The Herring Gull Larus argentatus complex has perplexed taxonomists for many years. The adults of the southern group of this complex, as defined by Cramp (1983), have yellow legs and have often colloquially been called Yellow-legged Herring Gulls. Recent studies have suggested that the southern Herring Gulls in the Mediterranean and west Asia areas are worthy of status as a separate species: Yellow-legged Gull L. cachinnans. This species occurs in two main forms: michahellis in the Mediterranean, here termed Mediterranean Yellow-legged Gull, and nominate cachinnans, now termed Caspian Gull, in the Black and Caspian Seas area (Klein, 1994; Garner & Quinn, 1997; Garner, Quinn & Glover, 1997; Jonsson, 1998). Further east two further forms Baraba Gull barabensis (Panov & Monzikov, 2000) and Mongolian Gull mongolicus (Yésou, 2001) have recently been described in detail.

The classification above omits the gulls of the southern group found on the eastern Atlantic coasts from northern Spain southwards, through Portugal, Morocco and Macronesia to Mauritania. In this paper such gulls are termed Atlantic Yellow-legged Gulls. A number of forms of Atlantic Yellow-legged Gull have been described of which the best known is atlantis (Dwight, 1922) of the eastern Atlantic archipelago (that is Macronesia, including the Azores, Madeira and the Canaries). On the Iberian Atlantic coast, preliminary forms established are lusitanius in south-west Portugal (Joiris, 1978) and cantabricans in northern Spain (Teyssèdre, 1983,1984; Dubois, 1987; Carrera et al, 1987; Yésou, 1993; Burger & Goedfeld, 1996). However, two sources, Snow & Perrins (1997) and Bermejo (1999), consider that all the birds on the Atlantic Iberian coast are of a single distinguishable form, termed lusitanius and Atlantic Iberian Yellow-legged Gull respectively. Populations on the Atlantic coast of Morocco have been considered by some authors, for instance Stegmann (1934) and Bannerman (1963), to be similar to atlantis but many accounts include them under michahellis with no apparent justification. Urban, Fry & Keith (1986) describe pale and dark forms found in north-west Africa, attributing them to michahellis and (probably) atlantis respectively, and Barlow, Wacher & Disley (1997) describe atlantis as found in Gambia in west Africa.

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