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Arctic Tern flight identification paradigm
Discovery Island
September 20, 2016

Every field guide I have states that Arctic Tern has a thin black line on the trailing edge of the under wing, due to the black tipped primaries. The field guides state that the upper wing is uniform pale gray. It seems like the field guides just keep parroting the same old field marks, but if one looks at these photos (and many other photos on the internet, which I just did), it is obvious that this black line can be seen on the upper wing and sometimes not seen at all, even on the under wing. It may be a useful field mark but certainly should not be relied on by itself for identifying Sterna terns in flight without additional characteristics, such as the gray belly and red bill. I have read several accounts on the bird list serves where Arctic Terns are identified solely by this feature, often at great distance, in flight with no further corroborating details. In addition, it is very easy to confuse with Forster's Terns using that field mark for ID, without a very good, close-up view or a good photo to look at.

The three photos below are the same bird.






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