Control of black-backed gulls to protect rare braided river birds

Date:  29 October 2018

DOC is overseeing the control work, with the support of Environment Canterbury as part of the Hurunui Waiau Water Zone Committee’s flagship project supporting threatened bird species.

Black-backed gulls are known to prey on the eggs and chicks of black-fronted tern/tarapirohe, black-billed gull/tarāpuka, wrybill/ngutu pare, and banded dotterel/tūturiwhatu — all endemic threatened birds that breed on this braided river.

Zone Committee Chair John Faulkner said Canterbury’s braided rivers, including the Hurunui, are unique. “They support many important fish, animals and plants. The birds that depend on these rivers are declining,” he said. “That is why we are supporting this important control operation.”

DOC Science Advisor Kerry Weston said control of the abundant black-backed gulls was being trialled on the Hurunui River after monitoring showed more than two thirds of black-fronted tern nests there were preyed on last breeding season.

“We had only eight black-fronted tern chicks survive on the Hurunui last season with 77% of nests destroyed after being preyed on by either mammalian or avian predators.”

“A recent study on the Waitaki River found black-backed gulls were the main predator of black-fronted tern nests there, so we know they will be having an impact in the Hurunui where there are high densities of black-backed gulls.”

The black-backed gull control trial will involve comparing the breeding success of the threatened bird species this summer in locations where gulls are controlled and an area where they are not. If successful it will be extended to the Waiau River next year.

Ecological consultancy Wildlife Management International will carry out the control work by targeting black-backed gull colonies over a 30 km section of the riverbed between the upper and lower Hurunui gorges for several weeks.

Landowners living next to the areas where gulls will be controlled have been informed and will be contacted before the operation starts.

The toxin alpha-chloralose, an anaesthetic compound registered for black-backed gull control, will be mixed with margarine on bread bait and hand laid within gull colonies.  All uneaten baits and gull carcasses will be removed for disposal within 24 hours. Warning signs will be in place at entry points to the operational areas.

Alpha-chloralose bread baits and gull carcasses are toxic to domestic animals, as well as people, if eaten. People are advised to avoid the control areas while signs remain in place and not to take their pets into these areas.

Background information

Canterbury’s braided rivers are a national stronghold for black-fronted tern/tarapirohe with an estimated 60% of the breeding population. Likewise, New Zealand’s only endemic gull, black-billed gull/tarāpuka, is largely found on South Island braided river beds with a colony of more than 1000 birds found on the Hurunui last summer.

Wrybill/ngutu pare is also mostly found on South Island braided rivers, while the banded dotterel/tūturiwhatu is more widespread in New Zealand but is also in decline.

Southern black-backed gull/karoro is abundant throughout New Zealand and has reached unnaturally high densities in some areas due to food supply from human sources. This species is also common at similar latitudes in southern Australia, South America, southern Africa and subantarctic islands.

The joint Hurunui Waiau Water Zone Committee, Environment Canterbury and DOC project to enhance braided river bird habitat on the Hurunui and Waiau rivers also includes bird surveying and monitoring and clearing vegetation from some river islands to create open safe areas for birds to breed.

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