adult survival

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It is easy to forget that research on the loons of Crow Wing County, Minnesota has been underway for over a decade. To be sure, this work has been spotty. From 2011 to 2014, Kevin Kenow and his USGS team placed geotags on a few dozen adults on four medium-sized lakes in the county. From 2015 through 2017, he shifted his efforts to the Whitefish Chain, where he captured 68 individuals, including 36 territorial adults.

Kevin’s goal was to determine migration and wintering routes of Minnesota loons, which he did after recovering many of the geotags placed on loons’ legs. Although his study was short-term, Kevin’s loons lived on. Each summer and fall they nested and reared young, foraged to build up their reserves for migration, staged on the Great Lakes, and made long overland flights to the Gulf of Mexico. Each spring they molted their feathers and made return trips back to the Whitefish Chain to restart the cycle.

When our Minnesota Loon Project began in 2021, we relocated many of the loons Kevin had banded 4 to 6 years before. We were quite thorough — obsessive, even — in our efforts to do so. At the time I regarded the USGS banding effort as fortunate for us, since it gave us a head start in our efforts to mark all territorial pairs on the Chain.

But Kevin’s marked loons have not merely reduced our loon marking workload. Kevin’s birds are charter members of the Minnesota Loon Project. The survival of these inaugural adults since the years Kevin’s team marked them provides our first multi-year snapshot of adult loon survival in Crow Wing County.

The data provide an unconventional snapshot. When one conducts a mark-recapture study, one normally searches diligently for all marked individuals during the years immediately after marking. This strategy produces data on annual return rate, which provides an estimate of annual survival. But we lack data on return rates from 2018, 2019, and 2020. So we must do the best we can to extract information from Kevin’s birds despite multiple years with missing data.

Fortunately, this is not rocket science. If “r” is the annual rate of return, then r2 is the probability of being on territory two years after banding, r3 is the probability of still being present three years later, and so on. Recognizing this, we can easily project how many of the 36 territorial adults that Kevin banded in 2015, 2016, and 2017 should have still been on territory in 2021. If annual rate of return were 90%, we would have expected to see 20.5 of Kevin’s loons in 2021. At 85%, the expectation is 15.1. If the annual rate of return were 80%, then we should have seen 11.0 loons. In fact, our exhaustive search turned up 13 of Kevin’s loons. So this places our rough estimate of annual loon survival for the Whitefish Chain at 82.5%.

To my knowledge, ours is is the first long-term estimate of adult loon survival from Minnesota based on a marked population. This is rather shocking; loons are well studied in the U.S., have been marked in at least ten states….and are the state bird, for goodness sake! In any event, this preliminary estimate gives us a ballpark figure for adult survival that we can compare with more robust estimates from other states.

A figure of 82.5% for Minnesota survival is lower than we would like. This long-term number based on Kevin’s birds, though, is slightly higher than the separate return rate of 51 Crow Wing County adults we banded in 2021 and looked hard for in 2022: 80%. For comparison, we have robust estimates of survival from a study done 15 years ago that included data from New England (88%; data from 1994-2001) and Wisconsin (87%; data from 1991-2001). We can also compare with longer-term survival rates from our well-known Wisconsin Study Area, which, again, were 86 to 87% for both males and females. In short, early data from the Minnesota Study Area indicate a percentage of adult survival in the low 80s, which is below the rates in the upper 80s we have grown accustomed to seeing in Wisconsin and New England.

The data from Minnesota so far only provide a glimmer about the loon population in Crow Wing County. However, these low survival estimates do bring to mind a worrisome downward trend in loon numbers for the region that can be seen in the 2021 Minnesota Loon Monitoring Report. But, really, it is early days. We need more data. Furthermore, the status of a loon population is not dependent upon adult survival alone. Low adult survival can be offset by a high reproductive rate. So we will have to spend at least two more years tracking return rates of marked loons and measuring breeding success before we can pull them together into a model that will tell us (preliminarily) how Crow Wing loons are doing. Still, if I am being honest, I wish the survival numbers were a bit higher.

Thanks to Katy Dahl, who photographed the Cross Lake-Arrowhead Point loon pair after we banded them in 2021. The male in the foreground with his bands out of water was spotted a few days ago just north of Minneapolis.

If, like us, you are concerned about the persistence of loons in Minnesota, consider a donation to support our field efforts. We run a lean program. Funds donated to the Loon Project do not pay overhead, administrative costs, or salaries for staff or senior personnel. They pay only field costs like: 1) stipends to keep student field workers alive, 2) travel costs to, from, and within our study areas, and 3) supply costs such as for colored leg bands and canoe paddles. Thanks!

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Although I am stuck in California teaching for another few weeks, others have been hitting the lakes. Based on thirty or so lakes from which I have had reports — mostly Linda’s work, but also a few lake residents — we have an early read on the return rate of adult loons from last year.

Let me explain. April and May are exciting months for the Loon Project, because we hustle from lake to lake to see which of our banded adults have returned and which have not. In a typical year, the vast majority of our study animals have managed to survive the winter, navigate fall and spring migrations successfully, and take possession of the territories they occupied the previous year. The figure hovers around 80 to 90%. A high rate of annual survival is vital to our population. The low reproductive rate of loons is sufficient to sustain the population only because most adults survive each year.

During years when I am able to steal away from my pedagogical commitments, I find these “censusing” visits oddly thrilling. On my first lake visit of the year, I fancy that the male and female both pause for a moment, wheel in my direction, and think, “Where’s he been?” This might not be pure imagination; after all, I have been observing most of these individuals for a decade or more.

Last year seemed an exception to the typical high rate of adult return. Fully a quarter of the adults that we left behind safe and sound in the early fall of 2019 failed to come back in the spring of 2020. While some of these adults had merely been evicted from their territories, most were dead. Coupled with our recent finding of population decline in northern Wisconsin, the low 2020 return rate weighed on my mind last spring. On the other hand, return rates bounce around. So I tried to avoid jumping to the conclusion that adult survival was going downhill.

Recent reports from the lakes this year have placed concerns about adult survival front and center again. Each of Linda’s almost-daily census visits seems to bring fresh news about a missing adult or breeding pair. Early on, Linda reported that a new male had replaced the long-time resident male on Manson. Okay, that happens, I thought. Nothing to worry about. The Deer Lake female, freshly marked in 2020, also turned up missing. A Halfmoon Lake visit brought no better news; the 2020 female from there was gone as well. Hildebrandt and Julia, always occupied by pairs and frequent chick producers, were vacant. Linda’s trip to Nokomis Lake was most devastating of all. Towards the eastern end of the lake, both members of a long-term pair with a consistent record for rearing chicks were AWOL. And Linda turned up only one unmarked loon from the entire 2200-acre Nokomis flowage, which usually supports three breeding pairs.

He did not know it, but Al from West Horsehead produced the straw that broke the camel’s back. His report from this morning that the 8-year-old West Horsehead male had been replaced by a 6-year-old male from neighboring East Horsehead hit me especially hard. The sample had become large enough that I could not longer deny the pattern. Looking at the number of returns right now, we have found only 21 of 31 adult loons that should be on territory. Now, we will track down some of these missing birds. A few will be alive and breeding on a seldom-visited lake near their old territory. And that will give us a momentary lift. But an adult return percentage in the low-70s, as we are seeing for the second straight year, will not sustain our breeding population for long.