The kopeka has overtaken the kakerori as the Cook Islands' most threatened endemic bird.
The kakerori, or Rarotonga flycatcher, has had its threatened status reduced to vulnerable by the International Union for the Conservation of Nature.
The Cook Islands Natural Heritage Trust director, Gerald McCormack, puts the number of kakerori at around 500, and the number of kopeka, which is also known as the Atiu swiftlet, at around 440.
He says the kakerori numbers are not being naturally maintained, but have been elevated by a rat control programme in the Takitumu Conservation Area.
Atiu is also being used to maintain reserve populations of the kakerori, because the island is free of ship rats.
Mr McCormack says while the kakerori is flourishing because of the conservation effort, there is a need to look at how to boost the numbers of kopeka.