The dramatic decline of a seabird on St Kilda is to be investigated by Glasgow University and the islands' owners, the National Trust for Scotland.
The great skua is also known to prey on puffins
The archipelago 41 miles off Benbecula is home to the largest population of Leach's storm petrel in Europe.
But its numbers have been falling and it is suspected another bird, the great skua, or bonxie, is responsible.
The university has calculated that skuas are eating up to 14,000 Leach's storm petrels a year.
A PhD student will research the relationship between the birds in the hope of understanding the problem.
The petrel colony, which makes its homes in burrows, had numbered 40,000 pairs, but the population appears to have almost halved since 1999.
Bonxies are also a rare species and up to 60% of the world's population breed in Scotland.
They have recently colonised St Kilda and the population has built up to more than 100 pairs.
The RSPB describes the great skua as "an aggressive pirate of the seas" which harasses other seabirds in an effort to steal their food.
The bird is also known to prey on puffins.
Richard Luxmoore, NTS head of nature conservation, said the birds' vulnerable status posed a complex challenge.
He said: "What should we do when one globally rare seabird is being decimated by another?
St Kilda is home to Europe's largest population of Leach's storm petrel
"The answer, in the short term, is to make sure of the facts and so we will be appointing a PhD student to research the relationship between skuas and petrels."
The project will run until 2009.
Professor Robert Furness, from the Institute of Biomedical and Life Sciences at Glasgow University, said a shortage of fish had turned one bird against another.
He said: "The huge quantities of fish discarded by fishermen in the seas around Scotland have been an important factor in the growth of bonxie numbers.
"Now that fisheries are in decline the bonxies are increasingly having to kill other seabirds as food."
St Kilda has dual World Heritage status and is a renowned natural environment.
The last of its inhabitants were evacuated to the mainland in 1930 after the islands' reliance on imports of food, fuel and building materials became a massive strain on the residents' way of life.