For such an important record we were eventually able to confirm the ring number with the finder and this was indeed a bird from Northumberland: DB67406 had been ringed as a chick on Coquet Island in 2002. With just one previous record of a 'possible' in Chicago in 2010 (details here), this bird could well turn out to be the first Sandwich Tern record for the USA.
DB67406 was seen by biologist Jeff Spendelow, who studies the use of staging sites by Roseate Terns in the Cape Cod area of southeast Massachusetts. It was first seen on one of his study sites on 31st July, but it wasn't until 21st August that Jeff was able to read its ring, with it also later seen at nearby Chatham on 7th September (in red on the map below). Several other intrepid American birders managed to paddle out to the islands to see the bird, but it was hardly 'twitchable'! Photographs of the bird do also show many of the features used to seperate Sandwich from Cabot's Tern, but you can't argue with a bird ringed as a chick in Europe!
Interestingly, there is an equally unusual record the other way round, with a Cabot's Tern from the USA being found dead in the UK (in green on the map below). NAW 110386842 was ringed as a chick at Beaufort, in 1984 and was found dead in November 1984 at the rather bizarre location of Newhouse Wood in Herefordshire. It was reported independently (as a tern/gull) by two observers so is genuine, and is the first record of Cabot's Tern in Europe. There have since been further records in Europe (including a possible Cayenne Tern in Wales), but this remains a most bizarre first.
View Sandwich and Cabot's Terns in a larger map