One of the birds below has just been added to the British list, the other is a Sandwich Tern. Can you tell them apart? Quite possibly not, but at least their mothers can - and have apparently done so for countless millennia, according to genetic research which has led to the BOURC's recommendation to split Cabot's Tern Sterna acuflavida (for a summary of other important BOURC taxonomic announcements, see here).
The phrase 'armchair tick' will be familiar to many, and on paper at least this is what Cabot's Tern should be. It has occurred in Britain, and also in The Netherlands. Unfortunately, however, the records involve ringed birds that were picked up dead - the British individual somewhat bizarrely by a Forestry Commission ranger at Newhouse Wood, Herefordshire, on 28 November 1984, and the Dutch bird prior to that on 23 December 1978 in Zeeland. Both were found as first-winters, and had been ringed as chicks at different locations in North Carolina, USA. So the only two Western Palearctic records - so far - involve corpses.
More attention will surely be paid to Sandwich Tern identification in future, even if it will prove testing. The most comprehensive paper is by Martin Garner, Ian Lewington and Jason Crook, and was published in Dutch Birding back in 2007 (29: 273-287). It goes into great detail on the separation of Cabot's (or American Sandwich Tern as it is known in the paper) from Sandwich Tern; less problematic would be the appearance of the variably yellow-billed Cayenne Tern, which has also been included within Cabot's by the BOURC (subspecies eurygnatha).
No time or space to go into detail here on the separation criteria for the two main protagonists - you can dig out your old Dutch Birding or, failing that, read a summary in Dutch here, or hope that those kind birders in Holland upload free PDFs of the issue in due course (as they have done for many previous volumes). Better still, you can subscribe to this excellent publication, and then next time you won't miss out.
But back to my original question: the answer is that the Cabot's Tern is the top photo, with the Sandwich Tern immediately below it. They were both taken this year, respectively on 1 August in Yucatán, Mexico, and 25 June at Rye Harbour NR, East Sussex. Compare, for example, structure (including bill) and the extent of dark pigmentation in the wing-tip. And to finish with, here's an adult and juvenile Sandwich Tern from Minsmere RSPB, Suffolk, on 1 July 2009 - juvenile plumage probably being the most distinct between the two species.