What should conservationists do when one species they are keen to protect is flourishing at the expense of another species which is just as precious?
By Huw Williams
The Great Skua is also known to prey on puffins and lambs
That's the dilemma facing scientists in the islands of St Kilda, 40 miles into the Atlantic, west of the Outer Hebrides.
The population of Leach's Storm Petrels that breed there has been declining fast over recent years.
And the chief suspect in the mystery is another sea bird - the Great Skua, or Bonxie, which is blamed for killing thousands of Leach's Petrels every year.
Walking down Mullach Mor - high above Village Bay on St Kilda's main island, Hirte - you quickly come into what's called Bomb Alley.
It's called that because the Great Skuas - or bonxies - fly directly at you, at waist height. They veer off at the last minute, and skim over your head. At least, that's the aim.
"I know someone who got knocked out by a bonxie," Sarah Money said.
She's the National Trust for Scotland's seabird ranger on St Kilda, and she was my guide.
"The bonxie got it wrong, and knocked him out completely," she added, "and when he woke up there was a bonxie lying unconscious next to him as well."
The Great Skua's tactic is a magnificent sight. But it's meant to be frightening. And it is.
They're large, long-billed birds, with dark brown plumage and white flashes on their wings. And they're blamed for killing as many as 15,000 Leach's Storm Petrels every year.
The petrels are black and white; about the size of starlings; spend most of their lives at sea; and are nocturnal when they come to land to breed. St Kilda is home to the only significant colony in Europe.
It seems likely that human intervention may be at least in part responsible for the problem.
Professor Bob Furness from the University of Glasgow told me fishing boats have historically discarded so many under-size fish that there's been a "huge increase" in the numbers of scavenging sea birds and predators, such as the Great Skuas.
But now over-fishing and climate change is producing great shortages of basic prey species - like sand eels - in the Skuas traditional breeding grounds. So more and more are expanding out into new territory, such as the islands of the St Kilda archipelago.
"The pairs which have moved to places like St Kilda have essentially been getting away from this big problem of a lack of food," Professor Furness said.
The birds have moved to places where there is a huge seabird population, on which they can feed.
St Kilda is home to Europe's largest population of Leach's Storm Petrel
"So, what's happening on St Kilda is perfectly natural and normal.
"The problem is that there are far more bonxies in the general environment of the north of Scotland than is really sustainable," Prof Furness added.
About 90% of all the Leach's Petrels in Europe breed on St Kilda, so if the colony is declining in size it really matters. But we can't just kill the Great Skuas, and make the problem go away.
Richard Luxmore, the head of nature conservation at the National Trust for Scotland, said: "If anything we care more about the bonxies, because in global terms they're rarer than the Leach's Storm Petrels."
That's the dilemma at the heart of this story.
"What should we do when one globally rare seabird is being decimated by another?," Mr Luxmore said.
The answer, in the short term, is to make sure of the facts. So NTS and the University of Glasgow have secured funding for a three year programme, to research exactly what's going on.
But, if humans are responsible for the increased numbers of bonxies which have led to this problem, might we have to intervene to correct it?
Prof Furness says we might.
"In Shetland there's a kind of parallel situation," he explained.
"For many decades crofters in Shetland have hated bonxies because a few of them attack lambs.
"Their strategy is not to go out and shoot all the bonxies. But if they know there's one particular rogue pair, and they remove them, the territory will be occupied by another bonxie and the chances are it'll feed on something else."
"So, potentially it might be possible to stop the birds by feeding on Leach's Petrels by selectively removing those individual Skuas which specialise on eating petrels."
And, if the thought of culling bonxies proves unacceptable there may be other less drastic options.
"You can change the behaviour of petrels in colonies by playing tape-recordings of a bonxie or a Great Black-backed Gull, or some other predator, the petrels become very quiet and secretive.
"So, you might be able to manipulate the predator/prey relationship, perhaps by alerting the petrels to the fact that there are bonxies here."
If the Great Skuas that have been killing petrels become less successful, Prof Furness argues, they might specialise instead on other species.
Back on St Kilda, Sarah Money showed me a pellet of indigestible petrel feathers and bones that a Great Skua had regurgitated.
Dramatic proof, if proof were needed, that what's going on at the moment is not sustainable.
It's hoped that once the research is completed, it'll not only clarify the facts, but also contain pointers for practical action on how to manage the situation in the future.