Cormorants are highly successful aquatic birds, instantly recognizable by their dark plumage and distinctive upright posture when perched. Belonging to the family Phalacrocoracidae, these species are found across most of the globe and are renowned for their exceptional diving abilities. Their efficiency as underwater predators makes their diet and feeding habits a frequent subject of study.
The Cormorant’s Primary Aquatic Diet
The diet of cormorants is overwhelmingly piscivorous, consisting almost exclusively of fish. They are generalist, opportunistic feeders, preying on whatever small to medium-sized fish are most readily available. In marine environments, this includes schooling fish like herring, sardines, and gobies. In freshwater habitats, they commonly target species such as perch, minnows, and shad. The precise species composition varies widely depending on the bird’s location, species, and the time of year.
Cormorants generally consume smaller, slower-moving fish that are easier to pursue and capture underwater. Studies of Double-crested and Neotropic Cormorants show that the majority of their prey is often less than 12 centimeters in length. While larger fish are occasionally taken, the average catch size minimizes the energetic cost of the hunt. For Great Cormorants, the average prey length is less than 20 centimeters, though they can swallow fish up to 50 centimeters long.
The preference for smaller fish means cormorants primarily feed on species that hold little commercial or recreational value, such as various forage fish. However, when local conditions present an abundance of commercially significant fish, such as during stocking events or smolt runs, their diet can temporarily shift. Beyond fish, cormorants supplement their diet with other aquatic animals when fish stocks are low. This includes crustaceans, mollusks, small amphibians like tadpoles, and eels.
Specialized Hunting and Feeding Behaviors
Cormorants are built for underwater pursuit, propelling themselves primarily with their large, fully webbed feet. Their legs are set far back on their bodies, providing powerful thrust for swimming, but resulting in an awkward, upright gait on land. Upon spotting prey, the bird dives from the water’s surface, often with a slight leap for a streamlined entry. They are efficient divers; species like the Great Cormorant can descend to depths of 45 meters, and the Imperial Cormorant has been recorded at up to 80 meters.
The bird’s plumage is adapted for diving, possessing less preen oil than most other waterbirds. This lack of waterproofing causes feathers to become waterlogged quickly, reducing buoyancy and allowing the bird to sink and pursue prey more effectively. Smaller prey is often swallowed swiftly underwater, while larger fish are brought to the surface to be manipulated and consumed. The prey is grasped with the long, thin bill, which ends in a strong, downward-pointing hook that secures slippery fish.
Following a foraging session, the cormorant must dry its saturated feathers to regain insulation and the ability to fly efficiently. This necessity explains the frequently observed behavior of standing with wings fully outstretched, allowing the sun and air to dry the plumage. This highlights the evolutionary trade-off between flight and their specialized diving capability.
Consumption Rates and Dietary Flexibility
The quantity of food consumed by a cormorant is substantial, generally ranging from 10 to 25 percent of its body weight daily. A large species, such as the Great Cormorant, consumes around 500 grams (about 1.1 pounds) of fish each day to meet its metabolic needs. This consumption rate is not static and is influenced by factors such as the bird’s species, seasonal changes, and whether it is feeding young during the breeding season.
The bird’s diet exhibits significant flexibility, as they are generalist predators that readily adapt to the most abundant local prey. This opportunistic approach means that shifts in local fish populations, such as the introduction of a new forage species, can cause a corresponding change in the cormorant’s menu. A population may switch from primarily consuming one type of fish to another if the latter becomes more prevalent.
The high consumption rate and dietary flexibility can lead to conflicts with human interests in specific locations. While high cormorant numbers may represent a minor impact on large, healthy fish stocks, their concentrated feeding in smaller, enclosed areas like aquaculture facilities or during fish migration can result in locally significant predation. This predation causes concern for fisheries managers, especially when the birds focus on economically or ecologically important species.