Common loons nest almost exclusively on the shores of freshwater lakes, building simple mounds of vegetation right at the water’s edge. Their nesting sites are dictated by a body built for diving rather than walking, and the lakes they choose tend to be clear, relatively undisturbed, and stocked with small fish. But the details of where and how loons set up their nests reveal a surprisingly particular bird, one whose reproductive success depends on water levels, human behavior, predator pressure, and even the clarity of the lake itself.
Why Loons Nest at the Water’s Edge
If you have ever seen a loon on land, you know it looks uncomfortable. Their legs are set far back on their bodies, close to the tail, which makes them phenomenal underwater swimmers but terrible walkers. A loon trying to move across solid ground does an awkward belly-shuffle that leaves it vulnerable to predators and exhausts it quickly. This anatomical trade-off means loons cannot nest inland the way ducks or geese sometimes do. They need to be able to slide directly from the nest into the water, so virtually every loon nest sits within a body length of the shoreline.
The preferred spots are small islands, marshy hummocks, floating bog mats, or sheltered peninsulas where the bird can approach and leave the nest from the water without hauling itself overland. Islands are especially prized because they provide a buffer against land-based predators like raccoons, foxes, and mink. On lakes without islands, loons settle for shoreline spots that offer good sightlines and quick water access, but these nests tend to be more vulnerable.
What a Loon Nest Actually Looks Like
Loon nests are not elaborate structures. They are low, roughly circular piles of dead reeds, grasses, sedges, and other aquatic vegetation scraped together into a mound. The birds sometimes add mud to the base. The whole thing typically sits just a few inches above the waterline, which makes it effective for a bird that needs to slide on and off but dangerously susceptible to flooding if water levels rise even modestly.
Both the male and female contribute to building the nest, and construction can happen quickly, sometimes in just a day or two. A pair may build more than one nest platform before the female lays eggs, apparently testing sites before committing. The finished nest holds one or two eggs, olive-brown and lightly speckled, which blend well with the surrounding vegetation. Two eggs is the norm; clutches of three are rare.
Incubation lasts roughly four weeks, with both parents sharing the duty. The sitting bird presses low against the nest, its dark back blending into the marsh. Despite this camouflage, the incubating loon is alert and will slip off the nest into the water at the first sign of danger, sometimes covering the eggs with vegetation before leaving.
Choosing the Right Lake
Not every lake attracts nesting loons. Across their breeding range in the northern United States and Canada, loons tend to pick lakes that are clear enough for them to hunt visually, large enough to support a territory, and deep enough to hold a healthy population of small fish. A lake under about eight hectares (roughly 20 acres) is usually too small to support a breeding pair, though there are exceptions on especially productive waters.
Water clarity matters for a specific reason: loons are visual predators. They locate fish by sight while diving, so murky water directly limits their ability to feed themselves and their chicks. A long-term study of 127 lakes in northern Wisconsin found that chick body mass tracked strongly with July water clarity, the month when chicks are growing most rapidly. Adult body condition also correlated with clarity, though less dramatically. The researchers concluded that declining water clarity, driven partly by climate change, can cascade through freshwater ecosystems by reducing the foraging efficiency of top aquatic predators like loons.1Ecology. Climate change-associated declines in water clarity impair feeding by common loons
Loons are also remarkably loyal to their chosen lakes. Adults often return to the same territory year after year, sometimes for a decade or more, and they frequently reuse the same nest site or build a new nest within a short distance of the previous year’s location. This site fidelity means that a lake that loses its suitability, whether through development, pollution, or habitat degradation, may lose its loons permanently rather than seeing the birds simply shift to a different spot on the same lake.
