Grey-backed Tern vs Great Crested Tern

In short

Juvenile and immature Great Crested Terns show a distinctive, strongly variegated upperwing pattern with dark bars, especially visible in flight—Grey-backed Tern immatures lack this feature and are much plainer above. For adults, the most reliable field mark is the bill: Great Crested Tern has a long, thick, yellow-green bill, while Grey-backed Tern’s bill is slimmer and dark. Great Crested Tern is also much larger and bulkier, with a shaggier black crest and a paler, battleship-grey back compared to the Grey-backed Tern’s clean grey mantle. Both species have white underparts, but the facial pattern and structure are key in any season.

Identification tips

1. Juvenile/immature upperwing pattern: In flight, look for a strongly variegated upperwing with dark bars and a dark inner flight feather bar in Great Crested Tern immatures—Grey-backed Tern immatures are plain above. This is the single best mark for young birds.
2. Bill color and shape: Great Crested Tern has a long, thick, yellow to greenish-yellow bill (never orange or black), visible at any age. Grey-backed Tern’s bill is slimmer and dark. Bill color is reliable even at a distance with good light.
3. Size and structure: Great Crested Tern is much larger, with a longer bill, heavier head, and bulkier body. Grey-backed Tern is small, slim, and compact. Use this when both species are present, but beware of scale illusions when alone.
4. Crest and head shape: Great Crested Tern shows a shaggy, glossy black crest that extends well behind the head, especially in breeding plumage. Grey-backed Tern lacks a shaggy crest and has a neat, smooth head profile.
5. Back color: Great Crested Tern has a battleship-grey back (darker and duskier), while Grey-backed Tern’s back is a cleaner, paler grey. This is subtle but can help in good light.
6. Facial pattern: Grey-backed Tern has a distinctive black eye-line from bill to nape, giving a 'spectacled' look. Great Crested Tern has a white forehead and a black cap or crest, but lacks the through-the-eye line.
7. Underparts: Both are white below; this is not a useful separator.
8. Wing pattern in adults: Both have grey upperwings, but Great Crested Tern’s are duskier and broader; Grey-backed Tern’s are narrower and lighter.
9. Perched posture: Great Crested Tern’s long bill and crest stand out when perched, while Grey-backed Tern looks more delicate and neat-headed.
10. Vocalizations: Loud, harsh 'kerrak' and 'korrkorrkorr' calls point to Great Crested Tern, especially in colonies. Grey-backed Tern is quieter and less raucous.
11. Colony context: Large, noisy, dense colonies on open sandbars or beaches are more likely to be Great Crested Tern. Grey-backed Tern is less likely to be in such settings.
12. Seasonal head pattern: In winter, Great Crested Tern’s crown becomes white with a peppered black crest; Grey-backed Tern keeps the black eye-line.
13. Association with other terns: Great Crested Tern often associates with other large terns (e.g., Lesser Crested, Royal), and stands out by size and bill. Grey-backed Tern is less likely to be in such mixed groups.
14. Flight style: Great Crested Tern flies with strong, deep wingbeats and a heavier jizz; Grey-backed Tern is more buoyant and agile.
15. When in doubt: Focus on bill color and thickness, head crest, and—if the bird is young—the presence or absence of upperwing barring.

Size

Great Crested Tern is substantially larger: adults measure 46–49 cm in length with a 125–130 cm wingspan and weigh 325–397 grams [wikipedia]. Grey-backed Tern is much smaller and lighter, with a more delicate build. This size difference is striking in mixed flocks or when seen side-by-side, but can be harder to judge at a distance or alone.

Plumage

• Juvenile/immature plumage: Great Crested Tern juveniles and first-winters have a boldly patterned upperwing with dark bars and a variegated look, including a dark bar on the inner flight feathers. This pattern persists into early immature plumages. Grey-backed Tern immatures are much plainer, lacking strong barring or variegation on the upperwing, and show a more uniform grey back.
• Adult breeding plumage: Great Crested Tern adults have a white forehead, black shaggy crest, and battleship-grey back and inner wings. Grey-backed Tern adults have a smooth grey back, a black eye-line from bill to nape, and lack the shaggy crest; their head pattern is 'spectacled' with a black line through the eye.
• Non-breeding/winter plumage: Great Crested Tern’s crown becomes white with a peppered black crest and mask, while Grey-backed Tern maintains the black eye-line and grey back. Both species have white underparts year-round.

Behaviour

Both species are typical terns in flight, but Great Crested Tern appears heavier, with a longer, thicker bill and a bulkier head. Its flight is strong and direct, often with deep wingbeats. Grey-backed Tern is more buoyant and agile, reflecting its smaller size and lighter build. Perched, Great Crested Tern’s long bill and shaggier crest are obvious, while Grey-backed Tern looks neater and more compact.

Habitat

Both are coastal and island-nesting terns, but Great Crested Tern is more likely to be seen in large, dense colonies along open coastlines, sandbars, and offshore islands, often with other large terns. Grey-backed Tern is less common and more localized, often found on remote islands and less likely to be in large mixed flocks. Context can help, but is not always diagnostic.

Voice

Great Crested Tern is highly vocal, especially at colonies, giving a loud, raucous, crow-like 'kerrak', a 'korrkorrkorr' at the nest, and a hard 'wep wep' in flight. Grey-backed Tern’s voice is less well described in the sources, but it is not noted for such loud, harsh calls. If you hear the characteristic Great Crested Tern calls, it is a strong clue.