It is often said of field biologists that we resemble our study animals. I guess it is true. No, I am not aquatic. Nor do I subsist on a diet of fish, crustaceans, insect larvae, and the occasional mollusk. I did not even engage in a dangerous battle to secure my mate and territory. But, like loons, I enjoy being alone.

One of the joys of my profession is the time that I spend alone in a canoe, watching loons and taking in the beauty and simplicity of their lives. When your world is distilled down to watching the sky for other loons and bald eagles, chasing fish under water, and preening from time to time to take care of your feathers, life seems pretty straightforward. During those moments when I am with loons, their few concerns are all that matters. At such times, the headaches of keeping a major research project afloat, supporting a young field staff, repairing or replacing broken equipment, publishing scientific papers, and sharing engaging stories, photos, and video via social media vanish.

Loons would seem to gain even more than I do from avoiding crowds, especially at this moment. As a migratory species that winters along oceanic coasts, summers on northern lakes, and uses a variety of lakes and rivers in between, common loons appear at great risk from the current outbreak of highly pathogenic avian influenza. After all, waterfowl like ducks and geese, which share these waterbodies with loons, are known to be important hosts for the virus. Yet to date, loons seem to have avoided the epidemic of HPAI that has decimated other aquatic birds in the United Kingdom and eastern North America. How have loons dodged this juggernaut? Mostly by breeding solitarily, instead of gathering in dense breeding colonies on oceanic islands, where the virus spreads quickly via saliva, respiratory droplets, and feces.

Loons’ ability to avoid massive mortality events from HPAI is welcome news. After all, they already have had to contend this year with a late ice-out that has delayed their reproductive efforts and a higher-than-usual population of Simulium annulus, the black fly that singlehandedly makes May a miserable month. Yet some pairs have remained steadfast. At long last this week, several breeding pairs in Minnesota and Wisconsin Study Areas have hatched chicks, like the ones in the photo above from Ossawinnamakee Lake (photo by Keith Kellen). Maybe things are beginning to turn around!

Each spring I feel my adrenaline level rise as we carry out the annual census of returning loons. This practice seems mundane, at first glance. During the census, we simply visit all loon territories and identify each territorial loon we find from its colored leg bands. But since I have gotten to know many of my study animals quite well, I wait with bated breath to learn whether Clune (the famously tame male on Linda Grenzer’s lake, whom I have known since he was a chick) comes back. I feel almost as strongly about Clune’s son, who settled 6 km away, on tiny Virgin Lake. I even have a soft spot for the comically skittish female on Silverbass Lake. She routinely appears down at one end of this long skinny lake, seems to wait for us to paddle in her direction from the other end, and then races by us underwater and reappears at the end of the lake we just vacated. She is so notoriously hard to approach that her very skittishness has become a useful identifying trait. In Minnesota, I was anxious to see whether the young male of unknown identity on Lower Whitefish — who nested rather recklessly on a water-logged artificial nesting platform exposed to the powerful west wind and waves — would return from the winter and try that move again or learn from his mistake and seek a more protected location. (I am happy to report that all four of these loons are back this year.)

Apart from the relief or dejection we feel when we spot our familiar study animals — or don’t — loons’ tendency to return provides critical scientific information. A tally of the proportion of all adult breeders that returned from the wintering grounds in the spring tells us about survival between late summer of the previous year and early spring of the current one. Of course, territorial eviction muddies the water. That is, a loon can either fail to return to its previous territory because it is dead or because a competitor has driven it off of its territory and forced it to move elsewhere. So we must be cautious in interpreting return rates. Still, they provide us with a crude metric of survival.

Let’s look at return rates throughout the study. What is clear from a quick inspection of the graph below is that loons in the Wisconsin Study Area have fluctuated in their tendency to return, coming back at a rate of over 90% in great years and just above 70% in dismal ones. (Minnesota study loons returning in 2022 also fell within this window, as the graph shows.)

