カスピセグロカモメ 対 キアシセグロカモメ

The most reliable field separation between Caspian Gull and Yellow-legged Gull, especially in immature birds, is the structure: Caspian Gull shows a longer, slimmer bill, longer legs and neck, and a more attenuated, elegant look, while Yellow-legged Gull is bulkier and more compact. In first-winter birds, Caspian Gull often shows paler underparts and a distinctive set of pale lines across the wing coverts, whereas Yellow-legged Gull juveniles have a darker, more heavily marked wing and a bold black tail band. Adults are trickier, but Caspian's paler legs (often pinkish) and longer, more parallel-sided bill contrast with the bright yellow legs and shorter bill of Yellow-legged. Both species can be similar in size and general coloration, so careful attention to structure and age-specific plumage is key.

1. First-winter/juvenile wing coverts: Caspian Gull shows two pale lines across the greater and median coverts, forming a distinctive double bar; Yellow-legged Gull lacks these, with more uniformly dark and heavily marked coverts. This is most reliable in birds less than one year old.
2. Tail pattern in juveniles: Yellow-legged Gull has a bold, well-defined black tail band contrasting with a paler rump; Caspian Gull's tail band is usually less sharply defined and the rump is paler, but this can be subtle.
3. Head and neck streaking: Caspian Gull juveniles have a pale head with dark streaking on the nape, giving a 'hooded' look from behind; Yellow-legged Gull juveniles have a paler, less streaked head overall.
4. Underparts: Caspian Gull first-years are paler below, often with a washed-out look; Yellow-legged Gull is more heavily marked and generally darker on the underparts.
5. Bill shape: Caspian Gull has a long, slender, parallel-sided bill with a gentle curve; Yellow-legged Gull's bill is shorter, deeper, and more robust. This is visible in all ages but most helpful in adults and older immatures.
6. Leg color (adults): Caspian Gull's legs are pale pink to yellowish; Yellow-legged Gull's are bright yellow. This is a classic adult mark, but beware of faded or muddy legs.
7. Eye color: Caspian Gull often has a small, dark eye; Yellow-legged Gull's eye is paler, often with a red orbital ring (visible at close range).
8. Structure/jizz: Caspian Gull looks longer-legged, longer-necked, and more elegant, with a sloping forehead and attenuated rear; Yellow-legged Gull is bulkier, with a more rounded head and compact body. This is best judged with direct comparison or experience.
9. Wing tip pattern (adults): Caspian Gull's outermost primary has a large white tip and a white 'tongue' up the inner web; Yellow-legged Gull has more extensive black with fewer white spots. This is subtle and best seen in good flight views or photos.
10. Back color: Both have grey backs, but Caspian Gull is slightly paler than Yellow-legged Gull. This is only useful in direct comparison.
11. Seasonal head color: Yellow-legged Gull adults are much whiter-headed in autumn, while Caspian can show more streaking, but this is variable and not always reliable.
12. Bill color (immatures): Both species have dark bills in first year, but Yellow-legged retains a black tip into second winter; Caspian's bill becomes paler more quickly.
13. Flight impression: Caspian Gull in flight appears longer-winged and more graceful; Yellow-legged is broader-winged and more compact.
14. Habitat context: If in urban western Europe, Yellow-legged is more likely; in eastern wetlands or steppe lakes, Caspian is more probable, but overlap is increasing.
15. Pitfalls: Both species are variable and can hybridize with Herring Gull and each other; always use a combination of marks, not a single feature.

Both species are large gulls and overlap in size, but Caspian Gull averages slightly heavier (body mass around 1154g [wikidata]). Caspian Gull measures 56–68 cm in length with a 137–155 cm wingspan, while Yellow-legged Gull ranges from 52–68 cm in length and 120–155 cm in wingspan. In the field, size alone is unreliable due to overlap, but Caspian Gull often looks longer and more slender, while Yellow-legged Gull appears stockier and more robust. Bill, wing, and leg measurements also overlap, but Caspian's proportions (longer bill, legs, and neck) give it a more stretched appearance.

In first-winter and juvenile birds, Caspian Gull has a pale head with dark streaking on the nape, pale underparts, and a greyish back. The greater and median coverts show pale tips, forming two pale lines across the wing—these pale covert bars are a key mark in flight and at rest. Yellow-legged Gull juveniles have a paler head and underparts than Herring Gull, but are generally darker and more heavily marked than Caspian, with a bold black tail band and dark flight feathers. Their wing coverts are less neatly patterned and lack the Caspian's double pale lines. By second winter, Yellow-legged Gulls are mostly adult-like but retain some patterned coverts and black bill tip, while Caspian Gulls also approach adult plumage but maintain a cleaner, more washed-out look. Adults of both species have grey backs (Caspian slightly paler than Yellow-legged), but Caspian's wing tips show a large white tip to the outermost primary and a white tongue on the inner web, while Yellow-legged has more extensive black with fewer white spots. Leg color is a strong adult mark: Caspian's legs are pale pink to yellowish, while Yellow-legged's are bright yellow. Eye color can help: Caspian often has a small, dark eye; Yellow-legged's is paler with a red orbital ring.

Both species are omnivorous and opportunistic, often seen scavenging at rubbish tips, harbors, and urban areas. Caspian Gull tends to appear more elegant and upright, especially when alert, due to its longer legs and neck. In flight, Caspian Gull can look more graceful and elongated, while Yellow-legged Gull appears more compact and powerful. Both can be aggressive at breeding colonies and around food sources.

Yellow-legged Gull is widespread in southern Europe, the Mediterranean, and parts of the Middle East and North Africa, often breeding in coastal colonies, urban rooftops, and sometimes inland. Caspian Gull breeds further east, especially around the Black and Caspian Seas, but both species can overlap in winter and at migration stopovers, particularly in central and eastern Europe. Habitat alone is not diagnostic where ranges overlap, but Yellow-legged is more urban-adapted in the west, while Caspian is more associated with open wetlands and steppe lakes.