The gyrfalcon at a rabbit burrow at Ness on Lewis
Volcanic ash from the eruption in Iceland has been suspected of driving the world's largest species of falcon to the Western Isles.
The gyrfalcon, which has white plumage and can have a wingspan of about 5.2ft (1.6m), has been seen on Lewis.
RSPB Scotland believes the bird of prey may have flown from Greenland to seek shelter from the volcanic ash cloud.
The falcon could also be hunting geese which have also appeared in unusually large numbers in the Hebrides.
Martin Scott, the RSPB's Western Isles conservation officer, said he was "very excited" by the raptor's appearance and expected twitchers to flock to the island.
He said the falcons have a circumpolar distribution, meaning they breed in Greenland, Iceland, Arctic Canada and Norway. Larger populations are found in Greenland, but the bird could also have come from Iceland.
The RSPB officer said: "There are only a couple of sightings each year of gyrfalcons in Scotland.
"We think this one has come here as a result of the volcanic dust cloud, which seems to have also attracted a large number of geese too."
Mr Scott added: "The gyrfalcon preys on the geese so it has plenty of food available.
"There are a large number of brent and pinkfooted geese here at the moment which should be in the Arctic Circle, but are being held back by the volcanic dust cloud."
Last summer, it was reported that a 2,500-year-old nesting site used by gyrfalcons was discovered on a cliff in Greenland.
The nesting site is still continually used by the species and is the oldest raptor nest ever recorded.
Three other nests, each over 1,000 years old, were also found, one of which contained feathers from a bird that lived more than 600 years ago.