Friday, 24 February 2017

The 'omissus' trap

Just when you thought it couldn't get any more confusing, along comes omissus. Anybody who does the gull roost at Pennington Flash will know the difficulties of picking out a yellow-legged gull. The idea that adult yellow-legged gulls can be identified simply by a mantle shade of grey which is somewhere between herring and lesser black-back is almost laughable at this time of year when there are lots of Scandinavian Herring gulls 'argentatus' about which also have mantles somewhere between herring and lesser black-back. Of course in contrast to yellow-legged gulls, adult argentatus have pink legs so problem solved if you see one out of the water (not easy at Pennington, but perhaps on the spit or a buoy). Not quite!

It turns out that herring gulls from the north east Baltic (omissus) have not only yellow legs, but their mantle colour is darker than the British herring gull (but lighter than argentatus). I've no idea how common these birds are in the UK, possibly even rarer than yellow-legged gulls, but obviously these are a potentially serious pitfall when claiming an adult yellow-legged gull. The bird in the photographs landed on the P&O ferry in Rotterdam harbour in the Netherlands.

The taxonomic status of omissus is unclear, and it's not certain if it should be regarded as a race of herring gull or not. There is an interesting article on yellow-legged herring gulls on the gull research website here.

In truth, a combination of features are required to clinch the identification of even an adult yellow-legged gull and apart from leg colour these include head shape, bill shape, mantle shade, wing length and mirrors. Trouble is I know the theory, but putting it into practise in the field is not easy and like many other birders, I find the identification of yellow-legged gull very difficult.

Rotterdam harbour.

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