body condition

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Following a long summer of capture, marking, and field observation, we have a new tranche of loon data from Wisconsin and Minnesota. The picture in Wisconsin does not change greatly from year to year. There we already had 32 years’ worth of research findings before 2025. But each successive year in Minnesota — where our research began in 2021 — increases our understanding of that population immensely. And with our improved knowledge of Minnesota loons, the status of the loon population across the Upper Midwest is coming into focus.1

Three demographic parameters together dictate whether a population of animals is increasing, decreasing, or stable, These factors are: 1) survival of breeding adults, 2) reproductive success, and 3) young adult survival. Recent measurements have shown us that the Wisconsin population is declining. And we know very well which of these parameters is responsible for the decline. If we compare our growing dataset in Minnesota to the trove of data we have from three decades of research in Wisconsin, we can learn whether or not Minnesota loons are headed in the same direction.

First, let’s look at survival of adult breeders. It should not be surprising that the most important single indicator of population dynamics (i.e. whether a population is stable, increasing or decreasing) is the rate of survival of its adult members. There is good news from the Wisconsin Study Area. The survival rates among territorial females and males both have been stable for the past three decades (Figure 1). This finding implies that once loons reach adulthood, they survive and hold their territories well. The decline that we are seeing in the Wisconsin population, then, must come about because of problems that occur before loons settle on territories.2

Figure 1. Annual survival rates of adult breeders on territories.

What about survival of territorial breeders in Minnesota? From measurements in 2022, 2023, and 2024, it appeared that adult survival in Minnesota might be lower than that in Wisconsin (look at these years in Figure 2, below). However, each year we get a better “read” on these numbers because our sample of loons becomes larger and more representative of the overall population. So the 2025 adult survival numbers are the most reliable ones we have to date. As you can see from Figure 2, there is no evidence for a

Figure 2. Survival rates of adult breeders in Wisconsin and Minnesota from 2022 to 2025. (Sample sizes are shown above each bar.)

difference in survival rates of territorial adults between Wisconsin and Minnesota. Minnesota, like Wisconsin, is seeing good adult survival. Again, this is good news!

Now let’s turn to reproductive success in the two states. Since we learned recently that the silver spoon effect is strong in loons, we know that we must look both at quality and quantity of loon chicks produced to get a good sense of how well a population is reproducing.

First let’s look at quantity. As Figure 3 shows, chick production in each region fluctuates greatly from year to year according to ice out date, severity of black flies, water clarity

Figure 3. Chicks fledged per territorial pair since 1995 in the Wisconsin Study Area and from 2021 to 2025 in the Minnesota Study Area. (Dotted line shows the trend in Wisconsin.)

in July, and other factors. Overall, however, chick production has decreased significantly in Wisconsin during the past three decades. Adult breeders are simply not producing as many offspring now as they did 30 years ago.

While Wisconsin data show a clear decline in number of chicks produced, it is too early to discern a trend in Minnesota. Chick fledging rate simply bounces around too much from year to year to see a pattern. We can say that chick production is at a similar level in the Wisconsin and Minnesota study areas. However, note that 2025 was a banner year for chick production in Minnesota and a poor one in Wisconsin.

Next we need to look at the quality of loon chicks that Wisconsin and Minnesota are producing. Our recent work has shown that chicks that fledge at low weights are much less likely to survive to adulthood and produce chicks themselves than are their heavier peers. Chick body condition has been falling for the past few decades in Wisconsin (see Figure 4, below).

Figure 4. Average body condition (mass divided by age) of chicks in Wisconsin from 1998 to 2025 and in Minnesota from 2021 to 2025. (Trendline shows Wisconsin pattern.)

The five years of data we have on body conditions of Minnesota chicks are not as many as we would like, but the numbers are consistent. Chicks fledge in Minnesota at similar — or even slightly worse — body condition than those in Wisconsin (Figure 4). We can infer that Minnesota is suffering from the same challenging chick-rearing conditions that have plagued Wisconsin (probably declining water clarity).

The third and final piece of the puzzle that we need to understand population dynamics is the survival of young adults. These birds are the breeders of the future that have not yet settled on territories. They range from three to about six years of age.

