I was on pins and needles last week when I visited Cunard Lake. Cunard, some may recall, is the 45-acre lake where two strapping six-week-old chicks and their parents fell victim to a devastating series of events in the last few days of July 2018. On July 27th one of the chicks swallowed an angler’s bait and hook, which kept the youngster from feeding itself and led to its death within five days. On July 29th, the Cunard male found himself challenged for territorial ownership, lost the battle, and vacated the lake. In the wake of that eviction, the surviving Cunard chick too vacated the territory and blundered into nearby Hasbrook Lake, where a different pair was rearing two slightly younger chicks. The lone positive that emerged from those events was that the Hasbrook pair let the huge refugee join their family and fed it thereafter as one of their own.

Naturally, I was intensely curious to observe the aftermath of this territorial carnage. Even what little we had seen in the few weeks following the loss of the Cunard family was tantalizing. The lake was left vacant. The evicted male, the male that had evicted him, and the female from the lake all went missing. The lack of loon activity in Cunard was odd, particularly in the case of the victorious male. This male had scouted out Cunard, observed the chicks, challenged the male breeder for this proven territory, and defeated him in battle, only to desert the lake after driving away the former owner and seizing ownership.

My visit to Cunard a week ago was anticlimactic. While I had expected to see the usurping male paired with the original Cunard female — the normal aftermath in  cases of eviction — both original residents (a 15 year-old female hatched on nearby Woodcock Lake and a 13 year-old male reared on North Nokomis) were back and acting as if nothing had happened. It was a complete territorial reset. To be sure, there is a slight chance that the usurper had died during migration or over the winter. But it is far more likely, given the 94% annual survival rate of young males in their prime, that he had simply changed his mind about settling on Cunard.

We have seen this behavior before. I had always expected that evicting a territory owner and settling on its territory would occur in one fell swoop, but it is not so. On numerous occasions, nonbreeders (either male or female) have taken the great risk of challenging an owner in battle, defeated the owner, and then not settled on the territory they had invested so heavily to win. Why?

We can only speculate at this point. One peculiar behavioral pattern that might help us understand the failure of usurping loons to settle is natal habitat imprinting. Natal habitat imprinting refers to the strong tendency of loons — both male and female — to choose breeding lakes that closely resemble the lake on which they were reared. There could be a variety of benefits to settling on a lake like the one you were raised on, such as having similar prey to eat, availability of similar nesting areas, or simply being able to use one’s natal lake as a model of what sort of breeding area to look for, which might speed up the territorial search. At this point, let’s take natal imprinting as a given, since we know it occurs strongly in this species, and ask the obvious question: “Why would a loon not decide if a territory was to its liking before engaging in a dangerous battle to win it?”

As is so often the case, I can merely state a plausible hypothesis based on what I have observed of loon behavior. Territory owners, of course, are intolerant of intruders. Thus, while a loon that attempts to evict a resident from its territory is likely to have visited the territory on numerous previous occasions, the strong tendency of the breeding pair to drive out all intruders shortly after they enter prevents non-residents from learning much about the territory. As a result, once it has defeated the resident and taken ownership of its territory, a usurper might find (to it chagrin) that its hard-won territory does not resemble its natal one as closely as it had seemed. This hypothesis will be difficult to test, but not impossible. It predicts that many nonbreeders will battle for and win territories that are similar to its territory in obvious ways (like overall size) but that usurpers that find that their new lake is unlike their natal one in more subtle ways (like available prey species) might not stay.

So, we must keep working. In the meantime, enjoy my photo of a new pair that has settled on Swamp Lake. The female (on the right), evicted from Prairie Lake in 2015, seems gung-ho to settle and breed. Let’s hope the male (left) finds Swamp similar enough to his natal lake, East Horsehead, to stay.

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…..in 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.)

2019-05-17 09.45.12

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.


After the blur of our early-season census, we are left with a massive whiteboard showing all territories and their current status. Sometimes I find it pleasing to gaze at the board and enjoy what we have accomplished already this year. But this year’s whiteboard reveals a worrisome pattern: a decline in the number of occupied territories in the study area. Those territories occupied in May 2018 but on which we have not yet found a pair this year number twelve: Bullhead, Dorothy, Johnson, Mercer, Minocqua-East, Minocqua-South, Minocqua-Huber Bay, Muskellunge (Oneida County), Nokomis-North, Pier, Swamp, and Wind Pudding-East.