Perhaps the most striking pattern is the lack of concordance between return rates of each sex. In other words, knowing that it is a bad year for male survival tells us nothing about female survival. True, there are some years in which male and female survival seem to go hand in hand — look at 2005-2009, for example. But male and female rates seem to run in opposite directions between 2010 and 2017. Overall, there is no statistical tendency for male survival to be correlated with female survival.

We can draw an important — though tentative — conclusion from the fact that male and female survival do not vary in concert. Major loon mortality events outside of the breeding season do not seem to drive annual loon survival strongly. If major die-offs during the non-breeding period (i.e. winter and migration) were a major cause of loon mortality, then male and female numbers should be correlated, because the sexes use similar migratory routes and winter quarters and should suffer in parallel each year.

The most interesting and potentially worrisome pattern we could spot in the annual return rate data would be a decline in survival of either males or females. As you can see from the color-coded dotted lines, female return rate has actually shown a slight rise over the past 29 years. On the other hand, male return rate has declined slightly, though not significantly, during this period. Still, since we already know that males are struggling to maintain optimal body mass in the Upper Midwest, it is disconcerting to see male survival decrease in a way that seems consistent with the mass loss.

Of course, while making the rounds of territorial pairs, we also notice if a territory is vacant or occupied by a lone adult after having supported a breeding pair the previous year. And therein lies a bit more troubling news. Ten of 118 Wisconsin territories that were occupied in 2021 are now vacant or inhabited by loners. We have also recorded two new territories in lakes not used for breeding last year, so the net loss in territories is only eight. Still, this was not the picture we wished to see in a population that has been on a downturn. (Though we are only learning about the Minnesota Study Population, it appears that only one territory among seventy or so that we have visited so far fell into disuse this year after having been occupied last year.)

Let’s put aside worrisome population patterns and turn to news of the moment. It is early June in the North. This is a time of great hope for loons. A few breeding pairs in our Minnesota and Wisconsin study areas — like the Lower Hay pair in the photo — were fortunate enough to dodge both black flies and raccoons and are on the brink of hatching young. Many more have rebounded from early setbacks and renested. If we are lucky, we still have the potential for a good crop of chicks in both regions. Lacking any more effectual means of bringing this about, I will keep my fingers crossed.

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.

He was the biggest, healthiest juvenile we caught in Minnesota last year. The Rush Lake-Northeast chick was so independent on July 16th, when we first attempted to catch the family, that we could not relocate him after capturing and banding his parents. We shrugged, returned the following night, and had better luck. At 2900 grams, “Copper-White”, as he became after banding, was 300 grams heavier than the second-heaviest chick we caught last summer and almost certainly a male.

Considering the risky environment inhabited by juvenile loons, it is a mistake, I have found, to become attached to them. So, with the exception of the “Miracle Chick” — a juvenile on Squash Lake in 2012 that lost his father at three weeks, watched his mother quickly re-pair with a new male, but still got enough food and attention to fledge — we have tried to avoid this practice. Still, Copper-White became lodged in my mind. I had great hopes for him. If any juvenile had a chance to fledge, migrate, and come back in a few years as an adult, it was Copper-White.

Large size and good body condition, it seems, are not enough to protect a loon in his first few months of life. Last Friday, the National Loon Center got a report of a loon hemmed in by ice on on Cross Lake. They raced out to check the bird, and Mike Pluimer snapped the photo above.

It was alarming enough to hear of a loon still on the breeding grounds in mid-December. By this time, loons from the Minnesota population should have arrived in Florida and begun adjusting to a saltwater diet. Our hearts sank a bit further to see the bird’s plight. Resting in a tiny pool of open water surrounded by encroaching ice, this juvenile was clearly in dire straits. Why had he failed to migrate south with others of his species? Something must have gone horribly wrong.

Following heroic efforts on the part of the Crosslake Fire Department, Copper-White was caught and transported to Wild and Free Rehab Center in Garrison. Terri and Richard, who live on Rush Lake and watched the chick grow from its earliest days, reported that the captured bird was strangely docile — another worrisome sign.

