Antarctic Terns on Kangaroo Island: some background info and thoughts re

Subject: Antarctic Terns on Kangaroo Island: some background info and thoughts re origins by John Cox.
From: "Mike Carter" <>
Date: Sun, 10 Sep 2006 21:04:50 +1000
From: "John Cox" <> Sent: Sunday, September 10,
2006 1:56 PM

> While the 3 September report on 'Birdpedia' of 10+ Antarctic Terns at Cape
> du Couedic, Kangaroo Island, was a surprise, we knew a desiccated specimen
> had been found on nearby South Casurarina Island in November 1982 and
> another was reported to have been seen at the Cape in November 2001.
> Therefore, when Chris Baxter phoned on 1 September to say he had seen
> red-billed terns at the same location, Colin Rogers and myself were off to
> the Island next morning.
> After meeting Chris, we arrived about mid-day at Admirals Arch by the Cape
> to find a stiff easterly wind blowing over the rocks on which the terns
> roosted the day before. When it became obvious from the weather conditions
> that they were not going to show up, we instead decided to scan through a
> large flock of terns noticed roosting on the landward side of North
> Casuarina Island. To do this it was necessary to sit behind a sheltering
> wall to eliminate windshake and peer through tripod-mounted scopes set at
> 40x magnification. This way we could see the birds quite well, but
> recognized from the outset that the extreme range would make it very
> difficult to identify birds with any confidence. But, for the want of
> anything better to do, we persisted.
> It was only a short while before two red-billed terns were detected among
> the hundreds of Crested and White-fronted Terns. Then a third was found,
> these birds were studied for a long time. They occasionally flew up,
> over the water or to another perch, but soon our voiced descriptions of
> birds became oddly contradictory. The reason for this, as we soon found
> was that we were looking at different birds and indeed there were more
> three red-billed terns present. As the afternoon progressed the number of
> all tern species increased as more and more came in to roost and
> we had ten red-billed terns staked out in the flock. Although ten was a
> definite number, we thought there were 12 but movements in the flock
> precluded a higher tally.
> During the afternoon we became increasingly confident that these
> terns were indeed Antarctic Terns and NOT Arctic, although doubts
> occasionally crept in because of the extreme range. Nonetheless, we had
> able to see all of the identifying features and determined that most were
> transitional plumage stages with five adults being mostly in non-breeding
> plumage, three adults in near breeding plumage (though all had small
> of white above the bill), and there were two immatures.
> Finally we became certain that these birds were Antarctic Terns and, after
> more than five hours of viewing them through scopes, the fading light
> induced us to leave.
> Hoping to get photos, Colin and I returned to the Cape next morning. The
> wind had changed to a strong south-westerly and the rocks by Admirals Arch
> were now in a sheltered position - with Crested and White-fronted Terns
> perched on them! We amused ourselves sea-watching while waiting in hope
> an Antarctic Tern to arrive, and at 11.30 one was spotted flying over and
> then to land on the rocks. Without doubt an Antarctic Tern and Colin
> about 75 photos during its 20 minutes stay.
> The twitch was on! Over the next few days many people saw and photographed
> the Antarctic Terns, with David Harper obtaining a particularly fine
> of pictures of five birds on 6 September. Colin and I met with David at
> home on 9 September to review all available photos taken by them and Mike
> Carter. It was ascertained that all of the birds photographed were without
> doubt Antarctic Terns. Of two initially troublesome photos: the one taken
> Mike of a flying bird was later reanalysed by him to be an Antarctic,
> 8 September), and we agree; and another taken by David of an immature bird
> (possibly 2nd winter?) shows among other features that its recent moult
> of primary feathers precludes any possibility of it being an Arctic Tern.
> It is not the purpose or place to describe the birds here, but Mike (by
> phone and email 8 Sept) has raised a few points:
> Size: Mike said the KI Antarctics were noticeably smaller than
> Terns, but I had always thought that White-fronted was the larger bird
> anyway, and when viewing the KI Antarctics believed the size discrepancy
> be a non-issue. Certainly more slender White-fronted has larger repeatable
> measurements than Antarctic, and some works state Antarctic is the smaller
> bird (eg NZ Field Guides), but what is this about weights? Pringle (1987)
> said White-fronted is c 160 grams whereas Mike, citing HANZAB, rates
> Antarctic as weighing 140-160 grams, heavier than White-fronted at 130
> grams. On reading HANZAB it is obvious the given weights are from few
> and depend on the condition of the individual. So back to basics. Gill
> (1967, Proc. US Nat. Mus. 123: 3605) cites the original weights of six
> Antarctic subspecies tristanensis from Amsterdam Island taken on the same
> day to range from 124.5 to 158.7 grams (av 141.1). So which bird is really
> the larger?
> Long tail streamers: Some KI birds had outer tail feathers extending
> the tips of the closed primaries. This feature was noticed by us and shows
> well in some photos. Typically Antarctic Terns have the tail extending
> slightly beyond the primaries but those from St Paul and Amsterdam Islands
> have a longer more deeply forked tail than other subspecies (Watson 1975,
> Birds of Antarctic & Subantarctic). The subspecies tristanensis is also
> typified by its longer culmen and legs (Gill 1967), and indeed the KI
> had big bills and long legs. It depends on the authority cited, but
> tristanensis is usually regarded to be the northern form breeding on South
> Atlantic islands and the Indian Ocean islands of St Paul and Amsterdam.
> Origin: Questions have been raised about the origin of the KI birds. The
> short answer is that we do not know, but we can speculate. Given that the
> Antarctic Terns are smaller than White-fronted, but have long tail
> streamers, large bills and long legs, it seems reasonable to hypothesise
> that they are very similar to tristanensis of St Paul and Amsterdam
> It would be interesting to examine the beach-derelict from WA.
> also winters in South Africa. We could, tounge-in cheek,
> conjecture that the KI birds come from an unknown breeding colony on the
> many poorly-explored islands off the coast of southern Australia.
> John Cox
> 28 Devon Drive,
> Salisbury,
> South Australia 5108

John sent this to a discussion group originally formed to debate the ID of
the mystery tern seen off Port Fairy last July and now absorbing this new
development. He gave me permission to post on BIRDING-AUS. I'd like to say I
endorse all the comments he makes. One other point, if they are migrants
from elsewhere, according to Tony Tree who studies the wintering population
in South Africa, some birds would already have returned to their breeding
islands and the remainder would depart in September.

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


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