アカボシカルガモ 対 カルガモ

The most reliable field mark to separate Indian Spot-billed Duck from Eastern Spot-billed Duck is the color and pattern of the speculum in flight: Indian shows a green speculum with a broad white band at the base, while Eastern has a blue speculum (sometimes described as blue-green) and lacks the broad white band. At rest, Indian Spot-billed Duck (mainland subspecies) has a distinctive red spot at the base of the bill, which is absent in Eastern Spot-billed Duck. Eastern is also overall darker and browner, with plumage more like Pacific Black Duck. Both are similar in size and structure, and both sexes look alike, but careful attention to wing pattern and bill base color will usually clinch the ID. Juveniles of both are duller and browner, so flight views and bill color are especially important for immature birds.

1. Speculum color in flight (all ages): Indian Spot-billed Duck shows a green speculum with a broad white band at the base; Eastern Spot-billed Duck has a blue speculum and lacks the broad white band. This is the most reliable field mark, especially in flight or when preening.
2. Bill base color (adults, especially males): Indian Spot-billed Duck (mainland) has a red or orange-red spot at the base of the bill; Eastern Spot-billed Duck lacks this red spot. Females may have a less obvious spot, and it is absent in some Indian subspecies (haringtoni), so use with caution in border regions.
3. Overall plumage tone: Eastern Spot-billed Duck is darker and browner overall, with body plumage more like Pacific Black Duck. Indian Spot-billed Duck is paler, especially on the head and neck.
4. Bill tip: Both have a black bill with a yellow tip, so this is not diagnostic.
5. Leg color: Both have bright orange to coral-red legs and feet; not useful for separation.
6. Juvenile appearance: Juveniles of both are browner and duller, but the speculum color and bill base mark (if present) remain useful. In dull birds, prioritize wing pattern and bill base.
7. Resting birds: At rest, look for the white tertials forming a stripe on the side in Indian Spot-billed Duck, which may be less obvious in Eastern.
8. Sex differences: Both sexes are similar; male Indian Spot-billed Duck has a more obvious red bill spot, but this is not always reliable for sexing or separating species.
9. Eclipse plumage: Neither species has a distinctive eclipse plumage; adults retain similar appearance year-round.
10. Flight jizz: Both species are similar in flight, with no noted differences in wing shape, primary projection, or tail.
11. Habitat and range: Indian Spot-billed Duck is resident in the Indian subcontinent; Eastern Spot-billed Duck breeds in East/Southeast Asia and is migratory. If seen outside the overlap zone, range is a strong clue.
12. Group behavior: Indian Spot-billed Duck is often in pairs or small groups; Eastern Spot-billed Duck is more likely to be seen in small flocks, especially outside breeding season, but this is not diagnostic.
13. Voice: Both sound like Mallard; do not use voice for separation.
14. Hybrid caution: Both can hybridize with Mallard, especially in eastern Russia, so odd individuals may show mixed features. Use multiple marks for confident ID.
15. Pitfalls: Beware of the absence of the red bill spot in some Indian Spot-billed Duck subspecies (haringtoni) and the duller appearance of juveniles in both species. Always prioritize speculum color and bill base in combination.

Both species are very similar in size, matching Mallard proportions: 55–63 cm in length, 83–95 cm wingspan, and adult body mass around 1195 g (Indian Spot-billed Duck; Eastern is in the same range per Wikipedia). No meaningful difference in size or structure is noted between the two.

• Juveniles/immatures: Both are browner and duller than adults, but the key difference remains in the speculum color when the wing is spread: Indian Spot-billed Duck (all ages) shows a green speculum with a broad white band at the base, while Eastern Spot-billed Duck has a blue speculum and lacks the broad white band. Juvenile bill color is less bright, but the red spot (Indian) or its absence (Eastern) is still a clue in older juveniles.
• Adults: Indian Spot-billed Duck (mainland) has a red spot at the bill base, a green speculum with broad white band, and is paler overall. Eastern Spot-billed Duck lacks the red spot, has a blue speculum, and is darker and browner, with body plumage closer to Pacific Black Duck. Both have a black bill with a yellow tip, orange to coral-red legs, and a scaly pattern on the body. Both sexes are similar, and neither has a distinctive eclipse plumage.

Both species are dabbling ducks, feeding mainly by dabbling in shallow water, often in the evening or at night. Indian Spot-billed Duck is usually seen in pairs or small groups and can take off nearly vertically from water. Eastern Spot-billed Duck is more gregarious outside breeding season, forming small flocks. Both are capable of diving when threatened, especially during moult. No major behavioral differences are noted that aid field separation.

Indian Spot-billed Duck is a resident breeder throughout freshwater wetlands of the Indian subcontinent and is non-migratory. Eastern Spot-billed Duck breeds in East and Southeast Asia, is migratory, and winters further south. Both favor freshwater lakes and marshes in open country. Overlap is limited to far eastern India and Southeast Asia, so range is a strong clue except in overlap zones.

Both species have calls similar to Mallard: females give a loud quack, males a deeper, quieter sound. No diagnostic difference in voice is noted between the two.