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| Caspian Gull is a large gull but slender in its proportions. Compared to a Herring Gull, it is of similar size or slightly larger (some males very large) but appears longer winged, longer necked, longer legged and longer billed. |
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| On a standing bird, there is a smooth line along the back and into the long narrow blade-like primaries, and the profile of the back is often straight and unbroken without a “step” between the tertials and primaries, in contrast to Herring Gull which has a significant tertial "step". Be aware that in cold and/or windy weather, gulls adopt a more hunched and horizontal posture. |
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| In flight, the dark primaries and secondaries are separated by a weak “window” made up of pale grey on the inner webs of the inner primaries. The brown outer greater coverts form a second block of brown in front of the brown secondaries along the trailing edge of the wing, separated by a distinct white line made up of the pale tips to the coverts. |
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The tail of first-winter birds is white at the base with a strong black terminal band, which may “fray” into finer black barring towards the white tail base. On fresh birds there may in fact be narrow white tips to the tail feathers.
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| Caspian Gull has a distinctive posture when calling. Herring and Yellow-legged Gulls keep their wings closed when giving the “long” call, with heads either raised or lowered. Caspian Gulls have a strong tendency to partially open their wings in an “albatross posture” when performing its unique hoarse nasal long call. |
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| The underwing of first-year birds is usually very pale, appearing white from a distance, although in fact the coverts are intricately marked with pale brown. Even where the coverts may be slightly more heavily marked, the axillaries (armpit) are mostly white. |
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Caspian Gull in first winter plumage is a rather neat gull being mostly white on the head neck and breast, largely grey on the back, brown and white on the wing coverts, and dark black-brown on the primaries. First-winter birds usually show limited head-streaking, being very white on the forehead, crown, cheeks, throat and upper breast. The lower breast and flanks are more smudged with greyish brown markings but are still predominantly white. There may be a light dusky patch made up of diffuse streaking around the eye, and the nape frequently shows narrow longitudinal streaking that can be very sharply defined in extent, forming an obvious “shawl” on the nape and neck sides. The bill is usually all-black on first-winter birds, although the base may start to fade to dull greenish-black in late winter; the bill may rarely show some pink in the base, but this is rather uncommon. The bill is often slender on first-year birds and may show little gonydeal angle. The small dark eye and its positioning on a white head gives Caspian Gull its characteristic gentle expression.
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Caspian Gull in first-year plumage shows a lot of grey in the feathers of the mantle and scapulars, usually with narrow dark-shaft streaks and a brown sub-terminal crescent, forming an anchor mark. The markings on the mantle can be quite broad and bold but they are often rather fine and delicate, allowing the scapulars in particular to appear largely grey. The wing coverts in first-winter Caspian Gull are often a rather uniform earth-brown, with fine pale fringing and tips, and lacking strong patterning or notching. The outer greater coverts in particularly are often solidly brown with pale outer fringes and slightly wider white tips and this can produce a quite solid block of brown on the closed wing, demarcated by a line of white tips from the usually blackish primaries. The tertials are blackish brown with narrow white edging and a somewhat broader white tip, sometimes referred to as a “thumbnail”, often split by a dark shaft. They do not show the notching along the edges that is a feature of Herring Gull. Beyond the tertials on the folded wing, the primaries are black without any white tips or mirrors and form a long narrow projection giving the gull a tapered streamlined appearance and the folded primaries form a very sharp narrow point.
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| Although subtle differences in proportions of the head are rather difficult to portray in words, it is clear that Caspian Gull has a distinct “look” about its face and head. The classic Caspian Gull has a small “pear-shaped” head on a long slim neck, with a long sloping forehead, flat crown and angular nape. The small head, small eye, slender bill and long gape line give it a gentle smiling “facial expression”, without the sharp and aggressive look of a Herring Gull. |
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| Cachinnans has a characteristic posture, standing tall on long legs with the wing-tips often dropped below the horizontal. Often they seems to have a trim belly and a full high breast bulging just below the throat (although of course other gulls may adopt a similar stance). |
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| The underwing of first-year birds is usually very pale, appearing white from a distance, although in fact the coverts are intricately marked with pale brown. Even where the coverts may be slightly more heavily marked, the axillaries (armpit) are mostly white. |
