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The most reliable field mark between Great Black-backed Gull and Lesser Black-backed Gull is overall size and bulk: Great Black-backed is much larger and more massive, with a heavier build and thicker bill. For immatures, both show scaly brown upperparts, but Great Black-backed juveniles are bulkier with a paler face and a more contrasting, patterned tail. Adults are best separated by leg color (Lesser has yellow legs, Great Black-backed has pinkish), mantle shade (Great Black-backed is much darker), and overall size. In flight, Great Black-backed appears broader-winged and more imposing, while Lesser is slimmer and more agile. Always consider age and season, as immature plumages are similar but differ in details of tertials, tail, and structure.

1. Size and bulk: Great Black-backed Gull is much larger and heavier than Lesser Black-backed. Use this first when both are present, but beware of lone birds or distant views.
2. Leg color (adults): Adult Great Black-backed has pinkish legs; Lesser Black-backed has yellow legs. This is a key mark in spring/summer, but mud or lighting can obscure leg color.
3. Mantle and upperwing shade (adults): Great Black-backed shows a blackish upperwing and back, darker than Lesser Black-backed's dark grey to blackish (depending on race). In mixed flocks, Great Black-backed stands out as the darkest-mantled large gull.
4. Bill structure: Great Black-backed has a thick, powerful bill, noticeably deeper and heavier than Lesser Black-backed's slimmer, more tapered bill. This is visible both at rest and in flight.
5. Juvenile tertials: Juvenile Lesser Black-backed has more solidly dark (unbarred) tertials, while Great Black-backed's tertials are patterned and more contrasting. Use this mark in first-year birds, especially at rest.
6. Tail pattern (juveniles): Great Black-backed juveniles have a white tail with zigzag bars and a broken blackish band near the tip; Lesser Black-backed's tail is less distinctly marked. This is best seen in flight or when preening.
7. Facial pattern (juveniles): Great Black-backed juveniles have a paler face and nape, contrasting with the darker crown and body. Lesser Black-backed is more uniformly dark.
8. Head color in winter (adults): Lesser Black-backed shows a greyer head in winter, while Great Black-backed remains whiter-headed. This helps in non-breeding season.
9. Primary mirrors (adults): Lesser Black-backed has smaller white 'mirrors' at the wing tips compared to Great Black-backed, but this can be subtle and often requires close views.
10. Flight style: Great Black-backed appears broad-winged, heavy, and steady in flight; Lesser Black-backed is slimmer, more agile, and buoyant. Watch for overall jizz, especially in mixed flocks.
11. Behavior: Great Black-backed is more likely to be seen dominating other gulls, pirating food, or actively hunting birds and mammals. Lesser Black-backed is less aggressive.
12. Habitat and range: If in North America, Great Black-backed is more expected in the northeast and on rocky coasts; Lesser Black-backed is increasing but still less common, especially inland.
13. Voice: If heard, Great Black-backed's call is much deeper and more resonant than Lesser Black-backed's.
14. Aging confusion: Both species take four years to mature and show similar scaly juvenile plumage. Always check tertials, tail, and structure before relying on color alone.
15. Pitfalls: Lighting can make mantle color and leg color unreliable. Size is best judged with direct comparison. Immature birds are the most challenging; focus on tertials, tail, and bill structure for best results.

Great Black-backed Gull is the largest gull in the North Atlantic, noticeably outsizing Lesser Black-backed Gull in all respects. While Lesser Black-backed averages around 817.5 g [wikidata], Great Black-backed is substantially heavier and bulkier (though exact mass not given here, it is always described as much larger). In the field, Great Black-backed appears massive, with a thick neck, deep chest, and a heavy, powerful bill. Lesser Black-backed is large but slimmer, with a more graceful, less hulking build. Side-by-side, the size difference is obvious, but at a distance or with lone birds, structural cues become more important.

Juvenile and immature birds of both species have scaly, checkered black-brown upperparts and streaked heads, but there are key differences. Juvenile Great Black-backed Gulls show a paler face and nape, a white tail with zigzag bars and a broken blackish band near the tip, and a brownish-black bill with a white tip. Their tertials are patterned, and the overall effect is more contrasting and 'clean' compared to Lesser. Juvenile Lesser Black-backed Gulls have a similar scaly pattern but are best told by their more solidly dark (unbarred) tertials. Both species take four years to mature, but Great Black-backed's development is slower, and its immatures remain bulkier and more streaked. Adults are more straightforward: Great Black-backed has a blackish upperwing and back, pinkish legs, and a very large bill; Lesser Black-backed has dark grey to blackish upperparts (depending on subspecies), yellow legs, and a slimmer bill. In winter, Lesser Black-backed shows a greyer head, while Great Black-backed remains whiter-headed.

Great Black-backed Gull is notably aggressive, often behaving more like a raptor than a typical gull, hunting and killing prey up to the size of ducks and even other gulls. It is dominant in mixed flocks and often seen pirating food from other birds. Lesser Black-backed is less aggressive and more likely to scavenge or forage in flocks. In flight, Great Black-backed appears powerful and steady, while Lesser Black-backed is more agile and buoyant. Both species are opportunistic, but Great Black-backed's predatory behavior is distinctive.

Both species occur along North Atlantic coasts, but Great Black-backed is more likely to be found in northern Europe and northeastern North America, often on rocky coasts and islands. Lesser Black-backed breeds in Europe but is increasingly common along the east coast of North America, especially in winter. Lesser is more migratory, wintering as far south as West Africa, while Great Black-backed is more sedentary except for far northern breeders. In mixed flocks, Great Black-backed is usually the largest gull present.

Great Black-backed Gull has a deep, 'laughing' cry (kaa-ga-ga), with a distinctly deeper tone than most other gulls. Lesser Black-backed Gull's calls are not described in detail here, but are generally higher-pitched and less resonant. Voice is not usually the primary field mark, but the deep, bovid-like quality of Great Black-backed's call can help in close encounters.