Arrow points out where Copper-White’s right wing was sheared off at the metacarpal bone by a boat propeller. (Photo courtesy of Wild and Free Rehab, Garrison, MN.)

It took little time for Katie, the vet at Wild and Free, to diagnose Copper-White’s problem. The end of the loon’s right wing had been sliced off some time ago by a boat propeller, rendering him incapable of flight. Unlike many hawks and owls, loons’ size and need for open water make them impossible to keep alive in captivity. The only option was to euthanize this bird.

Alas, I have no cheerful anecdote to cushion the blow. We are disheartened to lose a healthy, strapping juvenile loon to a boat strike. But boat strikes that injure loons are a fact of life in the Upper Midwest. We lost a healthy adult male even more tragically two years ago in Wisconsin. The only comfort here is that boat strikes occur infrequently enough in the Upper Midwest that they do not contribute meaningfully to loon mortality. At the moment, that is cold comfort.

Science, of course, is cumulative. Solving scientific riddles generally requires multiple studies by diverse authors using a broad range of scientific techniques. In fact, scientific conclusions are more compelling when they rest upon findings from many scientists using different techniques and with different backgrounds.

So I was quite anxious to learn what Kevin Kenow had found. Kevin, a scientist with the U.S. Geological Survey and also a member of the Scientific Loon Council at the National Loon Center, has studied common loons for about as long as I have. But while I have focused on the behavioral ecology of loons exclusively and established two fixed study populations for this purpose, Kevin has collected data on many species of migratory water birds, tackled questions related to human impacts and conservation, and worked across a variety of states and waterbodies in the Upper Midwest and beyond.

Kevin’s just-published article pulled together data from loons in Michigan, Wisconsin, and Minnesota. In contrast to my low-tech methods, Kevin’s team used satellite transmitters and geolocator tags to either: 1) track loons’ migratory movements in real time or 2) reconstruct their movements, following recovery of the geotags.

Kevin’s team confirmed several migratory pathways that we had known about or suspected through recoveries of dead birds. First, adult loons from the three Upper Midwest states typically “stage” on Lake Michigan in the late summer and early fall, before departing southwards to the Florida Gulf Coast. Second, juvenile loons (those only a few months of age) do not visit the Great Lakes prior to migrating south. Third, first- and second-year loons that are too young to return to the breeding grounds instead migrate northwards in the Atlantic, summering off of the Canadian Maritime provinces. The quirky patterns in loon migration and wintering behavior are important. They make it clear that a loon’s survival to the breeding stage requires that it survive and remain healthy across a period of many months, a variety of water bodies, and a number of geographic areas.

But one of the patterns that Kevin’s team identified loomed above the rest. They found a high rate of mortality among first- and second-year loons, especially in the spring. If you are following this blog and have an excellent memory, you recall that this finding appears to dove-tail with a recent one of ours. We found that “floaters” — the segment of adult loons that are two to five years old and are looking for breeding territories — have been disappearing at a high rate. Floaters alone, you might recall, account for most of the decline we have detected in the Wisconsin breeding population.

Do Kevin’s findings of high first- and second-year loon mortality solve the riddle of what is ailing the loon floater population in Wisconsin? Unfortunately not. In fact, we had long suspected that young adults would die at a higher rate than older adults, because they are less experienced. High mortality among young adults is a common feature of avian populations everywhere. But these new findings might help narrow down the “period of vulnerability” in the life history of loons. And our findings of high young adult mortality in the last decade combined with those of Kevin’s team might tell us where to look. Perhaps conditions along the Florida Gulf Coast and/or the Atlantic have deteriorated recently, causing higher than normal mortality during this trying time of life for young loons. This is only one hypothesis, and it will require rigorous testing. Thanks to Kevin’s team’s cool recent findings, though, the wheels of science are turning.

By the way, follow us on Instagram, if you like, at @loonproject. The whole LP team works together to post cool, informative “loonstagrams”!

