I had a hint that the territory was in flux on my May 29th visit. Though it was late morning, there was a persistent intruder on Towanda. Intruders sometimes visit in late morning, and do not always depart quickly, but I filed this observation away as a worrisome sign that the Towanda pair might not be fully in control of their territory. I relaxed a bit when, on my June 15th visit, the pair had deposited two eggs at a safe location in the southeastern bay. Whatever had happened early in the year, I thought, the pair seemed to have put it behind them. Pairs do not reproduce in the midst of territorial instability.

Yet the breeding female and two intruding adults were locked into an intense round of circle-dancing and excited diving at the northern end of the lake on June 24th, when I had again drawn Towanda on my circuit list. Oblivious to that unease, the male incubated the eggs quietly at the southern end. Well, I reasoned, the female is in a tussle, but the male is far away and unaware of the action. Once he takes his next turn off the eggs, perhaps he will use his size, aggressiveness, and voice to drive off any residual intruders. On Martha’s visit five days later, calm indeed seemed to have descended on the lake; the male foraged casually while the female sat on the eggs. Likewise Annie reported nothing unusual on July 6th: the male was back on the nest, and the female foraged and swam on the surface nearby. Moreover, these last two visits occurred shortly after dawn, when intrusions peak. The absence of intruders on these two early-morning visits seemed a good indication that the pair was firmly in control.

The last two uneventful visits to Towanda had prepared us poorly for what Allison encountered today. From the look of the nest (see Allison’s photo), all seemed well. Clearly both eggs hatched right on schedule. But no chick was with the loon pair, and the male’s behavior was odd. He seemed exhausted and barely budged when the wind carried Allison’s canoe to within a few meters of his resting spot. As she sensed something amiss and worked hard to nail the male’s bands, she came to an important conclusion: this was not the right male. Somehow the territorial male, banded 12 years before, had been replaced by a new, younger bird just at about the time that new chicks had disappeared.

We can infer what happened on Towanda this past few days, because we have seen it many times before. In the wake of a successful hatch, the long-term male breeder had been evicted by a young whipper-snapper, and the whipper-snapper was this male who seemed determined to rest and recover. The young male had likely killed the chicks, as male lions do when they take over a pride that is raising the offspring of other males, and as we have seen before in loons. His bands revealed that the male was a seven-year-old, hatched and reared on Arrowhead Lake in Woodruff, just a few miles south of Towanda. Allison noticed something else strange; the young male was unwilling or unable to open his right eye. Exhaustion and damage to the head or neck of a loon almost always indicate a prolonged and recent territorial battle. We are left to wonder: if the winner of the contest is in this kind of shape, what does the loser look like?

This contest fits the profile of terminal investment by male loons. That is, the original 17+ year-old male resident might well have poured all of his energy into holding his territory — possibly dying the in the process — especially since he had two chicks to rear that would have contributed greatly to his lifetime reproductive fitness. But the day comes for most breeders — male or female — when a fit, determined youngster intrudes that is able to overwhelm you in a territorial battle. According to our measurements of males a different ages, seven-year-old males have reached peak condition and offer a stiff challenge to older males — even determined ones. At least the old fellow made the youngster work for it!

Our paper that describes basic features of senescence has been accepted for publication Journal of Avian Biology. With the lightning-fast turnarounds and early views that the public is now granted to scientific articles, you can search for the paper and read an advance copy…months before copy-editing and proofing of the final version is done. Let me know if you find any typos!Screen Shot 2017-03-10 at 12.47.06 PM

The paper describes findings that I have been blogging about for some months now. First, both sexes of loons senesce (begin to die at a high rate) once they reach their mid-20s. Second, at first blush it seems that the sexes do not differ substantially in the senescence pattern. Third, this paper looks only at territory holders, which are the creme de la creme of adult loons, because they have not only survived to adulthood, but also claimed a territory and produced chicks there. Thus, this group of birds analyzed does not include the many adults who tried but failed to settle on a territory or settled briefly but did not reproduce. Fourth, old males (but not females) suffer a decline in territory resettlement after being evicted from a territory. Finally, we present in the paper preliminary evidence that suggests male might increase or at least maintain high breeding success at advanced age, while it seems that females fall into reproductive decline. So there is a glimmer of possible terminal investment by males (increased investment by animals near death) at which this paper hints. If you have followed my blog, you know that we have data from a separate analysis that deals more directly with the possibility of terminal investment by males.

That is all I have for now. I have just finished hiring the four field staff members for this year. They are a strong bunch and include one of our seasoned hands from 2016. Since we are on the verge of ice-out already, I have gotten the crew hired none too soon. By the time most of us arrive in May, nesting will be well underway. No matter. We are accustomed to scrambling to keep up with the loons.

See you out on the lakes!


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”).


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


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


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”:


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:


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.)