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.


LMG_9628 Manson Lake Male Yodeling

Loons do not settle on territories as we think they should. Traditional models in long-lived animals maintain that hopeful young individuals should be systematic in settling on territories. By current theory, a young loon should explore a certain region within proper habitat, find several territories that might be suitable for breeding, and then routinely monitor those potential breeding spots, waiting for a vacancy to occur. During this exploratory period, it is thought, the young loon gains familiarity with this small cohort of territories that will lead to a competitive advantage in territorial battles with other would-be settlers once a territorial slot opens up. The “foothold hypothesis”, as I call this model, is quite pleasing and logical. What’s more, there is evidence that many territorial animals gain territories in this manner. Loons do not.

We got another reminder of the quirky territorial settlement pattern of loons this past week, when Linda and Kristin scoured the study area and ID’d the pairs that had taken possession of the lakes we monitor. Among these settlers were many familiar faces — including a male on Townline Lake that has been in possession of the territory since 1994 and a female on West Horsehead who has bred there with a series of different males since 1995. One of the surprises was a 9 year-old female hatched on Rock Lake in Vilas County who settled on Manson, replacing a female that had bred on Manson for a dozen years. Owing to Linda’s careful observations, we know this Rock Lake female as a frequent intruder during 2014 and 2015. But she did not intrude into Manson Lake, where she eventually settled; instead she intruded repeated onto nearby Muskellunge Lake! Thus, our expectation that the Rock female was laying the groundwork for settlement on Muskellunge was not fulfilled.

There are several possible reasons why loons often do not settle on lakes that they seem to prefer. One of the most obvious is that settlement is not merely a matter of finding a desirable territory.  A loon bent on settling must also contend with the current resident on a territory where it hopes to settle. So a young nonbreeder that visits Territories A, B, and C might prefer Territory A but be prevented from settling there by a healthy and aggressive territorial resident of the same sex. In that case, the nonbreeder might end up settling on Territory B or Territory C. The Rock female is fortunate; Manson Lake, where she has settled, is one of the most productive territories in the study area. So even if she could not take possession of the territory she seemed to prefer, her future breeding prospects are bright.

You can read more about our testing of the “foothold model” for territory settlement in this blog post, which is based on a paper published in Animal Behavior. E-mail me if you would like a pdf of the paper.

The crisp photo above is by Linda Grenzer. It shows the Rock female performing a wing flap on Manson, her new breeding lake, while her mate, an 18 year-old male, yodels in the foreground.

My mother-in-law came to visit us in California last week. She is an avid follower of my blog (!), so I was excited to learn what she thought of my most recent post. She hated it. That is, she said of my report that loons get old:”I could have told you that!”. Naturally, I was deflated. To think that 23 years in the field had produced a result deemed pedestrian by my mother-in-law!

While one might argue that she is family and should have been blindly supportive of my work, Joanne is right, in a sense. As humans, we are accustomed to old age and deterioration of the elderly. But, as I tried to explain, senescence is not the rule in all animals. Birds are unusual, in fact, as they exhibit relatively late and gradual senescence compared to mammals of similar body size. So the striking and rather sudden senescence that I reported recently is mildly surprising for the taxonomic Class Aves. Still, I think I agree with Joanne that it is not terribly shocking!

But there is more. The blog where I reported senescence in adult loons was based on an analysis that pooled male and female individuals. Since then, I have analyzed the sexes separately. The results are striking. As the figures below show, the senescence that I reported for the species as a whole (measured by decreased survival) is driven purely by males. While males and females that have been on territory from 1 to 14 years survive at a rate of 95% annually, males with 15 or more years on territory only survive at a rate of 58%. (Old females show a very modest decline in survival to about 91%.) Since males and females that settle on territories are almost always 5 years old or older, we can say with confidence that territorial males in their twenties drop like flies; females, in contrast, are survivors.

young males and females do not differ in survival rate

old males die at a much higher rate then females

I don’t know if my mother-in-law will be impressed by these data. For the moment, I must be content in the knowledge that I have found a strong and highly unusual survival pattern. As a behavioral ecologist, this stunning disparity leads to several other questions. Among them are: 1) Do older males exhibit any other evidence of deterioration such as in territory defense, chick production or body condition? and 2) Does the high mortality of older males cause the adult sex ratio to swing towards females such that females are forced to wait years before finding a mate? Rest assured that I am exploring these possibilities with great enthusiasm.


