Birds that possess the ability to both fly and swim underwater represent a rare evolutionary achievement. A true dual-mobility bird must demonstrate sustained, powered flight for migration or foraging and also possess the capacity for effective underwater propulsion and diving. The existence of these few species highlights a remarkable biological compromise between the opposing demands of aerial and aquatic life.
The Biomechanical Challenge of Dual Mobility
The physical laws governing movement in air and water are fundamentally opposed. Air favors low body mass and large wing surface area to generate lift. Water, being approximately 800 times denser, demands a streamlined body shape and propulsion mechanisms designed to overcome drag and manage buoyancy. A bird built for efficient flight is generally too buoyant to dive deep, while a bird built for diving is often too heavy for sustained aerial travel.
This conflict is evident in the skeletal structure and wing design. Typical flying birds have highly pneumatic, air-filled bones to reduce weight, but diving requires denser bones to counteract buoyancy. Dual-ability birds must compromise, possessing bones that are neither extremely light nor extremely dense. Wings optimized for aerial lift are long and thin, while those best suited for underwater propulsion are short, stout, and paddle-like.
Key Physical Adaptations for Air and Water
Dual-ability birds employ specialized biological solutions to manage the competing demands of flight and diving. One effective adaptation is their plumage, which must provide both insulation and waterproofing. The primary waterproofing mechanism is the intricate, interlocking mesh created by tiny hooks and grooves (barbicels) on the feather barbs. This dense, water-repellent barrier creates a mesh with gaps so small that water cannot easily penetrate due to surface tension.
To maintain feather integrity, these species possess a well-developed uropygial gland, or preen gland, located at the base of the tail. The oily secretion is spread across the feathers during preening, not primarily for waterproofing, but to condition them and keep them supple. This conditioning ensures the delicate interlocking barbicels remain intact, which is essential for water repellency.
Diving birds often compress their plumage or exhale before a dive to reduce trapped air volume, decreasing buoyancy and making the initial descent easier. The morphology of the limbs further reflects their dual function, depending on the method of aquatic propulsion. Wing-propelled divers use their intermediate-sized wings as hydrofoils, essentially “flying” through the water to pursue prey. Foot-propelled divers, such as loons and grebes, have powerful legs set far back on the body, providing maximum underwater thrust but making them awkward on land.
Notable Examples of Dual-Ability Birds
The Auk family, which includes Murres, Guillemots, and Puffins, are premier examples of wing-propelled dual-ability birds, found mainly in the North Atlantic and North Pacific. These birds use short, stiff wings to “fly” underwater with powerful strokes, allowing them to pursue fish and crustaceans to significant depths. Their wing design results in a high wing-loading ratio, meaning they must beat their wings rapidly in the air, incurring one of the highest flight costs for their body size among all birds.
Diving Petrels, which inhabit the Southern Ocean, are the ecological counterparts to Auks, sharing convergent traits like short wings and an upright posture. These small, plankton-feeding seabirds also use their wings for underwater propulsion, exhibiting a characteristic “whirring” flight low over the waves. Their reliance on diving means they approach land only for breeding, spending the vast majority of their lives at sea.
Shearwaters, named for their habit of gliding close to the wave tops, are remarkable long-distance flyers that undertake some of the longest migrations on Earth. They combine surface feeding with pursuit diving, using their long wings to swim underwater to depths of 200 feet to catch fish and squid. Their ability to use their wings for both dynamic soaring flight and aquatic swimming is a testament to the evolutionary compromise.
Loons and Grebes are notable examples of foot-propelled divers, utilizing large, webbed or lobed feet for powerful thrust both at the surface and submerged. The adaptations for diving, including their heavy, non-pneumatic bones and posteriorly positioned legs, make them excellent swimmers but poor flyers. They require a long, running start across the water’s surface to become airborne, but are strong fliers once aloft, which is necessary for their seasonal migrations.