As my family and friends will tell you, I am judgmental. When an event happens that could be attributed to mindless error, I am inclined to view it, instead, as deliberate selfishness or irresponsibility. I derive my hypercritical worldview in part from my profession. As a behavioral ecologist, I presume that much of the behavior we see in animals (including humans) has evolved in order to promote their evolutionary fitness. Put another way, I assume that a good deal of animal behavior is selfish — evolved because it allowed the ancestors of living individuals to survive better and leave more offspring than others of their species.

The presumption of selfishness is a helpful touchstone in my field. It provides a starting point when one is interpreting a new and unexpected behavior pattern. For example, if I notice a new soft call emitted by female loons during courtship, I am apt to hypothesize that this call might help mates synchronize their breeding activities so that each will be prepared to do its share of the incubation duties, once eggs are laid. (Such synchronization, which involves rising prolactin levels in the blood, has proved crucial to successful breeding in many species of birds.) So the presumption of selfishness can  be a useful prism through which to understand animal behavior.

A week ago, the folks at REGI learned of an event that pushed even my cynical viewpoint to the limit. Following a report from a lake resident, they found an injured loon on Metonga Lake, which is just south of Crandon, Wisconsin. After Linda and Kevin Grenzer captured the loon (pictured in Linda ‘s photo above) and the REGI team examined and x-rayed it, they learned that it had been shot at close range with a shotgun and had lead shot throughout its body. Despite efforts to save the unfortunate shooting victim, it died in their care. The story might have ended there, except that the loon was banded.

Since Metonga is outside of our study area — some 20 miles east of our southeasternmost lake — we do not know the lake at all. Sleuthing by Linda and me revealed that this oval 2000-acre waterbody supported two breeding pairs in 2018. According to the loon ranger, both pairs hatched chicks this year, although only one of the pairs fledged their two hatchlings. Most important, neither pair contained a banded individual. Thus, the shooting victim was not a member of either resident pair.

Some of the circumstances surrounding the tragic shooting make sense. As many of you know, breeding loon pairs become restless in September and October, often leaving their territorial lakes. Moreover, large, clear lakes like Metonga are favorite spots for wandering adults to visit, as they forage intensively and lay down fat stores to fuel their southward migration. So it is not at all surprising that a breeding adult from a neighboring lake — as we presume the victim was — would be found on Metonga. Finally, virtually all of the loons that we band that show up that far from our study area are females, because females are the more dispersive sex. (On average, females settle 24 miles from their natal lake, while males settle 7 miles from their birthplace.)

The identity of the shooting victim allows us to speculate about its tragic end. When I looked up the band colors and partially-obscured USGS band number that Linda provided, I learned that we had banded this female nine years ago as a chick on Bear Lake in Oneida County. We have not seen her since. The father and mother of this female were among the most approachable loons in the study area. (The male still holds the territory there, as he has since 2001 or earlier.) As Chapman student Mina Ibrahim showed a year ago, tameness (the minimum distance that a resting loon will permit a canoe to approach before diving) is similar between parents and offspring. So it is almost certain that the dead female was a tame individual, like both of her parents.

If our simple inference is correct, then this incident has exposed one hazard of extreme tameness in loons. While the vast majority of humans who approach loons closely are merely curious and would never dream of harming them, an occasional human might do so. It is easy to reconstruct the chain of events that led to the shooting. In the opening week of duck season, a hunter got an easy shot at a duck-like diving bird and took full advantage.

This analysis might well be correct, but it has one hitch. Loons are so well-known across the heart of their breeding range that they can scarcely be confused with ducks. None of the species of ducks that a hunter in northern Wisconsin would be looking to bag is patterned much like a loon. Furthermore, all duck species in the area are far smaller than loons and are prone to fly, not dive, when approached by humans. And since we know that the hunter blasted this loon from very close range, it is even more difficult to believe that the incident arose from a misidentification.

