Antarctic Terns: The Kangaroo Island birds found by Chris Baxter at Admirals
Arch 1 Sept., confirmed by John Cox and Colin Rogers (CR, with photos of one)
2-3 Sept. 2006.
Additional photographs were obtained by Terry & Lyn Gould on 4 Sept. and an
extensive range by David Harper (DH) and myself (MC) on 5 Sept.
Thoughts on the identification; the pros, cons, and apparent anomalies.
Knowing the quality of the observers making the ID and a glance at CR’s picture
sent by email and I was on my way. While watching the birds I had no doubt that
they were Antarctic Terns as claimed and not the only other possibility, Arctic
Terns. They were exactly like I’d seen in South Africa and in pictures from
there. Brilliant deep, (post-box) red legs on all birds (~10 different
individuals), bills of several being the same colour, wholly red, whilst the
bills of others were between wholly black and a mixture of red and black. But
the bills were not as heavy as NZ region birds I’ve seen and photographed. Legs
were short but not as short as Arctic but were stronger looking, more robust.
So I left glowing with the joy of a successful twitch.
But then I thought that the above depends much on impressions and subjectivity.
Had I sufficiently considered the alternative? Two birds had tail streamers
which extended well beyond the tips of the folded wings and that did not fit
with my expectations nor, as I’ve since checked, with the literature, at least
for birds in the Australasian quadrant of the globe. One of these referred to
here as bird 1 was photographed by DH is now posted on ABID. The bird has a
white forehead and black at the base and tip of the bill. Breeding plumage of
the other bird, which had even longer streamers, was more advanced as it had a
wholly red bill and a black forehead with a white patch on the fore-crown.
Also, the birds were noticeably smaller than the accompanying White-fronted
Terns to a surprising degree. HANZAB rates Antarctic Terns as weighing 140-160
gms, heavier than White-fronted at 130 gms! Very odd! Another oddity was the
narrow but detectable black outer web to the outer tail feather in bird 1
visible in DH’s photo and on one of mine on a bird landing. Antarctic’s are
supposed to lack that feature.
So what was required was something more concrete. So I studied a shot of mine
of a landing bird that has spread wings thus revealing the diagnostic primary
pattern, hopefully shortly posted on ABID. The near wing is viewed dorsally but
as the primaries are spread and there is some back lighting, the feather
pattern, in particular p10 & p9, is clear but could be distorted a little by
rotation of the feathers. All the primaries look fresh. By comparison with the
diagrams on p.690 in HANZAB vol. 3, I initially reckoned they were a better fit
for Arctic Tern. Antarctic Tern is shown with the dark line bordering the shaft
on the inner web of p10 wider than in Arctic Tern and the remainder of that web
tinged grey not white as in Arctic and in the KI bird. This is shown well in
the drooped p10 in the near wing of the bird in the second pic posted by DH on
ABID today. Similarly according to HANZAB, p9 has the dark line adjacent the
shaft on the inner web wider than the outer web whereas in my pic it appears
quite narrow as shown (and as I know to be true) for Arctic Tern. Danny Rogers,
author of the HANZAB text, has today kindly studied numerous photographs of
skins of Antarctic Terns taken for preparation of that text and states that
perhaps contrary to the HANZAB diagrams, the KI birds match his series of
Antarctic Tern study skins!!! He has transferred the photos to a disc, which he
will send me. But P10 as revealed in my photo and in DH’s bird 1 on ABID has a
feature which I believe to be diagnostic of Antarctic Tern. The dark line
bordering the shaft on the inner web expands in width only slightly as it nears
the tip of the feather in an even graduation. In Arctic Tern the expansion of
the width of that line is greater and steeper to form more of a blob at the tip.
Other objective evidence is the state of wing moult clearly demonstrated in
bird 1. P9 is old and darker than the other visible primaries. The shorter
primaries (p8 not yet visible) are comparatively fresh and still growing in.
They are so short that the tip of p9 is exposed revealing the sharp and narrow
hookback diagnostic of Antarctic Tern. Wing moult in Arctic Tern does not
advance until they reach the ice pack so it would be impossible for one to be
just completing wing moult at this time. Most other birds present had completed
wing moult and this individual seems late but according to HANZAB, not too late
as they finish Sept.-Oct.
So when this exceptional live record for Australia comes to BARC, I will
accept. Staggering that it is a flock and not a lone vagrant!
30 Canadian Bay Road
Mt Eliza VIC 3930
Ph: (03) 9787 7136
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