The population on St Kilda accounts for about 94 per cent of the species, but the numbers on Dun, one of the archipelago's islands, halved from 1999 to 2003.
The National Trust for Scotland, which manages St Kilda, joined forces with carpentry and joinery stu-dents at Inverness College two years ago to create nesting boxes for the seabirds and three pairs of petrels are raising chicks this season.
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One, named Squeak, has been closely monitored and his weight has already increased by 40 per cent in just two days.
Gina Prior, St Kilda's seabird and marine ranger, said: "Given what seems to be an increasingly precarious situation for this rarest of seabirds, it was vital we found out more about their breeding patterns and the possible causes of population decline.
"This is no easy task as Leach's storm-petrels are quite difficult to observe because they breed in hollows and cracks in the rocks and only return to their burrow after dark."
Ms Prior said that new boxes were being designed and would be in place for the 2015 breeding season.
"Hopefully the results will be well worthwhile," she added.