Where have all the custard-heads gone? 5 facts on our rare mōhua

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With the affectionate nickname of custard-head, it should be easy to recognise the mōhua (yellowhead). Their bright yellow plumage donning their heads should be a giveaway, right? Wrong. Did you know mōhua have an avian impersonator? Here we take a look at these fascinating birds, their interesting quirks, and the biggest challenges to their survival.

Mōhua on a branch.
Yellowhead. Adult Anchor Island. Image credit: Oscar Thomas

1. Mōhua, or look-alike?

Mōhua can be confused with the similar-looking male yellowhammer. Can you tell them apart?

Maybe not – because the easiest way to identify the mōhua from its introduced look-alike isn’t by a photo. It’s by knowing the type of landscape you’re seeing them in.

These birds don’t live in the same habitats. Mōhua don’t leave their beech forests and yellowhammers don’t enter them – preferring open country. So if you’re in a beech forest in the South Island and you see a flash of yellow amongst the trees, you might have just spotted a mōhua.

A yellowhead and yellowhammer side by side.
Yellowhammer or yellowhead? Image credit: Charles J. Sharp (via Wikimedia Commons) and Leon Berard

2. A clever hiding spot turned death trap

A mōhua coming out of its tree hole nest
A mōhua in its tree hole nest. Image credit: Michael Eckstadt (via naturephoto.co.nz)
A mōhua coming out of its tree hole nest
A mōhua in its tree hole nest. Image credit: Ron Enzler

Mōhua nest in tree holes –  a defence mechanism they evolved to hide from our native birds of prey such as the ruru (morepork) and kārearea (falcon). But when stoats and rats were introduced into Aotearoa New Zealand, this clever nesting spot became a death trap.

For mōhua, the female takes sole responsibility for sitting on her eggs in a tree hole nest. For the three weeks that the female is incubating her eggs, she is in danger. There is no escape route for her when rats and stoats invade her nest. And when they attack, they will kill her along with her eggs and chicks. Because of this, the gender of mōhua populations are often highly skewed, with more males than females.

To make matters worse, mōhua nest later in the spring than most other forest birds, which means they are still nesting when stoat numbers hit their peak in the summer. 

But, there’s good news. Unlike other threatened species, mōhua can reproduce at a high rate. Each year mōhua can lay up to four eggs and can raise two broods. That means that when effective predator control is in place, mōhua populations recover extremely well. Just 14 mōhua lived in the Landsborough Valley before predator control began. In a survey done last year, that number has soared to 485 birds.

3. A balancing act

When you’re foraging around a tree looking for insects to feast on, how do you keep your balance? With your tail, of course – if you’re a mōhua.

Mōhua have rigid tail feathers that they use to prop themselves up as they feed. Over time, their tail feathers become worn and can end up looking like bald spines. That’s when they’re moulted.

Mōhua using its tail to prop itself against a tree while foraging
Mōhua using its tail for balance. Image credit: Ron Enzler

4. Mōhua raise imposter chicks

A mōhua and cuckoo on a branch together.
A cuckoo chick and its mōhua adoptive parent. Image credit: Maddie and Jason Van De Wetering
Koekoeā flying.
Koekoeā. Image credit: Christopher Stephens

Every spring, mōhua are the unsuspecting victims of some very lazy parents. 

Koekoeā (long-tailed cuckoos) take a very hands-off approach to their parenting. They’re brood parasites, meaning they take no part in raising their young. Instead, they will search for a host’s nest, lay their one egg and fly away.

Mōhua parents are none the wiser to the imposter egg laid in their nest. They will incubate and raise it as if it was their own. Once the koekoeā chick hatches it will kick the mohua’s eggs or chicks out of the nest. 

The koekoeā chick quickly dwarfs its adoptive parents. By adulthood, it will weigh four times as much as its parents. As you can imagine, feeding this big chick keeps both parents busy collecting insects.
There’s no denying that mōhua are getting a raw deal, but are koekoeā having a big impact on their population? Koekoeā are native summer migrants to New Zealand, so they’ve evolved this parasitic relationship with their hosts over a long period of time. There is no evidence that these lazy parents have a significant impact on mōhua. The big threats for mōhua are habitat loss and introduced predators.

5. Where have all the mōhua gone?

Mōhua were once a common bird. When European settlers arrived, they observed mōhua flocking together in large groups, their chattering calls echoing in forests across the South Island.

But colonisation was not kind to our mōhua. Large parts of the forests they lived in were logged and became farmland. With less habitat, mōhua populations shrank. And having evolved no defences to them, mōhua were easy prey to introduced stoats and rats. 

By 1900, mōhua were quickly disappearing from our forests. 

Mōhua are now absent from 75 percent of their historic range. They live in small scattered populations throughout the South Island and on island sanctuaries. 

These rare birds, with a total population of less than 5,000, are unique to Aotearoa New Zealand. The loss of this bird in New Zealand would be a loss for the world.

Mōhua on a branch.
Mōhua. Image credit: Leon Berard

How can you help out our mōhua?

If you’d like to see New Zealand’s mōhua population grow, the best way to help is by making their habitats safe.

If you live in an area close to a mōhua population, you can do this by setting traps in your backyard. You can also get involved with your local trapping group such as the Routeburn Dart Wildlife Trust in Southland and the Central Otago-Lakes Forest & Bird in Makaroa Valley. 

Find your local trapping group by using our map. You can also donate to the Mōhua Charitable Trust.