natal site matching

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Despite 31 years of field research, I have seen very few dead loons. The scientist within me should know better, but I think my rather limited exposure to loon mortality has led some part of me to presume that adult loons die on migration or on the wintering grounds, not during summer months. Spring and summer, after all, are times of renewal, of bountiful food and comfortable living conditions. It seems inconceivable that loons could perish in the warm, friendly environs of the Northwoods.

But this is an anthropocentric view. While most humans that we encounter on the lakes are relaxed and smiling, loons — and all other non-human species for that matter — are in a constant struggle to ward off predators, parasites, and pathogens and keep themselves and their young alive. Loons are not on vacation from April to October; they have merely traded one set of hazards for another.

Four members of the Wisconsin loon team were reminded of the incessant fragility of loons’ lives during a visit to Katherine Lake in late May. Split into teams of two in separate canoes for training of the new field team, we first paddled to the traditional nesting site on a small island towards the southern end of the lake’s main bay. In case nesting was under way, we circled at a distance. As we completed our brief circumnavigation, our brains struggled to make sense of a visual anomaly on the west side of the island. It was Ethan who first pieced things together. “There is a dead bird,” he remarked.

After approaching to check on Ethan’s assertion, we were greeted with a macabre spectacle. Amidst a vast mound of loon feathers lay remnants of the Katherine male, a well-traveled individual with a fascinating life history. A few feet away from his carcass was a nest containing a single intact egg.

There was no need to round up the usual suspects. An eagle, it seemed, had surprised the incubating male, ended his life, and closed the curtain on Katherine’s chances for breeding success for the year. The telltale plucking of the avian carcass after the kill clinched it.

Yet this is not the end of the story. Blithely uninformed about the recent horror that had taken place on the lake, two young loons — an eight-year-old female and a seven-year-old male — had quickly paired up and made their own plans. Even as we mourned the violent passing of the old resident male, these two individuals foraged and preened calmly about the lake, apparently savoring the prize they had inherited.

Young settlers carefully choose the lake on which they wish to breed on the basis of its overall size relative to their natal lake. I was cheered to see that the new settlers on Katherine were well-suited to the vast size of their new territory. The eight-year-old female was from Two Sisters-East, a good-sized lake not far south of Katherine; the seven-year-old male was from massive Lake Tomahawk, a short distance northwards. Still, there seemed little chance that this confident new couple would attempt to nest in 2023.

When Emily sent me the photo above, I was doubly surprised. With the disturbing sight we had observed on Katherine still etched in my mind, I had crossed the lake off the list of territories where chicks might be produced. Moreover, I had never dreamed that the new pair would choose to nest on the precise spot where an eagle had ended the life of and then feasted upon the old male. From Emily’s photo, you can see vestiges of the attack. The incubating bird (the eight-year-old female in this case) sits placidly on a new set of eggs, oblivious to the smattering of her predecessor’s feathers that surround her.

There is something exhilarating about unbridled and heedless optimism. Stuck as we often are in the past and present, it is often difficult to see what good might come down the road. And I have to say that the brazen breeding attempt of the new young Katherine pair has changed my outlook. If an inexperienced loon couple can dare an eagle to attack them — and even more so if they can pull off a hatch — perhaps a breeding season that began wretchedly can end on an up note.

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Pickiness is familiar to humans. We all invest considerable time and energy when looking for a home where we might live for a decade or more. But who knew that loons were equally choosy? Elaina and I have had a fresh reminder this past week of how finicky loons can be.

Sure, we have crunched the numbers. We know from the analysis that Mike Palmer and I ran six years ago that loons show a strong statistical preference for lakes that resemble their natal one in size and acidity. The figure above — though cropped for artistic effect — shows that both males (yellow arrows) and females (orange arrows) tend to settle on a breeding lake of similar size to their natal lake.

Knowing that loons eventually settle as breeders on lakes similar to their natal ones does not, unfortunately, tell us much about the exact means by which they choose a lake. That we must learn about by noting the sequence of lakes that a young loon visits, how long it stays on those lakes, and with whom it interacts while there. Although we have published a preliminary paper that dispels one popular idea about how loons might try to claim a first breeding territory and have long known that loons are strongly attracted to territories that are proven chick producers, we have quite a ways to go to nail the process down completely.

When we are out in our canoes, of course, we cannot see broad patterns. We simply note, for example, that a certain banded loner was at the north end of Birch Lake during the hour or so that we visited. At the end of each day, we check the database to learn the identity of any banded loon whose legs we saw clearly. Since we are tired, the discovery of a banded ABJ (“adult banded as juvenile”; i.e. a loon that we originally banded as a chick) that no one on the study has ever seen before elicits little more than a grunt of satisfaction. Then we ready ourselves for the next day in the field.

