Both species migrate late, sometimes just before freezeup.
Flock movements are rapid, often erratic, usually in compact groups.
Hens are silent; drake lesser scaup purr; drake greater scaup have a discordant scaup, scaup.
Goldeneye
Common--Length--19 in. Weight--2-1/4 lbs.
Barrow's--Length--19 in. Weight--2-3/4 lbs.
These are active, strong-winged fliers moving singly or in small flocks, often high in the air. Distinctive wing-whistling sound in flight has earned the name of whistlers.
Goldeneyes generally move south late in the season; most of them winter on coastal waters and the Great Lakes. Inland, they like rapids and fast water.
Barrow's goldeneye, predominantly a Westerner, is less wary than the common goldeneye.
Hens of both species are look-alikes.
Drakes have a piercing speer-speer--hens a low quack. Both are usually quiet.
Bufflehead
Length--14-1/2 in. Weight--1 lb.
Stragglers migrate south in mid-fall, but the largest numbers move just ahead of freezeup. Most flocks in feeding areas are small--5 or 6 birds, with more hens and immatures than adult drakes.
Very small size, bold black and white color pattern, and low, swift flight are field marks. Unlike most divers, they can fly straight up from a watery takeoff.
Largest concentrations are on both seacoasts and along the Gulf of Mexico. Inland, they will remain as far north as open water permits.
Usually silent. Drakes squeak and have a guttural note; hens quack weakly.
Ruddy
Length--15-1/2 in. Weight--1-1/3 lbs.
The ruddy duck often dives or swims away from danger rather than flying. When flying, their small wings stroke so fast they resemble bumblebees.
They are early to mid-fall migrants.
Drakes often cock their tails upright at an angle, the only species to habitually do so.
Both hens and drakes are silent in the fall.
Red-Breasted Merganser
Length--23 in. Weight--21-1/2 lbs.
These birds winter most abundantly in coastal waters, including the Gulf of Mexico, and to a lesser extent, the Great Lakes.
Their flight, strong and direct, is usually low over the water. They are difficult to distinguish in flight from the common merganser.
Voice: Seldom heard.
Ducks At a Distance: a Waterfowl Identification Guide · The Wunder Library — complete classics, free to read, with narration.