The Sandwich Tern Sterna sandvicensis  in the pictures A-B-C (see below) was found by me on 26th June 2006 along the Krammer-Volkerak, in the Rhine-Meuse delta in the SW Netherlands. The tern was actively moulting it's inner primaries which according to current knowledge is unusual for European Sandwich Terns to occur in June. Could this bird therefore be an American Sandwich Tern Sterna sandvicensis acuflavida? Previously, a first winter American Sandwich Tern, ringed in North Carolina was found dead on the Noord-Beveland coast also in the Dutch delta on 23rd December 1978. In view of last year's extremely intense hurricane season which brought so many American birds to Europe American Sandwich Terns could also have drifted across the Atlantic. 

   According to Malling Olsen & Larsson (1994) the American Sandwich Tern differs from the European Sandwich Tern Sterna s.sandvicensis in that the white border along the inner web of the outer 3-4 primaries is very thin in acuflavida and much wider in sandvicensis. The primary pattern of the Caribbean race eurygnatha looks similar to acuflavida in most skins and pictures I have seen  (though Millington 2006 claims the white primary fringes are a little broader in eurygnatha than in acuflavida) but has a yellowish or orange-yellow bill (see pictures of skins in the National Nat.His.Museum in Leiden). The difference in primary pattern can only be seen in fresh unworn primaries. The race acuflavida is the smallest. According to Malling Olsen et al. juvenile plumages of acuflavida and eurygnatha are more or less similar, both have a yellowish bill, and they differ from sandvicensis  in that the tertials have a broad dark centre. Thanks to Martin Reid's excellent pictures taken in Texas one can see that acuflavida is strikingly different in juvenile cq. first winter plumage from sandvicensis and should such a bird reach Europe it could easily be identified!              
  

   Ginn & Melville (1983), say that in sandvicensis postnuptial moult is complete and that primary moult begins from mid July to the end of August in the UK, and therefore also in the other countries along the North Sea as this is one population. They found primary scores of 14 to 24 in August and 23 & 24 on 11th September meaning that at most the inner 5 primaries are involved. They also say that moult may be suspended during migration to Africa and furthermore that moult in African winter quarters is complex as the inner primaries may be renewed again. In all it seems that the outer 3 to 4 primaries are being renewed only once a year which may result in the white border along the inner web having been worn off by the time they return to the breeding colonies! Fortunately some winter in the southern North Sea during the last two or three decades so that a journey to Africa to see freshly moulted outer primaries is no longer neccessary. 

   No description of the timing and length of moult in the other races have been found. To me however the outer primaries of the lone bird I photographed in June look fresh and unworn and I think therefore it's primary pattern is typical for acuflavida.

   On the 13th July very few individuals in the colony of Schouwen in the Dutch Delta I checked showed signs of primary moult. It is not easy to determine what stage of primary moult a tern is in while alive and flying around. I have photographed these birds here in The Netherlands during July to November and the pictures show great individual variation in primary moult (see picture sequence). I also took pictures of juveniles and they too show considerable individual variation in moulting from juvenal into first winter plumage (see picture sequence). An adult acuflavida photographed in Florida in December by Pekka Nikander (picture 52 in Malling Olsen et al.) has most of it's outer primaries renewed, the new primaries have indeed a narrow white border similar to the fresh outer primaries in the adult birds photographed by Arie and Peter de Knijff in February 1989 and by me on 3rd November 2006 along the Brouwersdam in The Netherlands (see pictures)!

                   
   I send a request to ID Frontiers  for information on primary moult and pictures of American Sandwich Terns for comparison. So far no news on moult but several people were kind enough to help with pictures and I thank Lisa Applebaum, David Cagnolatti, David Muth and Martin Reid for their kind help. My request still stands so I invite anyone with information or pictures to contact me. Furthermore I thank Hein van Grouw (Naturalis,Leiden) and Peter de Knijff for their help.
 Norman Deans van Swelm

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