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I have been back in California for some months now, my research in Wisconsin a distant memory. A stroll to the end of the pier at Newport Beach changed that. Several of my study species — unbanded animals that probably belong to breeding populations from Alaska, British Columbia, or Alberta — were foraging contentedly off the pier’s end, as fishermen cast their lines all about them. The fishermen appeared to avoid casting near loons, so I was not alarmed by what I saw. Loons are usually adept at avoiding fishing lines (though not always). Furthermore, an ecologist would not be surprised to see loons sharing a fishing hole with anglers, since they are competitors for the same small fish — mostly smelt and mackerel.

Common loons were not the only species diving and pursuing small fishes among the sea lions and occasional pod of common dolphins. It was a treat to find this juvenile red-throated loon out in the waves as well.

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Back home in Irvine I am crunching data, as I do habitually in the off-season. I feel some urgency at the moment, as I am about a week away from having to give a talk on my findings at the Winter Animal Behavior Conference in Colorado.  I will be more positive in a week or so, but I have already confirmed senescence in two different respects. First territorial common loons older than 20 abruptly begin to show much higher year-to-year mortality (roughly 20% annual mortality from 20 on; only 6% mortality up to 20 years). Second, territory holders 20 and older also stand a much greater chance of losing their territory through eviction (again a 20% rate of loss) than do those younger than 20 (12% rate of loss). In the coming days I will explore whether the sexes differ in these respects and whether old territory holders make any behavioral adjustments to this apparent decline in health and fighting ability.

Several months have passed since Gabby Jukkala’s and my article was accepted for publication in the Journal of Avian Biology. We have been anxiously tapping our feet while the wheels turn and our article comes out in the journal. This has just happened. You may now view our article here.  Gabby and I are thrilled that: 1) our article has been selected as “Editor’s Choice” for this issue of the journal, 2) Linda Grenzer’s nice photo of the female on her lake with a chick from 2015 is the cover photo for the issue (and a second is featured in the blog spot), and 3) the journal has included extra information about us and our article on their website here.

I have already described the findings we report in the article, so I will not rehash those here, but do take a look at the article, which the journal is making available free of charge, since it is “Editor’s Choice”. It is a very small honor, in fact. Still, these days I am often on the Newport Pier, as that is a good local birding spot and I must prepare for the Ornithology class I am teaching this fall. Whenever a member of my study species wanders nearby, as it forages for mackerel or smelt, I find myself smiling a bit more strongly than before.


There are three main reasons why I do field research. First, I love the outdoors. While it is unpleasant and inconvenient to many, the field is my briar patch. Second, I love observing and being with animals and gain new insights by watching loons up close and not just looking at data in my office. And third — well, I enjoy looking at data in my office. Complex puzzles and contradictions concerning loon behavioral ecology often become comprehensible when I look hard the numbers and run statistical tests.

Yesterday I had one of those Eureka! moments during data analysis. For the past several months, I have been looking at factors that: cause loons to: 1) be evicted from their territories and 2) disappear from their territories (that is, die). Some months ago I shared with folks that having your mate get evicted puts you at risk for eviction yourself. That is an interesting pattern and one that I continue to explore. But yesterday, I conducted the most sensitive and powerful analysis to date on the potential impact of age on rate of eviction and death in territorial residents. The  results were clear: age has a substantial impact on both eviction and death rates. Specifically, old loons are at risk for being booted off of their territories and they are even more strongly at risk for dying. The pattern was a bit tricky to detect, because very young adults are also at risk for eviction and death. So loons — like humans — have a “prime of life”. Loons of 4 to 6 years of age are still developing and improving in condition; at this age, they have a rather high rate of eviction and death. They reach prime condition about age 7 or 8 and remain in good condition until about 20, then condition falls off again, exposing them anew to a high rate of eviction and death.

The figure above shows the shape of the pattern with respect to age and death rate, when other factors are controlled. Sorry for the arcane title of the Y axis, but here is how to interpret the pattern. The curve shows how often territorial loons die, higher values indicating a higher death rate. The values are higher for young ages and old ages. Thus, the “U” shape shows that young and old loons die at a higher rate than do loons of moderate age. If you look closely, you can see that old age has a much stronger, more consistent impact on mortality than does young age. So senescence is very clear from the graph.

