in addition to thefeatures noted for Atlantic Yellow-legged Gulls atlantisin general , the following 17 further features, in general, are common to adult Southern Atlantic Yellow-legged Gulls:
darker mantles than those of michahellis.
a noticeably dark trailing edge on the underwing, due to an extensive dark area on the primaries and a clear dark subterminal area on the secondaries.
larger size than graellsii by 1-10% in terms of a number of measurements. Indeed they are quite similar to argenteus in overall size.
sturdy structure, with heads looking large for the body, stout necks, strong bills, thick legs and a full chest broad at the base.
bills with a pronounced gonydeal angle giving a thick appearance to the terminal third of the bill and a steep angle at the culmen which gives a rather blunt-ended appearance.
nearly always (90% of cases) a large white mark on P10; in 43% of cases this appears to be in the form of a complete tip, though this figure is undoubtedly exaggerated by wear.
infrequently (22% of cases) a white mirror on P9.
extensive black on the outer primaries giving a large dark triangle from the tip of P10 to the visible base of P10 on the leading edge and the outer web of P6 on the trailing edge (87% of cases); no pale edges show when the primaries are fully spread.
a complete black band on P5 in about 75% of cases; in 40% of such cases the band is thin; 25% of birds have a less distinct mark on P5 such as a broken band or a spot.
P4 is usually unmarked but rarely (9%) carries a small black spot on the outer web.
outside moult, bright yellow legs, often a rich ochre; in moult, usually (90%) dull ochre legs with a small minority (10%) also with pinky-yellow feet,
outside moult, bright yellow bills, carrying a large red gonydeal spot often extending just on to the upper mandible; in moult, generally (90%) dull yellow bills.
in moult, most (72%) show a slight hood on the head from extensive spots around eyes, forehead and crown; the remainder (28%) have a more definite hood with heavy speckling on the front, sides and top of the head and a brown wash on the nape
bright scarlet red orbital ring.
long calls like a 'tinny' graellsii, being shriller and less resonant than those of graellsii; mew calls close to graellsii.
early period of moult with adults in August typically having P1-P6 new, P7 growing, P8 missing, P9-P10 old.
incubation typically in April or early May.
Within the Southern Atlantic Yellow-legged Gull populations, a number of regional variations can be detected:
Mantles: slightly paler on south Moroccan coast.
Size: slightly smaller in south Morocco where they are only slightly larger than graellsii.
Structure: a significant proportion are distinctly lightweight in the eastern Canaries (20-50%) and Madeira and those in the western Canaries (Bakker, 2000) tend to have finer bills with a more tapering tip.
Primaries: in the eastern Canaries, while the black triangle on the wingtip is normally extensive, 20% have a slightly indented black area. In the western Canaries and Madeira only 10-16% show a mirror on P9. In the western Canaries and Madeira 20-32% show only a small mirror on P10 and no mirror on P9.
Calls: in most of the eastern Canaries (Lanzarote, Fuerteventura and Gran Canaria) and Madeira long calls are generally argenteus-like but perhaps slightly deeper and mew calls also tend to be argenteus-like. In the western Canaries long calls are slightly deeper being closer to graellsii.
Moult: in Madeira, western Canaries and north Morocco the heads can appear quite dusky in moult with obvious speckling around the eyes, on the forehead and on the crown.
Note: there appears to be a cline in the Canaries, at least in the calls, with western birds having calls like graellsii, central birds (Tenerife) having shrill graellsii calls and eastern birds having calls closer to argenteus. In this cline from west to east, birds also show a tendency for more white and less black in the wingtip and for less pronounced head markings in moult.