What Do We Know about the Minnesota Loon Population?

I have said a number of times that we do not know how the Minnesota loon population is doing. That blanket statement is misleading. In fact, two well-organized efforts to gather data on Minnesota loons — both run by the DNR and staffed by armies of citizen scientists — have been under way for decades. These massive efforts have given us glimmers of information about the status of loons in the state that I will summarize here.

The Minnesota Loon Monitoring Program

Begun in 1994, the Minnesota Loon Monitoring Program relies upon volunteers to count loons within six regions in the state and produces a summary report every five years. A second project, the Volunteer LoonWatcher Survey, also run by the DNR, began in 1979. Since the MLMP aligns closely with one important goal of our Minnesota work — assessing the status of the Minnesota loon population — I will limit my comments to that survey.

Loon density (number of loons per 100 acres of lake), according to the Minnesota Loon Monitoring Program. (From 2020 report by Krista Larson; Minnesota DNR. Reproduced with permission.)

What Do the MLMP Data Show?

Of course, using volunteers — some without boats and binoculars — to measure loon numbers increases uncertainty in measurement. But despite inevitable fluctuations in measurements from individual regions and years, the MLMP survey seems to be an effective tool for estimating loon populations. So it is reasonable to look at MLMP data and expect to see meaningful patterns.

One of the first patterns you notice in the MLMP survey data is the noise within it. The true density of loons on lakes within each region of Minnesota should not vary much from one year to the next, because loons are long-lived and reproduce at a low rate. Yet the MLMP data show huge fluctuations in loon density from year to year, especially in Itasca, Otter Tail, and Becker Counties. That substantial scatter in the data is important, because it makes interpretation difficult.

Second, despite the noise, it is clear that the density of loons varies greatly between regions. The DNR’s analysis shows only one loon per 100 acres of lake in Kandiyohi County (southwestern part of the state) but three or more loons per 100 acres in Itasca County (northern part). The four other surveyed regions — Becker, Otter Tail, Aitken/Crow Wing, and Cook/Lake — have loon densities that fall between these two extremes. Differences in density across the state are significant, because they help us identify regions of particular importance to a species. With apologies to loon lovers in southern Minnesota, if loons are three times as dense in Itasca County as in Kandiyohi, then Itasca is a much higher conservation priority. (This sort of “triage” perspective is the bread and butter of conservation biology.)

Third, population trends — the aspects of the survey in which we are most interested — are dimly visible within the data, despite year-to-year scatter. According to the DNR analysis, two regions — Cook/Lake and Itasca — have seen small declines in loon density since 1994; one — Otter Tail — has seen a small increase; and the other three regions have experienced no significant change. These conclusions highlight one difficulty we face in assessing the MLMP results. If there are two bits of bad news and one bit of good, what do we conclude about Minnesota’s loon population as a whole?

According to the DNR summary, “MLMP results suggest that Minnesota’s loon population remains stable with an average of 2 loons per 100 acres of lake across all six Index Areas.” It would be pleasing to conclude, as this statement does, that: 1) there is one statewide population pattern and 2) that this overarching pattern could be encapsulated so simply. But the DNR’s summary seems to gloss over some worrisome trends in the data.

What Do the Data Show for the Past Ten Years?

In light of the brevity of the DNR’s summary, it seems worthwhile to take a deeper dive into the MLMP. One oddity of the 2020 DNR report is that it uses 1994 to anchor the trend line. Why 1994? Simply because this was the inaugural year of data collection. But we are most interested in the trend over the past decade, because that gives us a better sense of what is happening now. Over the past ten years, Crow Wing/Aitken and Itasca regions both appear to have suffered sharp declines in loon density; Cook/Lake has declined slightly; and Becker and Otter Tail regions have been more or less stable. Only Kandiyohi County provides good news, as it appears to have greatly increased in loon density in the past decade. But since loon density in Kandiyohi remains far below that in Crow Wing/Aitken and Itasca, the good news from Kandiyohi does not even begin to offset the disappointing findings from Crow Wing/Aitken and Itasca.

What is the Real Take-Home about Minnesota Loons?

We cannot reach any firm conclusion about the status of Minnesota loons based on the Minnesota Loon Monitoring Report. There is simply too much scatter in the data for that. However, careful inspection of recent findings reveals worrisome downward trends in two vital loon population hubs. I take these troubling signs seriously. With support from the National Loon Center in Crosslake, I am accelerating my effort (begun in 2021) to establish a large marked study population in one of these two hubs — Crow Wing County. In the next few years, we will produce estimates of adult survival, reproductive success, and other demographic parameters to construct a new population model for the region. Our fine-grained analysis will indicate whether the downward trend suggested by the MLMP data is real and sustained or whether those of us who wish to conserve loons in Minnesota can breathe a great, collective sigh of relief.