Threats to yellowhead
Today, there are about 30 existing populations. They are divided into three groups - those east of the main divide, small scattered Fiordland populations, and Southland/Otago hill-country populations.
Mohua have also been transferred to several offshore islands - Chetwode Islands in the Marlborough Sounds; Breaksea, Chalky and Anchor Islands in Fiordland; and Ulva Island in Paterson Inlet, Stewart Island.
Ship rat
The populations east of the main divide seem to have suffered the greatest declines. The previously strong population in the Eglinton Valley (in Fiordland) suffered a major collapse in 1999-2001, nearly to the point of local extinction. This was mostly due to unusually high numbers of ship rats, which predated on adults and chicks. Ship rats are excellent climbers, taking not just chicks and eggs but also incubating females that are sitting in nest holes, high in trees with no escape route.
Beech masting
The ship rat population explosion was likely to have been caused by the occurrence of two beech masting seasons in a row, along with mild winter temperatures in between. This meant the rats had an abundant food supply, with no cold winter to bring numbers down before another bumper breeding season. Stoat control in the valley may also have reduced predation on the increasing rat population.
This double beech mast phenomenon also occurred in other eastern mohua-populated valleys, causing the extinction of the Mount Stokes population, and major declines in the Hawdon, Hurunui and Dart populations.
Those populations in the Southland hill-country seem to be the most stable. These include the Blue Mountains, the Catlins forest and Rowallan/Longwoods forest (near Tuatapere). These forests did not experience the same heavy beech seeding, and so remain under less predation pressure from rats or stoats.
Saving mohua from stoats and rats in South Westland
Monitoring shows that continuous stoat trapping and occasional use of biodegradable 1080 poison has boosted the population of endangered mohua in the Landsborough Valley in South Westland.
Elsewhere this unique little bird is completely defenceless against rats and stoats. Rebecca Wilson explains:
Video by Trakabat
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