If you are a loon aficionado, you know that male loons often die in the course of defending their territories, while females rarely do so. How does the new finding of senescence fit in with lethal contests among males? On the one hand, it fits, because senescence might make male loons “desperadoes”, who fight hard for their territories because they have no future to lose. On the other hand, the pattern of senescence cannot by itself explain male-only fatal battles, because females and males both senesce. Oh well….I am always looking for another excuse to look at data in my office.

Since the inception of the study, we have known that some adult loons permit a canoeist to approach to within 5 meters without alarm, while others become uneasy and dive at a distance of 30 meters or more. Over the past several years, we have worked hard to quantify such variability in “tameness”. Our efforts are motivated by the belief that — in a region well-known for human recreation — tameness must matter. That is, it seems inconceivable that loons’ survival rate and reproductive success are not impacted by the way they respond to humans.

At first blush, I would expect loons to have higher fitness (i.e. be able to survive and breed more successfully) if their tameness reflects the lake they inhabit. That is, loons that are very tame should fare well on lakes where humans are numerous and often approach loons closely. A skittish loon on a lake with abundant human traffic would spend a great deal of time and energy avoiding humans and might have to spend more time foraging to compensate for the extra energy expenditure. A skittish individual on a busy lake might even become distracted by humans and pay too little attention to eagles, which occasionally attack adult loons and often attack chicks. On the other hand, shyness towards humans should have no impact on fitness if it occurs in a loon that occupies a remote lake.

Tameness is surprisingly vexing to measure. While it is easy to see that loons vary in approachability by canoe, it is another matter to assign a number to the degree of approachability they show. One obstacle to measurement is simply that of measuring distances accurately across water. Another is the problem that we seek to know exactly at what approach distance a loon dives to avoid a canoe; once this critical distance has been reached, the loon has left only its wake on the lake’s surface for us to measure! After numerous trials, however, Seth Yund, a Chapman student, and I have found a technique that seems to work that requires use of a highly accurate laser rangefinder — and a lot of patience. In July we began to collect measurements on each banded loon in our study population, and this work will continue into the fall and in future years. (By the way, the technique requires paddling slowly in a canoe towards a resting loon until it dives, while taking constant measurements. Since the process must only be carried out once or twice per loon, it involves very mild disturbance. We have found that loons quickly resume normal behavior after we take a tameness measurement.)

It will be some months before we begin to see if our quantification of tameness is stable and consistent enough to constitute a useful behavioral measure. At that point, we can begin to test our preliminary hypothesis that a loon’s tameness should be correlated with amount of human usage on its lake. Since we have many parent-offspring pairs in the population and follow individuals throughout their lives, we can envision asking questions about the heritability of tameness and its constancy over time. We hope that tameness will become a rewarding topic of research for us. Perhaps our ability to quantify this behavioral characteristic will permit us to foresee negative impacts that increasing human-loon contact will have on our population and help recommend ways to minimize such impacts.



A few days ago, I was conducting observations on the lake where Linda Grenzer lives when she snapped this photo of me, the two pair members on the lake (the two loons closest to my canoe), and five territorial intruders. Linda’s picture captures beautifully what transpired during my visit of an hour and a half. Better yet, the photo shines a spotlight on an enduring question that drives much of my research on loon territoriality: what is the purpose of territorial intrusion. This is a question that we have only half-answered.

We do know that many intruders visit in order to learn whether or not a territory has chicks; if they detect chicks, they are more likely to intrude in the following year and attempt to evict the breeding pair member of their sex. (This makes some sense, as the presence of chicks is an indication of good nesting sites and plentiful food for young.) We also know, from a recent analysis, that loons practice natal-site matching; that is, they attempt to settle as a breeder on a lake that is similar in physical size and in pH to the lake they were reared on. So undoubtedly some intruders must be learning about lakes where they intrude so that they can settle on one similar to their natal lake.

