For a long time, many people, including me, thought Common Loons mated for life—that nesting pairs observed on the same lakes, year after year, were the same individuals returning over and over. This was put to the test by fitting individuals with unique combinations of colored leg bands, which can be seen when loons lift their legs out of the water, something they do fairly often. It became obvious that Common Loons do not mate for life. It turns out that on average they have about four different partners every 10 years. Reality TV stars might be full of drama, but loon reality is even better!
If you feel cheated learning that seemingly pure-and-devoted Common Loons are not always together for life, then perhaps you will find solace in knowing they are faithful during a particular breeding season. DNA analysis shows Common Loon chicks belong to the adults tending the nest. Once a pair is together for the season, there is no “sneaking around next door.” This is unlike many other species of birds where up to half of offspring belong to adults other than those tending the nest.
But divorces and switching partners only break the surface of the loon story. After Common Loons fledge, they migrate south to coastal marine areas where they overwinter for several years—using their salt glands to handle eating ocean fish. They return to the breeding grounds at about three to five years of age and begin searching for a breeding territory. One of the ways they achieve this is through “hostile takeover,” which means they injure or outright kill existing territory owners. Territorial battles between competing Common Loons can be extremely violent and are rarely observed, but over the course of their long lives—some live to see 30—they can accumulate quite a few battle scars.
Remarkably, both males and females have, on average, seven bill punctures in their sternums from being stabbed during past territorial scraps with other loons (one bird had 24 punctures). Oddly enough, males have more reason to fight, sometimes to the death, because for unknown reasons males choose the nest sites, so they have to learn where the safe nest sites are. Such hard-earned knowledge is so important to a male’s lifetime reproductive success that it’s apparently worth dying for.
So much for floating around without a care in the world. Divorce, murder, violence—who knew? And there’s more. The deeper you dive into the secret lives of Common Loons, the stranger and more fascinating it gets. In addition to defending their territories against other loons, they occasionally attack and sometimes kill just about any other water bird, including over a dozen species of ducks, grebes, a Canada Goose (Branta canadensis), and even a Bald Eagle (Haliaeetus leucocephalus). But despite the violence, they are also great communicators and sometimes quietly mingle with others of their kind.
Common Loons have four major call types that have different meanings, literally a “loon language.” They even recognize each other as individuals: Play a recording of an established local male and nothing much happens, but play the call of a stranger from miles away and all males in the neighborhood vigorously respond. And in late summer, Common Loons sometimes form “social gatherings,” where up to dozens of individuals peacefully float around close together, gazing into each other’s eyes. Nobody knows exactly why, although researchers have determined that participants include breeding and non-breeding individuals of many different ages, so there are probably a number of different reasons for attending. There is much more to understand about the natural history of these remarkable birds. Unfortunately, the same can be said for the state of Common Loon populations.
Conservation Issues
Up until the early 2000s, the number of breeding Common Loons was stable or increasing in North America. They appeared to be recovering well from overhunting and acid rain caused mostly by air pollution from burning fossil fuels, which interferes with fish growth, reproduction, and survival, and ultimately results in fewer fish for loons to eat in more acidic lakes. Bird protection laws and changes in public attitudes solved the overhunting issue. There have been large reductions of up to 90% in the pollution that causes acid rain, with acidity declining and fish increasing in some but not all lakes. For a time, it appeared we had little to worry about with regard to the Common Loon.
Then in 2013, scientists at Birds Canada, including me, demonstrated for the first time that Common Loons were producing fewer chicks over time across southern Canada. The pattern in productivity was similar for Common Loons across Ontario, as reported in a 2020 landmark study led by Dr. Kristin Bianchini, in which I participated. These findings were based on tens of thousands of observations made over multiple decades by more than 4,000 volunteers taking part in the Canadian Lakes Loon Survey delivered by Birds Canada, a major success story. Starting in about the early 2000s, productivity declined on average to roughly half that observed in previous decades, and years with especially high productivity (averaging > 0.65 young per pair per year) stopped occurring altogether at that time. Productivity across southern Canada and Ontario has remained at this lower overall rate ever since.
