Dozens of bird species across at least a dozen families regularly swim beneath the surface of oceans, lakes, and rivers, almost always to catch food. Penguins, auks, cormorants, loons, grebes, diving ducks, shearwaters, and even a handful of songbirds called dippers all propel themselves underwater using one of two basic strategies: flapping their wings as though flying through the water, or kicking their feet. The split between wing-propelled and foot-propelled diving has shaped everything from bone density to feather microstructure, and it turns out to be one of the most dramatic examples of convergent evolution in the bird world.
Wing-Propelled Divers
Penguins are the most recognized underwater swimmers, and for good reason. They generate thrust on both the upstroke and the downstroke of their wings, a pattern more like how fish use their fins than how most birds fly through air.1Journal of Zoology. Kinematics of swimming of penguins at the Detroit Zoo This double-stroke power delivery makes them remarkably efficient. Emperor, King, and Adélie penguins can even insert unpowered gliding phases between wingbeats, coasting through the water to conserve energy over a broad range of speeds.
Auks and their relatives, collectively known as alcids, include puffins, murres, and razorbills. They also fly underwater using their wings, but with a twist. At shallow depths where buoyancy pushes them back toward the surface, alcids generate forward thrust on both the upstroke and downstroke. Below about ten meters, as water pressure compresses their trapped air and buoyancy drops, they shift to producing thrust mainly on the downstroke.2Journal of Experimental Biology. Swim speeds and stroke patterns in wing-propelled divers: a comparison among alcids and a penguin Penguins, by contrast, keep powering through both strokes regardless of depth, likely because their wings have been reshaped far more thoroughly for underwater use.
A less expected member of this group is the dipper, a small songbird found along fast-flowing streams on several continents. Out of more than 6,000 species of perching birds, only the five species of dippers use their wings to swim underwater. They share some of the same adaptations as penguins and alcids, including changes to flight musculature, modified feathers, and increased body fat, but their skeletal modifications are minimal compared to full-time aquatic birds.3PubMed Central. Convergent evolution in dippers (Aves, Cinclidae): The only wing-propelled diving songbirds Dippers are shallow divers, rarely going deeper than a meter or two, and they walk along riverbeds as much as they swim. Still, the fact that they evolved wing-propelled diving independently from penguins and alcids says something about how strong the selective pressure is when food sits on the bottom of a stream.
Shearwaters round out the wing-propelled group. These oceanic birds spend most of their lives on the wing, but several species routinely plunge beneath the surface to chase fish, using their long narrow wings to propel themselves to respectable depths.
Foot-Propelled Divers
Cormorants swim underwater by paddling with both feet at the same time, in a gait that alternates between powerful kicks and long glides. Because they retain the light, air-filled body of a flying bird, they are highly buoyant near the surface and must angle their body upward relative to their direction of travel to counteract the tendency to float back up.4PubMed. Submerged swimming of the great cormorant Phalacrocorax carbo sinensis is a variant of the burst-and-glide gait This angled posture creates extra drag, but it is the price of keeping wings light enough to fly.
Loons and grebes are also foot-propelled, and they represent one of the oldest lineages of diving birds. The body plan for foot-propelled diving appeared in the Cretaceous period, in an extinct group called the Hesperornithiformes, toothed seabirds that had already lost the ability to fly. Today, loons, grebes, cormorants, and several duck species have all independently converged on a similar approach.5Palaeogeography, Palaeoclimatology, Palaeoecology. Morphometric comparison of the Hesperornithiformes and modern diving birds
Among ducks, the difference between a dabbler and a diver shows up clearly in how they handle buoyancy. In a comparison of mandarin ducks (dabblers that rarely dive) and pochards (committed diving ducks), the pochards tilted their bodies head-down at a pitch angle of about 16 degrees to overcome buoyancy, while mandarins stayed nearly level. The pochards also used simultaneous foot strokes rather than alternating kicks, and each foot generated roughly twice the vertical force of a mandarin’s foot, enough to counteract about 65 percent of their buoyancy per stroke compared to just 21 percent for the dabblers.6The Journal of Experimental Biology. Underwater paddling kinematics and hydrodynamics in a surface swimming duck versus a diving duck For a dabbling duck forced underwater, staying down takes enormous effort. For a pochard, the body has already been tuned for it.
Why They Go Under
The overwhelming reason birds swim underwater is to eat. Fish, squid, crustaceans, aquatic insects, and bottom-dwelling invertebrates all live below the surface, and a bird that can reach them has access to food that surface-feeders cannot touch. King penguins demonstrate how far this can go. Tracked birds made deep dives exceeding 100 meters during the day and shallow dives at night, following the vertical migration of their preferred prey, small lanternfish. The deepest recorded dive was 304 meters, and the longest lasted nearly eight minutes.7Ecological Monographs. Diving Behavior and Energetics During Foraging Cycles in King Penguins Swim speeds averaged about 2.1 meters per second, and the birds adjusted their descent rate depending on target depth, dropping faster on deep dives than on shallow ones.
