Individuals fly rapidly, and feed by diving from the surface to pursue aquatic animals underwater, using serrated bills to capture slippery fish.

They migrate each year from breeding sites on lakes and rivers to their mostly coastal wintering areas, making them the only species in the genus Mergus to frequent saltwater.

The red-breasted merganser was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the current binomial name Mergus serrator.

[2][3] The genus name Mergus is a Latin word used by Pliny and other Roman authors to refer to an unspecified water bird.

The common merganser displays more contrast between the darker head and lighter breast and has a light chin patch not seen on the red-breasted.

[10] The fastest duck ever recorded was a red-breasted merganser that attained a top airspeed of 100 mph (160 km/h) while being pursued by an airplane.

Threats include habitat loss through wetland destruction, exposure to toxins such as pesticides and lead, and becoming bycatch of commercial fishing operations.