While many birds are known for flight, some species have evolved to master the aquatic realm, forsaking flight entirely. These birds offer a compelling look into evolutionary trade-offs, where one ability is enhanced at the expense of another.
The Most Famous Example
Penguins are the most recognized example of birds that swim but cannot fly. These flightless birds primarily inhabit the Southern Hemisphere, with the largest populations in Antarctica. There are 18 species, ranging from the small Little Blue Penguin to the impressive Emperor Penguin, which can stand up to four feet tall.
Penguins are distinguished by their upright posture and black and white plumage, which provides camouflage in the water. They spend a significant portion of their lives in the ocean, foraging for krill, fish, and squid. While they return to land for breeding and nesting, they are exceptional swimmers and divers.
How They Master the Water
Penguins have evolved to become exceptional swimmers, a process that explains their inability to fly. Unlike flying birds with lightweight, hollow bones, penguins have dense, solid bones. This increased bone density reduces buoyancy, allowing them to dive to impressive depths and remain submerged for extended periods.
Their wings have evolved into powerful, rigid flippers, shorter and flatter than those of flying birds. These flippers propel penguins through the water with remarkable speed and agility, much like “flying” underwater. Strong pectoral muscles attached to a pronounced keel on their sternum provide the power for these strokes. Their streamlined, torpedo-shaped body also minimizes water resistance, enhancing their efficiency in aquatic environments.
Their dense, short feathers overlap like roof shingles, creating a waterproof and insulating layer. This feather structure, combined with a layer of blubber beneath their skin, traps air and provides insulation against frigid water temperatures.
Penguins also produce a waterproof oil from a gland near their tail, which they spread over their feathers during preening, enhancing their water repellency. Their legs, positioned far back on their bodies with webbed feet, primarily function for steering and propulsion underwater, contributing to their powerful swimming and diving abilities.
More Birds That Don’t Fly
While penguins are the most well-known, they are not the only birds that have lost the ability to fly while retaining or developing strong swimming skills. The Flightless Cormorant, found exclusively in the Galapagos Islands, is another compelling example. This species has significantly reduced wings, making them incapable of flight, but it compensates with strong legs and webbed feet, allowing it to dive effectively for food in shallow waters.
Certain species of steamer ducks, native to South America, also fall into this category. Three out of the four known species of steamer ducks are flightless, with wings too small to allow for functional flight. Instead, they use their wings like paddles to skim rapidly across the water’s surface, a behavior that earned them their name.
An extinct example is the Great Auk, a large, flightless seabird of the North Atlantic. Despite being unrelated to modern penguins, the Great Auk shared similar adaptations, using its short, flipper-like wings for powerful underwater propulsion. These examples highlight that the evolutionary trade-off of flight for enhanced swimming has occurred independently in various avian lineages, underscoring the power of natural selection in shaping species to their specific ecological niches.