Last updated 05:00 09/02/2014
A predator plague expected to hit the Marlborough Sounds this year poses a serious threat to native birds, says a Department of Conservation manager.
Marlborough Sounds services manager Roy Grose said beech forests in the Sounds would be "under siege" as a predicted beech mast would fuel a massive hike in the number of rats and stoats.
In a mast year beech forests produce a higher than normal amount of seeds. Rats and stoats feed on the seeds and turn on native birds such as fantail, grey warbler, tomtit, bellbird and tui when the seeds run out.
The situation was critical, Mr Grose said.
"A fantail has a clutch of two or three chicks during spring compared to rats who breed at 20 or 30 times faster than any native bird," he said. "These native birds are heavily preyed on by rats. For instance, rats will go into fantail nests, eat the eggs and chicks. Our native bird species don't have a chance, it is a forest under siege."
The last widespread mast in 2000 led to the yellowhead (mohua) all but disappearing from the Marlborough Sounds. The yellowhead features on the $100 bill.
Numbers of the bird on Mt Stokes, near Kenepuru Head in the Marlborough Sounds, dropped from 130 to only five.