On Thursday, Birdwatch Magazine editor Dominic Mitchell found an interesting gull on the tip adjacent to the RSPB reserve at Rainham. He immediately identified it as Britain's first Slaty Backed Gull from the western Pacific. However his initial images caused some concern due to the apparent paleness of the bird.
A number of birders saw the gull on Friday, and confirmed the identification, though the bird did not seem to be as dark as expected-somewhat between Yellow Legged and graelsii Lesser Black Backed instead of almost black. Naturally this dictated our Saturday destination.
Rainham are very good, and opened up two hours early at 0730, and by the time we arrived just after 0800, parking was limited to the approach road verge (they also opened up the normally closed road by the railway). It was a bit of a slog along the Thames footpath due to the wind but we eventually arrived to find around a thousand birders lined up on the bank along the fence. I managed to find a spot in with a group from Belgium and stared scanning. Some one then fainted prompting urgent calls for a doctor-and eventually an ambulance arrived.
Did not take long to find a good candidate first winter Caspian Gull, and soon after a number of Yellow Legged Gulls were found, several first winters and and adult. Had a bit of a scare when i saw an all white bird with bright red legs , but it soon became apparent that it was the well known leucistic Herring Gull bearing the ring number SH1T.
Over the course of five hours we found one or two birds that could have been Slaty Backed on colouration, but structurally they were all wrong and were either large pale Lesser Black Backs or very dark Herring/Yellow Legged types (the light was poor and viewing through the fence and vegetation added to the difficulties in judging colour). One did look very good, but it was largely hidden from view and we were not able to determine what it really was. Every now and again I would scan the flying throng with my bins and one or two birds were seen with a wide white trailing edge, but the requisite wing tip pattern was never seen.
After showing people some images on Surfbirds, someone got very excited about a dark headed Lesser Black Back and as in the nature of these events a number of others became convinced it was the Slaty Backed but we were able to rule it out on size and structure. We attracted a crowd though.
Just after noon, the tip closed down and this caused many of the gulls to leave, but a number settled down in front of us,including a fine adult Caspian-looking slightly better than a third winter bird I had seen distantly earlier.
Rumours started to pop up that the bird had been seen on the Thames, and also on a pool somewhere, and it eventually emerged that at least three small groups of birders had claimed to have seen the Slaty Backed, but news was never released.
We decided to call it a day and got back to the car at 1400-it was very tiring trying to stand on the narrow muddy bank and viewing through he scope was very uncomfortable and bad on the back. Having the 500mm lens did not help.
Still it was an interesting day, saw some good gulls and got in some practice on Caspians which is always useful. Nice to see a large contingent from across the Channel too.