A contribution by Pete Corson of Quality Conservation Limited.
Pictured: Tūī feeding on flax on Ahuahu Island. Photo by Pete Corson.
Red-crowned kākāriki, kākā, bellbird, tūī, New Zealand dotterel and pāteke have thrived over the last few years on pest-free Ahuahu/Great Mercury Island. This is a great reminder of the value of pest-free islands and a thank you to everyone for helping New Zealand’s offshore islands stay pest-free.
In March 2010, a team of ecologists visited Ahuahu to check out the conservation values. This led to conversations, consultation and ultimately a joint project between the island’s owners and the Department of Conservation to eradicate ship rats, cats and kiore from the island in 2014. The project was declared successful in 2016. A first full round of bird monitoring occurred in early 2015. More recently, shortly before the COVID-19 lockdown and 10 years after the first ecological assessment, a team of volunteers, as well as DOC and Great Mercury Island staff remeasured the same survey areas to find out what has happened. The results have been really encouraging.
Pāteke (brown teal) have been a standout. Pāteke were not observed on Ahuahu between 2010 and 2016, but in early 2017 a relaxed flock of nine of the birds were seen on one of the farm ponds. These birds are now going ballistic. The census indicated a minimum of 163 pāteke on the island currently, with flocks of up to 30 birds.
Chattering red-crowned kākāriki, racing through the forest like small fighter jets with a seeming disregard for the bird observers’ safety, have shown the largest increase of any of the forest birds, now 13 times more abundant. Tūī and bellbird (korimako), which were clustered together for monitoring, and kākā have more than doubled in numbers. These birds are now producing a great cocophany on the island. Encounter rates of bellbird and tūī are at the moment about 35 per hour compared to 15 per hour in 2015.
High pitched New Zealand dotterel (tūturiwhatu) were also measured on the beaches, estuary and sand dune areas in 2018 as well as at the time of the eradication in 2014. The numbers vary seasonally, but were recorded as 38 adults and 16 chicks in late 2018, up from only nine birds in 2014. That is more than one per cent of the total New Zealand dotterel population.
Introduced birds such as blackbirds, song thrush, chaffinch, dunnocks and magpies increased between 2015 and 2020. There was a strong response in the large pine forest area on Ahuahu for both native and introduced birds but interestingly, introduced birds decreased in the native forest areas between 2015 and 2020.
What is clear is that the response of birds on Ahuahu has been great over the last five years while the island has been free of cats and rats. The island is certainly getting louder. This survey has been a good check to make sure the work done continues to deliver on the pest eradication project’s objectives – to reduce the biosecurity threat to the other Mercury Islands, to allow Ahuahu’s ecosystems to function without cats and rats, to have a place for threatened species and to have a place where people can engage with conservation. The island has been accessible to the public since the owners, Sir Michael Fay and David Richwhite, took ownership in the late 1970s.
Monitoring will continue on Ahuahu. Seabird monitoring is due to occur later this year and vegetation plots and lizard surveys are planned in the future.
There has been over half a million trap nights on Ahuahu over the last five years to check for pests, but the best defence is if pests never get there. Everyone going to, or near, any of the Mercury Islands is urged to check their boats and gear for pests.