The bimodal pattern in King penguin dives is telling. Most dives were either shallow (under 40 meters) or deep (over 100 meters), with few in between. This matches the known vertical distribution of their prey, which cluster near the surface at night and sink to deeper water during the day. The birds are not exploring at random. They track their food through its daily movements with impressive precision.
Not all diving birds chase individual fish. African penguins gain a measurable advantage by foraging in groups, especially when targeting schools. Penguins catching fish from schools had roughly a 1.9-fold increase in catch rate compared to catching lone fish, and when birds foraged in groups against schools, that efficiency jumped to about 2.7 times higher.8PubMed Central. Group foraging increases foraging efficiency in a piscivorous diver, the African penguin The mechanism seems to be collective disruption of schooling behavior. Fish schools are a defensive formation, but multiple penguins attacking from different angles break that formation apart, making individual fish easier to catch.9Ardea. African Penguins Spheniscus demersus, Bait Balls and the Allee Effect
How the Body Is Reshaped for Diving
Underwater swimming demands a fundamentally different body than flying through air. Birds that dive frequently show modifications in their bones, muscles, and feathers that reflect how seriously evolution has committed them to life beneath the surface.
Bone density is one of the clearest markers. In the humerus, femur, and tibiotarsus, species with stronger diving abilities have denser, more compact bones. Flightless penguins sit at the extreme, with the highest bone compactness of all, which makes sense because they no longer need lightweight bones for flight and benefit from the extra weight to counteract buoyancy.10PubMed. How does the inner structure of the limb bones of aquatic birds relate to their locomotor abilities? Among wing-propelled alcids, the forelimb bones have cortical walls roughly 22 percent thicker than those of non-diving relatives, and the flightless members of that lineage developed fully osteosclerotic (abnormally dense) forelimbs.11PubMed. Osteological histology of the Pan-Alcidae (Aves, Charadriiformes): correlates of wing-propelled diving and flightlessness
Foot-propelled divers show dramatic changes in leg anatomy instead. Highly diving species tuck the thighbone and upper leg tight against the ribcage and under the abdominal skin, streamlining the body into a torpedo shape. Several key leg muscles shift in position and size: muscles that stabilize the hip and knee move along the shinbone, gaining leverage through longer moment arms, while the ankle and toe flexors that actually power each kick become exceptionally large.12PubMed Central. Comparative hindlimb myology of foot-propelled swimming birds
Feather structure is equally telling. Aquatic birds have shorter body feathers than land birds, and the difference comes specifically from a shorter downy base. This reduced down section limits the amount of air trapped in the plumage, which helps control buoyancy. At the same time, the outer portion of each feather has a denser arrangement of tiny interlocking hooks, increasing water resistance at the surface layer.13Functional Ecology. A phylogenetic comparative analysis reveals correlations between body feather structure and habitat Cormorant feathers are especially interesting. They entrain a thin layer of air within their feather microstructure that acts as a waterproof barrier at shallow depths, but that barrier breaks down in deeper dives. Once the bird surfaces, however, the feathers spontaneously dewet because, from a surface-energy standpoint, the dry state is the more stable configuration for water on these feathers.14PubMed Central. Quantification of feather structure, wettability and resistance to liquid penetration This may explain why cormorants are often seen holding their wings open after diving: the posture helps overcome any remaining droplets clinging to the plumage and speeds up the return to a dry, insulating state.
Preen oil contributes to waterproofing as well, though not quite in the way most people assume. Rather than directly making feathers waterproof through a chemical coating, preen oil encourages an even arrangement of the tiny barbs and hooks that physically interlock to seal the feather surface. The coating is extremely light, just enough to maintain the optimal microstructure for water repellency.15PubMed Central. Light to intermediate oil sheens increase Manx shearwater feather permeability
Holding Their Breath
A bird chasing fish at 200 meters depth needs to hold its breath for minutes at a time, and the physiological machinery behind that is impressive. Two oxygen-carrying proteins do most of the heavy lifting: hemoglobin in the blood and myoglobin in the muscles. Among North American ducks, sea ducks (the deepest and longest divers) had significantly higher concentrations of both hemoglobin and myoglobin than dabbling species. The myoglobin concentration in the main leg muscle used for paddling turned out to be the single strongest predictor of how long a species could stay submerged.16Journal of Avian Biology. Blood‐ and muscle‐O2 storage capacity in North American diving ducks
Wing-propelled divers show the same pattern in different muscles. Among shearwaters and petrels, the deeper-diving species had significantly higher total oxygen stores. Sooty shearwaters carried about 46 mL of oxygen per kilogram, flesh-footed shearwaters about 49 mL, while grey-faced petrels, which dive less, stored only about 41 mL. The myoglobin in their flight muscles, which double as diving propulsion muscles, correlated strongly with diving ability and was sufficient to keep those muscles running aerobically throughout a typical dive.17Marine Biology. Diving adaptations of migratory seabirds: oxygen stores in Procellariiformes highlight a key role for pectoral myoglobin in determining dive behaviour
Heart rate also drops during dives, a response called diving bradycardia. This slows the rate at which oxygen is consumed and redirects blood flow toward the brain and vital organs. Smaller penguins and most diving ducks can sustain normal aerobic metabolism throughout a dive simply because their oxygen reserves are large enough relative to their body size and dive duration. But larger species face a harder problem. Emperor penguins and other deep divers likely reduce their metabolic rate during dives, possibly through regional cooling of less critical tissues, and may rely partly on anaerobic metabolism, processing the resulting lactic acid after surfacing.18PubMed Central. Physiology of diving of birds and mammals
The Tradeoff Between Flying and Diving
A wing that works well underwater makes a poor wing for flying through air, and vice versa. This biomechanical conflict sits at the heart of one of the most-discussed questions in seabird evolution: why did penguins lose the ability to fly? The leading hypothesis is straightforward. As a wing becomes better shaped for propelling a body through dense water, the energy cost of using that same wing to fly through thin air rises steeply. In thick-billed murres, which still fly and dive, flight costs were measured to be extremely high, consistent with biomechanical models predicting that a wing optimized for diving becomes energetically ruinous in air.19PubMed Central. High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins At some point, as body size and diving commitment increase, the cost of maintaining flight capability exceeds the benefit, and natural selection favors giving up flight entirely.
