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
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
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
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).
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).
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.