Loons eat fish. That is the short, honest answer for all five species in the family, and it has been their primary strategy for a remarkably long time. A fossil loon skeleton from the early Oligocene, roughly 30 million years old, was found with fish remains still in its stomach, confirming that the lineage has been a specialized fish hunter since deep in the evolutionary record. But the specifics of what ends up in a loon’s diet, how it gets there, and what can go wrong along the way paint a far richer picture than “they dive, they eat fish.”
The Fish That Fill a Loon’s Diet on Breeding Lakes
On their freshwater breeding grounds, common loons eat a surprisingly diverse range of fish. A study of common loons in Minnesota used DNA analysis of fecal samples and detected 15 different fish species in their diet. Yellow perch, mimic shiner, and cisco were the most prominent prey across the study lakes.1Canadian Journal of Zoology. The relative importance of cisco (Coregonus artedi) in the diets of common loons (Gavia immer) among a set of cisco refuge lakes in Minnesota Yellow perch is a particularly consistent part of the common loon’s diet across many regions; it shows up in feeding studies from the Great Lakes to Atlantic Canada. These are generally small to mid-sized fish, the kind loons can swallow whole underwater or at the surface after a brief bit of manipulation.
Loons are not rigidly attached to one prey species, though. They are opportunistic within their fish-eating niche, targeting whatever is abundant and the right size at a given lake. In some regions, sunfish, minnows, or trout may dominate. The common thread is that loons hunt mid-water and bottom-dwelling fish that can be chased and captured in an underwater pursuit or ambush.
What Loons Eat in Winter
Most loon species migrate to saltwater or coastal environments for winter, and their diet shifts accordingly. Common loons wintering in coastal Louisiana were studied using stable isotope analysis of blood samples, which acts as a chemical diary of what the bird has been eating. The isotopic signatures placed these loons squarely in coastal estuarine habitats, and their readings were statistically indistinguishable from those of brown pelicans sampled in the same area, a species known to feed almost exclusively on fish.2BioOne Complete / Waterbirds. Trophic Position of Wintering Common Loons (Gavia immer) in Barataria Bay, Louisiana Following the Deepwater Horizon Oil Spill In other words, even in coastal marine waters, loons remain dedicated fish predators.
The types of fish shift, however. In marine environments, loons target species like flounder, herring, sea trout, and other nearshore fish. There is also some evidence from stomach content studies that crustaceans and other invertebrates occasionally appear in the winter diet, particularly for common loons feeding in bottom habitats. A comparison of common loons and yellow-billed loons during the non-breeding season found differences in their gastroliths, the small stones loons swallow to help grind food. Common loons had more gastroliths, suggesting they eat harder, spinier bottom-dwelling fish and crustaceans, while yellow-billed loons had fewer, consistent with feeding on softer pelagic fish.3Ornis Norvegica. Trophic ecology of Yellow-billed Loons (Gavia adamsii) and Common Loons (G. immer) during the non-breeding season
Gastroliths and How Loons Process Their Food
The gastrolith detail is worth lingering on because it reveals something about loon digestion that most people do not realize. Loons deliberately swallow small pebbles and stones, which sit in their muscular gizzard and help crush hard prey items like crayfish shells or spiny fish bones. This is not unusual among birds, but in loons the behavior has a dangerous side effect: loons sometimes pick up lead fishing sinkers or fragments of spent ammunition mixed in with the gravel on lake bottoms. Once ingested, lead is absorbed into the bloodstream and can poison the bird.4PubMed Central. Lead exposure biomarkers in the Common Loon Lead poisoning from ingested tackle remains one of the leading causes of adult loon mortality in parts of New England and eastern Canada, and it is entirely a consequence of how loons eat.
How Loons Hunt Underwater
Loons are among the most accomplished diving birds on the planet. Their entire body plan has been sculpted by evolution for underwater pursuit. Their legs are set far back on the body, which makes them famously clumsy on land but extraordinarily efficient in water. When swimming underwater, loons paddle both feet simultaneously in a lateral stroke, at a rate of roughly 1.8 strokes per second. This motion combines ankle flexion with rotation at the knee, generating lift-based propulsion. Loons adjust their speed by changing how long each power stroke lasts, and they use a rhythmic head-bobbing motion that researchers believe helps stabilize their vision while chasing prey.5PubMed. Foot-propelled swimming kinematics and turning strategies in common loons
The anatomy behind this is striking. In highly diving birds like loons, the upper leg bones are tucked tight against the ribcage and hidden beneath abdominal skin, streamlining the body into a torpedo shape. The muscles that power the feet are disproportionately large compared to non-diving birds, particularly the ankle and toe flexors that drive each propulsive stroke. Meanwhile, certain hip and knee muscles have shifted position along the leg bones, giving them longer lever arms that help hold the joints stable under the immense forces of repeated underwater paddling.6PubMed Central. Comparative hindlimb myology of foot-propelled swimming birds
All of this means that a loon chasing a fish underwater is not merely “swimming after it.” It is deploying a pursuit system honed over tens of millions of years, capable of rapid acceleration, tight turns, and sustained chases at depth. The loon’s eyes are adapted for underwater vision, and the bird typically spots its target from the surface by dipping its head below the waterline before committing to a dive.
