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Sunday, 22 September 2013

The conundrum of Orange-billed Tern identification

I returned home from holidays yesterday to see a few great birds have
turned up, not least the orange-billed tern found by a fellow Larne man in
Kerry.  I have received the following email from Tony Tree, who has
studied terns for many years in South Africa. He has kindly agreed to let
me share his thoughts on the current minefield of tern taxonomy on IBN.
Makes for interesting reading I'm sure you'll agree.

 "I see that there have been several sightings of Elegant Tern in the
south recently. How are these birds treated in UK/Ireland? We have a good
batch of recent records but having had a colour-ringed bird from France,
which has proved on DNA to be a hybrid with Sandwich, we are no longer
accepting this bird on the SA list. When the first bird turned up I
questioned it being a genuine vagrant but our resident ticker-in-chief was
adamant that it was. Shortly after that a second bird turned up at the same
site but he was adamant that there was only one despite one of our
foremost ornithologists photographing the two on the same morning. Last
summer three different birds turned up at one site in Namibia, one of which
was the ringed bird that was proven hybrid on DNA. As there are several in
Sandwich colonies in France and Spain now which appear to be asymptotic
throw-backs to the ancestral form it is impossible to distinguish these from
the genuine article. We have also had the same problem with Lesser
Crested Tern on the west coast (and even once in my area but that was
also colour-ringed from the same French colony) but I have also seen
genuine F1 hybrids as well as these asymptotes in the W Cape and
Namibia. The Elegant is a short-distance migrant from the Pacific coast of
North America and so genuine vagrants are rarely likely to appear in the
Atlantic and even more rarely likely to reach us here in SA. But with the
addition of the long-range migratory gene from the Sandwich its
occurrence is much the more likely. I see that they are also hybridising
merrily with the Cabot's Tern in Mexico!

Just as a matter of interest - I read that the Cabot's Tern and Elegant Tern
(both distinct species) are very closely related genetically. This then throws
an interesting question when one starts looking at terns from around the
world. The fairly recent separation of Little and Least terns for instance -
they look almost identical and yet are sufficiently genetically diverse to be
cited as separate species. Just to make things interesting and pertaining to
my long term study of the Antarctic Tern we are starting to get some
strange findings. We have two forms visiting us that are morphologically
identical (one from the Indian Ocean and one from the Atlantic). But the
Indian Ocean bird which is quite different morphologically from the more
southerly Indian Ocean birds is identical in DNA with them. Hence a
probable colour/size cline from north to south. But the really amazing thing
is that the Atlantic form is genetically so far off (much, much further apart
that the Elegant and Cabot's Tern) from all the others in the Indian Ocean
and Antarctica that it is possibly a species new to science (this still needs to
be tested more thoroughly). That the morphological differences are almost
non-existent is concurrent with the Sandwich and Cabot's tern which are
genetically well differentiated apparently but totally impossible to separate
in the field (and probably in the hand unless they were ringed as chicks).
So you can never twitch a Cabot's in UK/Ireland unless you can read it's
ring number!! I also suggested to Prof Alan Baker in Australia many years
back that they should look at the Australian Gull-billed Tern versus the
form visiting from Asia (we did take some blood samples but these
disappeared into his cavernous fridges back home and never surfaced
again!). I do believe that they are separate species as well and certainly
have several morphological features enabling easy separation in the hand,
and also in the field when you get your eye in". <<<

Neal Warner