オオハシアジサシ 対 アメリカオオアジサシ

要点

The most reliable way to separate Large-billed Tern from Royal Tern is by bill and leg color: Large-billed Tern has a strikingly large yellow bill and yellow legs, while Royal Tern has a stout orange-red bill and black legs. In all plumages, Large-billed Tern is shorter-tailed and more compact, with a unique jizz among terns. Immature Royal Terns show darker marks on the wings and a yellower bill, but never match the yellow-legged, yellow-billed look of Large-billed. Both species can show white foreheads outside the breeding season, so focus on bare part color and structure for positive ID. Size is not reliable at a distance, but Royal Tern is heavier and longer overall.

識別のコツ

1. Bill color and size: Large-billed Tern always shows a large, thick, bright yellow bill; Royal Tern's bill is orange-red (or yellowish in juveniles), never as bright yellow or as thick as Large-billed. This is the single most reliable mark at all ages.
2. Leg color: Large-billed Tern has yellow legs and feet, while Royal Tern's are black. This is visible when birds are perched or standing.
3. Tail and structure: Large-billed Tern is short-tailed and compact, with a 'chunky' jizz; Royal Tern is longer, more streamlined, and longer-tailed. In flight, Large-billed appears more front-heavy due to its bill.
4. Crown and crest: Both species have a black crown in breeding, but Royal Tern's is shaggier and extends into a crest. In non-breeding, both develop white foreheads, so this is not always reliable.
5. Juvenile/immature wing pattern: Juvenile Royal Tern shows darker gray to blackish marks on the wings, which Large-billed Tern lacks. If you see a young tern with dark wing markings and a yellowish bill, Royal is likely.
6. Bill shape: Large-billed Tern's bill is not only yellow but also proportionally larger and thicker than Royal Tern's, which is stout but more pointed and slightly decurved.
7. Underparts and upperparts: Both species are pale gray above and white below; this is not a useful separator.
8. Habitat context: Large-billed Tern is almost always found over rivers and freshwater lakes, especially inland South America; Royal Tern is coastal and marine. If you see a large tern inland in South America, Large-billed is much more likely.
9. Voice: If you hear a short, clear 'kree' or rolling whistle, Royal Tern is likely. Large-billed Tern's call is not described here, so absence of these calls is not diagnostic.
10. Size: Royal Tern is much heavier and longer, but this is hard to judge unless both are together. Use structure and bill/leg color first.
11. Non-breeding confusion: Both species can show white foreheads in non-breeding plumage. In this season, rely on bill and leg color, not crown pattern.
12. Perched birds: Look for yellow legs and bill (Large-billed) vs black legs and orange-red bill (Royal).
13. Flight jizz: Large-billed Tern in flight looks front-heavy and short-tailed; Royal Tern is longer, more elegant, and less 'top-heavy.'
14. Pitfalls: Juvenile Royal Tern's bill can appear yellowish, but never as bright or thick as Large-billed; always check leg color and structure.
15. Geographic range: In North America, Royal Tern is expected on coasts; Large-billed is a rare vagrant. In South America, Large-billed dominates inland, Royal on coasts.

大きさ

Royal Tern is substantially larger and heavier than Large-billed Tern, with a bill-to-tail length of 45–50 cm and a wingspan up to 135 cm, compared to Large-billed Tern's length of about 38 cm [wikipedia]. Royal Tern's weight (350–500 g) is also much greater than Large-billed Tern's (235 g) [wikidata]. In the field, this size difference can be hard to judge unless the birds are side by side, but Royal Tern will generally appear longer-winged and bulkier, while Large-billed Tern is more compact and short-tailed.

羽色

Juvenile and non-breeding Royal Terns show darker gray to blackish marks on the wings and a yellower bill, while Large-billed Tern juveniles are not described as having such wing markings. Adult Large-billed Tern is pale gray above with a black crown and ear-coverts, white underparts, and a large yellow bill; in non-breeding plumage, the crown and forehead become paler. Royal Tern adults have a black shaggy crest in breeding, which recedes to a patchy or white forehead and crown in non-breeding plumage, with black remaining on the nape. Both species show white underparts and pale upperparts, but only Royal Tern develops extensive blackish wing markings as a juvenile. Leg color is a constant: yellow in Large-billed, black in Royal. Bill color is diagnostic: yellow in Large-billed, orange-red in Royal (yellower in juvenile Royal, but never as bright yellow as Large-billed).

行動

Both species are typical terns in flight, but Large-billed Tern is notably short-tailed and compact, with a heavy, prominent bill that gives it a distinctive profile. Royal Tern is longer-winged and more streamlined, with a stouter, slightly decurved bill. Large-billed Tern is often seen over rivers and lakes, while Royal Tern is more marine and coastal in its habits. Perched, Large-billed Tern's yellow legs and bill are obvious, while Royal Tern stands taller on black legs with a more upright stance.

生息環境

Large-billed Tern is found primarily along rivers and freshwater lakes in South America, east of the Andes and north of the Pampas [wikipedia]. Royal Tern is a coastal species, found along marine shorelines throughout the Americas, rarely inland. If seen away from the coast, Large-billed Tern is much more likely, especially in South America; Royal Tern is rare inland except as a vagrant.

声

Royal Tern gives short, clear shrills, often described as 'kree' or 'tsirr,' and has a longer, rolling, more melodious whistle. No distinctive vocalization is described for Large-billed Tern in the provided notes, so voice is not a reliable separator unless Royal Tern's shrill calls are heard.