The toughest incursion season yet
The pest-free islands of Ipipiri, Bay of Islands, are becoming a mecca for birdlife. Since the eradication in 2009, tomtit, fantail and tui are thriving in the absence of predators, and the new arrivals of toutouwai (North Island robin), tieke (saddleback) and popokotea (whitehead) have found sanctuary in their new home with successful breeding seasons reported. With the proximity to the mainland and accessibility of these islands for everybody to enjoy, these safe havens are constantly monitored and managed to defend from invading predators.
Every year there is a high risk period, the incursion season between December and June, where young dispersing rodents and stoats can find their way out to the islands, swimming the short span ocean from the mainland. There is also a high potential for rodents to hitch a ride on visiting boats, with camping gear and food stores. The summer influx of recreational visitors, with busy campgrounds, compounds this risk to the islands.
This year has been a particularly bad season for incursions, and has challenged us. In February, a regular fortnightly check of surveillance traps revealed two Norway rats trapped, on Poroporo and Round Island, and a tracked card on Okahu Island (the first rat sign detected there since the eradication). This triggered an incursion response operation to ascertain the extent of rodent presence and to target the rodent on Okahu. Using rodent detection dogs and extra snap traps, tracking tunnels and waxtags, the information gathered over the following months yielded the presence of more rodents on Urupukapuka, Poroporo and Okahu islands. These were targeted with traps and poison baits, and to date 9 rats have been caught, all adult Norways. The last rat was caught on 10 May in Urupukapuka Bay, Urupukapuka, a Male Norway. It has been a tireless effort from all involved. As we go into winter, it is more important than ever to keep a finger on the pulse.
As part of the incursion response operation, the dead rats are sent away for analysis to ascertain age, breeding status and to profile their DNA for comparison with local populations and each other. This information is critical for making operational decisions on how we target the situation, helping to understand potential source populations and relatedness between invading individuals, where breeding could have occurred. Interestingly, the majority of rodents caught this season have had DNA markers that differ from any local mainland population sampled. This highlights how easy it is for rodents to find their way to new places, potentially over large distances, aided by human vectors like boats.
We have not been alone in this fight with incursions this season. Across country in several pest-free environs like islands and predator-fenced sanctuaries, there have been a high number of incursions detected; for mice, rats and stoats. This could be a reflection of an unusually mild autumn and the extended warm weather that we are experiencing, leading into winter. Although, this highly productive weather pattern boosts food sources and encourages good breeding seasons for our threatened bird species, it is also very favourable for our predators. This can mean an extended breeding season in habitats and a higher density of young predators dispersing, presenting a higher risk.
With the colder months approaching, the good fight continues to provide a safe sanctuary on the islands for the bird song to flourish. There has been a huge effort put into this incursion season, and the ongoing support and vigilance of the local public and visitors to the islands is crucial; checking for rodent signs and presence in any gear and vessels heading out to the islands. Every little bit reduces the risk.