On Sunday 6th at just before 5pm Bird Guides reported that a juvenile Black Tern, present for its second day at Longham Lakes, had been re-identified as an American Black Tern. There have been only 14 previously accepted records in Britain and this would be the first for Dorset.
As American Black Tern is not currently treated as a full species and it was already getting late I convinced myself to wait until the following morning. The tern remained on the lake until dusk and thankfully it was reported again from 8:44am the next morning.
I parked in the Bridge House Hotel car park near Ferndown and walked to the south east corner of south lake. I could see quite a few birders scattered along the western edge and particularly in the north western corner. I managed to pick up the tern in my binoculars and then in the scope. I had good views, the dusky flanks and underwings were obvious even at long range. The tern was clearly favouring the western edge of the lake and so I walked around to the other side. I joined a group of photographers and we had excellent views as the American tern patrolled the western edge of the lake. It was clearly catching good numbers of insects mainly from the water surface and there were also good numbers of House Martins over the lake still finding insect sustenance.
The features which separate this race from our own Black Tern were clearly evident – consistent neat uniform dusky flank panel merging into dark broad breast-side patches and also darker underwings. This compares to the gleaming white underparts and white underwing coverts of our juvenile Black Terns.
American Black Tern is currently treated as subspecies Chlidonias niger surinamensis although it is probably worthy of serious consideration for treatment as separate species. There are clearly diagnostic features but as their breeding distributions are non-overlapping there is no easy way to assess potential reproductive isolation.