I then called in at Rainham late morning in the hope of finally catching up with the Yellowhammer and Corn Buntings reported recently. That game of hide-and-seek is ongoing, but will hopefully be won another day. While there, I set off in search of both the Serin and the aberrant female European Stonechat, and after a bit of searching found both along the bank 100-200 metres west of what has become known as the viewing mound, overlooking the west end of the RSPB reserve and Wennington Marshes. Another birder there, a non-local, remarked that he still needed Yellow-legged Gull for his year list, so I walked him back round to the car park and about 300 metres beyond, from where it is possible to look south along the foreshore towards Coldharbour Point, and east down river towards the Dartford Crossing.
From this vantage point, at about 1pm, I began scanning to try and find him his year tick, and after about 10 minutes, on the falling tide, I picked up an interesting gull drifting away from me towards the bridge; it was probably at least 200 metres away when I first saw it. It was a large gull with smooth, ash-grey upperparts, and when it turned side-on the contrasting black primaries with white tips were clearly visible. The underparts were clean white, but the head was densely streaked all over, the streaks forming a neat, cleanly demarcated and 'full' hood. The bill was yellow with an obvious red spot towards the tip. The distinctive look of this bird struck an immediate chord with me: atlantis Yellow-legged Gull, a taxon I've seen thousands of times on numerous trips to the Azores. Both graellsii and intermedius Lesser Black-backs and argenteus and argentatus Herring Gulls were present in the vicinity for comparison, as was a third-winter michahellis Yellow-legged Gull. The mantle shade of this last bird was almost identical to the hooded gull, though in obvious contrast it was white-headed.
The atlantis type had a density and distribution of streaks that matched perfectly the typical pattern of well-marked adults which I've seen and photographed on numerous occasions in the Azores, most recently one month ago; in all the many michahellis Yellow-legged Gulls I've seen over the years, none has ever shown this extensive complete hood of streaks. Unfortunately, however, I couldn't see any meaningful detail of the primary pattern at that range and angle, and, while trying to get the other birder onto it, I lost it from view as it drifted away down river.
Immediately I phoned Howard Vaughan at the visitor centre. He was busy but called almost straight back, and on hearing the news went to look for the bird - and amazingly, he found it! He called me back to say it had flown from the river towards the Dartford side, but from where I was standing - now with Andy Tweed and Paul Whiteman - we couldn't relocate it. Fingers crossed someone else can pin it down tomorrow and obtain better images and a full description.