Conservation groups have vowed to step up efforts to eradicate the American mink from the Western Isles.
The American mink preys on ground-nesting birds
The pledge came after a study found initial controls had helped protect native birds and wildlife.
Monitoring work by RSPB Scotland found Arctic terns were breeding more successfully in areas where the mink had been trapped and shot on the Uists.
Supporters of the five-year project hope it will be extended across the Western Isles in 2006 or by early 2007.
The breeding success of Arctic terns was studied at selected sites in the Uists in 2004 and 2005 and on Lewis and Harris in 2005.
Nest survival of Arctic terns was found to be more than three times higher on the southern archipelagos than in Lewis and Harris.
The first phase of the Hebridean Mink Project, which began with the aim of eradicating the species from the Uists and drastically reducing numbers in South Harris, should be completed by the middle of next year.
Funding has yet to be secured for the £3m second phase, which would target the thousands of mink thought to be roaming Lewis and Harris.
Mink, a mammal related to Britain's native weasel, preys on ground-nesting birds, particularly in the breeding season.
It is not native to the UK but has been living wild on Lewis and Harris since the 1960s and spread to the Uists more recently.
It is believed that they were released or escaped from mink farms which were no longer commercially viable.
Martin Scott, RSPB Scotland conservation officer for the Western Islands, said the eradication of mink would benefit both protected birds and the local economy.
"Mink are a problem for fish farmers, anglers and crofters, not just wild birds," he said.
"Mainly on Lewis, where mink are not currently trapped, we have seen heavy casualties in tern colonies by these marauding predators.
"Allowing mink to remain on Lewis will not only condemn terns within that archipelago to this fate, but also those on the Uists when mink inevitably return following cessation of control."
The Hebridean Mink Project's chairman, David Maclennan of Scottish Natural Heritage, said the eradication of mink from the Uists looked likely to be achieved on time.
However, he added: "If we stopped now, sooner or later mink would again make their way over the Sound of Harris to the Uists and pose the same serious threats.
"We now have enough information to plan the second phase which aims to eradicate mink from Lewis and Harris.
"There's no doubt it is costly, but not as costly in the long run, particularly in terms of our native wildlife, as allowing mink to remain."