Braving the foul weather I walked down to the dam hoping for a storm driven migrant. As I scanned the central area of the lake I immediately saw 3 Black Terns. This was good, but not a year tick. As I watched the terns I noted that one bird seemed to have a darker mantle and paler wings. This encouraged me to move to the centre of the dam and get closer views. I was also then able to see that the bird had a white rump and that it seemed slightly smaller than the 2 Black Terns. I realised that it was now worth returning home to check the Collins guide. I did this and also contacted R.M. Returning to the lake we now checked to see if the bird lacked a dark breast patch. Again it seemed to fit all the requirements . I decided to contact S.L. and by 11.20 he had seen and confirmed the bird as a juvenile White Winged Black Tern. A county, Thornton and year tick for me and a lifer for some of the 30 or so birders who came to enjoy some very close views. The right hand image shows the W.W.B. as the closer bird and a Black Tern behind. The lack of a dark breast patch is clear in both photos. My poor quality photos were taken with a hand held Panasonic DMC-TZ10, cropped and zoomed, so you get some idea of how close the bird was.
A third Black Tern joined the group later. While watching them at about 17.15 in the company of A.F. a Peregrine spooked the terns and they flew off high. At first we thought they were leaving , but they returned and were still present when I left at 18.00.
My second Thornton tick for the day was a Grey Plover. A.F. identified it on call and we both had clear views of the black armpit patches as it flew over and disappeared to the east.