DOC staff and Wildlife Management International contractors spent three days counting toanui and their burrows on the small isle, also known as Saddle Back Island. The results from the January survey have just been released.
Toanui are medium to large-sized sea birds often caught unintentionally by commercial and recreational fishers. Classified as nationally vulnerable, their population is falling with just 12,000 breeding pairs found on offshore islands in New Zealand.
Initial estimates suggest toanui on Motumahanga Island number between 500 and 600 pairs – a big increase on the 100 -200 pairs found in a 1990 survey.
DOC Principal Science Advisor Graeme Taylor said the results were a pleasant surprise. The birds are 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,” he said.
“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.”
The team also caught and banded 50 toanui close to the campsite at night and re-trapped two birds banded on a trip in 1997.
Camping on the island was a mission, Mr Taylor said. Finding a gap in the trees for a tent was not easy and tree roots and rocks on the ground make for a rough sleep. At night the birds are very vocal.
“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!”
The team found the common diving petrel population on the island has declined substantially since the 1990s when several thousand pairs were thought to be present. Competition with the shearwaters might be displacing this species.
There is also quite a significant breeding colony of white-fronted terns on the western cliffs and a smaller colony of red-billed gulls. Landing on the island is by permit only and subject to strict biosecurity procedures to ensure plant and animal pests don’t make their way onto the islands.