Taupō's black-billed gulls are a rare but stubborn treasure.

The critically endangered birds are mainly found in braided rivers in the South Island but their North Island cousins sometimes choose challenging places to nest, such as on buildings and boats.

Department of Conservation (DoC) senior ranger biodiversity Sarah Tunnicliffe says this nesting season was particularly challenging, for birds and boat owners alike, when the gulls decided to nest on boats in Motuoapa Marina, located at the southern end of Lake Taupō.

Only about 5 per cent of the black-billed gull (tarāpuka) population are found in the North Island, including the population at the southern end of Lake Taupō. They are New Zealand's only endemic gull and the most threatened gull species in the world.

For the past five years, a colony of 350 or so have nested at Tokaanu Marina, at the southern end of Lake Taupō. In 2016 they were discovered nesting in a gravel pile at Motuoapa Marina, while the marina was under construction, after efforts were made to move them on from nests they established at Tokaanu Power Station in 2014.

"They like nesting at the Tokaanu Power Station because they have a ready food source and a 360-degree view so they feel secure as they can see airborne predators," Sarah says.

She says DoC is not sure why most of the black-billed gulls abandoned their nests at Tokaanu Wharf this year but suggests it was because of human interactions, avian or animal pests. In December last year, 100 gulls started building nests on boats in the Motuoapa Marina.

"When they start nesting they are absolutely determined not to move."

Under the Wildlife Act 1953, DoC has authority to move the gulls' nesting material, but no one else is allowed to interact with them.

Sarah describes the nests as "a few bits of grass and a twig or two". Over a three-week period in December DoC staff, working with the harbourmasters' office, moved the nests off the privately owned boats.

In a multi-phase operation designed to keep the birds onside, the nests were initially moved to pontoons borrowed from the DoC's fisheries team and located on an empty marina at Motuoapa. The pontoons were then rowed out and anchored outside the marina.

"The birds seemed to accept this, as it was a small distance from their preferred nesting site.

"The birds were squawking and watching us, and it was only through our persistence they moved."

Some days, they were moving more than half a dozen nests. Every effort was made to move the nest before the mother bird had laid an egg but Sarah says some nests did contain eggs and some nests were probably abandoned because of the move.

"On the other hand, eggs laid on boats tend to roll around as the surface isn't flat. No doubt some eggs were lost this way.

"Much of the black-billed work is guesswork on our part, and we are learning more each year."

A population count of the adult black-billed gulls at Motuoapa Marina now shows 300 adults, and Sarah says they think the colony split earlier in the season. There were 150 - 200 adults at Tokaanu Wharf until they abandoned that site (and also abandoned their eggs) over the Christmas period. DoC is happy the colony at Motuoapa has produced 60 chicks and from a population point of view, the nest-moving exercise was a success.

The breeding season is coming to an end, with the last few eggs hatching in the next few weeks and the oldest chicks getting their first adult feathers and moving around without their parents.

Motuoapa boat owners were helpful, with Sarah saying many put up anti-bird nesting devices such as hawk kites, flutter tape, water sprinklers and nets.

"Boat and house owners' reactions ranged from 'what can we do to help?' to 'take them away, the non-stop squawking keeps us awake and we can't stand the poo and nest mess'."

Boat owners and the local Motuoapa community were interested to learn black-billed gulls are freshwater gulls, not seagulls.

"Some of the residents were really interested and would help us by counting the birds and the eggs, and put up weekly reports on their community social media page."

The black-billed gulls nest in different places each year, and there is no way of knowing what location they will choose until they start pairing up in spring.

"Once they have homed in on an area to nest, the adults and newborn chicks have an inane sense to repeat nesting in this area."

Credit: NZHerald.co.nz