Birds have evolved many adaptations that allow them to swim effectively. While not all species of birds can swim, many aquatic and marine birds are excellent swimmers. In this article, we will explore the anatomy, physiology, and behavior that enables swimming birds to propel themselves gracefully through the water.
Buoyancy
One of the most important factors that enables swimming birds to stay afloat is their buoyancy. Buoyancy is an object’s ability to float in water and is determined by its density. Objects less dense than water will float, while objects denser than water will sink. Birds have lightweight, pneumatized bones that are filled with air pockets. This bone structure decreases their body density, allowing them to float more easily.
Birds also have special chemical adaptations that increase their buoyancy. Their feathers are coated with preen oil that waterproofs them. This prevents water from penetrating into the feather structure, which would otherwise cause them to become waterlogged and sink. Many aquatic birds also have more feathers than terrestrial birds, which increases the volume of air trapped against their bodies to provide lift.
Streamlined Body Shape
Birds that swim frequently have evolved hydrodynamic body shapes that reduce drag in the water. Their bodies are often laterally compressed, or flattened from side to side, which presents a smaller profile area in the direction they move through water. Species like penguins have smooth contours with very little surface protrusions. Their wings act like flippers, providing lift and thrust with less resistance than feathered wings.
Some birds, like puffins, use their wings to “fly” underwater and propel themselves forward. Other adaptations like webbed feet increase surface area to generate more propulsive force with each stroke. Anatomical structures like these allow swimming birds to move through the water efficiently with minimal energy expenditure.
Feather Waterproofing
As mentioned earlier, birds’ feathers play an important role in maintaining buoyancy. But they serve other functions for swimming as well. Feathers provide insulation that retains body heat since water conducts heat away from the body much faster than air. They also protect birds from getting hypothermia in frigid ocean environments.
Preen oil is secreted from the uropygial gland near the base of the tail and spread over feathers during preening. This oil coats feathers with a waxy, hydrophobic substance to prevent water saturation. Some aquatic birds, like loons, supplement preen oil by using their beaks to spread an oily substance secreted from glands around their tail coverts.
Density of Preen Oil
Researchers have found that preen oil density and composition differs across species depending on how frequently they interact with water. Birds that dive and swim often produce more viscous, dense preen oil. This adaptation provides superior waterproofing and insulation underwater.
For example, one study analyzed the preen oil of great cormorants and found it had lower levels of monoester waxes. This altered the intermolecular cohesion forces and created a denser layering effect on feathers. The preen oil was also found to contain higher levels of triglycerides, which are more viscous and unsaturated than other preen oil lipids.
Leg and Foot Anatomy
A bird’s legs and feet provide the propulsive force for swimming. Webbed feet are common in water birds and help increase surface area for paddling. The extent of webbing is related to a species’ aquatic lifestyle – birds that swim often and dive, like cormorants, typically have fully webbed feet. Meanwhile, those that just dabble in water, like mallards, may only have partially webbed feet.
Foot size and paddle shape also vary. Grebes have the largest feet relative to their body size of all birds, with wide lobes on each toe. These adaptations make their feet more effective for pushing against the water. Sea ducks have pointed lobes and small webs, providing velocity and reduced drag while swimming underwater.
Leg position is another important anatomical factor. Birds that swim primarily on the water surface, like geese, have legs positioned centrally on their bodies for optimal paddling. Diving birds tend to have legs positioned farther back to propel themselves while pursuing underwater prey.
Leg Muscle Composition
A key physiological adaptation is the muscle composition in swimming birds’ legs. Research has found the leg muscles of diving birds, like cormorants and grebes, contain more fast oxidative (FOG) muscle fibers than terrestrial fowl. FOG fibers are fatigue-resistant and support sustained swimming. The higher proportion of these fibers provides a metabolic advantage for swimming long distances.
Behavioral Adaptations
Along with anatomical features, swimming birds display specialized behaviors and movement patterns to help them expertly navigate through water:
- Many species use their wings to propel underwater, alternating strokes like a flying motion.
- Foot paddling entails alternating front and back strokes. Synchronously paddling both feet together maximizes thrust.
- Diving birds often employ a “leapfrog” pattern – thrusting with their feet while underwater, then using wings for aerial propulsion to the next dive point.
- Some aquatic birds run along the surface to build momentum before taking flight. This allows them to clear the water using only their wings.
- Birds conserve heat by tucking their feet and heads into their plumage while floating.
- Whirring their wings while sitting creates a vortex of air within feathers to speed evaporative drying.
Instinctual swimming movements like these enable birds to successfully forage and evade predators in their aquatic environments.
Physiological Adaptations
Along with anatomical and behavioral features, birds have a number of physiological adaptations that facilitate oxygen storage and delivery while swimming and diving. These include:
- Increased blood volume – Provides greater oxygen carrying capacity.
- Higher hemoglobin concentration – Increases oxygen binding in the blood.
- Greater myoglobin concentration – Stores oxygen in muscle tissue.
- Ability to selectively vasoconstrict arteries – Prioritizes blood flow and oxygen to the heart and brain.
- Increased oxidative enzyme activity – Supports muscle endurance during extended swimming.
Many diving birds, like penguins and puffins, also have adaptations that allow them to reduce their metabolic rate and slow their heart beat while submerged. This oxygen-conserving physiology enables them to stay under for longer.
Examples of Swimming Birds
Many birds across diverse taxonomic groups have adaptations that enable swimming capability. Some examples include:
Seabirds
- Penguins
- Albatrosses
- Shearwaters
- Auks (puffins, guillemots, murrelets)
- Gulls and terns
- Pelicans
- Cormorants
- Frigatebirds
- Boobies
- Loons
- Grebes
Waterfowl
- Swans
- Geese
- Ducks
- Mergansers
Waders and Shorebirds
- Plovers
- Oystercatchers
- Stilts
- Avocets
- Cranes
- Rails
- Coots
- Herons
- Egrets
- Ibises
- Spoonbills
- Flamingos
This list demonstrates the remarkable diversity of birds that have adapted in unique ways to swim, wade, and dive in aquatic environments all over the world.
Conclusion
Swimming birds rely on a combination of specialized anatomical structures, physiological processes, and behaviors that enable efficient movement through the water. While not all species swim frequently or skillfully, birds in aquatic habitats possess adaptations like webbed feet, waterproof plumage, and oxygen storage abilities that aid swimming capabilities. Understanding how birds are so adept at an activity typically associated with mammals and fish provides insight into the remarkable diversity of avian biology and ecology.
