A rat plague currently sweeping the country will almost certainly be followed by a stoat plague, with potentially disastrous results for Northland's kiwi population.

Northland pest control firms are reporting a surge in the number of rats this winter — some say they are twice as busy as this time last year — while conservation groups such as Paihia's Bay Bush Action say they are being inundated with a ''rat tsunami''.

The cause is thought to be a mega-mast, when many species of native trees mast, or produce large quantities of fruit and seeds, at the same time.

Rodents gorge themselves on seeds, breed prolifically, then invade people's homes and prey on native wildlife once the seeds are gone.

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Andrew Mentor, Kiwi Coast co-ordinator for the Mid North, said conservation groups around Northland were reporting large numbers of rats.

''The traps aren't keeping up with them,'' he said.

Rats competed with native wildlife for food and took eggs and fledglings from their nests. Ship rats hunted in the trees while Norway rats prowled the forest floor.

Even more worrying than the rodents, however, was the expected boom in stoats, which depend on rats for up to half of their food.

''As rodent numbers go up, stoat numbers also go up. When the rats die off in mid-winter the stoats will switch over to birds.''

About 95 per cent of kiwi chicks around New Zealand were killed by stoats between August and October each year, Mentor said.

Stoats also prey on ground-dwelling birds such as banded rails, bitterns, marsh crakes and pāteke. Photo / supplied
Stoats also prey on ground-dwelling birds such as banded rails, bitterns, marsh crakes and pāteke. Photo / supplied

The voracious hunters also preyed on ground-dwelling birds such as banded rails, bitterns, marsh crakes and pāteke, and anything in its nest.

Northland is a stronghold of the North Island brown kiwi. It is also one of the few places in the country where kiwi survive in areas densely populated by humans, such as Russell Peninsula and parts of Kerikeri. The Purerua Peninsula has a particularly high kiwi density.

■ Anyone who wants to help fight the expected stoat plague can contact the Northland Regional Council for traps or advice, start a community trapping programme, or join an existing group. For more information go to kiwicoast.org.nz.