Call me cynical, but I believe that the hunter who killed this loon was not foolhardy, as generous and forgiving people might believe, but rather purposely wicked. Of course, this conclusion further erodes my opinion of other humans. What kind of person deliberately shoots a loon?

To most people, Memorial Day weekend is both a sacred and joyous occasion. It is a time to remember those who have given their lives for our country. And it is a holiday that lets us gather around the barbecue with friends or enjoy an extra day of rest followed by a short work week. In the northwoods of Wisconsin, Memorial Day often brings a hint of summer’s warmth at a time when we are not quite free of the clutches of winter. Memorial Day convinces us that summer will return to the north.

Since 1993, when I first starting studying loons in northern Wisconsin, I have dreaded Memorial Day. On this holiday weekend, throngs of anglers bolt from their southerly homes for the northwoods to throw a hook in the water. Meanwhile, loon pairs that have managed to fight off black flies, eagles, and raccoons are well into the four-week incubation period. Memorial Day is the time when hopeful loon parents and hopeful human anglers collide.

I used to gird myself for the disturbance to nesting loons that humans caused each Memorial Day weekend. Fishermen and boaters commonly disregard or do not see loon nests and venture close to them, driving loons off of nests for a time. But such incursions now seem innocuous. They seldom cause great problems for loons, who sometimes complain but dutifully jump back on their nests after boats have moved off.

Now that we have better connections to the local community, I see that the substantial danger posed by humans to loons on big fishing weekends is not from flushing off of nests but from fishhooks and monofilament line. On Memorial Day weekend, the loon pictured in Linda’s photo, a female that reared a chick on Nokomis Lake in 2010, was hooked in or near its throat while it foraged on Nokomis. As ugly as it is to look at the silver hook buried in its throat and the local swelling that resulted, the Nokomis female might recover. She appears to be hooked externally, and Linda reports that she dives strongly.

A second female fell prey to an angler’s lure this Memorial Day. This bird, the mother of many recent chicks on East Horsehead Lake, apparently swallowed a lure or bait used by a fisherman. Initially Linda found that this female was severely impacted, often trying to jump onto the shoreline, as loons do when seriously injured. Nelson and I raced up to East Horsehead to help Linda and her husband, Kevin, try and capture this bird and transport her to a wildlife rehabber for treatment. But our efforts were in vain. The bird had bounced back and begun to dive normally, despite the fishing line protruding from its bill. Having ingested a lure, this bird’s long-term prospects are rather dim. She will certainly die if she swallowed a lead sinker.

These two cases illustrate a vexing paradox often faced by those of us trying to protect wildlife: animals commonly become injured in a way likely to kill them eventually but not so catastrophically that immediate capture is possible. So we must wait and monitor them until creeping hunger or infection reduces their mobility sufficiently for us to grab them and see to their injuries. These are most unpleasant and heart-wrenching vigils. Moreover, these occasions often end badly, if the animal becomes compromised beyond the point of recovery before it can be captured and treated. Still, knowing that a grave injury of this kind has occurred gives us a chance.

An encounter with fishing tackle ended quite badly for the East Horsehead male last year. Although it was not reported to us until a few days ago, last year’s East Horsehead male — the long-time mate of the female who swallowed a lure a week ago — became hopelessly ensnarled in monofilament line last August. Based on our records, we surmise that he succumbed to this entanglement sometime after August 10th, as we observed the female alone caring for the chicks on our two visits after that date. (Since the chicks were 11 weeks old by late August, they likely survived to migrate south. That, at least, is a relief!)

We were disappointed to hear only now about the unpleasant entanglement and death of the East Horsehead male. Unlike the two females, this male was probably immediately compromised enough by the fishing line that we could have captured him and cut him loose in good condition and with no harm to his survival prospects. Indeed, we were able to save a female on Perch Lake from a similar predicament in 2010. Since the East Horsehead male’s plight was never communicated to us, he had no chance.