Before we panic at what seems an alarming number of lakes where territories have winked out, let’s consider a few other possibilities. First, we surely missed a pair or two on our original census because they were off foraging on another lake that we had not seen them visit previously. This actually happened on Spider, where I observed no pair on May 4 but where Elaina just today found last year’s pair with a nest. Second, at least a few of these now-vacant lakes will probably support pairs soon — or at least by the end of the season. Loons are notoriously slow to occupy vacant lakes and even, in some cases, to replace dead breeders. Third, it is a normal part of loon population ecology that the species ceases to breed on a few lakes each year. In order for the breeding population to be stable, of course, such losses must be offset by settlement of new pairs on territories that did not exist before. In fact, new territories — vacant in May 2018 but occupied in May 2019 — have popped up on McCormick, Buffalo, and South Blue lakes, owing to settlement by a four year-old male, a six year-old male, and a displaced thirteen year-old male with unbanded females. We will surely find a few more new territories in the coming weeks. But even those projected additions will not be sufficient to offset the apparent number of lost territories this year.

Perhaps we should look more closely at individual lakes to learn what has happened to cause territory loss. Bullhead is a mystery, as neither pair member has been seen; the same is true of Dorothy, Johnson, Mercer, Minocqua-South, Nokomis-North, and Pier. In the remaining five lost territories, though, we can see that male has simply not returned. In each of the five, the female has come back from migration, settled on the territory, and has found no male with which to breed. Broadly, the pattern is not surprising; it is what we expect in an adult breeding population that is female-biased. As I have noted many times in this blog, males senesce and die much earlier than females, and this causes a persistent dearth of the former and a surplus of the latter among adults wishing to breed. Could we be seeing the leading edge of some die-off among male loons that will push the population into decline? It is possible but not likely.

Let’s try to match up this year’s apparent decline in territories with likely causes. While scientists project the long-term withdrawal of loons from northern Wisconsin owing to climate change, and loons each year face slightly greater human impacts from angling, boating, and shoreline development, negative impacts of these factors should occur gradually, not in one or two years. So the net loss of six or seven territories we have seen this spring (after accounting for the gain of three territories and a few pairs missed) seems too great to be a harbinger of a larger population decline. Thus, I will tentatively conclude that this year’s losses are a blip — a short-term downward bounce in number of territories caused by chance and/or a temporary downward shift in breeding success.

Could the decline come back to loons’ nemesis, the black fly Simulium annulus? Possibly. Looking back across years, in fact, we see the dreadful breeding year of 2014, caused largely by the prolonged explosion of black flies in that year. Male chicks that would normally have hatched in 2014 would be at their crucial settlement age of five right now. So we might speculate that the “missing” five year-old males — males that would have slid into many of the territory openings for male breeders that are evident this year — are the cause of this year’s decline in active territories.

If my speculation is correct, we should expect a sharp increase in the rate of territory settlement (i.e. a net increase in territories within the study area) in 2020 and 2021, because those years fall five years after the productive breeding years of 2015 and 2016. We shall see. In the meantime, perhaps I can dispel some of the negative feeling caused by all this talk of territory decline by showing you this year’s nest on Hanson Lake. Though she took a short incubation break two days ago, which allowed me to snap this photo, the female soon jumped back on the eggs. She seems determined to do her part to help reverse the current downward trend.

I have been out on the lakes for the past fifteen days. This time of year, we race around to all of our study lakes from the previous year – and a few more where we suspect new pairs might have settled – and see who is on territory. We do not dawdle; our task is to identify the pair present on each territory and move on to the next. It is an exciting but frantic annual ritual. While we usually observe the breeding pair on each territory for an hour, a minute will suffice during scouting, because we are merely confirming the identity of breeders, then moving to the next lake. Therefore, on a really good day – if the loons are easy to find and show us their bands – a scout might hit ten lakes.

