1. darker mantles than those of Mediterranean michahellis.
2. 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.
3. larger size than graellsii by 1-10% in terms of a number of measurements. Indeed they are quite similar to argenteus in overall size.
4. sturdy structure, with heads looking large for the body, stout necks, strong bills, thick legs and a full chest broad at the base.
5. 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.
6. nearly always (90% of cases) a large white mark on P10; in 45% of cases this appears to be in the form of a complete tip, though this figure is undoubtedly exaggerated by wear.
7. sometimes (25% of cases) a white mirror on P9.
8. 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; no pale edges show when the primaries are fully spread.
9. a complete black band on P5 in about 75% of cases; in one-third 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.
10. P4 is often unmarked but sometimes carries a small black spot on the outer web.
11. 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,
12. 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.
13. 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
14. bright scarlet red orbital ring.
15. long calls like a 'tinny' graellsii, being shriller and less resonant than those of graellsii; mew calls close to graellsii.
16. early period of moult with adults in August typically having P1-P6 new, P7 growing, P8 missing, P9-P10 old.
17. incubation typically in April or early May.

Within the Dark Atlantic Yellow-legged Gull populations, a number of regional variations can be detected:

1. Mantles: slightly paler on south Moroccan coast; slight purplish tinge in some south Portuguese birds.
2. Size: slightly smaller in south Morocco where they are only slightly larger than graellsii.
3. Structure: a significant proportion are distinctly lightweight in the eastern Canaries (20-50%) and those in the western Canaries and Azores (Bakker, 2000) tend to have finer bills with a more tapering tip.
4. Primaries: in the eastern Canaries, while the black triangle on the wingtip is normally extensive, 20% have a slightly indented black area. In the Azores, there is less white in the wingtip as P10 carries a smaller mirror (Chavigny & Mayaud, 1932) and P9 is usually unmarked, carrying a mirror in only 13% of adults (Teyssèdre, 1983). In south Portugal and the western Canaries only 10% show a mirror on P9.
5. Calls: in most of the eastern Canaries (Lanzarote, Fuerteventura and Gran Canaria); long calls are generally argenteus-like but perhaps slightly deeper and mew calls also tend to be argenteus-like. In south Portugal and the western Canaries long calls are slightly deeper being closer to graellsii. In Madeira the calls were thought by Teyssèdre (1983) to be deeper than those in the Canaries, perhaps making them close to graellsii.
6. 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. In the Azores (Garner, 1998; Jonsson, 1998), the head can appear quite fully hooded with dense and extensive dark streaking extending to the neck as well. The breast is, however, white except sometimes for a few spots on the sides (Bakker, 2000).

© Copyright Nick Rossiter 1999-2001.