But if I am correct that cool Aprils are most damaging to loon nesting — and this is a big “if”, as I am still exploring the result — then we might have cause for guarded optimism. A warming climate, while harmful globally in many respects, might provide a slight lift to nesting loons.