This explains why the largest alcids in the fossil record, like the great auk, also became flightless. It also explains why living alcids like puffins and murres are such famously clumsy fliers, with rapid wingbeats and high energy expenditure just to stay airborne. They sit in an awkward middle zone, good enough at both flying and diving to do each, but genuinely optimized for neither. Penguins resolved the compromise by committing fully to diving, and their wings became stiff, paddle-shaped flippers in the process.
Foot-propelled divers sidestep this tradeoff to some extent, because their wings remain dedicated to flight while their legs handle the underwater work. But they face their own version of the problem: legs positioned far back on the body for efficient paddling make walking on land difficult. Loons are practically helpless on shore, barely able to shuffle forward. Grebes are somewhat better but still ungainly. The body cannot be optimized for three different modes of locomotion at once.
Seeing Underwater
Hunting fish beneath the surface requires actually being able to see them, and the optical challenge is considerable. When a bird’s eye moves from air into water, the cornea, which does most of the light-bending work in air, suddenly loses nearly all its refractive power because water and corneal tissue have similar densities. In Australasian gannets, which plunge from the air into the sea at high speed, researchers found that the eyes compensate almost instantly, accommodating by more than 45 dioptres upon submersion.20PubMed Central. Visual accommodation and active pursuit of prey underwater in a plunge-diving bird: the Australasian gannet That is a massive shift. For comparison, a young human eye can accommodate by roughly 10 to 15 dioptres at most, and that ability declines with age. The gannet’s lenses are muscular enough to reshape themselves by three or four times that amount in less than a second, allowing the bird to actively track and pursue prey immediately after hitting the water.
Penguins and cormorants also have excellent underwater vision, though the specific mechanisms vary. Cormorants have unusually flat corneas even in air, which reduces the amount of refocusing needed when they submerge. Penguins, like gannets, rely heavily on powerful lens accommodation. In each case the solution is different in detail but identical in purpose: maintaining sharp focus on a moving target in a medium the eye was not originally designed for.
Threats Beneath the Surface
The same diving ability that lets sea ducks and other birds find food underwater also puts them in danger from human activity. Gillnet fisheries are a particularly serious problem for diving birds. Because these nets are nearly invisible underwater, diving birds swim straight into them and drown. For sea ducks alone, gillnet bycatch kills hundreds of thousands of individuals each year.21PubMed Central. Underwater hearing in sea ducks with applications for reducing gillnet bycatch through acoustic deterrence Researchers are exploring acoustic deterrent devices that could warn diving birds away from nets, but the approach depends on understanding what these birds can actually hear underwater, a question that had received surprisingly little study until recently.
Oil spills pose another distinct threat, one that specifically targets the feather adaptations diving birds depend on. When even a light sheen of petroleum contacts a seabird’s plumage, it disrupts the precise arrangement of barbs and barbules that creates the waterproof seal.15PubMed Central. Light to intermediate oil sheens increase Manx shearwater feather permeability The feather microstructure that normally repels water and spontaneously dewets after a dive becomes permeable, allowing water to soak through to the skin. For a bird that dives dozens or hundreds of times a day, this is catastrophic: waterlogged plumage means lost insulation, increased energy expenditure, and eventual hypothermia. Cormorants, which already tolerate partially wetted feathers and must dry their wings after diving, are somewhat more vulnerable to this chain of events than species with fully waterproof plumage, because their feathers already operate closer to the edge of their waterproofing capacity.
Underwater construction, shipping traffic, and seismic surveys add noise to the underwater environment that diving birds must navigate. While the direct effects of anthropogenic noise on diving birds are still being studied, the concern is that it could interfere with foraging by masking the sounds birds use to detect prey or orient themselves, or by driving them away from productive feeding grounds entirely. For species that are already declining due to overfishing and habitat loss, any additional pressure on foraging efficiency is unwelcome.