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The gull in the above images was photographed at Dosthill Lake, Warks, on February 15th 2010. On the water it caught the eye and its clean-cut appearance and the shade of grey of the mantle recalled Yellow-legged Gull. When it flew onto the bank, its legs were indeed yellow, while the underside of the primaries and secondaries was dusky grey, both features associated with michahellis. However, there are several features which are at odds with Yellow-legged Gull. I am grateful to Dick Newell and Peter Adriaens for subsequent discussion. In summary, both agree that this is not a Yellow-legged Gull and they favour a hybrid origin.

Features which exclude michahellis include:

• Aspects of structure, especially the delicate head and bill, which would be exceptional for michahellis
• A rather broad white crescent formed by the tips of the tertials

and more critically:

• Rather large white apical spots at tips of primaries
• A full white tip to outermost primary, p10
• No black band subterminally on p5

In combination, these features are incompatible with identification as Yellow-legged Gull. There seem to be two main possibilities:

• A yellow-legged Herring Gull of the type found in the Baltic (see 'Yellow-legged Gulls and yellow-legged Herring Gulls in the Baltic' by Lars Jonsson  [Alula, 1998, 4:74-100] )

• A hybrid, involving Herring Gull and one of Yellow-legged Gull, Caspian Gull or Lesser Black-backed Gull

The yellow-legged Herring Gulls from the Baltic are sometimes referred to as 'omissus' but this term is somewhat ambiguous as there are two apparently distinct phenotypes involved in yellow-legged Herring Gulls  i.e. individuals from the region centred on Norwegian Finnmark and individuals from the east Baltic/southern Finland region; the former are darker with a full white tip to p10 and generally no black band on p5 while the latter are a little paler and frequently show a black band on p5 (see Jonsson 1998 for fuller details).

Both Dick Newell and Peter Adriaens consider that a hybrid origin is most likely for the Dosthill gull, though Peter Adriaens notes that it does not resemble the hybrids he has documented in Belgium (most of which are LBB x YLG). [Useful data by Peter Adriaens, on hybrids observed in Belgium, can be found at site 1, site 2 and site 3.]

The head and bill-shape of the Dosthill gull are compatible, perhaps, with a female Lesser Black-backed Gull but the shade of grey of the mantle would be expected to be rather darker in a hybrid involving that species, while LBB also has black on p5.

The delicate head, relatively slender bill (lacking prominent gonys) and full white tip to p10 in the Dosthill gull might raise thoughts of Caspian Gull in its parentage but the steep forehead, pale iris, lack of black on p5, and extensively dark underside of the outer primaries are unlike cachinnans.

The shade of the mantle and the lack of black on p5 point to Herring Gull and the full white tip to p10 would also fit with argentatus. A hybrid between Herring Gull and Caspian Gull, or between Herring Gull and Yellow-legged Gull is an origin which must be given serious consideration but a pure yellow-legged Herring Gull of the 'omissus' type is also a possibility.

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