By Annie Hill
The original article in Russell Lights | Ngā Mārama o Kororāreka
Although they’ve lived alongside us for centuries, accompanying us as we expanded around the world, enjoying the bounty of our food supplies and homes, people generally dislike rats. Most Russellites will have encountered rats in or near their home, and when Predator Free Russell turn up offering to get rid of rats for free, they’re enthusiastically welcomed.
In Aotearoa~New Zealand, we have the dubious distinction of hosting three rat species, all introduced by humans, all having impacts on our biodiversity.
The first arrival was the kiore~Pacific rat, which originated in South-East Asia, and were carried across Polynesia as people migrated.
They appear to have landed with the first waka in the 13th century and were probably brought intentionally, to be used as food. Some people believe these rats are relatively harmless, but while they eat a largely vegetarian diet, they are omnivores, and more than happy to predate birds’ eggs and chicks, as well as lizards, frogs and insects. They were probably implicated in the extinction of six duck species, six flightless rails, two species of coot, and responsible for the demise of two species of wren, as well as an unknown number of gecko and skink species that became extinct before colonisation, along with four species of frogs that we know of.
The kiore was no match though for the Norway rat (rattus norvegicus), which arrived in the 19th century. Also known as water rats and able to grow to over 30 cm long (including the tail), they are larger and more aggressive than kiore, and soon displaced them. Kiore are now only found around Fiordland and some offshore islands. A small population is protected on Sail Rock, near Whangarei, as they are revered as a traditional food source. They would undoubtedly bounce back were the other two species to vanish.
Norway rats prefer damp habitats such as streams and mangrove swamps, but also flourish around human settlements. They’re efficient burrowers and their tunnels under a building will often go unnoticed. Very social animals, they live in colonies (known as a ‘mischief’ of rats), grooming one another and playing. They are prolific, and as a female can be concurrently pregnant and lactating, she can produce a multitude of offspring in a year: up to five litters with an average of seven babies, although 12 is not uncommon, which could mean 15,000 rats in one year.
This was (sadly) demonstrated on Henderson Island, where an attempt was made to eradicate the (Pacific) rat population. Although well over 100,000 were killed, it was estimated that 60 to 80 survived and the island is now overrun again.
In most of Europe, the ship or black rat (rattus rattus), originating in India, was the first invader, and the Norway rat, originally from China, followed behind. However, Aotearoa~New Zealand didn’t get any ship rats until the 19th century, probably when trade really became established. Old World ship rats prospered alongside human beings in many places, because they are naturally arboreal, so would make their homes in wooden houses with thatched roofs and lofts. They flourished in granaries and anywhere food was stored, but were mostly just considered a nuisance.
Norway rats arrived in England about 1730, apparently in timber ships from the Baltic (hence their scientific name), spreading rapidly up waterways and via coastal shipping. As they spread north, they dispossessed the ship rats, resulting in their effective extinction in the UK.
Here, although forest cover has declined from 80% in AD1000 to around 30%, this is still a lot of habitat for ship rats and they’re very much at home in our trees, forming nests there and preying on birds, their eggs and chicks. Urban dwellers are more likely (than rural dwellers) to see Norway rats, which are less comfortable in forests, but the ship rats are still more common. Predator Free Russell have caught over 2000 rats in two years, of which only 18 were identified as Norway rats.
While ship rats work through our forests, Norway rats, being burrowers, can wreak havoc on our offshore islands where seabirds nest. They are very capable swimmers, even underwater (they are known to feed on freshwater mussels) so if introduced to one island, they will probably spread to adjacent ones. They tolerate a wider range of conditions, probably because nesting in burrows means they stay warm in colder climates.
South Georgia was infamously overrun with rats that flourished long after whaling stations closed down and people left; and Campbell Island once had the densest population of rats on Earth prior to a successful eradication programme. Now, the once-rare pipit and teal are abundant, the subantarctic snipe re-established itself, and birds such as storm petrels are breeding even on the main island.
As soon as rats are eradicated, the difference is quickly noticeable. The trapping team in Russell have observed the difference in the number of miromiro (tomtits) and pateke (teal). In Wellington, there has been a 71% increase in native bird detections, since commencing elimination of Norway and ship rats, stoats, and weasels from the 1,000 ha Miramar Peninsula. Between 2017 and 2022, pīwakawaka (fantails) increased by 500%, riroriro (grey warblers) increased by 340%, and tūī increased by 72%; and the local community gains from rat-free homes and more interactions with our native fauna.
Maybe it’s not surprising that we are so often ‘fellow travellers’ with these intelligent animals that are as adaptable and resourceful as us.

Norway Rat Image: Rod Morris | Creative Commons