アフリカコガモ 対 キバシガモ

The most reliable field mark between Blue-billed Teal and Yellow-billed Duck is the bill color: Blue-billed Teal has a pale bluish bill (with a dark culmen and nail), while Yellow-billed Duck shows a bright yellow bill. In flight or at rest, Blue-billed Teal is much smaller and more compact, with a shorter neck and tail, and a distinctly spotted and patterned plumage, especially on the neck and breast. Yellow-billed Duck is larger, with a mostly plain grey body, darker head, and a green speculum bordered with white. Juvenile and female Blue-billed Teal can be duller, but always lack the plainness and size of Yellow-billed Duck. Both species have a green speculum, but the surrounding wing pattern and overall structure are different.

1. Bill color is the quickest check: Blue-billed Teal has a pale bluish bill with a dark tip, while Yellow-billed Duck always shows a bright yellow bill. This works for all ages and sexes.
2. Size and structure: Blue-billed Teal is much smaller and more compact, with a short neck and tail; Yellow-billed Duck is large, long-necked, and robust. Use this when birds are seen together or with known species.
3. Head and face pattern: Blue-billed Teal shows a dark brown crown and paler face with a blackish patch on the ear (adult males most striking, but visible in all ages to some degree). Yellow-billed Duck has a uniform dark head with no strong facial markings.
4. Breast and flank spotting: Blue-billed Teal (all ages, but especially adult males) has black spots on the neck and breast, sometimes extending to the flanks. Yellow-billed Duck is plain grey with no spots or streaks.
5. Speculum and wing pattern in flight: Both have a green speculum, but Blue-billed Teal's is bordered by narrow black and white bars and is more compact; Yellow-billed Duck's is larger, with broader white borders and a pale underwing. This is most useful in flight or when wings are stretched.
6. Glossy upperwing coverts: Adult male Blue-billed Teal has a greenish gloss to the wing coverts, lacking in females and juveniles, and absent in Yellow-billed Duck.
7. Underparts: Blue-billed Teal's underparts are spotted and vermiculated, especially toward the rear; Yellow-billed Duck is plain and unmarked below.
8. Leg and foot color: Blue-billed Teal has bluish-grey legs and feet; Yellow-billed Duck's leg color is not distinctive in the field.
9. Juvenile confusion: Juvenile Blue-billed Teal is duller but still shows some spotting and facial contrast; juvenile Yellow-billed Duck is plain and only slightly duller than adults.
10. Group size and behavior: Large flocks in open water are more likely Yellow-billed Duck; small groups or pairs in vegetated pools suggest Blue-billed Teal.
11. Habitat context: Blue-billed Teal is more likely in quiet, vegetated wetlands; Yellow-billed Duck is common in open, disturbed, or suburban water bodies.
12. Vocalizations: Loud quacks or whistles point to Yellow-billed Duck; Blue-billed Teal is mostly silent, with only soft mechanical calls during display.
13. Tail and neck length: Blue-billed Teal has a short tail and neck, giving a compact look; Yellow-billed Duck is longer in both.
14. Sex differences: Male Blue-billed Teal is more contrasting and glossy; female and juvenile Blue-billed Teal are duller but still patterned. Yellow-billed Duck sexes are similar.
15. When in doubt: If the bird is small, compact, spotted, and has a blue bill, it's Blue-billed Teal. If it's large, plain, and yellow-billed, it's Yellow-billed Duck.

Blue-billed Teal is a small duck, measuring 33–35 cm in length and weighing 53–288 g, with a wingspan of 58–69 cm. In contrast, Yellow-billed Duck is much larger, at 51–58 cm long (mallard-sized), and is noticeably heavier and bulkier in the field. The difference in size is often obvious when seen together or with familiar species for scale. Blue-billed Teal appears dainty and compact, while Yellow-billed Duck is robust with a longer neck and tail.

Juvenile and female Blue-billed Teal are duller than males but always show some degree of black spotting on the neck and breast, and a contrasting face pattern, even if less crisp than in adult males. The upperwing in all ages has a green speculum with narrow black and white borders, but the coverts are glossy greenish only in adult males. Yellow-billed Duck of all ages and sexes is mostly plain grey with a darker head, lacking obvious spots or facial contrast; its green speculum is bordered by white, and the rest of the wing is otherwise plain. Juvenile Yellow-billed Ducks are only slightly duller than adults, without strong patterning. In flight, Blue-billed Teal shows a compact body and short wings with a green speculum and white trailing edge, while Yellow-billed Duck shows a larger, broader wing with a green speculum and more extensive white borders, and a pale underwing.

Blue-billed Teal is generally less conspicuous, often seen singly or in small groups, and is less gregarious than Yellow-billed Duck. It is more likely to be found quietly dabbling or resting at the water's edge. Yellow-billed Duck is highly gregarious outside the breeding season, forming large flocks, and is more likely to be seen in open water or flying in groups. Both species dabble for food, but Yellow-billed Duck is more active and visible, especially outside the breeding season.

Both species share freshwater habitats such as lakes, marshes, and ponds, but Blue-billed Teal tends to prefer quieter, vegetated wetlands and is more likely to be found in smaller or temporary pools. Yellow-billed Duck is abundant in open country and larger water bodies, and is more tolerant of suburban or disturbed habitats, including golf courses and parks. Blue-billed Teal is generally less common and more restricted to less disturbed sites.

Yellow-billed Duck has a male whistle and a female quack similar to a mallard, which can be heard in flocks or during courtship. Blue-billed Teal is quieter, with males producing a soft mechanical 'burping' call during display, but is otherwise not very vocal. In the field, the presence of loud quacking or whistling is more likely to indicate Yellow-billed Duck.