アフリカコガモ 対 アカハシオナガガモ

The most reliable field mark between Blue-billed Teal and Red-billed Duck is the color and shape of the bill: Blue-billed Teal has a pale bluish bill, while Red-billed Duck shows a bright red bill. In flight, the upperwing pattern is also diagnostic—Blue-billed Teal (especially adult males) shows a green iridescent speculum with white and black borders, whereas Red-billed Duck reveals buff secondaries with a black stripe. Juveniles of both species are duller, but the bill color remains a key distinction. Both species have a contrasting dark crown and paler face, but the face pattern and underpart spotting differ. Size is also a clue: Red-billed Duck is noticeably larger and bulkier than Blue-billed Teal.

1. Bill color: Blue-billed Teal of all ages has a pale bluish-grey bill, while Red-billed Duck has a bright red bill (less vivid in juveniles but still reddish). This is the single most reliable field mark at all ages and distances.
2. Size: Red-billed Duck is much larger and bulkier (43–48 cm) than Blue-billed Teal (33–35 cm). In mixed flocks or side-by-side, this difference is obvious.
3. Upperwing pattern in flight: Adult Blue-billed Teal shows a green iridescent speculum with narrow black and white borders; Red-billed Duck has buff secondaries with a bold black stripe. This is visible in all ages but especially striking in adults.
4. Crown and face pattern: Both have a contrasting dark crown and pale face, but Blue-billed Teal (especially males) has a dark brown crown, paler face, and a distinctive blackish ear patch; Red-billed Duck has a blackish cap and nape with a pale face but lacks the ear patch.
5. Breast and flank spotting: Blue-billed Teal shows heavy black spotting on the neck and breast, with larger, less obvious spots on the flanks and abdomen; Red-billed Duck has a dull dark brown body scalloped with white, not spotted.
6. Sex differences: Blue-billed Teal males are more contrasting and glossy, with brighter wing and body markings; females and juveniles are duller but still show the pale bill. Red-billed Duck sexes are similar, with juveniles only slightly duller.
7. Tail and scapulars: Blue-billed Teal has dark brown to black scapulars and tail; Red-billed Duck's tail is not distinctive.
8. Leg and foot color: Blue-billed Teal has bluish-grey legs and feet; Red-billed Duck's leg color is not distinctive in the notes.
9. Jizz and posture: Blue-billed Teal is smaller, more delicate, and sits lower in the water; Red-billed Duck appears larger, longer-bodied, and heavier.
10. Group size: Red-billed Duck is often in large flocks, especially outside breeding season; Blue-billed Teal is more often seen in pairs or small groups.
11. Habitat context: Both species use similar habitats, but Blue-billed Teal may be more common in smaller wetlands or marshes, while Red-billed Duck prefers open water with dense vegetation for nesting.
12. Voice: Not a primary field mark, but Blue-billed Teal males may give a soft 'burping' call, while Red-billed Duck males give a 'whzzt' and females a soft quack.
13. Juvenile confusion: Even in dull juvenile plumage, bill color remains diagnostic—check for bluish in Blue-billed Teal and reddish in Red-billed Duck.
14. Speculum color: In perched birds with wings closed, look for a hint of green gloss in Blue-billed Teal (especially males); Red-billed Duck lacks this gloss, showing buff and black instead.
15. Pitfalls: Do not rely solely on the pale face and dark crown, as both species show this; always check bill color and, if possible, wing pattern in flight.

Blue-billed Teal is a small dabbling duck, measuring 33–35 cm in length and weighing 53–288 g [wikipedia]. Red-billed Duck is substantially larger at 43–48 cm in length [wikipedia]. This size difference is usually obvious in direct comparison or in mixed flocks, with Red-billed Duck appearing longer-bodied and heavier overall. Blue-billed Teal is also more delicate in build, with a proportionally shorter tail and smaller bill.

Juvenile and immature birds of both species are duller than adults, but the bill color remains a strong cue: Blue-billed Teal juveniles have a pale bluish-grey bill, while Red-billed Duck juveniles already show a red bill (though sometimes less vivid). In flight, adult Blue-billed Teal (especially males) display a green iridescent speculum on the secondaries, bordered by narrow black and white bars. Red-billed Duck, by contrast, shows buff-colored secondaries with a bold black stripe—this is visible in all ages. On perched or swimming birds, Blue-billed Teal males have a dark brown crown with a paler face and a distinctive blackish ear patch, plus heavy black spotting on the neck and breast. Females and juveniles are duller and less contrasting but still show the pale bluish bill. Red-billed Duck has a blackish cap and nape, pale face, and a dull dark brown body scalloped with white, lacking the heavy spotting of Blue-billed Teal. Both sexes of Red-billed Duck are similar, with juveniles being duller but still patterned as adults.

Both species are dabbling ducks and feed by dabbling or foraging on land, mainly in the evening or at night. Blue-billed Teal is generally sedentary but can be nomadic in response to drought, while Red-billed Duck is not migratory but will fly long distances to find suitable water. Red-billed Duck is highly gregarious outside the breeding season and forms large flocks, whereas Blue-billed Teal is less demonstrative and often seen in smaller groups. Breeding behavior is more subdued in Blue-billed Teal, with subtle courtship displays, while Red-billed Duck is not described as particularly demonstrative.

Both species are found in freshwater habitats in open country across southern and eastern Africa. Blue-billed Teal is resident in a broader range, including West Africa and Madagascar, and is more likely to be found in smaller wetlands or marshes. Red-billed Duck is abundant south of 10° S and prefers open water bodies with dense vegetation nearby for nesting. Both species can occur together, but Red-billed Duck is more likely to be seen in large flocks on open water.

Blue-billed Teal is generally quiet, with males producing a soft mechanical 'burping' call during display. Red-billed Duck is also quiet, but displaying males give a 'whzzt' call, and females have a soft mallard-like quack. Neither species is particularly vocal, so calls are not the most reliable field mark, but the 'whzzt' of Red-billed Duck may be heard in display.