Having read that northern Wisconsin loons are reproducing poorly and returning to the breeding grounds in very low numbers, many of you are probably wondering, “How widespread is the problem?”. Alas, most efforts to mark and monitor loon populations in other parts of the Upper Midwest have been fragmentary, short-term, and limited in scope. Lacking longitudinal data from other studies of marked individuals — the only kind of data that will permit a reliable assessment — we cannot say whether other populations in the Upper Midwest have experienced the same downturn as our study population.
Two points are worth making here. First, the loons that we study in Oneida County do not exist in an isolated pocket. Rather, they are part of a continuous swath of loons that stretches from central Wisconsin to the Great Lakes, and northwards across most of Canada. Moreover, loons exhibit the sex-specific natal dispersal pattern characteristic of birds generally: males settle to breed close to where they were hatched and reared; females disperse much greater distances. So the female breeder on your lake is likely to be tens or even hundreds of miles from where she grew up, like the current female from Two Sisters Lake, who was reared on Crab Lake in Vilas County, or the female on Manson, who grew up on Rock Lake, also in Vilas County — or the female that dispersed over 200 miles east and wound up in Antrim County, Michigan. Hence, the loons in northern Wisconsin are part of a vast interdependent network that stretches to adjacent counties, states, and provinces. Females raised in Oneida County breed in Price County, Wisconsin, Michigan, and even Minnesota, while females from those distant places provide breeding females back to Oneida County. The whole system relies upon a dynamic exchange of females across great distances. In short, the downturn in chick production in northern Wisconsin does not spell trouble merely for local loons, it means fewer females are available to breed in outlying counties and adjacent states.
The second point to make is that the reproductive downturn we are seeing is not a short-term pattern that seems likely to reverse course. The inexorable nature of the decline — the fact that the numbers have been slipping downwards steadily for the past two decades — implies that some relentless, slowly-worsening environmental factor has been at work that reduces the abundance of small fishes in northern lakes and will continue to do so in the coming decades.
I am sorry for all of my gloomy forecasts of late. I know: I have only made it worse here by stating that I think loons might be in trouble throughout the entire Upper Midwest. In truth, I am deeply worried. But I am also thinking of strategies that we might use to learn what is hurting the loons and even possibly turn things around. First, of course, we must understand the problem. If it is food, that is not entirely bad news, because humans have been altering fish populations in myriad ways for hundreds of years. By targeted manipulations of small fish populations in certain lakes that we observe closely, we might be able to pinpoint the cause of loons’ reproductive decline, design a strategy for reversing it, and put loons on the comeback trail.