Why Do Cormorants Spread Their Wings?

Cormorants spread their wings primarily to dry their feathers after diving. Unlike most waterbirds, cormorants have plumage that absorbs a significant amount of water, and holding their wings open to the air and sun helps shed that moisture. The behavior is one of the most recognizable sights on coastlines, lakeshores, and river pilings worldwide, and while it looks simple, the science behind it turns out to be more layered than you might expect. Several competing explanations have been proposed over the decades, and the evidence doesn’t point to a single tidy answer for every species.

Why Cormorant Feathers Get Wet at All

Most waterbirds, like ducks and geese, emerge from the water looking almost dry. Their feathers are coated in oils and structured to repel water thoroughly. Cormorants are different. Each body feather on a great cormorant has a loose outer section that gets soaked almost instantly and a tightly structured central portion that stays waterproof, trapping a thin layer of air against the skin.¹ So a cormorant’s plumage is only partly wettable, not fully waterproof the way a duck’s is.

This partial wettability isn’t a design flaw. It’s a trade-off. Waterproof feathers trap a thick layer of insulating air, which is great for warmth but terrible for diving. All that trapped air makes a bird incredibly buoyant, meaning it has to burn enormous amounts of energy just to stay submerged. Cormorants that allow their outer feathers to soak up water reduce their buoyancy significantly, making deep dives easier and less costly in energy terms.² The downside, of course, is that they come out of the water heavy and waterlogged, which is where the wing-spreading comes in.

What Their Preen Oil Does and Doesn’t Do

A common assumption is that cormorants simply lack the waterproofing oil that other birds have. That’s not quite right. Cormorants do have a uropygial gland (the “preen gland” near the tail) that produces oil, but its composition is dramatically different from a duck’s. When researchers compared the preen secretion of great cormorants to that of Muscovy ducks using mass spectrometry, the cormorant’s oil turned out to be a complex mixture of over a thousand compounds, while the duck’s secretion was dominated by just two specific esters. The duck’s oil was measurably more water-repellent.³

Here’s the surprising part, though: when researchers actually removed feather waxes from cormorant feathers and measured the effect on water repellency, the difference was small. The physical structure of the feather itself, not the oil coating it, appears to be the main factor controlling how much water gets in.³ So the old story of “cormorants lack waterproofing oil” has the right spirit but the wrong mechanism. The feather architecture is doing most of the work.

How Feathers Dry Themselves (And Why They Still Need Help)

Detailed physics work on cormorant feathers has revealed something counterintuitive. At the microscopic level, the tiny barbules on cormorant feathers are spaced in a way that, thermodynamically, favors a non-wetting state for plain water. In other words, a cormorant feather sitting on a table will naturally repel water droplets. The feather’s surface energy means that the “dry” configuration is actually the most stable one for water.⁴

So why do the feathers get wet during dives at all? Because the water pressure during a dive physically forces water past the feather’s defenses. Researchers calculated that the “breakthrough depth” for cormorant feathers, the depth at which water is pushed into the feather microstructure, is roughly one to four meters, which is toward the shallow end of typical dive depths.⁴ Since many cormorants regularly dive far deeper than that, their feathers are expected to be fully wetted on a typical deep dive.

Once the bird surfaces, the feathers “want” to return to a dry state, because that’s their thermodynamic equilibrium. But water droplets cling to the feather surface due to what physicists call pinning, essentially a kinetic barrier that slows drying. Spreading the wings open to airflow and sunlight helps overcome that barrier and speeds the process along.⁴ So the wing-spreading posture isn’t creating drying from scratch; it’s accelerating a process the feather structure already favors.

