I have previously seen this species widely in the USA and central America, here is a bird photographed in Mexico, although unlike the Fair Isle bird which appears to be a 1st winter, this is an adult in non breeding plumage:
Magnolia warbler, Mexico (Simon Colenutt)
The very same weather patterns as produced the influx of yellow-browed warbler of yesterday are clearly producing the same from the west, if one studies the synoptic chart shown on the blog from 21st a clear westerly airflow can be seen generated by high 1033 and low 990. This airflow clearly originating from the coast of the USA and Canada.
Also on the island today were a lanceolated warbler, Arctic warbler and a Blyth's reed warbler, a cliche, but truly an east meets west collection. Further afield this cliche was repeated on Shetland with a Swainson's thrush and Sykes's warbler on Foula and great snipe and buff-breasted sandpiper on mainland Shetland. How will Scilly respond? What about one of these:
Wilson's warbler (adult male), Mexico (Simon Colenutt)
The only British record being of a bird at Rame Head, Cornwall on 13th October 1985 in a similar plumage to the one above.