The birds’ breeding success has been recorded on Hirta, the main island of St Kilda, 112 miles out into the Atlantic from the Scottish mainland.
The island, owned by the National Trust for Scotland, is also home to a handful of defence workers who operate part of the missile test range based on Uist, 41 miles away.
The swallow’s nest was established in a garage operated by defence contractor, QinetiQ, who agreed to leave the doors open for the duration of the breeding season.
Swallows are regular summer visitors to St Kilda, but this is the first time that they have successfully raised young, said the NTS.
Meanwhile, efforts to monitor the breeding success of threatened Leach’s Storm Petrels received a boost when two chicks were successfully reared in artificial nest boxes.