A survey has found a big jump in the number of flesh-footed shearwater birds (toanui) on an island off New Plymouth.
Initial estimates suggest toanui on Motumahanga Island number between 500 and 600 pairs - a big increase on the 100 to 200 pairs that were found in a 1990 survey.
Department of Conservation principal science advisor Graeme Taylor said the results of the January survey, just released, were a pleasant surprise.
Mr Taylor said toanui were caught extensively in commercial fisheries both here and overseas, and he had expected the population to be lower.
"I was on the 1990 trip and the shearwater burrows were mostly at the southern end, and in patches on the western side.
"The rest of the island was dominated by a dense diving petrel colony. Now the shearwater colony has spread across the plateau and is the dominant species on the island."
Toanui are medium to large sized sea birds which are classified as nationally vulnerable, their population is falling with just 12,000 breeding pairs found on offshore islands in New Zealand.
DOC staff and Wildlife Management International contractors spent three days counting toanui and their burrows on Motumahanga, also known as Saddle Back Island.
The team caught and banded 50 toanui close to the camp site at night and re-trapped two birds banded on a trip in 1997.
Mr Taylor said camping on the island during the survey was a mission.
Finding a gap in the trees for a tent was not easy and the roots and rocks on the ground made for a rough sleep. The birds were also very vocal, he said.
"The calls of flesh-footed shearwaters are like cats fighting. Multiply that by a few hundred birds and you can sense that sleep was not easy."
Mr Taylor said the team found the common diving petrel population on the island had declined substantially since the 1990s when several thousand pairs were thought to be present.
Competition with the shearwaters might be displacing this species, he said.
There was also quite a significant breeding colony of white-fronted terns on the western cliffs and a smaller colony of red-billed gulls.
Landing on Motumahanga is by permit only and subject to strict biosecurity procedures to ensure plant and animal pests do not make their way onto the islands.