The Experimental Evidence for Wing Drying

The most direct experimental test of the wing-drying idea came from a study on captive great cormorants. Researchers controlled how long the birds spent swimming and whether they were fed fish beforehand, then measured how long each bird spread its wings afterward. The result was clear: wing-spreading duration tracked closely with time spent in the water. Birds that swam longer spread their wings longer. Whether the bird had eaten or not made no difference at all.⁵

That finding rules out one popular alternative: the “food-warming” hypothesis. Some researchers had speculated that cormorants spread their wings to absorb solar heat, warming their body core to speed up digestion of cold fish. If that were the case, you’d expect fed birds to spread longer than unfed ones, and you’d expect food intake to matter more than swimming time. Neither prediction held up.⁵

Field studies on double-crested cormorants and the flightless cormorants of the Galápagos also support the drying explanation. In both species, the behavior was observed only in wet individuals and was never accompanied by gular flutter, the rapid throat vibration that birds use to dump excess heat. If wing-spreading were about thermoregulation, you’d expect it to appear alongside other cooling behaviors, and it doesn’t.⁶

Thermoregulation Isn’t Off the Table Entirely

Despite the strong case for drying, the thermoregulation hypothesis hasn’t been fully buried. The trouble is that different species tell different stories, and lumping all cormorants together leads to confusion.

Double-crested cormorants, for instance, spread their wings sparingly and show no relationship between wing-spreading and either ambient temperature or solar intensity.⁷ That pattern looks like simple drying behavior with no thermal component. Flightless cormorants in the Galápagos show a more complicated picture. Their wing-spreading frequency increased with ambient temperature and decreased with solar intensity, which is the opposite of what you’d predict if the birds were trying to soak up heat to warm themselves. The researchers concluded that even in this species, drying was the primary function, though conserving metabolic energy after drying may be a secondary benefit.⁶

The real wrinkle comes from anhingas, close relatives of cormorants that also spread their wings. Anhingas exhibit the behavior far more frequently and clearly in response to thermal conditions: they spread their wings more when the air is cool and solar radiation is strong, which is exactly what you’d expect from a bird using the sun to warm up.⁷ The lesson is that wing-spreading in the broader cormorant-anhinga family probably serves different purposes in different species, and the relative weight of drying versus thermoregulation shifts depending on the bird’s ecology and environment.

Other Hypotheses That Have Been Floated

Beyond drying and thermoregulation, researchers have kicked around several other ideas over the years, including aiding digestion, social signaling, and parasite control.⁸ Most have come up short when tested.

The digestion idea, sometimes framed as “food-warming” or “gut-warming,” proposes that spreading wings to the sun warms the abdomen and speeds up processing of recently eaten fish. As noted above, the captive great cormorant experiment found no link between feeding and wing-spreading duration, which is a pretty direct refutation for that species.

Social signaling is harder to rule out definitively, because you can’t easily design an experiment that separates “I’m drying my wings” from “I’m signaling something to nearby birds.” Some researchers have noted that cormorants often spread their wings in groups, which could look like a coordinated social display. But since the behavior is tightly linked to having wet feathers in the species tested so far, any social function would most likely be secondary rather than the primary driver.

The parasite-control hypothesis got an indirect test in a study on mourning doves and their feather lice. The idea was that spreading wings in direct sunlight might improve preening effectiveness, perhaps because UV light weakens ectoparasites. But doves given the chance to preen in full sun didn’t preen more often or more effectively than those in shade, suggesting that sunlight exposure alone doesn’t boost parasite control through preening.⁹ That study wasn’t conducted on cormorants specifically, so it’s not a slam dunk, but it’s the best direct test of the UV-parasite link in birds that we have.

Why Drying Matters So Much for Cormorants

It’s easy to watch a cormorant holding its wings out and think, “just shake it off.” But wet feathers create serious problems for a bird that also needs to fly. Waterlogged plumage adds weight, increasing the energy needed to get airborne and stay aloft. There’s a well-documented trade-off in seabirds between being good at diving and being good at flying, and cormorants sit right on the edge of that compromise.¹⁰

Wet feathers also compromise insulation. The thin air layer trapped in the waterproof inner portion of the plumage helps keep cormorants warm, but any water that breaches that barrier pulls heat away from the skin. In cold water, this is a significant metabolic drain. The bird has to burn more calories to maintain its body temperature, which means it needs to catch more fish, which means more time underwater, which means wetter feathers. Breaking that cycle quickly by drying out between dives is important for energy balance.

Research on imperial cormorants illustrates the extremes of this trade-off. Species that carry a lot of plumage air face unusually high power costs when foraging in shallow, warmer waters, where the buoyancy of that trapped air is a real obstacle. Wettable plumage helps in those conditions. But in deeper, colder water, insulation becomes more valuable, and species with less wettable plumage have an advantage.² This helps explain why different cormorant species have evolved different degrees of plumage wettability depending on where they live and how deep they dive.

