But this individual cannot for certain be considered as a proven heuglini as there seems not to be any characters diagnostic for heuglini and even the late primary moult associated with heuglini can even occasionally occur in graelsii as well. The problem of separating graellsii and heuglini is complicated by the recent discovery that graellsii occur occasionally in Finland. This has been proven by the arrival in Finland of birds ringed as pulli in England, the Netherlands and Norway. Thus, only on range it is most likely to be a heuglini.
Features suggesting heuglini in these pictures are upper-parts grey-tone, the state of primary moult and general structure. The bird is obvious paler than typical fuscus and the primary moult has not yet started: P1-P10 still present, but primaries still in relative good condition in June. The primary moult of graellsii and intermedius usually commences in May and continues until November/December whereas heuglini moults later, commencing in June/July and often not being completed until January/February. The general structure, especially slender, long-legged and “elongated” look together with head-shape and bill is often a good clue for heuglini.
The size is larger than fuscus and proportions are rather more elegant than typical graellsii with longer primary projection. In graellsii the projection of the primaries beyond the tail is less than or equal to the tarsus length (measured from the centre of the knee joint to the ground on a standing bird).
There is only a relative small mirror on P10 situated further from the feather tip than in typical graellsii. There is no mirror on P9 unlike the average graellsii which has mirrors on both P9 and P10. There is black on 8 primaries, with the markings on P4 and P3 confined to the outer web. Both heuglini and graellsii can have black on a total of 6, 7 or 8 primaries, but a larger proportion of heuglini have black on 8 primaries than graellsii. Adult heuglini also have a greater tendency to have black markings on the primary coverts than graellsii.
Red spot is confined to lower mandible in contrast to graellsii and intermedius in which the red frequently extends onto the upper mandible in mid-summer. Differences in the frequency of red on the upper mandible (less frequent in heuglini) are due to overlap not particularly useful for identifying individual birds.
On probability it is a heuglini but it does not look like the average heuglini pictured from Oman and other countries of the Middle East. Structurally these large light mantled Finnish gulls look somehow intermediate between the stockier graellsii and the lanky heuglini. Could they stem from a hybridization zone with fuscus?
