Size: variable; in general smaller as only the same or slightly bigger than accompanying graellsii. However, a number of individuals (about 10% from 1993-94, 7% in 2001) are substantially bigger than graellsii and thought to be similar in size to Mediterranean Yellow-legged Gull, hence bigger than those in the central Canaries.

Mantle: the smaller birds are a paler grey than those in the Canaries but still have a distinct blue tinge. They are much paler than graellsii. The larger birds have paler mantles than the smaller birds, with a lead-grey tint.

Underwing: for both types, the dark trailing edge is much less obvious than in graellsii over both primaries and secondaries.

Structure: the larger birds appear sturdy alongside the smaller birds with heavier heads and necks, longer legs and stouter bills. Their heads are flat crowned with angular sides, hence less rounded. The larger birds also hold their bills pointing down at rest in hot weather whereas the smaller birds tuck their heads into their necks or hold them horizontally out in compact fashion.

Primaries: not greatly different for the smaller birds. There is slightly more black on P5 with as many as 92% (76%) carrying a definite band on P5. On the outer primaries there is slightly less black with 76% (87%) showing a solid triangle. There is slightly more white with 31% (23%) of birds carrying a mirror on P9. Dwight noted (1925 p.207) that the Mediterranean form of cachinnans on the western limits of its range has a tendency in the wing-tip for only one mirror (P10).

Calls: for the smaller birds, similar to those in Tenerife for both long and mew calls, lacking the guttural tones associated with Mediterranean Yellow-legged Gull.

Moult in adults: whiter heads for both sizes; showing less mottling than the Atlantic forms, as found in, say, north-west Morocco. The heads appear all-white at a distance although at close range a number show speckling around the eye and sometimes on the crown as well. The period of moult is similar to that of Mediterranean Yellow-legged Gull in south-west France (see later) and is slightly later than that in the central Canaries with a frequent pattern of P6 growing and P7 missing in mid-August and P7 growing and P8 missing in late August.

First-years: other than a tendency to be slightly heavier, not readily separable from those in the central Canaries, sharing in general dusky heads, grey-brown mantles (some with rusty tinge), gleaming white tails and indistinct pale primary panels. The tertials, however, show a broad white tip only to the rusty-brown centre as in Mediterranean Yellow-legged Gull.

Comment: structurally and in mantle colour and calls, the smaller birds are similar to Dark Atlantic Yellow-legged Gull except that their mantle shade is slightly paler, their heads are quite white in August and there is more likelihood of a mirror on P9, a slight indent on the black triangle on the outer primaries and a definite band on P5. The larger birds are similar to Mediterranean michahellis.