コシジロアジサシ 対 エリグロアジサシ

The most reliable field mark between Aleutian Tern and Black-naped Tern is the Aleutian's white forehead (in adults and juveniles), contrasting with the Black-naped's uninterrupted black eye-stripe and nape band. In flight, Aleutian Tern (especially adults) shows a dark bar across the secondaries, while Black-naped Tern's wings are pale with only a narrow black edge on the outermost primary. Both have black bills and legs, but Aleutian Tern is slightly heavier and has a darker, duskier look overall. Juvenile Aleutian Terns can be especially confusing, but their white collar and forehead, plus lack of a continuous black nape band, are key. Habitat is also a strong clue: Aleutian Tern is subarctic, Black-naped is strictly tropical/oceanic.

1. White Forehead (Adult Aleutian Tern): Adult Aleutian Tern in breeding plumage shows a conspicuous white forehead contrasting with a black cap. Black-naped Tern never shows a white forehead; its black band continues unbroken from the eye around the nape.
2. Juvenile Aleutian Tern: Juvenile Aleutian Tern has a white collar and extensive white forehead, which can be confused with juvenile Arctic Tern but not with Black-naped Tern, which lacks both features.
3. Dark Secondary Bar (Aleutian Tern): In flight, adult Aleutian Tern shows a strong dark bar across the secondaries (upperwing), present in both breeding and non-breeding plumages. Black-naped Tern lacks this bar, showing only a narrow black edge on the outermost primary.
4. Black Nape Band (Black-naped Tern): Black-naped Tern is named for its black band from the eye around the nape, forming a continuous loop. Aleutian Tern's black cap does not form a nape band; instead, it is interrupted by the white forehead.
5. Bill and Leg Color: Both species have black bills and legs, so this is not a useful separator.
6. Underparts and Mantle: Aleutian Tern has duskier, mid-grey mantle and underparts (especially in breeding adults), while Black-naped Tern is cleaner white overall with only pale silvery-grey on the wings and mantle.
7. Tail Shape: Both have deeply forked white tails, but Black-naped Tern's tail is especially long and often projects well beyond the wingtips when perched. Aleutian Tern's tail is long but not as exaggerated.
8. Perched Posture: When perched, Black-naped Tern's tail extends noticeably past the wingtips, giving a more attenuated look. Aleutian Tern's tail is long but usually not as strikingly so.
9. Habitat Context: Aleutian Tern is found in subarctic Alaska and eastern Siberia, rarely if ever in tropical waters. Black-naped Tern is strictly tropical/subtropical and oceanic, rarely seen inland or in cold climates.
10. Juvenile Pitfalls: Juvenile Aleutian Tern may lack the adult's dark secondary bar, but still shows the white forehead and collar. Juvenile Black-naped Tern is not described in detail but is not likely to show these features.
11. Non-breeding Aleutian Tern: In non-breeding plumage, Aleutian Tern loses the white forehead, but retains the dark secondary bar and gains a white-speckled crown. Black-naped Tern's head pattern remains consistent year-round.
12. Flight Appearance: Aleutian Tern in flight appears duskier, with a strong wing bar and darker underparts; Black-naped Tern is cleaner white and more silvery, with only a narrow black edge to the outermost primary.
13. Size: Aleutian Tern is slightly heavier and longer, but this is rarely obvious unless both species are seen side by side.
14. Foraging Behavior: Aleutian Tern may be seen hawking insects over freshwater, especially juveniles; Black-naped Tern is almost always seen over saltwater.
15. Range: If seen outside the subarctic or in the tropics, Black-naped Tern is vastly more likely. Aleutian Tern is not expected in the tropics or far offshore in warm waters.

Aleutian Tern averages slightly heavier (112 g vs. 106 g) and is a bit longer (32–39 cm vs. 30–35 cm), with a broader wingspan (75–80 cm vs. 66 cm). In the field, this difference is subtle and rarely diagnostic, as both are medium-sized, long-tailed terns. Both have deeply forked tails, but Black-naped's tail often appears even longer relative to its body, extending well past the wingtips when perched.

Juvenile Aleutian Tern has a white collar, extensive white forehead, white underparts, and a gray tail with white outer webs. It generally lacks a clear dark bar on the secondaries, but adults (breeding and non-breeding) show a strong dark bar across the secondaries in flight. Black-naped Tern, at all ages, is mostly white with a narrow black band from the eye around the nape, pale silvery-grey wings, and a white tail. Its only dark wing mark is a narrow black edge on the outermost primary. Aleutian Tern adults have a black cap with a white forehead, mid-grey mantle, duskier underparts, and black bill/legs, while Black-naped adults are cleaner white overall, with no forehead patch and a distinctive black nape band. In non-breeding Aleutian Tern, the white forehead bar disappears, but the secondary bar remains, and the crown becomes white-speckled. Black-naped Tern hatchlings are mottled brown, but this is rarely seen in the field.

Both species are agile flyers and feed mainly by surface-dipping for small fish. Aleutian Tern may also hawk insects over freshwater ponds, especially juveniles. Black-naped Tern is highly oceanic, rarely seen inland, and often forages far from shore. Aleutian Tern is more likely to be seen foraging nearshore, over rivers, or tidal flats, sometimes up to 50 km offshore. Both species have black bills and legs, and neither is a strong swimmer. In flight, Aleutian Tern may appear duskier and more robust, while Black-naped Tern looks cleaner and more buoyant, with a strikingly long white tail.

Aleutian Tern breeds in subarctic Alaska and eastern Siberia, favoring coastal colonies and foraging over shallow waters, rivers, and tidal rips. It is never found in tropical or subtropical waters. Black-naped Tern is strictly oceanic, found in tropical and subtropical parts of the Indian and western Pacific Oceans, and is rarely seen inland. If you are in the tropics or on a remote coral island, Black-naped is far more likely; if in subarctic Alaska or Siberia, Aleutian is the default.