アカリュウキュウガモ 対 リュウキュウガモ

The most reliable field mark to separate Fulvous Whistling-Duck from Lesser Whistling-Duck is the color of the upper tail coverts in flight: Fulvous shows a striking white band across the rump and black tail, while Lesser has chestnut upper tail coverts with no white. This difference is visible in all ages and is especially diagnostic when the birds are flying away. Fulvous is also larger and longer-legged, with a richer buff head and neck, while Lesser is more compact with a dark crown and a yellow to orange eye-ring. Both species are similar in overall shape and behavior, but careful attention to tail pattern, crown color, and size will resolve most field identifications.

1. White rump in flight (all ages): Fulvous Whistling-Duck shows a bold white band across the rump and black tail when flying away; Lesser Whistling-Duck never shows white here, instead having chestnut upper tail coverts. This is the single most reliable mark in any plumage.
2. Crown color (all ages): Lesser Whistling-Duck has a solid dark (blackish) crown, contrasting with the paler face and neck; Fulvous has a dark stripe down the crown and nape but not a solid cap. This is visible in perched and swimming birds.
3. Flank stripes (adult and juvenile): Fulvous shows pale (whitish) stripes on the flanks, especially when fluffed or preening; Lesser lacks these or shows only faint pale edges.
4. Eye-ring (adult and juvenile): Lesser Whistling-Duck has a distinct orange to yellow eye-ring, which Fulvous lacks. This can be hard to see at distance but is useful at close range.
5. Overall color tone: Fulvous is richer buff on head, neck, and breast, with a darker back; Lesser is more uniformly chestnut brown.
6. Bill and legs: Fulvous has a longer, grey bill and long grey legs, giving a lanky impression; Lesser is more compact with proportionally shorter legs.
7. Size: Fulvous is larger and bulkier, but beware overlap and distance effects—use only in direct comparison.
8. Perched jizz: Fulvous stands taller and more upright, with a longer neck and legs; Lesser is shorter-necked and more compact.
9. Flight style: Both fly with rapid wingbeats, but Lesser often holds its head below the body in flight; Fulvous appears more level.
10. Voice: Fulvous gives a clear, musical whistle ('kee-wee-ooo'); Lesser gives a repetitive, wheezy two-note call. Calls are helpful if heard well but can be confusing at a distance.
11. Habitat context: In South and Southeast Asia, Lesser is much more common, especially in urban wetlands; Fulvous is local and less likely in city parks.
12. Juvenile confusion: Juvenile Fulvous is paler below and duller overall but still shows the white rump in flight; juvenile Lesser is similar to adult but lacks the white rump and has a dark crown.
13. Sex differences: Both sexes are similar in field marks; Fulvous females are slightly duller and smaller, but this is not usually useful in the field.
14. Pitfalls: Do not rely on overall brown color or general shape—both are similar. Always look for the rump/tail pattern and crown color first.
15. When marks fail: In poor light or distant views, focus on flock structure and habitat, but do not attempt identification without a clear view of the rump or crown.

Fulvous Whistling-Duck is distinctly larger and heavier (average adult mass 682 g) compared to Lesser Whistling-Duck (525 g). Fulvous also appears longer-legged and more elongated overall, with a longer neck and bill. Lesser Whistling-Duck is more compact and slightly shorter in body length and wingspan. In mixed flocks or side-by-side, Fulvous will stand taller and look bulkier, but size alone can be hard to judge at a distance or in isolation.

• Juvenile and adult flight views: Fulvous Whistling-Duck always shows a bold white crescent or band across the rump, sharply contrasting with the black tail—this is visible in all plumages and ages. Lesser Whistling-Duck lacks this white; instead, its upper tail coverts are chestnut, blending with the rest of the back and tail.
• Crown and neck: Lesser Whistling-Duck has a noticeably dark (blackish) crown, even in juveniles, while Fulvous has a dark stripe down the crown and back of the neck but not a solid dark cap. Fulvous's head, neck, and breast are a richer buff (fulvous) color, while Lesser is more uniformly chestnut brown.
• Flank stripes: Fulvous shows pale (whitish) stripes on the flanks, which can help in perched birds; Lesser lacks these or shows only faint pale edges.
• Eye-ring: Lesser Whistling-Duck has a distinct orange to yellow ring around the eye, absent in Fulvous.
• Juvenile differences: Juvenile Fulvous is paler below and duller overall, but still shows the white rump in flight. Juvenile Lesser is similar to adult but with less contrast and still lacks the white rump.

Both species are often found in flocks around wetlands and can be seen perching on trees or resting on the ground. Lesser Whistling-Duck is especially gregarious and may be more likely to perch in trees or use tree holes for nesting. Both feed mainly on plant material in wetlands, dabbling and sometimes diving. Fulvous may be seen more often walking with a lankier, long-legged gait. Lesser is noted for being more nocturnal and often rests during the day.

Fulvous Whistling-Duck has a much broader global range, occurring in the Americas, Africa, and Asia, while Lesser Whistling-Duck is restricted to the Indian subcontinent and Southeast Asia. Both favor lowland wetlands with abundant vegetation, such as lakes, marshes, and rice paddies. In overlapping Asian regions, both may occur together, but Fulvous is generally less common and more local. Lesser is often abundant in urban wetlands and may be found in larger daytime roosts.

Both species have prominent whistling calls. Fulvous gives a clear 'kee-wee-ooo' whistle, often heard at night or in flight, and quarrelling birds give a harsh repeated 'kee.' Lesser Whistling-Duck produces a repetitive, wheezy two-note call, sometimes described as 'seasick,' and is very vocal in flight. The calls are similar but Fulvous is clearer and more musical, while Lesser is more nasal and wheezy. Sex differences in Fulvous calls exist but are subtle and not usually useful in the field.