Identification Pitfalls
If any of the forms of Atlantic Yellow-legged Gull were to appear in Britain, they would create potential pitfalls if current identification criteria for large gulls were applied. The Atlantic Cantabricans form is likely to cause considerable problems. For instance close scrutiny of an individual from this population from northern Iberia would show a gull appearing very much like a slim argenteus in some respects but with yellow legs. Such a gull might be considered erroneously as a yellow-legged Herring Gull, that is an aberrant Herring Gull, rather than a Yellow-legged Gull. Any argenteus-like gull in Britain with yellow legs could conceivably be cantabricans. This is also the only Atlantic form that could be confused with Caspian Gull as it shows extensive white in the wingtip on P10 and P9 and pale inner webs on the outer primaries. Caspian Gull’s dark eye, broad black band on P5, larger size and longer bill, wings and legs should prevent confusion on reasonable views.
Garner & Quinn (1997) depict wing-tip patterns for putative graellsii x argenteus hybrids showing less black on the wingtip than Mediterranean Yellow-legged Gull without in particular a broad black band on P5. Cantabrican Atlantic Yellow-legged Gulls and some Dark Atlantic Yellow-legged Gulls do have less black on P5 than Mediterranean Yellow-legged Gull and could be erroneously assigned as hybrid graellsii x argenteus on their criteria. An example of the dangers of labeling apparently non-conforming `yellow-legged gulls’ as hybrids is provided by the history of the species in North America. There, the individual, found at Quebec in 1973 (discussed earlier, see Likelihood of Arrival in Britain) was thought initially to be a hybrid smithsonianus x graellsii, but on closer examination was considered to be a Yellow-legged Gull of the race atlantis (Wilds & Czaplak, 1994). Until all Atlantic (and Asian) forms are fully documented, caution should be applied in assigning ‘difficult’ individuals as aberrants or hybrids of other taxa. Harris, Morley & Green (1978) suggested that hybrid graellsii x argenteus would be very rare: "Remarkably few hybrids are known in comparison with the numbers of mixed pairs that have resulted from cross-fostering in Dyfed and Cumbrian colonies; the hybrids must be at some great selective disadvantage".
Another potential pitfall is with yellow-legged argentatus from Scandinavia where Jonsson (1998) identifies two forms: the first from Finnmark, northern Norway; the second from the southern Baltic and eastern Finland. Both show a red orbital ring, a blue-grey tinge and extensive white on P10 (large tip) and P9 (large mirror). The former are relatively dark (as dark as Mediterranean michahellis) and usually have a small and narrow grey tongue on the inner web of P10 and black only to P6 with little black on P7 and P8. The latter are paler-mantled (only slightly darker than argenteus), the black extends to P5, there is little black on P6/P7, a long wing projection and usually a more extensive and lighter tongue on the inner web of P10. The Finnmark birds should be easy to distinguish from Dark Atlantic Yellow-legged Gull because of their limited black in the wingtip. The Baltic birds are more difficult to distinguish but their pale mantles, small amount of black on P7, large mirror on P9, extensive light tongue on the inner web of P10, relatively small orange-red gonydeal spot and large wing projection are very useful indicators.
Other features of Dark Atlantic Yellow-legged Gulls which may cause confusion are their short legs, their bluish tinge, the darkness of their mantles and the structure of the wingtip with P9 and P10 about the same length and P8 not much shorter than P9 giving a bunched effect of the three primary tips for P8-P10 on the folded wing. Their small size and their rounded heads should be less of a problem, probably falling within the normal range of variation expected for Yellow-legged Gulls (Garner & Quinn, 1997).