Minnesota Has a Higher Loon Density than Wisconsin

As we motored around the Whitefish Chain in late May, the thought crossed my mind. As I looked over Katy and Jordana’s field notes from Minnesota in June, it occurred to me again. And by the time we banded five tightly-packed loon pairs on Ossawinnamakee Lake in a single night in July, I had become utterly convinced: there are more loons per lake, on average, in our new Minnesota study area than in our long-time study area in northern Wisconsin.

Of course, it is one thing to have a gut feeling that a natural pattern is out there and quite another to demonstrate that the pattern is real. Indeed, having enough self-discipline to wait and test a hypothesis instead of blurting it out and selling it as fact is what separates science from…..well, something less than science.

So I examined our data from both study areas. If there are more breeding loons per acre of lake in Minnesota, the difference should be evident from a statistical analysis. I looked at all lakes between 165 and 740 acres in both study areas for which we have reliable data, divided lake area by number of breeding pairs, and ran a test. The result: In our Wisconsin study area, a loon pair’s average territory size is 282 acres, while in Minnesota an average pair occupies a territory of only 180 acres. This is a highly “significant” statistical difference, which means that the huge disparity seems to represent a real pattern, not just a chance result.

What does it mean? Having done the easy part — finding a difference — we are now faced with the thorny task of explaining it. Innumerable hypotheses leap to mind. (1) The slightly different climate of central Minnesota might support a denser breeding population than the northern Wisconsin climate. (2) Lake chemistry might be more favorable in Minnesota and thus explain the difference in density. (3) Minnesotans are somewhat more apt to put out artificial nesting platforms for loons (21 of 105 territories; 20%) than are Wisconsinites (23 of 216; 11%), which might support more loon pairs. (4) Predators might be more abundant in Wisconsin and/or food scarcer. (5) Minnesota lakes might be more convoluted in shape and thus contain more natural boundaries that allow coexistence of more loon pairs on the same area of water. (However, a quick glance at lake shapes suggests the opposite — that our Minnesota lakes are more round.) (6) Human harassment of loons might be more intense in Wisconsin. (I have not noticed any such pattern, however.) In short, we have lots of questions and no answers, at present!

Now, it is important to take a step back. We have under active investigation only about 10% of all Wisconsin loons. In this beginning phase of Minnesota research, we have only 1.4% of all Minnesota loons in our study. (The nice loon photo by Katy Dahl above shows only one of about 14,000 loons in the North Star State.) So our two study areas — especially the one in Minnesota — capture only a snapshot of a small subsample of each population. The overall statewide loon densities might be quite different. Still, the two study populations are similar in numerous ways, including latitude, degree of human lake usage, and deep affection of almost all lake residents for the species. The ability of Minnesota loons to live “shoulder-to-shoulder” tells us something profound, I think, if we can only ferret it out.