Anhingas and Darters Aren’t Doing the Same Thing

If you’ve seen a bird perched with spread wings near water and assumed it was a cormorant, it might have been an anhinga (also called a darter or snakebird). Anhingas look superficially similar and share the wing-spreading behavior, but the evidence suggests their motivations aren’t identical.

Anhingas display the behavior often and in clear response to weather: more frequently when the air is cool and the sun is strong. Double-crested cormorants, by contrast, display it sparingly with no consistent relationship to temperature or solar radiation.⁷ Anhingas have even more wettable plumage than most cormorants, and their body temperature regulation appears more dependent on external heat sources. So for anhingas, the spread-wing posture likely serves a genuine thermoregulatory function on top of any drying benefit.

This distinction matters because many older references and casual birdwatching guides treat “wing-spreading in cormorants and anhingas” as a single behavior with a single explanation. The researchers who documented the differences were explicit about the risks of generalizing from one species to the other. An explanation that works for an anhinga doesn’t necessarily apply to a double-crested cormorant, and vice versa.

The Galápagos Flightless Cormorant

The flightless cormorant of the Galápagos Islands is a fascinating edge case. It has stubby, vestigial wings far too small for flight, yet it still spreads them after diving. Since this bird can’t fly and doesn’t need dry feathers for takeoff, the behavior might seem pointless. But even flightless cormorants need to manage heat loss. The Galápagos Islands straddle the equator, but the waters around them are surprisingly cold, fed by the Humboldt Current. A flightless cormorant coming out of a dive is wet and cold, and spreading what remains of its wings to the equatorial sun helps it warm up and dry its remaining plumage faster.

Field observations showed that flightless cormorants, like their double-crested relatives, only spread their wings when wet and never combined the behavior with gular flutter.⁶ The temperature-dependent pattern of their wing-spreading (more frequent in warmer air, less frequent in brighter sun) may simply reflect that evaporative drying is faster in warmer, drier conditions, so the behavior gets the job done more quickly. The persistence of this behavior in a bird that no longer flies is a vivid illustration of how deeply rooted the drying function is. Whatever else wing-spreading might accomplish, it was apparently useful enough to survive the loss of flight itself.

Why This Behavior Keeps Generating Debate

One reason the question “why do cormorants spread their wings” has lingered in ornithology for decades is that the behavior almost certainly doesn’t have a single universal answer. Wing drying is the best-supported primary function across most cormorant species studied, but the thermal, digestive, and social hypotheses aren’t mutually exclusive alternatives. A behavior can dry feathers and incidentally warm the bird. A behavior driven by the need to dry can be co-opted for social signaling over evolutionary time. Biology rarely offers clean, exclusive explanations for complex behavior.

The other reason the debate persists is that cormorants are hard to study experimentally. Most of the controlled data come from captive birds, where conditions don’t perfectly mirror the wild. Field observations can show correlations between wing-spreading and environmental variables, but they can’t easily prove causation. And there are roughly 40 species in the cormorant family, distributed from the tropics to the sub-Antarctic, occupying habitats from coastal cliffs to inland lakes. Studying a handful of species and assuming the conclusions hold everywhere is, as one research team titled their paper, a “risk of generalization.”⁷ The behavior looks the same from a distance, but the ecological pressures shaping it can differ enormously from one species and one latitude to the next.

Oils and Alkanes Tell a Different Story

One detail from the feather physics research is worth flagging because it highlights how specific the cormorant’s solution is. Recall that the feather microstructure creates a state where water naturally beads off once the bird is out of the water. But that only works for water. When researchers tested the same feathers with other liquids, like alkanes and common oils, the feather’s surface energy favored full wetting. The feathers would soak up oil and stay soaked.⁴ This has real conservation implications. In an oil spill, cormorant feathers wouldn’t spontaneously shed petroleum the way they shed water. The very microstructure that lets cormorants be efficient divers makes them especially vulnerable to oil contamination, because the mechanism that normally restores their feathers to a dry state simply doesn’t work for hydrocarbons.