Underwing: the trailing edge on the underside of the wing is particularly obvious in fresh plumage in September.

Size: on balance smaller; larger ones obviously larger than graellsii but smaller individuals are similar to graellsii.

Structure: on balance slighter; although many show a sturdy structure with 30% looking distinctly heavy, another 50% are heavier in appearance than graellsii but less obviously large in isolation. The remaining 20% are more lightweight showing smaller heads and thinner necks in particular but all still show a stout bill, bold chest, short sturdy legs and broad wings at least up to the carpal. The latter are of a similar size to graellsii but are still heavier structurally showing less attenuation.

Primaries: tendency for slightly less black and slightly more white; in 20% of adults, the extensive black triangle is slightly indented with grey on the inner edge; slightly more (25%) show a mirror on P9. The white tips on the folded wings are small as found in other spring visits (see south Morocco).

Calls: generally appearing very different; in Lanzarote, north Fuerteventura (Corralejo) and Gran Canaria (Las Palmas) all long calls are much closer to argenteus with the same shrill clamour but perhaps at a slightly lower pitch. The mew calls are also similar to those of argenteus. At Las Palmas, long calls were delivered at 30 degrees in May but apparently at the more widespread 60 degrees in September. In Fuerteventura, away from Corralejo, the predominant (55%) long call is still close to argenteus but all mew and alarm calls are similar to those in the central Canaries. In south-east Gran Canaria the mew calls are also closer to graellsii and some long calls approach the shrill graellsii form. The frequency of calling in Lanzarote was more like that of argenteus, the birds being raucous and screaming readily when at all excited. Such behaviour was also noted by Bannerman (1914).

Moult in adults: in mid-May 36% of adults were missing only P1, a further 5% were more advanced up to P2 growing/P3 missing and 59% had still to shed any primary. In August the moult is slightly later on primaries than in the central Canaries and with less pronounced head markings.

Moult in first-summers: in early April some first-years show already a grey tinge to the saddle and a minority have P1 growing and P2 missing. By mid-May only 20% of first-years had yet to shed a primary, 25% were as advanced as P6 growing/P7 missing with the remainder evenly distributed in intermediate states. The birds appear to be paler here in spring and summer than those further west. There is a rapid lightening of the underparts and head during spring. In Fuerteventura from 7th-14th April, 85% had heavily streaked breasts and 100% dusky heads. By 14th-20th May in Gran Canaria only 25% had heavily streaked breasts and 70% dusky heads. Whether the relative paleness is due to the desert environment in the eastern islands or genetic factors is not clear. The pale examples of first-summer/second-winter shown by Dubois (2001) are therefore more typical of those in the eastern Canaries. However structurally they are unusually heavily-built and the long legs may even suggest Mediterranean michahellis. The first-winter for Fuerteventura shown by Small (2001) is more obviously Mediterranean michahellis: the very heavy bill, head and neck and long legs (equal to the height of the bird above the legs) all point in this direction. The structure is very different from that shown for the adults found. At Asilah in north Morocco from 20th-27th August, Mediterranean michahellis were regularly seen flying south with Audouin’s Gull L. audouinii. It is likely that some reach the Canary Islands closest to the Africa coast, particularly Fuerteventura and that some immatures may not return in the following spring.

Second-years: by late September the bare parts are less juvenile-like: the bills nearly always show a pale base extending 1/3 to 1/4 of the length of the bill and the legs are mostly a straw colour with few remaining flesh coloured.

They are significantly more advanced by April particularly in the bare parts. Most by now have a pale area on the bill extending to 2/3 of its total length and most have some yellow in the legs. The general appearance in flight is of contrast with the dark brown-black outer primaries, secondary bar and narrow carpal bar looking very dark compared to the grey scapulars, back and the lesser and median coverts. The greater coverts are often weakly mottled and the underwing coverts are much faded The more retarded individuals still show extensive mottling on the lesser and median coverts, a solid subterminal tail band, a small pink-yellow base to the bill and pink-yellow legs. The more advanced individuals have the bill as adult, except for a black mark on the upper mandible above the red gonydeal spot, pale ochre legs and a subterminal tail band which is beginning to break up, looking sparsely mottled rather than solid. At rest though the advanced birds still show some mottling on the coverts and a very prominent secondary bar. Such birds look almost adult when perched but in flight their immature status is very obvious with the striking brown-black areas on the wing.

In May advanced individuals show pale yellow bills with no black near tip, bright pale ochre legs, a diffuse tail band and a deep blue-grey mantle. The coverts remain mottled and the most obvious dark areas are the outer primaries and the secondary and carpal bars. Retarded individuals still show black on the terminal quarter of the bill, a less obviously clear mantle, more mottling throughout the wings and a solid tail band. Primary moult has not yet started in April or May indicating that the timing is similar to that for adults.

Moult in second-summers: in late September the timing of the primary moult is similar to that in adults with P9 and P10 growing. However, the heads are rather duskier than in adults with a distinct triangle extending back from the eye across the sides of the head. The crown, chin and forehead are all relatively pale but are still finely speckled so that the overall effect is rather dusky.

Third-years: in late September the bill can look quite dusky with a large dark indistinct gonydeal spot and other scattered dark speckles. The legs are usually a pale ochre but some show a pinky tinge, particularly on the feet. The wing tip is adult-like in the extent of black but the white is less developed with usually only a small mirror on P10 and none on P9. The mantle is basically blue-grey but the sheen is less bright than in adults with a number of small grey-brown markings. The tail is virtually all-white. In flight the upperside of the wings show mottling on the carpal, sometimes extending into a broad bar from the greater coverts to the carpal, and traces of a secondary bar. The underside shows remnant dark edges on the tips of the underwing coverts. At rest the remnant secondary bar is usually visible to some extent. In April some still show dusky marks on the head and dull bare parts. By May the head is white, the mantle is a cleaner blue-grey and the bill and legs are almost adult-like being bright yellow and a bright ochre respectively. The gonydeal spot on the bill is large and bright red. The wings and tail are as in September except of course that the feathers are now fully grown.

Comment: in some respects, particularly primary pattern (P10 and P5 markings), the Gran Canaria adults are closer to those of Tenerife but in structure, size and calls they were closer to those of the eastern Canaries and have accordingly been grouped with them.