I had the kind of scouting visit we dream of on Lake Seventeen on Sunday. Seventeen has beautiful clear water, but it is large (175 acres) and has multiple bays and convolutions. One can be certain that a visit there will require putting in the canoe and searching for twenty minutes or more for the birds. Three days ago I stopped by a home at the northern tip that is our access point, glanced at the lake as I habitually do, and was delighted to see the pair foraging thirty meters offshore. I grabbed my binoculars and ran to the water’s edge. Five minutes later I confirmed that the male was “Yellow/Silver, Green/Copper” and the female “Silver/Red, Pink/Red”. The loons were so close to my position that I even observed several small bluegill scatter and flee to within inches of shore as the female pursued them. For a moment I wondered what life must be like for a small fish that ventures occasionally into open water, where predators lurk. But my reflection was cut short; I had six more lakes to hit that day.

Few scouting visits go as my recent one to Seventeen did. Yesterday, in fact, I had one of those days when I think that loons might have gone extinct suddenly. The pair on Tomahawk-Little Carr territory was cooperative: I got their bands quickly and saw them building a nest in their usual spot. But that was the end of my good fortune. I was skunked on Bullhead Lake, where the pair was missing (for the third visit this spring), and the usual nesting location showed no signs of usage. Minocqua-North, an area where the Bullhead pair has foraged in previous years, was also vacant, though I found two patches with marshy hummocks that would make excellent nesting spots. In short, the banded male and female of Bullhead, consistent breeders and chick-producers since 2010, are at large. The same is true for Minocqua-South, where a banded pair first nested on an island in 2018. Johnson Lake, where the banded male was caught on a fishing line­­­­ last May, was empty as well. My next lake, Mercer, is one we dread, because both pair members are extremely skittish. A typical datasheet from Mercer following an hour-long visit might list the leg band colors of the male and female, respectively, as “Red or Orange?,light band?” and “banded?”. It is that bad. After an hour and 53 minutes of exhaustive searching yesterday, though, I would have been thrilled just to see a loon. As on the three previous lakes, I did not find the pair.

2019-05-12 06.14.19

Yesterday’s hours of fruitless scouting ended the first round lake visits with a whimper. All other days have gone far better, however. Linda’s great work at the southern boundary and my observations in the central and northern parts of the study area have clarified the picture of who is back and who is not. Though I was stung by the disappearance of many banded and well-established pairs — or possibly my failure to find them – I also can look back and smile at the return of the venerable female from Upper Kaubashine. Banded first with her chicks in Vilas County in 1993, she has been a most successful and itinerant breeder, having produced chicks with at least four different mates on four territories. She is at least 31 years old and probably older. So let’s lift a glass to this resilient bird (on left in photo with her larger mate). With luck, she will dodge the blackflies, raccoons, eagles, and fishing lures, and add two more offspring to her lifelong tally.


Early spring research takes a special kind of mettle. With winter merely loosening its grasp on the North, but not letting go entirely, many boat landings remain iced in — like the one Linda found at Hilts Lake today. Meanwhile, inconvenient portions of each lake become ice-free, and returning loons hang out there. In the photo above, for example, Linda is trapped at the public landing, far from an ice-free strip of lake on the northwest side. So the birds are, for the next day or so, inaccessible. Sometimes, too, entry roads are blocked or impassable, as Linda found yesterday on O’Day Lake, where one of our oldest females (marked in 1996) breeds. We will have to come back to these lakes when conditions are more favorable for our work.

Early-season census, which involves visiting lake after lake to verify IDs of all loons found, can be a frustrating business. Not only are conditions unpleasant, loons move around. As part of their ceaseless quest to stay a step ahead of raccoons, loon pairs that lost eggs in one year will check out adjacent lakes in early spring of the next year to search for better nesting sites. Thus, one can fight one’s way to a boat landing and dodge ice slabs to reach open water, only to find that the breeding pair is off scouting a nearby but unknown lake.

The upside of observations of loons in April and May is the excitement of seeing which of our marked study animals have returned. As many of my posts have made clear, we get to know and love our loons, especially the tame ones that permit us to approach to within a few meters and identify them from their bands easily. So each year I read with great anticipation the list of returning breeders that Linda, Nelson, and other scouts have found. These birds — and this does not feel like an exaggeration — are my friends.

This year I get to help with scouting. While normally I would be bogged down with teaching until the middle of May, my sabbatical this spring allows me to join Linda in the challenging task of identifying our returning breeders. Within a week, I will be zipping around from lake to lake, hoping at each stop that both pair members will be right off the boat landing, roll-preening to show off their leg bands. A few weeks later, Elaina, who worked with us last year, will join us as well. We are short-handed in 2019 and will not have a banner year for data collection, but the three of us will do what we can to hold down the fort.


