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The native birds took up residence at the Blakes Rd stormwater retention basin in Belfast in about August last year when work on the site had to stop because of the Covid lockdown.
Christchurch City Council stormwater and waterways team Leader Keith Davison said it seems the birds mistook the partially constructed stormwater basin for a braided river and started nesting in the area.
There were about 750 black billed gulls in the colony. Up to 30 pied stilts have also been nesting there.
Davison said because black billed gulls were classed as critically endangered at the time of their discovery in the retention basin, all work was stopped while the birds were nesting and new chicks were hatching.
In December, the status of the species was re-classed to ‘At Risk – Declining’ in a review of the New Zealand Threat Classification System.
"We’ve had to monitor water levels in the stormwater basin very carefully while the birds have been nesting there," Davison said.
"We could not let the water levels drop too low because the shallow muddy water was a habitat for insects, which the birds fed on.
"The water also acted as a moat, stopping other predators from attacking the nests.
"At the same time, we couldn’t let the water level rise too high because we did not want to flood the nests and drown the chicks."
Access to the retention basin was prohibited while the birds were nesting.
Council regional park rangers helped set up a protective trapline around the site to stop rats and other predators threatening the young chicks’ chances of survival.
The rangers are experienced in setting traps in sensitive environments and could quietly check the traps without disturbing the birds.
"Unfortunately, because of the measures that have been required to protect the birds, our contractor, Fulton Hogan, has been unable to complete work on the stormwater basin," Davison said.
"It has delayed completion of the project by a few months, but black billed gulls are fully protected native birds and protected by the Wildlife Act so they could not be disturbed once they had started nesting.
"Fulton Hogan has been very understanding of that and has done all it can to protect the site."
Davison said the council’s parks biodiversity team ornithologist says it is unlikely the black billed gulls will return to nest at the Blakes Rd stormwater detention basin as, by the time the next breeding season starts, the site’s environment will have changed.
Black billed gulls have previously been discovered nesting in the flooded foundations of a partially demolished building on Armagh St in the central city. They have also recently nested on a gravel island in the Charlesworth Wetland Reserve in Ferrymead and on mooring piles in a completely marine environment at Diamond Harbour.