How Human Activity Shapes Where Loons Nest
Loons are sensitive to disturbance during the breeding season. Boat traffic, shoreline development, docks, and foot trails all influence where a pair decides to place its nest. A study that used classification models to analyze nest site selection found that the distance to and density of human disturbance features could correctly distinguish actual loon nest locations from random shoreline points about 75 to 78 percent of the time. In other words, loons are clearly choosing sites that put distance between themselves and human activity.2The Journal of Wildlife Management. Effects of spatial disturbance on common loon nest site selection and territory success
Interestingly, the same study found no clear statistical link between disturbance variables and actual breeding success, possibly because the breeding seasons in question had unusually low success rates overall. The researchers interpreted this as evidence that loons manage disturbance by avoiding it: they pick nest sites far enough from human activity that the disturbance never becomes a problem. The concern, then, is not that loons nest near people and fail, but that increasing development pushes them into fewer and fewer suitable sites, eventually leaving no good options on a given lake.2The Journal of Wildlife Management. Effects of spatial disturbance on common loon nest site selection and territory success
This has practical implications for lakefront property owners and recreation planners. Maintaining undeveloped buffer zones on at least some portions of a lake gives loons the separation they need. Voluntary “loon nesting zones” that restrict boat speed and approach distance during May through July can help as well, particularly on busy recreational lakes where quiet shoreline is at a premium.
The Two Main Killers of Loon Nests
The majority of loon nest failures come down to two causes: predation and flooding. These pressures are so consistent across the species’ range that most conservation efforts are built around mitigating one or both.
Mammalian predators are the primary threat. Raccoons are the most common nest raiders in much of the loon’s range, but mink, foxes, skunks, and even black bears will take eggs opportunistically. Avian predators like eagles, crows, and gulls also destroy nests, though less frequently. The loon’s strategy of nesting on islands reduces mammalian predation significantly, but on lakes without natural islands, nests on the mainland shoreline face much steeper odds.
Flooding is the other major cause of failure. Because loon nests sit just above the waterline, even a modest rise in lake level can swamp the eggs. This is a natural risk on any lake, but it becomes a much bigger problem on reservoirs and regulated waterways where water levels can fluctuate suddenly due to dam operations, heavy rain events, or spring snowmelt. A nest that was safely above water on Monday can be underwater by Wednesday if a dam upstream releases water or a storm dumps several inches of rain.
Artificial Nesting Rafts
One of the most successful hands-on conservation tools for loons is the artificial nesting raft, a small floating platform anchored in a loon’s territory and topped with nesting material. These rafts address both major causes of nest failure at once. Because they float, they rise and fall with the water level, eliminating the flooding problem. And because they sit offshore, they are much harder for mammalian predators to reach than a shoreline nest.3Northeastern Naturalist. Guidelines for Constructing and Deploying Common Loon Nesting Rafts
The results have been encouraging. A study modeling the relationship between raft use and nesting success estimated that each successive increase in how consistently a pair used a raft was associated with roughly a 9 percent improvement in nesting success.4The Journal of Wildlife Management. Reproductive Advantages for Common Loons Using Rafts That may sound modest, but in a species that lays only one or two eggs per year and does not breed until age five or six, even a small bump in per-nest success compounds meaningfully over time.
Raft design matters. Effective rafts are typically about three to four feet square, built from a frame of untreated lumber with a base of hardware cloth or chicken wire, topped with a layer of sod and aquatic vegetation. They are anchored loosely enough to ride waves but tightly enough to stay within the territory. Poorly designed or poorly placed rafts get ignored. Loons will not adopt a raft that feels exposed or is positioned in the wrong part of their territory, so placement requires some knowledge of where the pair has historically nested or displayed territorial behavior.
Mercury and Reproductive Health
Beyond the immediate threats at the nest, loons face a subtler and more pervasive challenge: mercury contamination. Loons sit at the top of freshwater food chains, which means they accumulate methylmercury from the fish they eat throughout their lives. This bioaccumulation makes them one of the most mercury-exposed bird species in North America and has measurable effects on their ability to reproduce.
A study of loons in New York’s Adirondack Park found that females in the highest mercury exposure category fledged about 32 percent fewer chicks per year than females in the lowest category. For males, the reduction was even more dramatic, around 56 percent fewer chicks fledged.5Waterbirds. The Impact of Mercury Exposure on the Common Loon (Gavia immer) Population in the Adirondack Park, New York, USA The population modeling from that study indicated that highly contaminated loons had a growth rate barely above replacement level, while loons with low mercury burdens had a much healthier growth rate.5Waterbirds. The Impact of Mercury Exposure on the Common Loon (Gavia immer) Population in the Adirondack Park, New York, USA
Mercury does not destroy the nest directly the way a raccoon or a flood does, but it suppresses the entire reproductive pipeline. Contaminated adults may lay fewer eggs, incubate less attentively, or produce chicks that are less vigorous and less likely to survive. Because mercury levels in a given lake depend on factors like acid deposition, wetland drainage, and regional industrial history, some lakes that look perfectly suitable for loons based on size, clarity, and disturbance levels are actually population sinks where breeding pairs consistently underperform.