Several events were happening at once last June. I was about to turn 60. The entire college of science at Chapman was moving to a new building. We had just brought our loon research goals to a conclusion, which left me uncertain what questions to tackle in the future. Most important, the money was running out. These events conspired and left me thinking: should I keep studying loons? Or is it time to step back from field research and devote my energy to service to the University?

This question loomed over me for many months. When anyone asked, I was pushing ahead with plans to look at causes of aging in loons through our study of telomeres, but doubt nagged at me each day. Why continue? Have I answered all of the important questions? Should I just rest my back and write my book?

Strangely, I pulled out of this funk not through some unexpected statistical breakthrough or spellbinding discovery but by writing a grant proposal.

To me, grant proposals are a necessary evil. I do not enjoy marketing my ideas to colleagues in my field, who will weigh in with a thumbs up or down – perhaps for the wrong reasons. Selling ideas in an effort to secure funds seems shameless and mercenary. I became an academic in the first place partly to avoid such work. Yet to many scientists, grants are the bread and butter. Without funds to purchase equipment and supplies and hire personnel, most of us are quite limited in what we can study. So we write grant proposals.

Although I had pieced together the rudiments of an outline during this past summer, it was not until November that I started to turn that outline into the introduction and body of a proposal. Even then, I moved at an almost comically glacial pace. Each finished sentence, it seemed, was a great victory and warranted taking time off to recharge. I would find a clever opening phrase and birdwatch for two hours; locate a useful journal article and birdwatch for three. I was going nowhere.

Oddly, I gained momentum. As I added a reference here, ran a new statistical analysis there, a robust, compelling set of hypotheses began to emerge. In the process of constructing a document to convince other ecologists that I had ideas worthy of testing, I convinced myself that I had such ideas. What had begun as a hollow, pro forma exercise culminated in a thoughtful, executable plan for ten years of future loon research directed at the question of why loons settle and breed on small lakes that produce few offspring.

So I am back on track. Far from drifting numbly towards a new research season, I am energized and anxious to hit the lakes again. The sense of dread at facing a year of field work short‑staffed has lifted. Now I relish the challenge of keeping tabs on all of our study animals this summer with fewer field workers – as long as my back holds out.



We have known it for some time. Young loons looking for territories observe chicks on a lake and return the following year, hoping to evict one of the resident pair members and take over the breeding position. The effect is dramatic; the intrusion rate increases by 70% following a year with chicks.

From the viewpoint of a young floater (a young bird that has not yet settled), chicks are a boon. The mere sight of young encapsulates all of the information necessary to size up a potential breeding location. Instead of having to learn about breeding success by trial and error, the floater need only seize an existing territory that has proven to be a chick producer. (Of course, seizing a territory has costs.)

From the breeders’ standpoint, raising a chick is like painting a great big target on their backs. Those little brown fuzzballs look cute in the moment, but their presence portends many battles with floaters the following year. No wonder parents take steps to hide their chicks, when they can.

A week ago, during the writing of my proposal to NSF, I made another finding related to chick production and its impact on a territorial pair. In this case, I was examining our data related to aggression between breeders and floaters, which can take the form of grappling battles between two loons, lunges by one loon at the other, chases across the water, or severe feather damage to the head of the breeding bird — evidence of a recent violent encounter. In a few cases, I could infer aggression because a pair member was injured on its territory after having been healthy a few days beforehand. When I threw all of observed aggression or evidence of past aggression together and asked whether it was related to chick production, I found that more aggression occurs per visitor both in a year in which chicks are present and also in years following chick production.

Now you might wonder whether I have moved the needle here. We already knew that chick production causes a spike in territorial intrusions the following year; what extra information do we get from knowing that aggression also spikes? The answer relates to costs. Just visiting a chick-producing territory more frequently expresses interest in the territory. The fact that floaters elicit a greater rate of aggression per visitor tells us that floaters are willing to incur costs (i.e. the cost of injury) while visiting such territories. In short, floaters flock to successful loon territories — and they mean business when they do so.


It is only a glimmer — the kind of glimmer one often gets when eyeballing new data. But the implications of this small discovery are enticing.