But there must be more motivation for intruders to visit lakes defended by breeding pairs. Among the 5 intruders pictured in the photo, for example, were: a banded 9 year-old loon not known to be settled on a territory yet (i.e. a probable “floater”), an unbanded loon whose status is wholly unknown, and three banded loons known to be members of breeding pairs from neighboring territories. While the first two birds could plausibly be shopping for territories through chick detection or natal-site matching, the neighbors are unlikely to be doing so.

What, in fact, could neighbors stand to gain from intruding next door? Several hypotheses are possible here. Neighbors might gain by becoming familiar with other loons with which they might become mated in the future, if they both lose their current breeding positions and must settle elsewhere in the general area. Neighbors might also be trying to learn about the territory, which they might occupy in later years, providing one of the current pair members dies. Another possibility is that neighbors have no particular interest in the territory where they intrude but, rather, are intruding in order to draw attention away from their own territory (since loons on the water tend to attract flying loons to land and investigate). That is, intruders might be attempting to “decoy” loons away from their own territory so that others do not learn about it and attempt to settle there. Finally, neighbors might simply visit to forage in someone else’s territory, depleting the food supply there instead of at home.

An additional question raised by the photo is: what is the breeding pair’s response to intruders? One might expect that the breeders would react aggressively towards intruders, driving them out immediately so that they cannot learn about their chicks (of which there were two in this case, hiding our near shore and far from the intruders) or harm them. Yet this pair — typical of breeders — was tolerant of the intruders and allowed them to roam freely throughout the territory for over an hour. Are the breeders feigning nonchalance to reduce the likelihood that intruders will look closely for and detect the chicks? Or do large numbers of intruders pose a severe threat to territory ownership such that territory owners must tolerate them or risk losing their positions?

I hate to raise so many questions that we cannot answer immediately. Testing of most of these hypotheses for intrusion and defense towards intrusion is feasible in our population. For example, we can look statistically at the “decoy” hypothesis by seeing whether pairs that vacate their territories and intrude next door experience a lower rate of territorial eviction than pairs that remain on their territories faithfully throughout the season. And we can test whether pairs that attack and stalk intruders, rather than tolerating their intrusions peacefully, suffer a higher rate of territorial eviction, because they betray the presence of their chicks and place a target on their own backs.

Such statistical analysis requires large samples of lakes and intrusions, so it will take time. Meanwhile, we will have to enjoy the experience of tracking intruders and breeders around territories by canoe and wondering what peculiar combination of evolutionary interests of breeders and intruders could produce such flotillas.


As most of you know, most loons acquire their territories by force. That is, a non-breeding adult intrudes into a territory defended by a breeding pair, challenges the pair member of its sex, battles with that pair member and, if lucky, evicts that owner. (An evicted owner is sometimes killed in battle, especially if it a male, but more often simply leaves its former territory after a defeat and looks elsewhere to settle.) By the way, evictions (also called takeovers or usurpations) are about equally frequent in males and females.

We have now observed or inferred 226 territorial takeovers, and we are at a point where we can begin to look for other patterns in these data. I have been analyzing the data to learn whether one eviction leads to another. For example, if the territorial male on a lake gets evicted, does that place his mate at risk for losing her position as breeder as well? Based on 2207 breeder-years of data, the answer is a resounding “Yes!”. When a male is evicted from his lake, his mate is at substantial risk for losing her position. Female eviction also exposes male breeders to an increased threat of eviction, although the pattern is not as strong.

What can we make of this pattern? It is early, and many analyses remain to be completed, but here is an early read on this finding. Breeders are largely stuck with the danger of eviction and with the problem that their mate’s eviction threatens their own position. Females, which are about 20% smaller than males, are really in a bind, as their size probably prevents them from helping their mate avoid eviction — thus protecting themselves indirectly. On the other hand, males might be expected to chip in and help their mate drive off a would-be usurper, if doing so protected their ownership as well. Yet we have never observed a male teaming up with his besieged mate to drive off a potential usurper. If this happens, it must be rare. Indeed, males appear to be dispassionate observers of female battles for territorial ownership, despite the risk it poses to them. Why? Perhaps males are better off with a mate that fights strongly (such as a proven usurper), since having a vigorous mate ensures future years of eviction-free breeding. It is a bit sad and selfish to say it, but if your mate is vulnerable to eviction, it is probably best to let her fend for herself than to intercede and save her.