Confusing large white-headed, dark-backed gull at Rainham Landfill, London/Essex

See http://www.birdingetc.com/

Dominic Mitchell photographed this very odd, dark-backed gull at Rainham Marsh (Essex/London) yesterday afternoon and this very same bird was seen on a number of occasions today, as it commuted between Wennington Marsh Fields (on the RSPB reserve) and the Landfill.

I saw it very well this afternoon, initially in the field and then later as it wandered about the rubbish of the landfill from 1435-1515 hours. It certainly is a very intriguing gull and is either a 4th-year or older SLATY-BACKED GULL, a VEGA x SLATY-BACKED GULL intergrade or an exceptionally dark SCANDINAVIAN HERRING GULL.

Two features worry me about its appearance. It has unusually long legs, almost Caspian Gull-like, which are raspberry-red in colour (in dull light) and it has dark grey upperparts (not black). Other than these two anomalies, everything else suggests schistisagus. It could be that these features fall within the normal variation of Slaty-backed Gull but I don't know.

There is very limited parking near the landfill (perhaps for 20 cars maximum) so it will be preferable to park at the official Rainham Marsh RSPB reserve car park and then walk back west for 2 kilometres to view through the wire fence at the landfill. At the 1.3 km mark, the roosting gulls in the fields can be studied (this afternoon this field harbouring 5 different CASPIAN GULLS).

The landfill will be active until midday tomorrow but will then close until Monday morning. History has dictated that feeding gulls distribute far and wide on Sundays, so to be sure of a sighting, a visit tomorrow is essential. There are upwards of 5,000 feeding gulls at the tip.