How Deep and How Long
Common loons can dive deep when they need to. Tracking studies of loons wintering in the Gulf of Mexico and along the southern Atlantic coast recorded dives as deep as 50 meters in the Gulf, 34 meters in the Atlantic Ocean, and 26 meters in southeastern reservoirs.7Journal of Field Ornithology. Dive characteristics of Common Loons wintering in the Gulf of Mexico and off the southern U.S. Atlantic coast Dive duration was strongly linked to depth: deeper dives simply took longer. After particularly deep dives, loons spent more time resting at the surface before going under again, which makes sense given the physical demands of the effort.
Red-throated loons, a smaller species, tend to be shallower divers. One biologging study of a foraging red-throated loon recorded a maximum dive depth of 20 meters, with a mean depth of just 5.4 meters.8Marine Ornithology. First biologging record of a foraging red-throated loon gavia stellata shows shallow and efficient diving in freshwater environments This suggests that most foraging happens well above the maximum depth a loon can reach, with the deep dives reserved for when prey is scarce or concentrated near the bottom. Loons are capable of staying submerged for several minutes in extreme cases, but most foraging dives are shorter, targeted affairs.
Why Clear Water Matters So Much
Loons are visual predators, and that dependence on sight shapes nearly everything about where they can feed effectively. In clearer water, loons can spot prey from the surface before diving, target fish at greater distances, and pursue them deeper. Studies have shown that loons dive deeper and for longer in clear-water lakes, because they can see farther and are willing to chase fish at greater depths. In murky or turbid water, detection range shrinks and foraging becomes less efficient.9Waterbirds. Water Clarity and Diving Behavior in Wintering Common Loons
This connection between water clarity and feeding success has real consequences. A long-term study covering 127 lakes in northern Wisconsin from 1995 to 2021 used satellite imagery to track water clarity over time and compared it to the body condition of loons on those lakes. Chick body mass tracked July water clarity strongly, which makes sense because chicks grow mainly during that month and depend entirely on adults bringing them fish. Adult loons showed a weaker but still detectable relationship. The researchers concluded that both short-term and long-term declines in water clarity are strong predictors of reduced foraging efficiency, and that climate change is driving some of these clarity declines through increased runoff, algal blooms, and dissolved organic matter.10PubMed. Climate change-associated declines in water clarity impair feeding by common loons
This is one of those cases where a seemingly minor environmental change, water getting slightly browner or greener, cascades into a survival problem for a top predator. A loon does not starve because there are no fish; it can starve because it cannot see them.
Feeding Loon Chicks
Loon chicks cannot dive effectively for their first several weeks of life, so they rely entirely on their parents to catch fish and bring them to the surface. The energetic demands are enormous. A study of red-throated loon chicks in the Arctic calculated that if chicks were fed entirely on energy-rich least cisco, parents would need to deliver about 9 fish per day per chick, totaling roughly 438 fish over the entire rearing period. If the diet consisted of lower-energy rainbow smelt instead, that number jumped to about 17 fish per day, or around 842 fish total.11Oxford Academic. Fast and efficient: Postnatal growth and energy expenditure in an Arctic-breeding waterbird, the Red-throated Loon (Gavia stellata) The caloric content of the available prey species directly determines how hard the parents have to work, and in lean years or on lakes with low fish density, that workload can become unsustainable.
