Mergansers are specialized diving ducks recognizable by their streamlined bodies and unique bill structure, which allows them to capture aquatic prey efficiently. Unlike many waterfowl that graze on vegetation, mergansers have evolved a diet centered almost entirely on protein-rich animal matter. Their feeding strategy involves specialized physical adaptations and hunting behaviors that allow them to thrive in diverse aquatic environments. This focus on hunting beneath the water’s surface dictates their preferred habitats, seasonal movements, and the types of food they consume.
Merganser Species and Identification Traits
North America is home to three main species of these fish-eating ducks: the Common, Red-breasted, and Hooded Mergansers. The Common Merganser is the largest, typically preferring freshwater rivers and large lakes. Males feature a distinct, non-crested, iridescent green head and a white body. The Red-breasted Merganser is slightly smaller and is frequently found in coastal, saltwater environments during winter, recognizable by its spiky crest on both sexes. The smallest species is the Hooded Merganser, often found in wooded ponds and swamps, named for the male’s large, fan-shaped white crest bordered in black.
Primary Food Sources: The Merganser Menu
The diet of mergansers is predominantly piscivorous, centered on consuming fish, which often makes up over 75% of their total intake. They target small, slow-moving fish, typically measuring between 10 and 15 centimeters in length. Common prey items include sculpin, minnows, sticklebacks, sunfish, and smaller trout and salmon. Mergansers also supplement their diet with various aquatic invertebrates.
These supplementary sources include crayfish and other small crustaceans, which are important in coastal or brackish habitats. They also consume aquatic insects and their larvae, mollusks, worms, and amphibians like frogs and tadpoles. This flexibility allows them to sustain themselves when fish populations are less available or when foraging in shallower, insect-rich waters during the breeding season.
Unique Feeding Habits and Hunting Adaptations
Mergansers are sometimes called “sawbills” due to the unique structure of their bill, a specialized adaptation for gripping slippery prey. Their long, slender bill is lined with sharp, backward-pointing serrations, or lamellae, that function like teeth to hold fish firmly once captured. This tool is essential for securing prey that might otherwise slide free. They are visual hunters, relying on clear water to spot prey before diving.
The ducks propel themselves underwater using powerful webbed feet, positioned far back on their bodies to maximize thrust. They are capable of diving to considerable depths, sometimes reaching over 9 meters, and can stay submerged for up to 45 seconds while pursuing fish. Mergansers often employ a coordinated hunting strategy, especially the Common Merganser, where several birds form a line or semicircle. This group effort herds schools of small fish toward shallow water or banks, concentrating the prey for easier capture.
Habitat, Range, and Seasonal Diet Shifts
The availability of suitable food resources directly influences where mergansers live and migrate throughout the year. Common Mergansers, which prefer freshwater, often follow rivers and lakes south during winter to remain in open, unfrozen water where they can continue to fish. Red-breasted Mergansers frequently migrate to coastal, brackish, or saltwater environments, where their diet shifts to include more crustaceans and marine fish, such as herring and silversides.
Seasonal changes necessitate a shift in diet composition, particularly during the summer breeding season. Mergansers move to inland lakes and rivers to nest. While adults continue to eat fish, ducklings initially rely more heavily on smaller, easier-to-catch aquatic invertebrates and insect larvae. This ensures the young receive the dense protein required for rapid growth before transitioning to a fish-dominated diet around 12 days old.