Thursday, 15 September 2011

Cabot's Tern: an update

The essential Dutch Birding paper is also available as a PDF on the Birding Frontiers memory stick.
Thanks to Martin Garner, David Callahan and Tristan Reid for some useful follow-up on my last post on Cabot's Tern, that recent BOU TSC-recommended split from Sandwich Tern.

I mentioned that Martin was lead author on the essential ID paper for this Nearctic tern that was published in Dutch Birding back in 2007, and should add that in addition obtaining the back issue of that journal, the same paper is also available as one of 25 articles on the excellent value Birding Frontiers memory stick (order for just £11 including p&p here). You'll need Acrobat Reader (download free) to view the material, which includes a superb plate of Cabot's (aka American Sandwich) and Sandwich Terns by Ian Lewington.

Apparent Cayenne Tern Sterna acuflavida eurygnatha (Cemlyn Bay, Anglesey, 8 June 2006). Note the dull orange-yellow tone to the bill, the same colour also being visible on the knees and feet.
Meanwhile, David Callahan reminded me that, although it's not yet on the BOU's British list, there is a strong British claim of the eurygnatha subspecies of Cabot's Tern, known as Cayenne Tern. This is in addition to the accepted record of nominate Cabot's Tern, a recovery of a ringed first-winter found dead in Herefordshire in 1984, so both subspecies have apparently made it to Britain. The Cayenne Tern was present at the Welsh tern hot-spot of Cemlyn Bay in June 2006, and best photos I've seen of it were taken by Tristan Reid (visit his website here).

Apparent Cayenne Tern Sterna acuflavida eurygnatha (Cemlyn Bay, Anglesey, 8 June 2006). Cayenne Tern is distributed from the Caribbean southwards along the Atlantic coast of South America; Caribbean birds are said to have the yellow-orange bill toned greyer in the central and basal part (Olsen and Larsson 1995), rather like this.
Tristan tells me that, subsequent to seeing and photographing the bird, he sent images to Floyd Hayes, who has done extensive studies on this form in the Bahamas. Floyd was of the opinion that the bird fitted Cayenne Tern and said that, had he seen it in his study areas, he would have recorded it as such. I'm not sure that pure adult European Sandwich Tern has ever been known to have a yellow bill, although I have once recorded a juvenile with a strongly yellow bill (see British Birds 82 (1989): 414).

Apparent Cayenne Tern Sterna acuflavida eurygnatha (Cemlyn Bay, Anglesey, 8 June 2006). From this angle the bill appears more strongly yellow. In some Cayennes, the upperparts can apparently be slightly darker grey.
In a quick trawl of the literature I haven't seen any obvious comment on Cayenne Tern showing yellow on the knees, which the Anglesey bird clearly does. Is this problematic, or just not well documented? It seems logical that a bird with variably dark tones to the bill might have something similar happening with its legs too, but the closest mention I can find to this so far is "[Legs and feet] black with black or yellow soles; underside of heel also yellow in some birds. Foot occasionally all yellow or mixed black and yellow" (Shealer 1999).

Feedback on this or the current status of any claim for Cayenne Tern with the BOURC would be welcome - as a potential split in its own right, eurygnatha has the potential - if the record is accepted - to become a new British bird. In the meantime, thanks to Tristan Reid for allowing me to use his informative images.

ADDENDUM
Thanks to Ed Smith for drawing my attention to an image from Venezuela of a Cayenne Tern which, like the Cemlyn Bay bird, has similarly yellow areas on the legs - perhaps this is more a feature of this form than illustrations have hitherto suggested. There has also been recent discussion on ID-Frontiers of the identity of a Sandwich-type Tern photographed in Chicago, Illinois, USA, in September 2010 - Greg Neise makes a reasonably strong case for this bird being a European sandvicensis rather than a North America acuflavida (see here), but I guess that's one the committees will be wrangling over for some time.

* NB Watch out for a review of the Birding Frontiers memory stick in a forthcoming issue of Birdwatch.

References
  • Olsen, K M, and Larsson, H. 1995. Terns of Europe and North America. Christopher Helm, London.
  • Shealer, D. 1999. Sandwich Tern (Thalasseus sandvicensis), The Birds of North America Online (A Poole, Ed.). Cornell Lab of Ornithology, Ithaca. Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/405 doi:10.2173/bna.405

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