Why are mohua declining

Mohua were once one of the most abundant birds heard in the forests of the South Island, yet today most New Zealander’s and tourists to our country won’t be able to seen them. Why has the mohua disappeared from 75% of its former range and what threats continue to contribute to its decline?

Less forest = less mohua
When humans first arrived in the South Island of New Zealand most of the land was covered in podocarp-hardwood or beech forests and home to small birds such as the mohua. Today some large tracts of forest still remain along the West Coast and throughout Fiordland, but much of the forested areas from Marlborough, Canterbury, Otago and Southland have been logged and turned into farmland. The equation is quite simple that if you take away the forest you are also taking away the homes of forest birds - hence deforestation was a major cause of the historical declines of mohua across the South Island. Today native forests are protected from logging but mohua still continue to decline.

Evolving with no mammalian predators
For tens of thousands of years before humans arrived in Aotearoa the land was ruled by birds - the only land mammals were native bats. In this bird-oriented world small birds such as the mohua only developed defence mechanisms against larger birds like the New Zealand falcon (Falco novaeseelandiae), the morepork (Ninox novaeseelandiae) and the much larger Laughing Owl (Sceloglaux albifacies) that went extinct in the 20th Century. Nesting in holes in trees provided perfect cover from these large aerial predators!

The stoat is a major predator of mohua

Introduced predators - tipping the ecological balance
When the first Polynesians landed in Aotearoa around the 10th Century they brought with them the first mammalian predator – the kiore or Polyensian rat (Rattus exulans). Although an agile climber, the kiore targeted small ground dwelling birds and insects, rather than hole-nesters. With the arrival of the first Europeans the ecological balance of New Zealand was to be changed forever as a new wave of predators was liberated. Of these introductions the ship rat (Rattus rattus) and the stoat (Mustela ermine) are both excellent climbers and subsequently have had the greatest impact on the decline of small, hole-nesting birds such as the mohua.

Hole-nesting now a death trap
The once perfect nest site that was safe from native flying predators - a hole in a tree – now makes New Zealand’s native birds ‘sitting-ducks’ for introduced predators. Hole-nesters such as the mohua are particularly vulnerable to predation from ship rats and stoats as there are no escape routes from predators when you are sitting on a nest inside a tree. Cup nesting birds now have the ecological advantage as they can fly off when a predator approaches.

A female entering her nest in a tree is wary of any predators

(Photo: Michael Eckstadt)

To make matters worse for mohua, females are the worst hit as they solely incubate the eggs for 20 days before hatching, making them extremely vulnerable to predation during this time. Hence the sex-ratio of predated mohua populations is often highly skewed towards having more males than females, a sad tale if you are a male looking for a mate!

Behavioural characteristics contribute to mohua declines

Mohua are more vulnerable to predation by introduced mammals than many of New Zealand’s other bird species because of the five key behavioural characteristics listed below:

Mohua nest in holes. Nest predators not only eat mohua eggs and chicks but also incubating adults, which are unable to escape. Furthermore since only females incubate, nest predation results in a biased sex ratio with lots of males and few females.
Mohua have long incubation and nestling periods. Eggs are incubated for 20 days and chicks are in the nest for another 22 days after hatching before they take their first flight. This is about two weeks longer than most introduced passerines, significantly increasing the risk of chick predation.
Groups of mohua occasionally spend long periods feeding on, or close to, the ground. These groups are very noisy and, although there is no evidence of predation, they would make conspicuous targets for predators.
Mohua nest later in the spring than most other forest birds. They are still nesting when stoat numbers reach their summer peak.
Some mohua also roost in holes. It is unknown how common this activity is but it does increase the risk of mohua predation outside the breeding season.

Did you know? The New Zealand falcon and morepork still survive in our forests today but in much fewer numbers than mammalian predators and pose little risk to the survival of mohua as a species. In fact morepork too are in a gradual decline due to predation by stoats, loss of habitat and road deaths.