Mohua/yellowhead were once one of the South Island’s more abundant native forest birds, but a dramatic decline in their population over the last 30 years has meant mohua, endemic at the genus level, are all but gone from 75 percent of their range. The mohua is a hole nester and therefore especially vulnerable to the usual depressing repetoire of predators- rats, stoats and possums. “Mast” years, where an unusually heavy flowering of native trees leads to an abundance of seed, cause a spike in rat and stoat numbers. When seed supplies fall, they switch to birds, and mohua is in the firing line. Recognition of the particular significance of masting came around the time we got involved, and with added government money also, mohua’s status has improved to Nationally Vulnerable. Mohua work revolves around predator control and island translocations…
Whenua Hou Translocation
Our very first project. An insurance population of mohua was required on Whenua Hou/Codfish Island and the population on Breaksea was strong enough to be harvested.
Catching on Breaksea went well, and even the tieke/saddleback by-catch could be utilised, being dropped off on the way to Whenua Hou to populate another island in Fiordland. The transferred birds were breeding happily within months, and a survey after 6 years showed moderate population increases.
In additon to this project we assisted with an infrastructure upgrade. Mohua had earlier been moved from two mainland sites to an island in the Marlborough Sounds. Field workers needed assistance with some half-decent accommodation; the caravan they stayed in was well past its use-by date. A new hut replaced it and the caravan had a helicopter ride out to a barge bound for the mainland. The mohua there are hanging on, but not exactly thriving. Of course the hut gets use for other projects centered on this predator-free island as well.
Secretary Island Translocation
This project aimed to create a safe mohua population with excellent genetic diversity. The Dart area is thought to have the most diverse mohua population on the mainland and the near-elimination of predators from Secretary Island gave us an opportunity.
Predator control in the Dart and an excellent breeding season made for good mohua-catching. Hannah Edmonds and Barry Lawrence from DOC and their catching team netted 75 mohua in only two days in October 2008 and flew them to Secretary Island with no losses and the longest time a bird was kept captive being 7 hours.
A flock of mohua was sighted on Secretary a couple of months later and they are presumed to be breeding well.
Chalky and Anchor Surveys
There have been eight island translocations of mohua, and success has been variable, results possibly revolving around the types of forest the mohua were used to in the source areas, and what they encountered on the various islands. We thought some sort of follow-up was needed.
Ros Cole from DOC organised a 2009 survey by Bryce Masuda and Ian Jamieson from Otago University, of mohua populations on Chalky and Anchor Islands. Both received mohua in 2002 and anecdotally they were doing well. The survey found year-on-year population increases of about 40% on Chalky, and nearly 200% on Anchor. It was thought that both islands might be nearing carrying capacity for mohua. This was contrasted with slower growth on Codfish. Apart from the suitability of the varying forest types, one factor may be the presence of the threatened koekoea/long-tailed cuckoo, which parasitises mohua nests, on Codfish but not Anchor or Chalky. Indeed the two latter islands might now become candidates for koekoea translocations.
Funded by Fauna Recovery New Zealand ($47,000). Boat transport by Dart River Jet.