Page last updated at 02:34 GMT, Friday, 19 March 2010

Good news, bad news, and a mystery for conservationists

A Leach's Storm Petrel
The number of Leach's storm petrels in St Kilda had been falling sharply

By Huw Williams
BBC Scotland reporter

For some time there has been concern that the population of Leach's storm petrels on the islands of St Kilda was falling fast.

Another species of sea bird - the great skua or bonxie - was blamed for eating tens of thousands of petrels.

But both populations are internationally important.

So Glasgow University PhD student Will Miles was asked to solve the conundrum - what do you do when one species you want to protect is flourishing at the expense of another species which is just as precious?

Researcher Will Miles
Will Miles has spent three years researching the issue

After three years work he had just completed his thesis.

And there is some good news, some bad news, and a new mystery to solve.

The good news is the population of bonxies has peaked.

At the same time, there are more petrels than had been predicted.

Will Miles said that means "the conservation dilemma is slightly less severe".

The bad news is the bonxies are taking even more petrels than had been thought.

"We've estimated the predation rate to be round about 20,000 birds a year," Mr Miles explained.

"That is extremely high," he added.

A Great Skua
Great skuas or bonxies eat tens of thousands of Petrels a year

In fact, it's so high it could wipe the petrels out within a few years.

Will Miles said: "The Leach's storm petrel population would go extinct really quickly, if there wasn't an input of birds from elsewhere."

And that is the mystery. Where exactly are those extra birds coming from?

They could be from very large breeding populations in Iceland, or Newfoundland.

Researchers at the University of Plymouth hope to use DNA testing to answer that question.

In the meantime, Mr Miles says it would be a good idea to try to comprehensively survey Leach's storm petrel numbers on the islands of the St Kilda archipelago to establish exactly what is going on.



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