Fewer Territories in 2019: Why?

 

After the blur of our early-season census, we are left with a massive whiteboard showing all territories and their current status. Sometimes I find it pleasing to gaze at the board and enjoy what we have accomplished already this year. But this year’s whiteboard reveals a worrisome pattern: a decline in the number of occupied territories in the study area. Those territories occupied in May 2018 but on which we have not yet found a pair this year number twelve: Bullhead, Dorothy, Johnson, Mercer, Minocqua-East, Minocqua-South, Minocqua-Huber Bay, Muskellunge (Oneida County), Nokomis-North, Pier, Swamp, and Wind Pudding-East.

Before we panic at what seems an alarming number of lakes where territories have winked out, let’s consider a few other possibilities. First, we surely missed a pair or two on our original census because they were off foraging on another lake that we had not seen them visit previously. This actually happened on Spider, where I observed no pair on May 4 but where Elaina just today found last year’s pair with a nest. Second, at least a few of these now-vacant lakes will probably support pairs soon — or at least by the end of the season. Loons are notoriously slow to occupy vacant lakes and even, in some cases, to replace dead breeders. Third, it is a normal part of loon population ecology that the species ceases to breed on a few lakes each year. In order for the breeding population to be stable, of course, such losses must be offset by settlement of new pairs on territories that did not exist before. In fact, new territories — vacant in May 2018 but occupied in May 2019 — have popped up on McCormick, Buffalo, and South Blue lakes, owing to settlement by a four year-old male, a six year-old male, and a displaced thirteen year-old male with unbanded females. We will surely find a few more new territories in the coming weeks. But even those projected additions will not be sufficient to offset the apparent number of lost territories this year.

Perhaps we should look more closely at individual lakes to learn what has happened to cause territory loss. Bullhead is a mystery, as neither pair member has been seen; the same is true of Dorothy, Johnson, Mercer, Minocqua-South, Nokomis-North, and Pier. In the remaining five lost territories, though, we can see that male has simply not returned. In each of the five, the female has come back from migration, settled on the territory, and has found no male with which to breed. Broadly, the pattern is not surprising; it is what we expect in an adult breeding population that is female-biased. As I have noted many times in this blog, males senesce and die much earlier than females, and this causes a persistent dearth of the former and a surplus of the latter among adults wishing to breed. Could we be seeing the leading edge of some die-off among male loons that will push the population into decline? It is possible but not likely.

Let’s try to match up this year’s apparent decline in territories with likely causes. While scientists project the long-term withdrawal of loons from northern Wisconsin owing to climate change, and loons each year face slightly greater human impacts from angling, boating, and shoreline development, negative impacts of these factors should occur gradually, not in one or two years. So the net loss of six or seven territories we have seen this spring (after accounting for the gain of three territories and a few pairs missed) seems too great to be a harbinger of a larger population decline. Thus, I will tentatively conclude that this year’s losses are a blip — a short-term downward bounce in number of territories caused by chance and/or a temporary downward shift in breeding success.

Could the decline come back to loons’ nemesis, the black fly Simulium annulus? Possibly. Looking back across years, in fact, we see the dreadful breeding year of 2014, caused largely by the prolonged explosion of black flies in that year. Male chicks that would normally have hatched in 2014 would be at their crucial settlement age of five right now. So we might speculate that the “missing” five year-old males — males that would have slid into many of the territory openings for male breeders that are evident this year — are the cause of this year’s decline in active territories.

If my speculation is correct, we should expect a sharp increase in the rate of territory settlement (i.e. a net increase in territories within the study area) in 2020 and 2021, because those years fall five years after the productive breeding years of 2015 and 2016. We shall see. In the meantime, perhaps I can dispel some of the negative feeling caused by all this talk of territory decline by showing you this year’s nest on Hanson Lake. Though she took a short incubation break two days ago, which allowed me to snap this photo, the female soon jumped back on the eggs. She seems determined to do her part to help reverse the current downward trend.