In addition, the total adult Common Loons detected on Breeding Bird Survey routes has declined across southern Canada and Ontario since about the early 2000s. The numbers of breeding adults observed now is as low, or lower, than those seen during the aftermath of acid rain in the 1980s and 1990s. The recent decline in abundance is probably due, at least in part, to relatively low overall productivity since the early 2000s. Although the Breeding Bird Survey is generally considered to poorly reflect population trends in loons, it is still the best source of information on the status of the Common Loon across southern Canada, where trends have high precision and overall reliability is also considered high. See Figure 1 for details.
In 2020, longtime loon researcher Dr. Walter Piper at Chapman University and his assistants shocked the loon world by reporting similar results in northern Wisconsin, where abundance and productivity of Common Loons declined at rates comparable to those observed across southern Canada and Ontario over a similar time period. These researchers also found that chick mass declined across their 100-lake Wisconsin study area over the past several decades, which is disturbing because it’s well known that smaller and lighter fledglings have lower chances of making it to adulthood.
What was going on? So much for stable or increasing populations and having little to worry about. The “big money” question, of course, is why was the decline happening? If overhunting and acid rain are no longer significant issues for these birds, then what is the driver (or drivers) behind these disappearing loons?
The quick answer: We don’t know—yet. In 2021, the Loon and Diver Research Working Group was formed to tackle this very question. This international group of over 30 loon researchers is supported by the Loon Preservation Committee in the U.S. and Birds Canada (Walter Piper and I are co-chairs). Members of the group and others have considered dozens of mechanisms that might explain the decline, including: human disturbance; shoreline development; lake level fluctuations; poisoning by lead fishing tackle; commercial fishing bycatch; marine oil spills; offshore wind turbines; indirect, ongoing effects of acid rain; loon diseases and their complications; emerging contaminants, including flame retardants, PFASs, and pesticides; botulism in the Great Lakes; competition from cormorants; predation by eagles; fish mercury contamination; and climate change. All of these threats negatively influence Common Loons locally or regionally (on a particular lake or across part of a state or province), but only a few appear to be responsible for decline at broad scales, such as across southern Canada.
The ultimate large-scale culprit is likely climate change in at least two ways and probably a third. In the first pathway, increasing water temperatures and more frequent floods and droughts facilitate a series of complex steps that increase the movement of toxic mercury up the food chain and into loons. As top predators, Common Loons are especially susceptible to mercury poisoning because it gets more and more concentrated with every step up the chain. Once inside loons, mercury interferes with their nervous system, makes them lethargic, and causes them to raise fewer chicks to independence. Mercury is released into the atmosphere mainly during the combustion of fossil fuels and then makes its way into lakes in rain and runoff.
The amount of anthropogenic mercury released to the environment has been reduced over the years, but once mercury is out there it’s extremely persistent and can be taken up into food chains whenever conditions become favorable. Multiple datasets now suggest that fish mercury contamination is increasing in some loon breeding areas due to climate change, and the Canadian Lakes Loon Survey across Ontario and other datasets suggest that Common Loon productivity is lower in areas where mercury in loon forage fish is higher. However, assessing the strength of the influence of mercury on productivity is complicated, so the importance of mercury as a threat to breeding Common Loons is somewhat unclear.
In the second pathway, increasing extreme precipitation events wash more and more sediment into loon breeding lakes, where it reduces water clarity over time. As visual underwater predators, the ability of Common Loons to catch fish is impaired by low water clarity, meaning fewer fish are fed to chicks by their parents as water gets murkier. This negative effect of climate change on loons was first reported in 2024 by Walter Piper’s group in northern Wisconsin, where water clarity has declined over the years and chicks are now only ~80% of the mass they were a few decades ago, and where only half the number of young loons now return from overwintering areas to breed as adults compared to 30 years ago. It’s reasonable to assume that reductions in water clarity due to climate change are harming Common Loon populations elsewhere across the species’ breeding range.
