Antarctic Terns at Kangaroo Island: their identification

Subject: Antarctic Terns at Kangaroo Island: their identification
From: "Mike Carter" <>
Date: Sat, 9 Sep 2006 19:24:05 +1000
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!

Mike Carter
30 Canadian Bay Road
Mt Eliza    VIC     3930
Ph:  (03) 9787 7136

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