So, now, a plea. Please let folks know that angling casualties happen. We are anglers ourselves and understand this. But anglers who cut the line and flee the scene after accidentally hooking a loon — or observe a loon in distress and fail to report it — are turning a dangerous situation into a catastrophic one. As so often occurs, it is the cover-up, not the crime, that causes real damage. (I am happy to take reports of loons in distress at wpiper@chapman.edu.) Let’s try and have summer holidays in the northwoods bring to mind the events they were meant to commemorate, not the toll they exact on loons.

 

 

Science is a cumulative pursuit. That is, the thirst for knowledge is never fully quenched. Rather, we answer one question, only to reveal another puzzle or two more. And thus begins another search for answers. That is certainly how my 24-year-old loon project has gone. But sometimes we reach a point where a vexing question is finally laid to rest, and it feels as though we have made real progress. I am at such a point now.

Let me back up. If you have been following this blog, you know that senescence in old loons is a phenomenon we have recently discovered. Senescence, loss of body condition and decline in survival rate in aging individuals within a species, is all too familiar to me and other humans. During the past twenty years, many studies have reported senescence in birds, mammals, fish, and reptiles. So what? Well, we expect that animals that lose condition as they grow old will change their behavior in response. In other words, scientists have long predicted that senescing individuals should start to behave so as to leave more to their offspring and care less for themselves. To put it another way, old individuals should be willing to take a hit to their survival if it allows them to pour more resources into their young and help their young survive. This makes sense, of course, because old individuals reach a point where they stand little chance of surviving longer, so they would do well to give whatever they can to their offspring, which DO have a bright future. Animals that behave this way should leave more and healthier offspring, and thus this behavior should spread in populations. This very logical idea is termed “terminal investment”. Again we can all probably think of human parallels.

Terminal investment, which I have mentioned before, has become a central theme of the loon project, ever since we published a paper 9 years ago on fatal fighting of males. Terminal investment became interesting to us because it was the most plausible explanation for such lethal contests. Our reasoning was as follows. If males are willing to die to defend their territories, then they must reach an age at which they have little to lose. And if males have little to lose, this must mean that senescence hits males (but not females, which seldom battle to the death) very hard to the point where old males have little future to look forward to. In this case, it might make sense for them to fight like crazy to hold a territory for another year or two, rather than give it up easily and leave themselves nowhere to breed during their last year or two of life. So we have two clear predictions here: 1) males, but not females, must start to die off at a certain age, and 2) males beyond this age must still fight like hell for their territories. It is this clash of body condition and behavior among old males that might cause fatal fighting.

At the time we started to consider the terminal investment hypothesis as a means to explain reckless battling by males, we had almost no solid information on the ages of males in our population. With patience and tireless field work by dozens of us, we have now turned things around. Analysis of loons of varying age has shown us that many males hit the wall at age 15. First, and most important, they start to die at a high rate. You can see from the figure below that males (blue bars) are suffering higher mortality than females (red bars), whether they are on territory (Terr) or without one (floaters: “Float”).

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But males also lose mass at age 15, indicating loss in body condition, as shown here:

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Finally (and predictably), males get evicted from their territories at a high rate at age 15:

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Wow, males are really getting slammed after they pass the age of 15 years!

So all of these data tell us that the first prediction of the terminal investment hypothesis, abrupt senescence at a certain age in males but not females, is clearly met in loons. That age, surprisingly, is only fifteen. Females clearly remain strong, healthy, and vigorous well past age fifteen.

As hard as the first prediction of terminal investment was to test, the second prediction is even harder. You see, fights are common in loons if you take the perspective of a loon’s lifetime, but they are quite uncommon if viewed from the standpoint of human observers in canoes. In other words, most individual loons have engaged in several major battles during their long lives, but territorial battles are not common during day to day observations and often occur so quickly that we are not present to witness them.

