AT A GLANCE
- Common Name: Red-breasted Merganser
- Scientific Name: Mergus serrator
- Category: Birds
- Subcategory: Waterfowl
- Range: Circumpolar across northern North America, Greenland, Iceland, northern Europe, and northern Asia; wintering along temperate maritime coastlines
- Habitat Type: Clear boreal freshwater lakes, rocky rivers, and coastal estuaries in breeding season; sheltered coastal bays, fjords, and open marine waters in winter
- IUCN Red List Status: Least Concern
- Lifespan: 10 to 14 years in the wild; maximum recorded over 20 years
- Diet: Piscivorous (small marine and freshwater fish like herring, sandeels, sticklebacks, and sculpins; also crustaceans and aquatic insects)
- Average Size/Weight: 51 to 62 cm in length; 70 to 86 cm wingspan; 800 to 1,350 g in weight
- Key Defining Feature: Slender, crested ‘sawbill’ duck with a shaggy double-pointed crest, long narrow serrated red bill, and ruby-red eyes
Overview
The Red-breasted Merganser (Mergus serrator) is a sleek, rakish, and high-speed diving sea duck distributed across the boreal and maritime waters of the Holarctic. Famously belonging to the ‘sawbill’ group of waterfowl, it is equipped with a narrow, needle-like bill lined with backward-pointing serrations designed specifically for gripping slippery fish underwater. With its untidy, wind-swept punk-rock crest and low-slung, torpedo-like swimming profile, it cuts a dramatic figure across stormy ocean coastlines and clear subarctic lakes. Capable of swimming with astonishing agility beneath the waves, it is one of the premier piscivorous waterfowl in the northern hemisphere.
Biology and Physical Characteristics
Mergus serrator is characterized by an elongated, streamlined body and a relatively long, thin neck. The species exhibits striking sexual dimorphism in breeding plumage: the male possesses a glossy bottle-green head with a ragged, wispy double crest, a wide white neck collar, and a warm rusty-cinnamon breast streaked with black. His flanks are intricately vermiculated in black and white, and his back is pitch black. The female and non-breeding male display an attractive greyish body with a warm rusty-cinnamon head and a shorter crest that blends subtly into a pale throat. The bill in both sexes is bright coral-red to orange, long, slender, and furnished with fine, sharp, tooth-like serrations on both mandibles.
Behavior and Ecology
Red-breasted Mergansers are consummate pursuit divers. When hunting, they frequently swim with their heads submerged below the surface to spot schools of small fish before executing a swift, arching dive. Propelled by powerful, set-back webbed feet, they chase fish underwater at high speeds, using their serrated bills to snap up darting prey. They frequently hunt in cooperative flocks, forming long lines that herd schools of minnows or herring into shallow coastal bays before plunging into the trapped fish simultaneously. Pairs nest on the ground, carefully concealing their nests beneath thick heather, low spruce boughs, or rock crevices near water.
Conservation and Human Impact
The species is assessed as Least Concern on the IUCN Red List due to its expansive circumpolar distribution and robust global population estimated between 400,000 and 600,000 individuals. It is protected under the Migratory Bird Treaty Act and the African-Eurasian Waterbird Agreement (AEWA). Local threats include oil spills in marine wintering areas, entanglement in commercial monofilament gillnets, degradation of river water quality from industrial forestry runoff, and bioaccumulation of heavy metals from contaminated fish prey.
Interesting Factoid
The Red-breasted Merganser is one of the fastest flying ducks in the world, capable of reaching sustained level flight speeds of over 80 miles per hour during migration.