Fish-eating birds have developed specialized methods to hunt in water, a medium vastly different from the air in which they fly. The density of water creates significant physical challenges, requiring birds to overcome buoyancy and withstand immense impact forces. To successfully capture prey, these species must employ refined hunting techniques that rely on either high-speed momentum or powerful underwater propulsion.
The Two Primary Methods of Attack
Fish-diving birds fall into two distinct hunting categories based on their entry into the water and how they maneuver once submerged. The first method, known as plunge diving, involves a high-velocity attack from the air, relying on the force of impact and subsequent momentum to reach prey. Plunge diving is typically a surprise attack, allowing the bird to stun or quickly seize fish near the surface before its momentum is lost.
The second primary strategy is pursuit diving, where the bird enters the water from the surface or a short, shallow jump. Unlike plunge divers, these birds do not rely on aerial speed to penetrate the water column. Instead, pursuit divers are built for underwater endurance and agility, using their wings or feet for strong, sustained propulsion to chase and capture fish.
Key Examples of Plunge Divers
Plunge divers, such as the Northern Gannet, execute dramatic dives. A Gannet begins its attack from heights of 10 to 40 meters, folding its wings back to achieve a streamlined, arrow-like shape just before impact. It can hit the water at speeds approaching 90 kilometers per hour, using this high velocity to penetrate the surface and pursue fish down to depths of 30 meters.
The Brown Pelican uses a shallower, less vertical attack from heights of 10 to 20 meters. Upon hitting the water, the pelican’s large gular pouch opens, acting like a parachute to slow the bird down while simultaneously scooping up fish and water. This method focuses on stunning shoals of fish and capturing them immediately at the surface rather than deep pursuit.
Terns represent a smaller, lighter class of plunge diver, often hovering momentarily before dropping into the water. Species like the Caspian Tern use high aspect ratio wings for slow, controlled flight, allowing them to spot fish and execute a relatively shallow, less impactful dive. Their maximum dive speed is much lower, estimated around 26 kilometers per hour, which is sufficient for snatching prey near the surface.
Key Examples of Pursuit Divers
Pursuit divers sacrifice terrestrial mobility for superior performance underwater. Cormorants are foot-propelled divers that use powerful, webbed feet to chase fish down to depths that can exceed 45 meters. Their legs are positioned far back on their bodies, which creates excellent thrust underwater but results in an ungainly gait on land.
Cormorants possess specialized feathers that are less waterproof than those of most other seabirds, allowing the plumage to quickly become waterlogged. This saturation reduces the bird’s natural buoyancy, making it easier to sink and remain submerged without expending excessive energy.
Loons are highly specialized pursuit hunters that use their large, fully webbed feet for synchronous paddling beneath the surface. They can dive to depths of 70 meters or more and remain underwater for several minutes while hunting. The Loons’ legs are set extremely far back on their streamlined, dense-boned body, maximizing hydrodynamic efficiency for the rapid, coordinated foot strokes.
Grebes utilize a unique, specialized foot structure, featuring individual lobate toes rather than a continuous web. These stiff, asymmetrical lobes collapse on the recovery stroke to minimize drag and expand on the power stroke, functioning more like a slotted hydrofoil than a simple paddle. This adaptation, combined with their rear-mounted legs, makes them exceptionally agile underwater, though they are almost entirely confined to the water’s surface.
Specialized Biological Adaptations
For high-speed plunge divers, impact absorption is managed by specialized structures. Subcutaneous air sacs located in the head and neck inflate just prior to impact, acting as a natural airbag to cushion the brain and internal organs from deceleration. Plunge divers also feature reinforced skull structures and powerful neck muscles that contract to stabilize the neck upon water entry, preventing buckling. Species like the Gannet possess fused nostrils, which are covered by hard tissue flaps to prevent water from being forced into the respiratory system.
Underwater vision is facilitated across many diving species by the nictitating membrane, a transparent third eyelid that slides across the eye for protection and improved focus. Some birds can also rapidly change the shape of their eye lens to compensate for the difference in light refraction between air and water, maintaining clear vision for locating prey.
For pursuit divers, adaptations focus on buoyancy and propulsion. These include the denser bone structure found in Loons, which helps counteract the natural buoyancy of air-filled lungs. Many pursuit divers have a higher concentration of myoglobin in their muscles and an enhanced ability to slow their heart rate, which maximizes oxygen storage and conservation during extended submergence.
The rearward placement of the legs in Loons and Grebes increases the efficiency of their foot-propelled swimming. This positioning, while making walking difficult, allows the legs to generate maximum thrust along the bird’s longitudinal axis for a powerful, streamlined pursuit.