Slender-billed Gull vs Sooty Gull

In short

The most reliable field mark between Slender-billed Gull and Sooty Gull is the overall darkness of the plumage: Sooty Gull adults are strikingly dark above with a contrasting white belly, while Slender-billed Gull is pale grey and white throughout. Immature Slender-billed Gulls show a black tail band and dark wing areas, but never approach the deep sooty tones of Sooty Gull. Structurally, Slender-billed Gull has a long, slender, red bill and a long neck, giving it a distinctive 'elegant' jizz, while Sooty Gull is chunkier with a heavier bill. Habitat overlap is possible along some coasts, but Sooty Gull is strictly coastal and tropical, while Slender-billed Gull is more often found in lagoons and estuaries around the Mediterranean and northwest Indian Ocean.

Identification tips

1. Plumage Darkness: Adult Sooty Gull is dark brownish-black above with a white belly, while Slender-billed Gull is pale grey above and white below. This contrast is visible at all ages.
2. Juvenile/Immature Pattern: First-year Slender-billed Gull shows a black terminal tail band and dark areas on the wings, but remains overall pale; Sooty Gull immatures are dark and never show a clean black tail band on a white base.
3. Bill Shape and Color: Slender-billed Gull has a long, slender, dark red bill that appears slightly downcurved and tapers to a fine point; Sooty Gull has a heavier, shorter bill, often with a yellow tip, and never appears as fine or red.
4. Leg Color: Slender-billed Gull has dark red legs, while Sooty Gull's legs are duller and not as brightly colored.
5. Neck and Head Shape: Slender-billed Gull appears long-necked and small-headed, with a gentle, elongated profile. Sooty Gull is thicker-necked and blockier overall.
6. Head Color: Adult Slender-billed Gull has a white head year-round (sometimes with a faint pink flush in summer), while Sooty Gull's head is dusky or dark, blending into the mantle.
7. Primary Pattern in Flight: Slender-billed Gull shows black wingtips contrasting with pale grey upperwings; Sooty Gull has uniformly dark upperwings with a white trailing edge, much less contrast.
8. Breast Color: In summer, Slender-billed Gull may show a faint pink wash on the breast, never seen in Sooty Gull.
9. Tail Pattern: Juvenile Slender-billed Gull has a distinct black tail band; Sooty Gull does not show this feature at any age.
10. Habitat: Sooty Gull is strictly coastal and tropical, usually on coral islands or sandy beaches; Slender-billed Gull is more often found in lagoons, estuaries, and sometimes inland lakes.
11. Behavior: Sooty Gull is often seen scavenging around fishing boats and feeding on carrion or seabird eggs; Slender-billed Gull is more likely to be seen feeding on fish by dipping or probing mud.
12. Size and Structure: Slender-billed Gull is more slender and elegant, with a longer neck and bill; Sooty Gull is bulkier and heavier-billed.
13. Iris Color: Slender-billed Gull has a yellow iris, which may be visible at close range; Sooty Gull's iris color is not specified but is not a noted field mark.
14. Range: If seen far from the tropics or away from the Red Sea/Indian Ocean coasts, Sooty Gull is highly unlikely; Slender-billed Gull is more likely in the Mediterranean and northwest Indian Ocean, sometimes wandering further.
15. Pitfalls: Immature Slender-billed Gulls can look darker than adults but never approach the sooty tones of Sooty Gull. Always check overall darkness, bill shape, and habitat together.

Size

Slender-billed Gull averages about 255.5g in body mass, making it a mid-sized gull, slightly larger than Black-headed Gull. Sooty Gull's mass is not specified, but it is generally similar in size or slightly larger, with a heavier, more robust build. In the field, Slender-billed Gull appears more elongated and delicate, with a longer neck and bill, while Sooty Gull is more solid and broad-winged.

Plumage

Juvenile and first-year Slender-billed Gulls have a distinctive black terminal tail band and dark areas on the wings, which can be confused with other immature gulls but not with the deep, uniform sooty-brown upperparts of Sooty Gull of any age. Adult Slender-billed Gulls are pale grey above, with white head and underparts, black primary tips, and in summer, a faint pink flush on the breast. Their bill and legs are dark red. Sooty Gull adults are dark brownish-black above (mantle, back, and upperwings) with a sharply contrasting white belly and underparts. The head is dusky or dark, and the bill is heavier, often with a yellow tip. Immature Sooty Gulls are also dark but may show more mottling; they never show the clean pale look or pinkish tones of Slender-billed. In flight, Sooty Gull shows a dark upperwing with a white trailing edge, while Slender-billed Gull is pale with black wingtips.

Behaviour

Slender-billed Gull is often seen feeding by dipping and diving over water, sometimes probing mud, and is gregarious in winter. Sooty Gull is a predator and scavenger, frequently seen around fishing boats, feeding on offal, and also preys on eggs and chicks of other seabirds. Sooty Gull tends to be more coastal and is often seen loafing on coral islands or beaches, while Slender-billed Gull is more likely to be seen inland or in estuaries.

Habitat

Slender-billed Gull breeds and feeds in coastal lagoons, lakes, estuaries, and sometimes grasslands, mainly around the Mediterranean and northwest Indian Ocean, and is rarely far from land. Sooty Gull is strictly coastal, breeding on coral islands and foraging along tropical and subtropical shores of the Red Sea, Arabian Sea, and Indian Ocean. Sooty Gull is almost never found inland, while Slender-billed Gull can be seen at inland lakes and lagoons.