If you have been following my blog closely, you know that young adult survival of Wisconsin loons has seen the most dramatic decline among the three critical population determinants. That is, adult survival has held steady, and chick production has fallen somewhat, but the return rate of young adults to the breeding grounds has been abysmal — far below what it was a quarter century ago (see Figure 5, below).

Figure 5. Return rates of chicks to the breeding grounds 2 to 4 years after being banded as chicks in Wisconsin. (Data are missing for 2000 and 2007.)

We have been on pins and needles to see if this distinctive and rather alarming Wisconsin pattern is present also in Minnesota. Fortunately, our understanding of young adult survival has grown by leaps and bounds in Minnesota this year. Why? Because: 1) we started banding Minnesota chicks in 2021 and have done so every year since then, 2) most young loons return to the breeding grounds at three or four years of age in adult plumage, and 3) we regularly record identities of these young birds as intruders and loafers within our study areas. Thus, 2024 gave us our first glimpse at young adult survival in Minnesota using the crop of 26 chicks banded in 2021. And 2025 provided an even better window onto young adult survival there, since we could look at the return rate of 64 banded in 2021 and 2022 combined.

What do our findings show so far? In 2021, we banded 52 chicks in Wisconsin. Of these, 7 had returned as of 2025 (13.5%). We marked 28 chicks in Minnesota during 2021, and only one has so far been spotted as an adult (3.6%). For chicks banded in 2022, the numbers that have returned in Wisconsin and Minnesota, respectively, are 5 of 44 (11.4%) and 4 of 36 (11.1%).

These numbers tell a clear story. The percentage of young adult loons returning as adults in Minnesota is well short of that expected in a healthy population (a rate of about 41%). That percentage is also far below what we have seen in the past in Wisconsin (note the return rate in the 1990s and 2000s in Figure 5). In fact, the low return rate of young adult loons in Minnesota closely mirrors the dismal rate in Wisconsin.

In summary, it has taken five years to be confident of how the loon population in the Minnesota Study Area is faring. But our data now show that loons in Minnesota — at least those in Crow Wing and Cass counties, where we work — exhibit the same set of quirky demographic patterns that typify loons in Wisconsin and have set in motion a decline in the overall population there: 1) strong and stable adult survival, 2) poor reproduction in terms of both number of chicks and body condition at fledging3, and 3) a massive and diagnostic plunge in the survival rate of young adults (which are future breeders).

We have work to do.

1Thanks to Sheila Johnston, who took this photo of a molting adult loon on Gull Lake, which is just south of the Minnesota Study Area.

2I know. I just published a blog post in which I mourned the losses of many male breeders in the Wisconsin Study Area. I am still concerned about these losses. But in the long-term, which spans over three decades, adult males and females both have survived well. So I am hoping that the loss of several old, established male breeders this summer in Wisconsin was a blip.

3As noted earlier, it is too soon to tell from our data whether the number of chicks fledged is declining in Minnesota. We will gather those data over time. But we already know that Minnesota loon chicks are fledging in poor condition, just like Wisconsin loons. It is worth noting that the Minnesota Loon Monitoring Program, which has counted chicks across the state since 1994, reports a long-term and statewide decline in chick numbers.

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The Silver Spoon Effect occurs when individual animals that experience favorable circumstances early in life — like abundant food — enjoy long lives and produce many offspring when they reach adulthood. In essence, growing up well-fed places a young animal on a track to become successful as an adult.

The Silver Spoon Effect is strong in loons of the Upper Midwest. How do I know this? I have just done a massive analysis of factors associated with return to the breeding grounds among loons banded as chicks. I learned that chick mass (adjusted for age) is the single strongest predictor of both survival to breeding age and breeding success. That’s right: a loon’s mass as a chick accurately predicts how long it will live and how many young it will raise.

This does not mean that a loon chick that grows up without adequate food is guaranteed to die young and raise no chicks. Nor does it mean that a fat, healthy chick is certain to survive to adulthood and have many offspring. After all, it is a statistical pattern. But body mass in chicks is a very strong predictor of lifelong success. The graph below shows the effect.

Loons that did not survive to breeding age tended to be those with low mass as chicks (top row). Loons that survived to return to the breeding ground but never raised young (middle row) tended to be of intermediate mass as chicks. Loons that became successful breeders were mainly those that had been heavy as chicks.

