There are 17 species of resident breeding bats in the UK; six have been recorded in Shetland, but only one, Nathusius’s Pipistrelle, is termed a scarce migrant rather than a vagrant. On Fair Isle, bats are extremely rare visitors, although no doubt they are under-recorded due to their largely nocturnal nature.
The Nature in Shetland website lists only one record for Fair Isle, an unidentified pipistrelle on 3rd May 1989, although this database does not include bats not identified to at least species-pair level. Nick Riddiford (pers. comm.) considered that unidentified bat records probably averaged out at about one every five years on Fair Isle.
The most recent records were both in 2011, an unidentified bat seen over the Vaadal on 11th June and an individual found exhausted at Barkland in strong easterly winds and rain on 26th October. Following discussions with SNH and the Shetland Biological Records Centre, the latter was taken into care and identified as a Nathusius’s Pipistrelle Pipistrellus nathusii, before being released in better weather that evening when it had dried out and recovered its strength. It is likely that the 1989 record (and many of the other sight records) also refer to this species, the commonest bat to be recorded in Shetland (and the only species of pipistrelle to be confirmed). Nathusius’s Pipistrelle is a strongly migratory bat that was formerly considered a vagrant to the UK, although since the 1990s it has been discovered to be a regular winter visitor and breeding resident in small numbers (Bat Conservation Trust).
It should be noted that all bats are protected and should not be handled (unless to tend to an injured individual), but as potential carriers of rabies, anyone lucky enough to find a bat on Fair Isle should content themselves with getting pictures and not handling it.