The pattern is stark. As you can see from the graph below, loon pairs using artificial nesting platforms have produced a much higher rate of fledged chicks in the Wisconsin Study Area than in the Minnesota Study Area. The pattern was especially dramatic in 2021, when Wisconsin platform pairs reared twice as many chicks per platform as Minnesota pairs. But to compare study areas on the basis of a single year is unwise. Moreover, 2021 was a dreadful year for black flies in north-central Minnesota; most Minnesota nests started in May of last year were abandoned because of the blood-sucking pests. The current year provides a better comparison because flies were not severe in either state. Yet even when we carry out this “apples to apples” comparison by looking only at 2022 data, Wisconsin platforms look far more productive than their Minnesota counterparts. How can this be?
Let’s be very clear on one point. Lake residents in both Minnesota and Wisconsin are moving heaven and earth to help loons. Indeed, folks in both states who float platforms for loons commonly shift them from a first spot to a second and even a third, if doing so keeps nests safe from mammalian egg predators, eagles, waves, and curious humans. It makes no sense to suppose that Wisconsinites are better or more committed platform monitors than Minnesotans.
How then might we explain this curious cross-state disparity in platform success? Perhaps the difference can be attributed to lake size. Since more of the platforms that we study in Minnesota are found on large lakes, the lower rate of fledged chick production from platforms in Minnesota might simply result from higher wind and boat exposure on large lakes, not any state-to-state difference. Lake size, however, cannot explain better platform outcomes in Wisconsin. As the graph below illustrates, the proportion of hatched chicks that actually survives to fledging age is higher in Wisconsin both on large and small lakes. Furthermore, survival of hatchlings is, in general, a bit higher on large than small lakes. So having more large lakes in a sample should increase fledging success, not decrease it.
The new graph does shed some light on the platform pattern. Chicks seem to fledge better in Wisconsin at least in part because more hatched chicks make it to adult size. That is, part of the reason for greater fledging success at Wisconsin platform nests is high chick survival, not necessarily high nest survival.
Could it be that platforms are somewhat overused in Minnesota? In the past two years, 67 of 141 Minnesota nests (48%) but only 43 of 195 Wisconsin nests (22%) have been placed on platforms. Maybe in their zeal to support the state bird, some Minnesotans have lured loons to nest on lakes or parts of lakes that are unsuitable for rearing chicks. At present, this is only one speculative hypothesis to explain the rather low fledged chick production of Minnesota platforms. But it is certainly worthy of investigation. *
Featured photo by Woody Hagge
*I must quickly note that most platforms in Minnesota seem well-placed. Some clearly provide loons an opportunity to nest in locations that lack nesting habitat but where food is plentiful. Such locations are perfect for platforms and must help the loon population produce more chicks than it would otherwise.