The Loon Population is Declining in Wisconsin

I have put off writing those words from some weeks now. The patterns in my data were clear; every measure of breeding success was pointing downwards. If you have been following the blog, you might recall that number of chicks per pair has fallen sharply since I began studying loons in 1993. Although I had not reported it yet, loss of chicks after hatching has also increased significantly since I began my work. That is, many pairs hatch two young but lose one or both of them nowadays. Furthermore, even chicks that survive to five weeks of age are now in poorer condition (as measured by body mass) than in 1998 or 2006 or 2013. In short, breeding pairs in northern Wisconsin now raise fewer and less robust chicks than they did 25 years ago.

It was a combination of scientific caution and denial that caused me to delay describing the implication of these trends — and others that I have detected more recently. As a scientist, I am used to finding one pattern, drawing a tentative conclusion based on that single finding, and then being proven wrong by the next finding. Having had an initial conclusion reversed many times during my career, I was unwilling to sound the alarm without a stronger, more consistent set of findings. But the data I now have are consistent and come from multiple independent analyses. Furthermore, the data are fundamentally simple. My conclusions are not derived from complex models based on measurements of invisible particles detected by finicky high-tech devices. We are just counting and weighing loons here!

Still, you might wonder why my earlier caution has turned so suddenly to alarm. Let me explain. I followed a simple line of reasoning. If loon pairs are producing fewer and weaker chicks, then fewer chicks must be able to migrate to the wintering ground. And if fewer juveniles make it to Florida, then fewer should survive long enough to return to northern Wisconsin (which happens at 2 to 4 years of age) and look for a breeding territory of their own. So, declining chick production should result in a reduced population of nonbreeders (or floaters) in our study area, which are young adults looking to settle on their first territory with a mate. Since we mark chicks and obsessively reobserve them as young adults, we can test the idea that lower chick production has resulted in fewer floaters. The results are stark. After one adjusts for number of observer-hours spent looking for floaters each year, a dramatic pattern

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emerges. The population of floaters has plummeted. Look at the scale of the graph carefully. This is not a small decline. Between 1998 and 2006 the number of floaters seen per observer hour fell from 0.020 to about 0.010. From 2006 to 2015, that number has fallen still further — to about 0.006. In other words, we have seen roughly 1/3 as many floaters from the 2015 year class (which are 4 years old now) as we saw from the 1998 year class. In terms of percentages, we reobserved about 45% of all chicks banded in 1998 and 1999 much later as adults; we see only about 14% of banded chicks as adults these days.

So what? Floaters are nonbreeders. They contribute no offspring to the population. Do they really matter? There are not even enough territories to hold them. They are surplus individuals, in a sense. You might view the presence of any floaters at all as a positive sign that the population is bursting at the seams. Yes….but floaters are also the future. That is, floaters are loons not yet old enough to claim territories but waiting to fill in for dead breeders (or evict them forcefully). Without floaters, a breeding population cannot sustain itself, because, inevitably, breeders die and must be replaced.

Even now that I have a set of strongly suggestive patterns, I cannot be absolutely certain that loons are in trouble in Wisconsin. Perhaps the steep decline in floater survival simply means that the weak floaters now die off long before settlement, leaving the strong ones to replace dead breeders. Perhaps something very odd is happening in the north-central part of the state (more intense human recreational activity?) that is not happening elsewhere. So, let’s keep fingers crossed that this is a fluke of some kind. But I am convinced that we are seeing a worrisome pattern that is unlikely to be confined to my study area.

What does the future look like, if the floater population truly is declining markedly, as I suggest? Despite the strength of the pattern, the short-term effects might be subtle, because loons are long-lived. Imagine that families in your town or city suddenly began to have only 1/2 or 1/3 as many children as in years past. When would you notice a marked change in the community? You might — if you were very sharp — notice fewer large families at local playgrounds. Sometime later, you might notice the closing of a school here or there. But decades would pass before you would discern a loss of occupied homes in your neighborhood (presuming that kids in your town grow up to inhabit local homes).

So it will be with loons. On some lakes, the death of a pair member — especially a male, because males are in short supply — will leave a vacancy that is not filled by a floater. Some lakes that always supported a breeding pair will lose that pair and instead only see loons that come and go to forage. Large lakes with six perennial breeding pairs will, over time, see some territories “wink out”, leaving only three or four. In a few decades perhaps, even those territories will vanish. If I am correct, and unless this long-term pattern in the survival of young loons reverses itself, loons will ultimately disappear altogether from northern Wisconsin.