コサギ 対 セジロミゾゴイ

The most reliable field mark to separate these two is overall plumage: adult Little Egret is entirely white, while White-backed Night Heron is a dark, rufous-breasted heron with a distinctive white triangle on its back and a black head. Juvenile White-backed Night Herons show streaked breasts and white spots on the upperwing coverts, never matched by Little Egret at any age. Structurally, Little Egret is more slender with a long, thin black bill and black legs with yellow feet, while White-backed Night Heron is chunkier, with yellow legs and a shorter, heavier bill. In flight or at rest, these differences are clear if you focus on color contrasts and leg color.

1. Juvenile/immature wing and breast pattern: Juvenile White-backed Night Heron has a streaked breast and white spots on the upperwing coverts; Little Egret juveniles are plain white, sometimes with a few greyish or brownish feathers but never streaked or spotted. This is the fastest check for young birds in flight or perched.
2. Overall color: Adult Little Egret is entirely white, while White-backed Night Heron is mostly dark with a black head, rufous neck and breast, and a white triangle on the back. If you see any rufous or black on the head or breast, rule out Little Egret.
3. Bill color and shape: Little Egret has a long, slender, entirely black bill; White-backed Night Heron has a shorter, heavier bill (color not specified, but not black and slender like egret).
4. Leg and foot color: Little Egret has black legs with yellow feet (juveniles: greenish-black legs, duller feet); White-backed Night Heron has yellow legs. This is visible in both perched and wading birds.
5. Eye color: White-backed Night Heron has large red eyes with white eye-rings; Little Egret has a yellow iris with no prominent eye-ring.
6. Head pattern: White-backed Night Heron has a black head with a short crest (feather display), while Little Egret’s head is all white, with long, narrow nape plumes in breeding season.
7. Back pattern: The white triangular patch on the night heron’s back (from the scapulars) is unique; Little Egret’s back is plain white without any contrasting patch.
8. Breast and neck color: White-backed Night Heron has rufous (reddish-brown) neck and breast; Little Egret is white throughout.
9. Throat: White-backed Night Heron has a white throat contrasting with rufous breast; Little Egret is uniformly white.
10. Bare part color in breeding season: Little Egret’s lores turn red and feet may turn red at height of courtship; White-backed Night Heron’s lores are pale yellow.
11. Structure and jizz: Little Egret is more slender, longer-legged, and graceful; White-backed Night Heron is stockier, shorter-necked, and more hunched, especially when perched.
12. Behavior: Little Egret is active and diurnal, often foraging openly; White-backed Night Heron is secretive, crepuscular or nocturnal, and prefers cover.
13. Habitat: Little Egret is found in open wetlands, estuaries, and lakeshores; White-backed Night Heron is tied to wooded wetlands and forested watercourses.
14. Geographic context: Little Egret is widespread in Eurasia and Africa, expanding into the New World; White-backed Night Heron is restricted to sub-Saharan Africa. If seen outside Africa, Little Egret is far more likely.
15. Pitfalls: Beware of Little Egret juveniles with slightly duller legs and feet, but they never show the streaked or spotted pattern of young night herons. No age or morph of Little Egret shows a dark head, rufous breast, or white back patch.

Little Egret is slightly larger, with a length of 55–65 cm and a mass around 532 g [wikidata], compared to White-backed Night Heron's 50–55 cm (no mass given, but generally more compact and heavier-bodied for its length) [wikipedia]. The egret is longer-legged and more graceful, while the night heron appears stockier and shorter-necked.

Juvenile/immature birds are easiest to separate by wing and breast pattern: juvenile White-backed Night Heron has a streaked breast and white spots on the upperwing coverts, while Little Egret juveniles are plain white overall, sometimes with a few greyish or brownish feathers but never streaked or spotted. Adult Little Egret is pure white in all plumages, with long, narrow nape plumes and elongated scapulars in breeding season, and black legs with yellow feet (or greenish-black legs and duller feet in juveniles). White-backed Night Heron adults have a black head with a short crest, red eyes with white eye-rings, rufous neck and breast, white throat, yellow lores, and a striking white triangular patch on the back (from the scapulars). Their belly is whitish-brown and legs are yellow. No white egret shows this combination of dark head, rufous underparts, and white back patch. In all ages, the night heron’s upperparts are much darker and more patterned than the egret’s.

Little Egret is active and diurnal, often seen stalking or running in shallow water, sometimes with wings raised, and frequently feeds in open, exposed situations. It may also feed on land, chasing prey or scavenging. White-backed Night Heron is more secretive and typically crepuscular or nocturnal, often roosting during the day and foraging at dusk or night. It tends to stay close to cover and is less likely to be seen out in the open.

Little Egret is widespread in shallow wetlands, estuaries, and along lakeshores, often in open areas and sometimes in association with humans or livestock. White-backed Night Heron is found in sub-Saharan Africa, usually in dense, wooded wetlands or along forested watercourses, and is much more tied to cover and less likely to be seen in open habitats. Range overlap is minimal outside of vagrancy, but confusion may occur in zoos or collections.