ホオジロガモ 対 キタホオジロガモ

The most reliable field mark between Common Goldeneye and Barrow's Goldeneye is the shape and placement of the white facial patch on adult males: Common Goldeneye shows a round white spot below the eye, while Barrow's Goldeneye has a crescent-shaped patch at the base of the bill. Female Barrow's Goldeneye also stands out with a mostly yellow bill, in contrast to the Common Goldeneye female's mostly dark bill with only a yellow tip. Both species are similar in size and structure, but subtle differences in head shape, bill size, and gloss color on the male's head can help. Immature and female birds are especially tricky, but careful attention to bill color and head shape usually resolves confusion.

1. Adult male facial patch: The single most diagnostic mark: Common Goldeneye has a round white spot below the eye; Barrow's Goldeneye shows a vertical crescent at the base of the bill. This is reliable in all seasons for adult males.
2. Head gloss (male): Common Goldeneye males have a greenish sheen to the head, while Barrow's Goldeneye males show a purplish gloss. This can be subtle and depends on lighting, so use with caution.
3. Back pattern (male): Barrow's Goldeneye males have more black on the back compared to the more white-backed Common Goldeneye males, but this is subtle and best used as supporting evidence.
4. Bill size (male): Common Goldeneye (especially the North American subspecies) has a longer and thicker bill than Barrow's Goldeneye. This is subtle and best used in direct comparison.
5. Adult female bill color: Barrow's Goldeneye females have a mostly yellow bill, while Common Goldeneye females have a dark bill with only the tip yellow. This is the best mark for females.
6. Female head shape: Common Goldeneye females have a less rounded head profile compared to the more peaked or angular head of Barrow's Goldeneye females. This is subtle but useful at close range.
7. Eye color: Both species have golden-yellow eyes, so this is not a separator.
8. Juvenile/immature separation: No distinctive upperwing or flight pattern differences are described for young birds; rely on bill color and head shape, but expect more overlap and uncertainty at these ages.
9. Size and structure: Both species are similar in size and overall structure; do not rely on size for separation.
10. Habitat context: If you are in an area where Common Goldeneye is much more numerous, default to that species unless a clear Barrow's field mark is seen. Barrow's is more likely in urbanized estuaries and less likely to overlap with Common Goldeneye.
11. Behavior: Barrow's Goldeneye may be more territorial, especially among males, but this is not a reliable field mark for quick identification.
12. Vocalizations: Both are generally quiet; wing whistling is not diagnostic.
13. In flight: No distinctive differences in upperwing pattern or flight silhouette are noted in the sources; identification in flight relies on the same head and bill features as when perched.
14. Seasonal variation: All main field marks are present year-round; eclipse or non-breeding plumages are not described as confusing in the sources.
15. Pitfalls: Immature and female birds are most likely to be confused; always check bill color and head shape, and use multiple field marks when possible.

Both species are medium-sized sea ducks and are nearly identical in mass (Common Goldeneye: ~900 g, Barrow's Goldeneye: ~910 g) [wikidata]. Length and wingspan overlap considerably: Common Goldeneye males average 45–51 cm long with a 77–83 cm wingspan, while Barrow's Goldeneye males are about 49 cm long with a 70–73 cm wingspan [wikipedia]. In the field, size is not a reliable separator.

Adult males are best separated by the facial patch: Common Goldeneye has a round white spot below the eye, while Barrow's Goldeneye shows a vertical crescent at the base of the bill. The head gloss differs—greenish in Common, purplish in Barrow's—but this can be subtle and light-dependent. Male Barrow's Goldeneye also has more black on the back. Females are more challenging: Barrow's Goldeneye females have a mostly yellow bill, while Common Goldeneye females have a dark bill with only the tip yellow. Female Common Goldeneye also shows a less rounded head profile. Juvenile and immature plumages are not described in detail in the sources, so separation at these ages relies mainly on the same bill and head shape differences, though these may be less pronounced. No distinctive upperwing pattern differences are noted for immatures or adults in these sources.

Both species are diving ducks, foraging underwater for aquatic invertebrates and plant material. Barrow's Goldeneye is noted to be more territorial, especially among males, with displays and occasional fights along territory boundaries. Common Goldeneye is generally more numerous and less territorial. In flight, both species produce a whistling sound with their wings, but this is not a reliable separator between them.

Both species winter on open inland or protected coastal waters. Barrow's Goldeneye tends to use more urbanized, coastal estuaries and is less likely to overlap with Common Goldeneye, which is much more numerous. Barrow's is also more likely to nest in tree cavities and is considered more arboreal. In areas of overlap, Common Goldeneye is likely to be the default species.

Barrow's Goldeneye is generally quiet, vocalizing mainly during breeding and courtship with low squeaks, grunts, and croaks. Both species' wings produce a whistling sound in flight. No distinctive vocal field marks are noted to separate the two in typical birding situations.