But the data accumulate. A loner that Elaina, Evelyn, and Tarryn saw yesterday at Katherine Lake, “Green over Green Stripe, Pink over Silver” matched a bird that I had encountered 24 days prior on East Horsehead Lake and that Elaina had seen on June 7th of last year on Bearskin. Three sightings do not constitute a publishable pattern, but this is a familiar observation on the project: a loon appears repeatedly on lakes of a certain size. The fact that we have seen this female only on large lakes (200 acres or more) and that she was also reared on a large lake (Two Sisters) suggests that she has a natal habitat preference for large lakes. What’s more, her apparent favoritism towards large water bodies at the tender young age of four indicates that she has already narrowed her choices, even though she will not settle to breed for two to three more years.

Our recent sightings of G/Gs,P/S — when combined with those of the 383 other young loons we have observed in the process of scouting out breeding territories — will help us piece together the strategy that young loons use to zero in on a breeding territory. If G/Gs,P/S is typical, young males and females cherry pick certain kinds of lakes to visit within their home region, skipping over many intervening lakes that do not catch their fancy. In another case, B/B,Ts/S, a male, was raised on tiny Fox Lake (15 acres), visited mainly small and medium-sized lakes when searching for a territory, and settled on Schlect Lake (25 acres). Since G/Gs,P/S and B/B,Ts/S have completely opposite ideas of what a good territory looks like, they could both inhabit the same cluster of 20-30 lakes during the two-to-three-year pre-settlement period, both search feverishly for a territory, and never meet.

We are nearing the point where we will formally analyze visitation patterns such as I have described to learn more about how loons decide on a breeding territory. Our data include sightings not only of adults but also of young juveniles in the late summer and fall after they have fledged but before their first fall migration. It seems quite possible that a rough picture of its ideal breeding lake forms very early on in the brain of a young loon.

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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.

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LMG_8014 Fall Molting Loon

Some months ago, I reported with excitement that loons that leave their first territory — usually because they are evicted from it by another loon — have a strong tendency to settle on a second territory with similar water clarity to the one they left. It was a striking finding and one that would have added a fascinating story to ecologists’ limited understanding of how animals choose where to settle once they reach adulthood. That pattern, which initially seemed a robust one, has turned out to be trivial.

Let me explain. When loons are forced to shift from one territory to another (or do so voluntarily), they typically settle on a second territory very close to their first one. Often they move to the lake next door or even settle on a different part of the same lake. What we thought we had found was that those “resettlers” carefully choose to settle on second lakes of similar clarity to their first lakes. But such a pattern is tricky to show conclusively. In order to do so, one has to determine how similar 2nd lakes would have been to 1st lakes if loons had settled at random and compare the actual pattern to random settlement.

The problem we ran into was spatial autocorrelation. Spatial autocorrelation simply means that within a large area, two points that are physically close to each other resemble each other more closely than two points that are farther apart. It turns out that lakes of similar clarity tend to be clustered. That is, if one lake is clear, then its neighboring lake is likely to be clear also. The same is true of murky lakes. So while we thought that loons were purposely choosing to settle on second lakes of similar water clarity to their first ones, they were merely settling on lakes near their first one which happened to be similar in clarity to their first because of proximity. In other words, we have no evidence that choice of lakes is non-random.

It took us a long time to discover the error. In fact, Mike Palmer and I had written up this finding and submitted it for publication before we realized — after running a new analysis requested by a reviewer — that spatial autocorrelation was solely responsible for our result.

Naturally, I am feeling a bit like the female on Muskellunge Lake, photographed recently by Linda Grenzer. A month or so ago, this bird and I were on top of the world — she because she and her mate had just fledged two chicks (a bumper crop for loons) and was in her showy black and white formal attire; I because I had an exciting finding that I could not wait to share with colleagues. Now the luster of the recent past is gone. The female’s chicks have dispersed to fend for themselves, she has lost her gaudy summer plumage, and she is about to resume survival mode during migration and winter. I have lost my thrilling discovery and will have to pick myself up, dust myself off, and attack some other scientific question.

Luckily, the Loon Project supplies an endless string of questions. One that tantalizes me at the moment: do chicks that fledge and leave their natal lakes to forage on other lakes nearby favor those that resemble their natal lakes? Such a result, if it occurs, would dovetail nicely with our finding that the first lake a loon ever settles on as an adult (at 4-7 years of age) tends to be strongly natal-like in size and pH. This result (he adds quickly) is not confounded by spatial autocorrelation, because first lakes tend to be far from natal lakes. At any rate, I shall let you know what I find.