Large-billed Tern vs Gull-billed Tern

In short

The most reliable field mark between Large-billed Tern and Gull-billed Tern is the bill: Large-billed Tern has a very large, bright yellow bill, while Gull-billed Tern’s bill is shorter, thicker, and black. Both are robust, short-tailed terns with similar overall structure, but Large-billed Tern is slightly larger and has yellow legs and feet, versus the black legs of Gull-billed. Plumage differences are subtle in adults, but non-breeding and juvenile birds can be separated by crown and facial pattern. Habitat can help: Large-billed Tern is almost always found along South American rivers and lakes, while Gull-billed Tern is more cosmopolitan and often coastal.

Identification tips

1. Bill color and shape: Large-billed Tern has a very large, bright yellow bill that is longer and more slender than the short, thick, black bill of Gull-billed Tern. This is the most reliable field mark at all ages and seasons.
2. Leg and foot color: Large-billed Tern has yellow legs and feet, while Gull-billed Tern has black legs and feet. This is visible when birds are perched or standing.
3. Crown and facial pattern (breeding): In breeding plumage, Large-billed Tern shows a full black crown and ear-coverts, while Gull-billed Tern has a black cap but lacks the extensive ear-covert patch.
4. Crown and facial pattern (non-breeding/juvenile): Non-breeding Large-billed Tern has a paler crown and forehead but retains a black ear-covert patch; Gull-billed Tern loses the black cap entirely, showing a dark eye-stripe or mask instead.
5. Upperparts and underparts: Both species have grey upperparts and white underparts, so this is not a useful separator.
6. Tail and wing structure: Both are short-tailed and robust, but Gull-billed Tern may look broader-winged and more gull-like in flight. Tail shape is not strongly diagnostic.
7. Size: Large-billed Tern is slightly larger and heavier on average, but overlap is considerable; use only as supporting evidence.
8. Habitat: Large-billed Tern is almost always found along tropical South American rivers and lakes; Gull-billed Tern is more likely in coastal, saltmarsh, or even inland wet fields, especially outside South America.
9. Feeding behavior: Gull-billed Tern often hawks insects over land and fields, while Large-billed Tern is more strictly piscivorous and tied to open water.
10. Voice: Gull-billed Tern’s 'ker-wik' call is distinctive; Large-billed Tern’s call is not described in the sources.
11. Juvenile confusion: Juvenile Gull-billed Tern has a faint mask but otherwise resembles winter adults; Large-billed Tern juveniles are not described in detail, but the yellow bill should be present from an early age.
12. Geographic context: If seen in South America away from the coast, Large-billed Tern is much more likely; in coastal or non-South American settings, Gull-billed Tern is expected.
13. Perched vs. flying: Bill and leg color are easiest to see when perched; in flight, bill size and color may still be visible with good light.
14. Seasonal changes: Both species lose the full black cap in non-breeding plumage, but facial pattern and bill/leg color remain reliable.
15. Potential pitfalls: Do not rely on upperwing or tail pattern, as neither species shows distinctive age-specific flight marks according to available sources.

Size

Large-billed Tern averages about 38 cm in length and 235 g in mass [wikidata], making it slightly larger and heavier than most Gull-billed Terns, which range from 33–42 cm in length and 150–292 g in mass [wikipedia]. Both are robust terns, but Large-billed Tern is typically at the upper end of the size range. In the field, size alone is rarely diagnostic unless the birds are side-by-side, as overlap exists.

Plumage

Juvenile and non-breeding plumages are most useful for separation. Juvenile Gull-billed Terns have a faint dark mask through the eye but otherwise resemble winter adults, with a pale cap and little facial contrast. Large-billed Tern non-breeding adults show a paler crown and forehead, but retain a distinctive black ear-covert patch and yellow bill. In adults, both species have grey upperparts and white underparts, but Large-billed Tern’s black crown is more extensive in breeding plumage. The black cap of Gull-billed Tern is lost in winter, replaced by a dark eye-stripe. Legs and feet are yellow in Large-billed, black in Gull-billed. No distinctive age-specific upperwing or flight pattern marks are noted for either species in the sources.

Behaviour

Large-billed Tern is typically seen along wide rivers and lakes, often perched on sandbars or flying low over water. Gull-billed Tern is more versatile in feeding, often hunting insects over fields and even away from water, and is less tied to aquatic habitats. In flight, both species appear robust and short-tailed, but Gull-billed Tern may show broader wings and a more gull-like profile, while Large-billed Tern’s flight is direct and purposeful over water.

Habitat

Large-billed Tern is almost exclusively found along rivers and freshwater lakes in tropical South America, rarely straying far from these habitats. Gull-billed Tern is much more cosmopolitan, breeding in scattered localities across Europe, Asia, Africa, and the Americas, and is often found in coastal areas, saltmarshes, and even wet fields. In North America, Gull-billed Tern is largely coastal, while Large-billed is absent outside South America except as a rare vagrant.

Voice

Gull-billed Tern gives a characteristic 'ker-wik' call, which is quite distinctive. No specific vocalization is described for Large-billed Tern in the sources, so voice is not a primary field mark for separation.