You see, I have been looking at our data on tameness, Since 2014, our team (mainly Kristin, Seth, Mina, and Nelson) has measured tameness of loons by creeping up to birds resting on the surface. We do this by first measuring our distance to the loon with a rangefinder and then paddling slowly in its direction, taking distance readings every few strokes. The final distance reading — just before the loon dives to avoid us — is our measure of tameness. Determined in this way, tameness varies from well above 50 meters to less than 2 meters. (In fact, some of our marked birds, like the male in Linda Grenzer’s photo above, find our approach so unremarkable that they simply veer slowly out of our path, instead of diving.)

We can examine the origins of tameness in far greater depth than most other studies, because we have tameness readings on many sets of close relatives in the study area. In fact, owing to the duration of our study, the limited natal dispersal of many individuals (especially males), and our efforts to find adults that we banded as chicks, we now have tameness measured for 60 sets of relatives. These include Linda’s male (“Clune”) and his son, who breeds on tiny Virgin Lake; the notoriously skittish male on Oneida-East and his full brother on Hughitt; and the Bear female and her full brothers on Cunard and Gilmore (all three banded 13-15 years ago on North Nokomis Lake).

As the figure below shows, we have noted a strongly and statistically significant relationship in tameness between parents and offspring. This pattern implies that either: 1) offspring inherit their tameness from parents, or 2) parents teach their offspring to be tame or skittish during the chick-rearing phase (or both). Either way, similarity in tameness between adults and their young means that despite being measured on different lakes, many years apart, and at very different ages, tameness is stable within individuals and is largely fixed early in life. A loon’s degree of tameness is, in effect, part of its personality.

Screen Shot 2019-02-17 at 10.00.46 AM

Parent/offspring similarity in tameness is more than a hollow novelty. Since tame parents produce tame young (either via genetics or rearing environment), those young should respond to the habitat in much the same way as their parents. I am in the process of writing a proposal to the National Science Foundation to study, among other topics, the possible impact of loon tameness on habitat selection. Specifically, I wish to test the hypothesis that tame loons might be suited to lakes with lots of human recreational activity (generally large lakes) and skittish loons to lakes with limited human activity (generally small ones). If this logical hypothesis holds up, then pity skittish individuals. Since human activity is increasing, and even many small lakes now see frequent human usage, skittish loons appear to have a small — and shrinking — set of lakes on which they can breed. Moreover, the reduced chick production of small lakes might also doom skittish loons to poor breeding success, so that fewer skittish individuals are produced each year.

The long-term consequences of parents passing skittishness to their young and fewer offspring produced by skittish loons are easy to guess. Tame loons will produce a large proportion of all offspring in the northern Wisconsin population, and tameness should increase in frequency in coming decades to the point where skittish loons are hard to find at all. This vast behavioral shift might go unnoticed by most observers, since there will still be loons on the lakes. But to an ecologist, it is exciting to think that we might be on the brink of learning the precise mechanism by which a population of an important animal can become tame.



I have touched upon this theme before. A peril of longitudinal investigation is that one decides, after a period of time, that one understands the system. So it has been with the Loon Project.

For many years I have thought I had a good handle on territorial behavior. Indeed many aspects of the loon territorial system have become clear during the course of my work and are not in doubt. Both sexes usually fight to claim their territories and face the constant threat of eviction. Males, which establish strong ties to a territory through controlling nest placement and learning where the best nest sites are, fight harder than females, and sometimes die during territorial battles. Early senescence among males sets the stage for them to become very territorial and aggressive as they reach their declining years (their mid-teens in many cases), which seems a means to help them eke out another year or two on a familiar territory.

But I might have been off in my understanding of the role of lake size and body size in territorial behavior. I have always thought of breeding territories on large lakes as much sought-after, because large lakes have ample food for rearing chicks. (Small lakes, you might recall, run low on food for chicks, resulting in lower fledging success.) If large lakes produce more young, I reasoned, large-lake territories must be highly desirable. Competition must be fierce, then, for these territories. A recent analysis of territorial tenure — how long a male or female can hold onto their territory before getting evicted from it — has forced me to rethink the effect of lake size on territorial competition. The figure below is a plot of territorial tenure versus body mass for males on lakes smaller than 20 hectares (50 acres) in size (like Langley, whose current pair is pictured). As you can see, small males — especially those below 4600 grams — have very short stays on small lakes, in most cases, while large males — notably those heavier than 5000 grams — often enjoy very long territorial tenure. This pattern suggests that, contrary to my expectation, territorial competition is fiercer on small lakes than large.

