body condition

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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.