We know much less about the third potential pathway. Most Common Loons overwinter in nearshore coastal areas of the Pacific and Atlantic oceans and the Gulf of Mexico. Increasing ocean temperatures and resulting changes in ocean currents might be altering the distribution and abundance of their winter forage fish. Climate-warming-induced changes in food are causing widespread die-offs in various seabirds in some of the areas where loons overwinter, so it seems plausible that something similar might be occurring in loons. Adult Common Loons undergo a simultaneous wing molt during winter, which might make them particularly vulnerable to climate-induced changes in food abundance, since they are unable to fly long distances to find alternative forage during that period. Productivity and abundance appear to be declining more in Common Loon breeding populations that predominately overwinter in the Pacific and the Gulf compared to the Atlantic. This suggests that climate-warming-induced changes in winter forage are most problematic in these areas. There may also be additional climate-change-induced pathways that are threatening Common Loons waiting to be discovered.
Concluding Thoughts
Canada and especially Ontario are critically important breeding areas for an impressive 94% and 37% of the world’s Common Loons, respectively. It’s a huge shock to me and other loon researchers that abundance and productivity are declining in these strongholds. The Common Loon packs a higher emotional punch than many other species because of its popularity among the public and birders alike, so it’s really going to hurt if populations crumble to low levels. Members of the Loon and Diver Research Working Group are working hard to identify the mechanisms behind the declines so we can implement actions to reverse them.
Birds Canada has hired postdoctoral researcher Dr. Mathew Fuirst to analyze long-term Common Loon productivity data from across North America—at last check, an amazing 42,700 recorded breeding attempts across eight U.S. states and southern Canada. From this, we hope to identify new directions for conservation action. Visit https://www.birdscanada.org/loons to learn about actions we can all take now. Currently, the most likely mechanisms explaining the broad decline are linked to climate change, with unique suites of additional mechanisms operating at local and regional scales. Ultimately, we hope to identify regional sets of conservation actions to reverse the decline across the Common Loon’s breeding range to help ensure birders and all outdoor enthusiasts continue to have the opportunity to observe and enjoy these fascinating birds well into the future.
Acknowledgments
The author thanks Kristin Bianchini, Matt Fuirst, Walter Piper, and Ron Tozer for reviewing and commenting on the article. Thanks also to Kathy Jones, who has worked on the Canadian Lakes Loon Survey at Birds Canada for nearly 25 years, and Mark Mallory at Acadia University in Nova Scotia, who has co-written multiple loon papers with the author.
Suggestions for Further Reading
Bianchini K., D. C. Tozer, R. Alvo, S. P. Bhavsar, and M. L. Mallory. 2021. Canadian Lakes Loon Survey: Celebrating 40 Years of Conservation, Research, and Monitoring. Birds Canada, Port Rowan, Ontario, Canada. birdscanada.org/loons.
A non-technical, readable summary of the current state of Common Loons in Canada, plus a list of conservation actions people can take now to help loons.
Evers, D. C. 2014. Conserve the Call: Identifying and Managing Environmental Threats to the Common Loon. Center for Loon Conservation, Biodiversity Research Institute, Portland, Maine, U.S.
A highly readable and well-illustrated overview of threats to Common Loons.
McIntyre, J. W. 1988. The Common Loon: Spirit of Northern Lakes. University of Minnesota Press, Minneapolis, Minnesota, U.S.
Despite being decades old, this remains one of the best and most accessible overviews of Common Loon natural history and ecology.
Nunes, J. 2022. Loons: A Cry from the Mist. Produced by Yap Films, overseen by Blue Ant Media, and distributed by Blue Ant International.
This one-hour TV documentary chronicles a year in the life of the Common Loon and spotlights how the species is under threat.
Paruk, J. D. 2021. Loon Lessons: Uncommon Encounters with the Great Northern Diver. University of Minnesota Press, Minneapolis, Minnesota, U.S.
A delightfully entertaining update on recent loon science, reviewed in this issue of Birding magazine. A must-read for all birders—you won’t be disappointed!
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