Arctic Terns at Mordialloc, Victoria

To: <>, <>
Subject: Arctic Terns at Mordialloc, Victoria
From: "Mike Carter" <>
Date: Thu, 7 Oct 2010 21:44:24 +1100
I was unsuccessful in a quick look for the Arctic Terns this afternoon between 15.30 and 17.00. There were 130 Crested Terns on the large sand area on the north side of the mouth of the Mordialloc Creek and 220 on the rock platforms at Ricketts Point but no other terns. Don't yet know the time of day originally seen but as that is often important I suggest others look at other times and elsewhere. Since I got home, I've learnt that the first spot described above is where James Bailey saw the birds and that it was on 2nd not 3rd October. James is new to birding and not aware of whom he should contact re interesting sightings. Nor apparently in the amount of detail or precision desired in difficult confusion species.

Mat Gilfedder asked how often is this species seen in Port Phillip Bay. My guess is about once every five years over the last 50 years. I would have seen perhaps half-a-dozen but most were in the 1960's, 70's and 80's when I was looking intensively at terns. One of those was at the Eastern Treatment Plant, one was on Mud Islands and the others in the Sorrento/Portsea area.

Simon Mustoe asked why do I say that it is an Arctic Tern (the standing bird). Firstly because it has very short legs, far too short for a Common, White-fronted, Antarctic or Roseate Tern. Secondly, it has the dagger-like bill of an Arctic Tern, shortish and evenly tapered from base to tip. Head pattern is typical of the species in non-breeding plumage, i.e. tonsured or monk-like with white extending to rear of crown, black restricted to side of head and the nape. The six outer primaries are visible and are uniformly dark indicating they are of similar age. No other confusion species would show this in early October. Common would be closest with the four outers being dark. On the far wing the black line on inner shaft of outermost primary is narrow, about the same width as the shaft (wider in Common Tern). On the near wing the black hook-back (where the black extends backwards along the rear edge of the wing) on next-to-outermost primary is very narrow. Would be much broader in Common Tern.

The flying bird is also an Arctic Tern but is in a different plumage and from this dorsal view more difficult to ID. It is an adult that now has a black instead of a red bill but otherwise retains much breeding plumage. A ventral view would have made the identification of this bird easier but again it has an Arctic's bill shape. The primary and secondary patterns are the critical features. An adult Common Tern in October would have the four outer primaries contrastingly blackish. On this bird they are all grey and of similar age and wear. Otherwise I might have said that the extent of black along the trailing edge could suggest that of a Common Tern if I didn't know that the photo was taken in October and not May! The broad white tips to the secondaries is very significant.

Mike Carter
30 Canadian Bay Road
Mount Eliza  VIC 3930
Tel  (03) 9787 7136

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