Initial reports identified it as the scarce, but regular vagrant Little Bunting.
However when pictures were published on the internet that night the birding fraternity was sent all atwitter, with some local and mainland observers claiming it could be the even rarer Chestnut-Eared Bunting.
On Wednesday morning a small group of determined enthusiasts set about resolving the puzzle and after several hours chasing the sub-sparrow sized creature through the undergrowth they resolved the mystery.
This is only the second UK sighting of the Chestnut-Eared Bunting, the first seen on Fair Isle in October eight years ago, and the third ever European appearance.
One group of four birdwatching enthusiasts were inspired to charter a plane from York on Wednesday morning to add it to their list of sightings.
Paul Harvey, of the Shetland Biological Records Centre, said there had been debate across Britain on the internet about the bird’s true identity.
“Some people thought it was the rare species and some didn’t and today there were five or six people looking for it,” he said, including himself out at dawn.
The tiny bird should be in the Far East heading from Siberia to southern Asia for the winter.
Harvey said more rare species have been appearing in Shetland during the migration season in the past five or six years and no one knows why.
One theory is that the eastern species are expanding their ranges and starting to be seen as the spread westwards.
“We are getting cases now we would not have dreamed of about 20 years ago,” he said.
This week Fair Isle has seen another Siberian Rubythroat, the bird which caused a stir in Shetland last year when it made its eighth UK appearance.
You can see more pictures of the Chestnut-Eared Bunting on the Facebook pages for Shetland Wildlife and Nature in Shetland.