I was belatedly reading JTR Sharrocks Frontiers of Bird Identification which has a chapter on separating Marsh Tern species and noticed that two pictures I had taken identified one of the statements in the book.
'Both ( Black and White winged Black Tern) occasionally gave the appearance of having a slightly forked tail, but when fully spread the tail appeared square:'
If you compare the pictures above and below of the bird doing the same manouvre they seem to demonstrate the comment. What a shame such books cannot be kept with you at all times to help when you come up against a tricky I D but I'm afraid the backpack would get pretty heavy and the books pretty tattered!
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