Common loon chicks in Wisconsin were estimated to consume between about 160 and 380 grams of fish per day, depending on their age, with older chicks eating substantially more as they approach adult size.12Ornithology. Daily Energy Expenditures of Free-Ranging Common Loon (Gavia immer) Chicks Parents deliver fish one at a time, often quite small fish for young chicks and progressively larger ones as the chick grows. The size selection matters: a fish that is too large for a small chick to swallow can be fatal, and at least one documented chick death on an acidic lake in Ontario was attributed to the chick apparently trying to eat a food item that was too large, a consequence of the lake’s acidity having wiped out the smaller prey that would normally be available.13Canadian Field-Naturalist. Common Loon, Gavia immer, Breeding Success in Relation to Lake pH and Lake Size Over 25 Years
Acid Rain and Disappearing Prey
That connection between lake acidity and loon feeding is a broader problem. Acidification of freshwater lakes, driven by acid rain, systematically removes prey from the bottom of the food web upward. Invertebrates and acid-sensitive fish like minnows disappear first, followed by other species. For loons, this means fewer and larger fish remain. The result is that adults on acidic lakes have to work harder to find food, chick feeding rates drop, and breeding success declines.14PubMed. Impacts of acid rain on aquatic birds
On one pair of large acidic lakes in Ontario studied over 25 years, adult loons made unusually long dives while foraging for chicks, averaging 99 seconds, roughly double the typical foraging dive. The researcher attributed this to a shortage of appropriately sized food caused by the lake’s low pH.13Canadian Field-Naturalist. Common Loon, Gavia immer, Breeding Success in Relation to Lake pH and Lake Size Over 25 Years The loons were not avoiding those lakes, because the lakes were large enough to serve as viable territories. But the food web beneath them had been gutted, forcing the birds into a punishing foraging regime.
Mercury, Lead, and the Risks of Eating at the Top
Because loons sit at the top of the freshwater food chain, they accumulate whatever contaminants are present in the fish below them. Mercury is the most studied of these. Inorganic mercury enters lakes from atmospheric deposition and is converted by bacteria into methylmercury, a far more toxic form that builds up in animal tissues and concentrates at each step of the food chain.15PubMed. Spatial patterns of methylmercury risks to common loons and piscivorous fish in Canada A loon eating yellow perch that have already accumulated mercury from their own prey ends up with mercury levels many times higher than the water itself would suggest.
Research in Kejimkujik National Park in Nova Scotia measured mercury in both yellow perch and common loons and confirmed that mercury biomagnifies through the food web, with loons accumulating concentrations high enough to be reproductively toxic.16Hydrobiologia. Bioaccumulation of mercury in yellow perch (Perca flavescens) and common loons (Gavia immer) in relation to lake chemistry in Atlantic Canada Elevated mercury can impair loon reproduction, reduce egg viability, and alter behavior. Chicks are especially vulnerable because they are accumulating mercury from the first fish they eat, and the Wisconsin study that estimated daily fish intake also calculated that chicks consumed between 16 and 192 micrograms of mercury per day depending on their age and how contaminated the local fish were.12Ornithology. Daily Energy Expenditures of Free-Ranging Common Loon (Gavia immer) Chicks
Lead, as discussed earlier in the context of gastroliths, is the other major dietary toxin. Loons ingest lead fishing sinkers either directly or by eating fish that have swallowed tackle. Unlike mercury, which accumulates slowly, lead poisoning from a single ingested sinker can kill a loon within weeks. Several states and Canadian provinces have banned or restricted the sale of small lead fishing tackle specifically because of the threat to loons and other waterbirds.
An Ancient Hunting Strategy
The piscivorous lifestyle of loons is not a recent specialization. A partial loon skeleton from early Oligocene deposits in Germany, dating to roughly 30 million years ago, was found with preserved stomach contents containing fish remains. This provides direct physical evidence that loons were already specialized fish hunters at that early point in their evolutionary history, likely operating in marine environments.17Journal of Ornithology. A partial skeleton of a new fossil loon (Aves, Gaviiformes) from the early Oligocene of Germany with preserved stomach content The basic body plan, feet set far back, dense heavy bones for buoyancy control, powerful leg muscles, was already in place. What we see today on a Minnesota lake is essentially the same hunting system that was operating in European seas before the first apes appeared.
That deep evolutionary commitment to one way of feeding is both a strength and a vulnerability. Loons are phenomenally good at catching fish. But unlike more generalist waterbirds that can switch to seeds, vegetation, or terrestrial invertebrates when fish are scarce, loons have no backup plan. Their anatomy is so specialized for underwater pursuit that they can barely walk on land, let alone forage there. When the fish disappear, through acidification, contamination, invasive species, or declining water clarity, loons have no fallback. They either move to a different lake or they fail to breed. That rigidity is the trade-off embedded in 30 million years of perfecting a single hunting strategy.