It is hard to know how to feel about the Silver Spoon Effect. On the one hand, it seems cruel. One would love to think that a loon chick could overcome a rough start and turn its life around. But such a turnabout rarely happens. A juvenile that struggles to get enough food in its first month might make its first migratory flight to Florida. However our data show that such a loon has very low odds of surviving beyond its first few years.

On the other hand, loons reared with a silver spoon become the reproductive pillars on which the population’s persistence depends. Take the Pickerel-West male pictured in Hayden Walkush’s photo above. This male — “yellow over white-blue, auric red over silver” (Y/Wb,S/Ar in the table below) — was 8% heavier than average when we captured him on Tom Doyle Lake on August 3rd, 2013 with his parents and younger sister. He was then five weeks old. The momentum that his parents built for him gave him good odds of surviving to adulthood, settling on a productive territory, and rearing young himself. Indeed, he has already fledged six chicks on Pickerel with two different females despite being only eleven years old.

“Yellow over white-blue” is not exceptional. In the table below, you can see the list of all of our recent Wisconsin breeders that were marked as chicks. (Most are males because of the much shorter range of natal dispersal by males.) Pay particular attention to the right-hand column (“% above avg”). A “0” in this column indicates that this loon was of average mass for its age as a chick. If this were a random set of loons, the red numbers would all fall around zero; some a bit negative, some a bit positive. In fact, the table looks like something from Lake Wobegon: almost all territorial breeders in our study area were well above average mass at the chick stage. Sixty-five percent were absolute whoppers — 20% or more above average mass. This is a good illustration of the strength of the Silver Spoon Effect. (The pink-shaded cells show the few current breeders that were of below-average mass as chicks.)

Breeding lake, natal lake, band combination, sex, Chick Mass Index, and % difference from population average for current breeders in the Wisconsin Study Area.

The table shows something clearly. Territory settlers are the cream of the crop. Yes, there are a few overachievers — Harrison Flowage, Manson, Nose, Sherry, Silverbass and Soo. But their small number speaks to the strength of the pattern.

Leaving aside the fascinating and often brutal nature of silver spoons, let’s look at the implications of the pattern for loon conservation in the Upper Midwest. If you recall, the young adult “die-off” that we have found there is the most troubling aspect of the current population decline. Put simply, we are losing the vast majority of all young adults between the time they leave the breeding grounds as juveniles and return to it as adults 2 to 4 years later. This mortality must occur at some stage(s) of the life history of young loons — like migration or winter — that we have not studied well.

Average +/- standard deviation for Chick Mass Index from 1998 to 2020.

Here is the critical point. I have been suggesting that a decline in habitat quality along the migration route or on the wintering ground in Florida might be responsible for the die-off of young adults. But look at the graph above. Chick Mass Index has slid downwards steadily during the past quarter century. This means that the number of loons “fit” enough to survive to adulthood, claim territories, and rear chicks has also declined steadily. So loons are likely not dying because of environmental degradation on the migratory path or in Florida. Rather, loons themselves are of poorer quality than they were 15-20 years ago owing to limited food they received as chicks and can no longer survive the same challenges as well they used to.

Thus, the Silver Spoon Effect forces us to confront an uncomfortable reality: factors on the breeding grounds — during the chick-rearing period — are almost certainly contributing strongly to population decline. We cannot blame Florida.

But the silver spoon has a silver lining. Why? If we can improve lake conditions in Wisconsin and Minnesota so that loon parents can feed their chicks amply each July and August, we can help them raise fit chicks. And those fit chicks will grow up to become robust, successful breeders and strengthen the population.

So the answer, after all, lies in the lakes of the Upper Midwest. And my current push to discover the exact cause of water clarity decline in Minnesota and Wisconsin lakes suddenly takes on even greater importance.

$20,000 Match from a Wisconsin Foundation

If you have already donated to support our work, thank you! If have not yet done so recently, this would be a great time. Earlier this week we learned that a northern Wisconsin foundation will match every dollar raised from other sources up to $20,000. So every dollar that folks are able to donate will add $2 to our 2025 research fund.

DONATE HERE

We would dearly love to take advantage of this opportunity and field a strong research team next year. Thanks in advance, if you can help!