Territorial Behavior Around the Nest
Loon territories are large relative to the bird’s body size. A breeding pair typically claims an entire small lake or a substantial bay of a larger one, and they defend it aggressively from other loons. Territorial battles between males are intense and occasionally fatal. The iconic wailing and yodeling calls that loons are famous for serve partly as territorial advertisements, warning neighboring pairs and unpaired floaters to keep their distance.
Territory size depends on lake productivity. On a lake teeming with fish, a pair may hold a relatively compact territory because food is concentrated. On an oligotrophic lake with fewer fish, the territory expands. This means that the biggest, clearest, most “loon-appropriate” lakes are not necessarily the ones with the most breeding pairs. A smaller, murkier but nutrient-rich lake sometimes supports more pairs than a pristine lake three times its size, as long as water clarity stays above the threshold where the birds can still hunt effectively.
Young nonbreeding loons, sometimes called floaters, spend years prospecting on lakes occupied by established pairs, occasionally challenging territory holders. These intrusions spike during the nesting period and can cause real disruption. A sitting loon that leaves the nest to engage an intruder leaves the eggs exposed to cooling, sun, and predators. Prolonged territorial disputes during incubation can indirectly cause nest failure even without any direct contact with the eggs.
Timing and Geography of the Nesting Season
Across their North American breeding range, loons begin nesting as soon as the ice clears from their chosen lake, which can be as early as late April in the southern part of the range (the northern United States) or as late as mid-June in subarctic Canada. The window is tight: loons need roughly four weeks to incubate their eggs and another 11 to 12 weeks before chicks are ready to fly. Since they must migrate south before lakes freeze in autumn, a late start can mean trouble.
The breeding range of the common loon stretches from the northern tier of the United States, including states like Minnesota, Wisconsin, Michigan, Maine, New Hampshire, and Vermont, through most of Canada and into Alaska. Isolated populations also breed in Iceland and formerly nested in parts of Greenland and Scotland. Within this range, loons are generally found on lakes surrounded by boreal or mixed forest, though they also nest on tundra lakes in the far north.
If a nest fails early enough in the season, a pair will often renest, sometimes on the same site and sometimes at an alternate location within their territory. Renesting attempts typically produce only one egg rather than two, and they face tighter time pressure because the chick has fewer weeks to grow before migration. Late-hatched chicks are lighter and less developed at fledging, which reduces their chances of surviving the first winter.
An Ancient Preference for Northern Waters
The loon’s association with cold northern lakes is not a recent habit. Fossil evidence suggests that the family Gaviidae has been shifting its breeding range northward for millions of years. Early loon species in the Miocene lived in warm-climate regions of southern Europe. As global temperatures cooled during the Late Miocene, loons largely disappeared from the fossil record in lower latitudes, a pattern researchers interpret as a shift in their nesting range toward higher latitudes.6Paleontological Journal. A loon (Aves, Gaviiformes) in the Upper Miocene of Mongolia
A Late Miocene loon fossil discovered in Mongolia falls within the modern breeding range of the Arctic loon, supporting the idea that loons were already nesting at high latitudes by that time.6Paleontological Journal. A loon (Aves, Gaviiformes) in the Upper Miocene of Mongolia The implication is that loons are not simply generalist waterbirds that happen to live in the north. They are specialized for it, shaped by millions of years of co-evolution with cold, clear, fish-rich lakes. That specialization is what makes them so sensitive to changes in lake clarity, mercury levels, and habitat disturbance: they are adapted to a particular type of freshwater ecosystem, and when that ecosystem degrades, loons have limited ability to make do with something different.
This deep evolutionary history also helps explain why loons are absent from so many seemingly suitable lakes in temperate regions. The lakes may have the right size and fish populations, but they lack the low-disturbance shorelines, stable water levels, and clear water that loons evolved to depend on. Restoring loon populations to lakes where they have been lost is not simply a matter of protecting shoreline. It requires maintaining the whole package of conditions that these ancient diving birds need to nest and raise their young.