impact of lk size, male body size, on terr tenure

Let’s look at the same pattern on medium-sized lakes (20 to 80 hectares; or 50 to about 200 acres). You can see that the overall pattern is still evident, although it is weaker here, because a number of very large males (5400 to 5800 grams) have anomalously short tenure.

i2 mpact of lk size, male body size, on terr tenure

Finally, let’s inspect the data only for males on lakes larger than 80 hectares (200 acres). In contrast to my earlier hypothesis, large males are not holding their territories any longer on large lakes than are small males, as you can see from the plot below. Males of all sizes may enjoy long tenure on large lakes.

3impact of lk size, male body size, on terr tenure

How on Earth do small males hold their territories much longer on large lakes — which  seem much in demand, get more intrusions, and appear difficult to defend — than on small lakes, which get fewer intrusions and should be more easily held? I don’t know exactly how males hide in plain sight on large lakes, but it might have to do with the difficulty that territorial intruders have in simply finding a nesting pair and identifying nesting habitat on large lakes. Consider the Lake Tomahawk-Little Carr pair. This pair nests in a marsh at a well-hidden location. When one bird is incubating, its mate is usually far off in the wide open portion of Lake Tomahawk, which is many kilometers long and has an area of 1400 hectares (about 3500 acres). A male intruder might well find and socialize with the off-nest pair member on on the big water, but it would have no way of knowing that the mate of this loner was on a nest hidden far away in a marsh. Similarly, when the eggs hatch, the pair quickly leads the chicks to the main bay of the lake, far from the critical nesting area. Pairs with chicks provide an enticing cue to young males seeking territories, because the presence of chicks tells of the availability of nesting habitat. But a male intruder that encounters the Tomahawk-Little Carr pair and their chicks on the main bay of the lake would face the needle in the haystack problem in locating the precious nesting area that yielded the chicks. A dangerous battle might win the territory, but the knowledge of how to use the territory (that is, where to place the nest) would vanish with the old male’s departure. Hence, large lakes appear to be less valuable to males.

A male intruder bent on taking a territory likely to yield chicks in the future would be better-served by evicting a chick-rearing male on a small lake. Such an intruder would have a much smaller set of nesting areas to inspect and would likely find and use the nesting area that produced the chicks. Thus, we might expect stronger competition among males for small, easy-to-learn territories — a pattern that dovetails with the longer tenure that large, competitive males enjoy on small lakes, compared with small, easily-evicted males.

What about females, you might ask? Do large females on small lakes, like large males, have an advantage in holding their territories when compared with large females on large lakes? If my hypothesis is correct, and the value of a territory depends upon knowledge of safe nesting areas, then large female size should not be especially beneficial on small lakes. Indeed, any impact of female body mass on territorial tenure should be equal across all lake sizes. Why? Because females do not control nest placement in this species. An intruding female that evicts a breeding female with chicks and pairs with the breeding male would have access to that male’s knowledge of nesting sites on a lake of any size. As predicted, large size is no more beneficial to small-lake females than large-lake females. (Indeed, size has an overall weaker effect on competitive ability in females.)

So my post hoc hypothesis for the fierce territorial competition on small lakes holds for the time being. The explanation I have given is not the only one consistent with these data, by the way. In fact, the entire complex pattern described above might be explained by a completely different scenario. Large males might hold small territories longer simply because they are in better body condition. This is highly plausible, because mass is a good predictor of health and condition. Thus, only large males might be able to hold onto small lakes for a long time, because they are in good enough condition to survive in a habitat with limited food. Small males, by this logic, are already in sub-prime condition, so they are destined to have short territorial stays on lakes with limited food. In contrast, medium-sized and large lakes do not show the pattern, because food is not limited on them.

As you see, we have a long way to go to figure out exactly what the above graphs are showing us. Distinguishing between the “small-lakes-are-highly-competitive” and the “holding-small-territories-requires-good-body-condition” hypotheses will take some years. The difficulty of using what seem like beautiful, clear data to reach a firm conclusion provides a nice window on what it is to be a scientist.