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When humans search for answers to complex problems, we often spend a good deal of time casting about in the dark. And our quests for answers commonly conclude with limited success or total failure. At least that is how it goes for me.

I suppose my quests are nerdier than average. In my research, I try to answer questions like: “Why do young loons seek to settle on lakes that resemble their natal one?” or “Why do female loons not yodel like males?” Still, the vexing puzzles I face in my work are not wholly different from the gnarly ones I confront in everyday life. “Why does the cupboard door under the sink never close properly?” “Why do plants keep dying in the southeastern corner of our yard?” Regardless of their origin, life is full of riddles that are challenging to address — and rewarding to solve.

This fall, I have been investigating factors that influence masses of adult loons and chicks. Most predictors of adult male mass* are easy to understand or, at least, lead to highly plausible and testable hypotheses. Males show a long-term decline in average body mass that began thirty years ago and continues today. This result suggests that human encroachment of some kind has made it harder for loons to maintain optimal body condition — and is getting worse. Recently we also found that adult males have lower mass on territories with a high density of docks. Again, we can understand how more docks — and more angling — might hinder adult male foraging and lead to lower mass.

Likewise, most predictors of chick mass are easy to explain. Short-term water clarity affects chick mass strongly. We have known for over a year that lower aquatic visibility hinders loon foraging. And I was not surprised to learn that a chick’s mass is correlated with that of its mother. It stands to reason that body size would be heritable in loons, as it is in other species. Finally, like their parents, chicks show a long-term downturn in mass over three decades that hints at environmental degradation. These three statistical relationships make perfect sense.

In fact, a brain twister did not emerge from this recent analysis until I looked at the relationship of dock density and chick mass. You see, chick mass is positively correlated with dock density. That’s right: the more docks there are on a lake’s shoreline, the fatter the local loon chicks become! The result is astonishing for two reasons. First, it runs counter to logic; second it is opposite to the male pattern. How could more docks — and thus more boaters and anglers — help chicks gain mass, when the same factor is harming their male parents?

I checked and double-checked the statistics. I scanned the data for abnormalities. Many days of fruitless pondering passed. At last I hit upon an idea that was hiding in plain sight: maybe the anomalous pattern had to do with food. Now, the data on fish abundance in our study lakes is of poor resolution and contains many holes. In fact, there are not enough data from any of our 205 Wisconsin study lakes to compare fish abundance with chick mass directly. Still, scientists have collected reams of data on abundance and size of fishes statewide. So we know a good deal about broad patterns.

A recent long-term analysis of fisheries shows that small fishes favored by loons — bluegill, yellow perch, crappies — have been declining in average size across the state for 70 years. The decline in panfish sizes — a consequence of overfishing, according to fisheries biologists — provides a plausible explanation for the long-term loss of mass in adults and chicks. Loons, it seems, must work harder each year to capture enough panfish to sustain themselves. This broad decrease in panfish size in Wisconsin lakes, therefore, might explain why masses of chicks and their male parents have declined in recent decades.

But how do we explain why loon chicks gain mass in busy lakes while males lose mass? Remember, when young, chicks can only swallow very small fish (as Linda Grenzer’s photo shows). Even at five weeks, chicks consume fish smaller than those that adults do. This appears to be the key point. Young chicks ingest small fishes — the very size-class of fishes that thrives when larger fishes are removed from the lake by anglers.**

So I have solved a puzzle related to loon feeding ecology. Well, more accurately, I have generated a highly plausible hypothesis that is consistent with the data but will require further testing. For the moment, I am quite satisfied with this outcome. Because I am a scientist, and that is how we roll.

*We have a large sample of males than females, so our analysis of male mass is more robust.

**While we might celebrate that young loon chicks appear to benefit from human overfishing, this news is not cheering. The first five weeks of a chick’s life, after all, is only one critical phase that it must pass through. This year’s hatchlings are 13 to 18 weeks old now. Their diet is now identical to their parents’. Like their parents, 2023 juveniles are finding fewer large bluegills, perch, and crappies. So they too must spend more time foraging than they did 30 years ago. Could the dearth of large panfish on the breeding grounds help us understand why loon survival from the late juvenile stage to adulthood has declined precipitously in the Upper Midwest?

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

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

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

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

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

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

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

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