Ducks are air-breathing birds and cannot extract oxygen from water; therefore, they do not breathe underwater. Their ability to remain submerged for foraging relies on a sophisticated suite of physical and physiological adaptations. These adaptations allow them to overcome natural buoyancy, propel themselves efficiently, and manage limited oxygen reserves until they resurface.
Physical Mechanics of Submersion
A duck’s first challenge when diving is counteracting its natural buoyancy, which is largely due to air trapped in its plumage and air sacs. To reduce this flotation force, a duck actively presses its feathers close to its body, squeezing out the insulating layer of air. Diving ducks, in particular, may also exhale a portion of the air held within their respiratory air sacs, which further decreases their overall volume and density.
The dense plumage is protected by an oil applied from the preen gland, making the feathers water-resistant. This oily coating prevents the feathers from becoming waterlogged, ensuring the duck remains insulated and can dry quickly upon surfacing.
For propulsion, ducks rely heavily on their large, webbed feet, which act as hydrofoils to create thrust. The webbed structure expands on the power stroke, pushing water backward for maximum forward movement. On the recovery stroke, the toes fold together, minimizing water resistance as the foot is brought forward for the next push. This powerful, leg-driven swimming is especially pronounced in true diving ducks whose legs are set farther back on the body, functioning much like a ship’s propeller for efficient underwater travel.
The Avian Diving Reflex and Oxygen Management
The avian diving reflex is the most significant adaptation for underwater survival. This involuntary physiological response is triggered by the submersion of the bird’s head. The moment water touches the nostrils, the body initiates bradycardia, a dramatic reduction in heart rate. This immediate drop in heartbeats conserves the limited oxygen supply by slowing the overall metabolic rate.
Accompanying the drop in heart rate is peripheral vasoconstriction. This process constricts blood vessels to the extremities and non-essential organs. This action shunts the limited oxygenated blood supply away from the muscles, digestive tract, and skin, redirecting it primarily to the brain and heart. By prioritizing the most oxygen-sensitive organs, the duck maximizes the time it can remain functional underwater.
Ducks also possess enhanced internal oxygen storage capabilities, particularly within their muscles. Diving species have a higher concentration of the oxygen-binding protein myoglobin in their leg muscles. This myoglobin acts as a localized oxygen reservoir, allowing the leg muscles to continue working aerobically for a longer duration during the dive.
Once the localized oxygen supply is depleted, the exercising muscles must switch to anaerobic respiration. This is a less efficient process that results in the buildup of lactic acid. The duration of a dive is ultimately limited by how long the heart and brain can function on the conserved oxygen stores before the duck must resurface to replenish oxygen and metabolize the accumulated lactic acid.
Specialized Strategies of Diving and Dabbling Ducks
The physical and physiological differences among duck species are dictated by their foraging strategies, separating them into dabbling and diving ducks. Dabbling ducks, such as Mallards, primarily feed in shallow water by “tipping up.” They submerge only their heads and forebodies while their tail feathers remain above the surface. This behavior requires minimal use of the full diving reflex because their breath-holds are brief, lasting only seconds.
In contrast, diving ducks, which include species like the Canvasback and Scaup, fully submerge and propel themselves underwater using their powerful feet to reach food in deeper water. These ducks have legs positioned further back on their bodies, which grants them superior underwater maneuverability and thrust. Diving ducks rely heavily on the full physiological diving reflex to support their longer foraging expeditions, which can last 10 to 20 seconds or more.
The anatomical specialization for deep diving extends to the wings and body shape of these ducks. Diving ducks often have shorter, more streamlined bodies and smaller wings relative to their body mass, which reduces drag underwater. Their dependence on the water for foraging means they must build up speed by running across the surface to take flight, unlike dabbling ducks, which can spring almost vertically into the air.