Patience pays, however. Since we can draw upon 24 years’ worth of field observations, we now have a trove of observations that we can search for any evidence of aggression and territorial behavior. I made this search, looking for two kinds of evidence: 1) territorial yodels, which serve to communicate a male’s aggressive tendencies and willingness to battle, and 2) out and out aggression, in the form of battling, lunging, chasing and underwater attacks launched by territorial loons on intruders to their territories. I was simply asking “Do old male loons (above age 15) tend to maintain a high level of yodeling and aggressiveness towards intruders?”.  The answer is a resounding “Yes”:

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As you can see from the figure above, old males actually increase their tendency to yodel (yodels per intruder), compared to young males. Similarly, old males step up their aggression (see below) and contrast in this way with females, who show no increase:

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By the way, all of these patterns I have shown are “statistically significant” via tests that I have performed.

You cannot be as excited as I am about this set of results. No one is. But, as I mentioned, this is one of those rare cases where we have finally managed to answer a burning question to our satisfaction. Even better, the question is one that had been the foundation of my research funding from the National Science Foundation. So I can now report to them that I have found the holy grail! What makes this clear finding even more significant is that terminal investment is quite rare in vertebrates. Of the hundreds of species studied thus far, the only other one to show such a clear pattern of terminal investment is the California Gull. Appropriate, don’t you think?

(Photo by Woody Hagge.)

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Most of us think of adult females as the main care-givers and protectors of the offspring. I am reminded of this constantly during my work. Several times a year, when I chat with a lake resident about their loons, I hear them say “She was on the nest today” or “The mom was feeding them down at the south end” or “She hollered like crazy when the eagle flew near the chicks”. It is natural for humans to infer that the female takes the lead in breeding activities; after all, we are mammals. In almost all mammals, females protect the young within their bodies for many weeks or months before they are born and continue to care for the young by themselves after they are born. Lengthy gestation makes male parental assistance superfluous, so mating systems have evolved in which males occupy themselves in other ways. Most mammals are either polygynous (one male mated with multiple females) or promiscuous (rapid mating and the lack of a pair bond). Females, in such cases, are left to provide most or all of the parental care, because males are either engaged in seeking out new mates or are not around at all.

Birds are different. Laying of eggs “frees” females, in the sense that they are not physically connected to the eggs and are no better equipped to care for eggs or young than are males. In birds, therefore, monogamy and biparental care are the rule. Most male birds  mate with a single female and provide substantial assistance to her in rearing the young. Male loons, as I have noted in the past, incubate the eggs equally with females and actually provide somewhat more parental care for chicks. Thus, more often than not, when folks tell me that the female hollered at a menacing eagle, or the female was feeding the chick lots of minnows this morning, they have mistaken the male for the female. (This puts me in the awkward position of either correcting the mistake, at the risk of embarrassing my friend, or grinning good-naturedly and leaving the error unchallenged.)

I got one more reminder of male loons’ central role in reproduction during a statistical analysis this past week. I asked whether males or females show age-related changes in fledgling production as they mature from young territory holders to middle-aged to senescent adults. (Note that one bar is missing for each sex in the figure below: too few females settle by age 5 and too few males survive to age 24 to produce reliable estimates of reproduction for those age-classes.) Both sexes show an increase in fledgling production after their first few years on territory, a pattern seen in many animals. Females showsenescence-paper-figure-3

“reproductive senescence”, another widespread pattern, in that fledgling production declines near the end of life. What is surprising here is that males do not show a decline in fledgling production as they reach old age.

What is odd about the male pattern? We have growing evidence that male loons decline at a younger age and more severely than females do. Recent analyses have shown that males lose body weight as they age, and that males suffer a high rate of mortality, especially as they reach their late teens. And, of course, males engage in dangerous battles. So the capacity of old males to produce lots of chicks into their 20s runs counter to what we would expect based on male survival and body condition. How do they do it? We are still puzzling over the pattern, but the most likely explanation is that old males invest heavily in chick production — perhaps through extra feedings of chicks or an extended period of care — to crank out a few more chicks before the wheels come off completely. Hence, old male loons appear to make a “terminal investment” in breeding success. Of course, nothing is free. Terminal investment is a deal with the devil; high chick-rearing success comes at the cost of earlier death.