King Eider vs American Moorhen

In short

The most reliable field mark to separate King Eider from American Moorhen is overall structure and bill shape: King Eider is a large, heavy-bodied sea duck with a thick, sloping bill (males with a prominent yellow knob), while American Moorhen is a much smaller, slimmer marsh bird with a thin, pointed bill topped by a red shield. In flight or on water, King Eider shows a robust, rounded profile and sits low, whereas Moorhen is upright and often seen walking or swimming with a jerky gait. Plumage differences are stark in adult males but more subtle in females and juveniles, so pay close attention to size, shape, and bill details when plumage is not obvious. Habitat context is also a strong clue: King Eider is strictly coastal and marine, while Moorhen is found in freshwater marshes and ponds.

Identification tips

1. Size is a major clue: King Eider is more than three times heavier and much longer than American Moorhen. If the bird looks duck-sized or larger, it's not a moorhen.
2. Bill shape and color: King Eider has a thick, sloping bill (male with a huge yellow knob; female/juvenile with a black or grey bill), while Moorhen has a thin, pointed yellow bill with a red shield in adults. Juvenile Moorhens lack the shield but still have a slender bill.
3. Head and face pattern: Female/juvenile King Eiders show a buffy spot at the bill base and a buffy eye ring curving behind the eye, plus fine streaks on the face. Moorhen never shows these marks; its face is plain except for the red shield in adults.
4. Plumage texture and markings: Female/juvenile King Eiders have chevroned upperparts and streaked necks, while Moorhens are plain dark brown or grayish with a white undertail and no chevrons or streaks.
5. Adult male King Eider is unmistakable: Look for a black-and-white body and a multicolored head with a massive yellow knob above the bill. No moorhen ever shows this.
6. Juvenile confusion: Young King Eiders are greyish-brown but still larger and chunkier than any moorhen, with a thicker bill and chevroned patterning. Juvenile Moorhens are plainer brown and much slimmer.
7. Leg color: Moorhen has yellow legs; King Eider's legs are greenish-grey (female/juvenile) and not as long relative to body.
8. Behavior on water: King Eider sits low, dives frequently, and rarely leaves the water. Moorhen swims with a jerky motion, often flicking its tail to show the white undertail, and readily walks or runs on floating vegetation.
9. Flight style: King Eider flies strongly and directly over open water, often in lines or flocks. Moorhen's flight is weak and fluttery, usually just above the marsh or pond surface.
10. Habitat context: Eider is marine/coastal; Moorhen is freshwater/marsh. If you see the bird in saltwater, it's almost certainly not a moorhen.
11. Vocalizations: If you hear loud gargling or hissing, it's Moorhen. King Eider is usually silent.
12. Tail and undertail: Moorhen flashes a white undertail as it flicks its tail; King Eider's tail is short and not prominently marked.
13. Sexual dimorphism: Only King Eider shows strong sexual dimorphism (male vs female), while Moorhen sexes are similar except for the red shield in adults.
14. Seasonal changes: Young male King Eiders in late autumn may show some white on the breast and rump, but will still be much larger and heavier than any moorhen.
15. Mixed flocks: These species never flock together; if you see both, habitat and size will make separation easy.

Size

King Eider is dramatically larger and heavier than American Moorhen. Adult King Eiders average about 1617 g in mass and measure 50–70 cm in length with an 86–102 cm wingspan [wikidata][wikipedia]. In contrast, American Moorhen is much lighter (310–456 g), shorter (32–35 cm), and has a smaller wingspan (54–62 cm) [wikipedia]. Even a female or juvenile King Eider dwarfs any Moorhen. If you see the two together, the size gap is unmistakable.

Plumage

Juvenile and female King Eiders are warm brown with blackish chevrons above and fine streaks on the head and neck, plus a buffy spot at the bill base and eye ring. Juveniles are greyer-brown overall, and young males in late autumn start to show white on the breast and rump, but do not reach full adult male coloration for three years [wikipedia]. Adult male King Eiders in breeding plumage are unmistakable: mostly black body, white breast, and a multicolored head with pale blue-grey, green cheek, and a huge yellow bill knob. American Moorhen adults are dark overall with a white undertail, yellow legs, and a red frontal shield above the bill; the bill itself is thin and pointed. Juvenile Moorhens are browner and lack the red shield. In all plumages, Moorhen lacks the chevroned or streaked brown patterning of female/juvenile King Eider and never shows the eider's thick bill or any hint of a bill knob.

Behaviour

King Eider is a powerful diver, mostly seen swimming low on the sea or flying in strong, direct lines over water. It rarely walks on land and is never seen climbing through marsh vegetation. American Moorhen is a marsh specialist, often walking or running on floating vegetation thanks to its long toes, and can be seen swimming with a jerky, head-bobbing motion. Moorhen may upend in shallow water to feed, a behavior never seen in eiders.

Habitat

King Eider is strictly marine outside the breeding season, found in coastal Arctic and subarctic seas, rarely venturing into freshwater except on breeding grounds [wikipedia]. American Moorhen is a freshwater marsh bird, common around ponds, lakes, canals, and vegetated wetlands throughout the Americas [wikipedia]. If you are in a saltwater coastal environment, expect eider; in a reedy pond or marsh, expect moorhen.

Voice

American Moorhen is often heard giving a variety of gargling calls and loud hisses, especially when alarmed or defending territory [wikipedia]. King Eider is generally quiet, especially outside the breeding season; its calls are not typically heard in the field and are not a practical field mark for separation.