Experts studying the decline of a seabird have observed it being hunted at night by another rare bird.
Leach's storm petrel are smaller than the great skua
The National Trust for Scotland (NTS) has been recording "alarming" falls in Leach's storm petrels on St Kilda.
Researchers from Glasgow University have now calculated great skua, or bonxie, may be eating up to 14,000 of the smaller bird every year.
The hunting has been witnessed with the use of night vision equipment. The research will run until 2009.
NTS said the Leach's storm petrel colony on St Kilda, which it owns, is the largest in Europe and numbers about 40,000 pairs.
Bonxie are regarded as the chief suspect in its decline.
The predator is also rare and up to 60% of the world's population breed in Scotland, according to NTS.
They recently colonised St Kilda and the population has built up to more than 100 pairs.
Will Miles was awarded a studentship to carry out the research and is in the first year of a three-year PhD studying the predation of great skuas on petrels. He is being assisted by Liz Mackley, who is a masters student.
Richard Luxmoore, head of nature conservation at NTS, said: "Leach's petrels are much smaller and are so vulnerable to predation that they nest down burrows and only return to the island at night.
"One of the objectives of Will's study has been to find out how and when the skuas manage to catch the petrels.
"The use of night vision equipment has enabled Will to make some very interesting observations of skuas hunting in the petrel colonies in the middle of the night."