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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