Happy together

A deeper look into the co-dependent evolution of New Zealand’s unique native birds and plants.

We do love our birds in New Zealand. With good reason. Not only are we collectively named for our iconic and beloved kiwi, but our native birdlife is also remarkably unique thanks to our geographic isolation and varied landscape.

Though humans have not always been kind to native birds in New Zealand and our co-existing love of cats notwithstanding, we modern Kiwis (the non-feathered kind) have generally learned to celebrate and safeguard our unusual and often rare bird species.

Each year, devotees of different species engage in a practically Darwinian battle to crown a local Bird of the Year. And, from June 24 to July 2, avid bird lovers will be engaging in the 2023 New Zealand Garden Bird Survey, counting the birds that live and visit our gardens to help determine not just how many there are, but how their habits and habitats are being impacted by long-term changes in climate change, urbanisation and shifts in biodiversity.

This annual survey helps researchers write the latest chapters in the evolutionary history of our birds and their evolving environment.

That our native birds and native trees share an interdependent history should come as no surprise, given both have been steered by New Zealand’s geographically isolated island landscape.

Plant evolutionary biologist Janice Lord, an associate professor in the Department of Botany at Otago University, says one good example of how our native plants have adapted to the native bird life is in fruit size to help ensure seed dispersal.

“The trees rely on birds to spread the seeds around. We do have some big-seeded plants like the tawa and taraire that rely on kererū for fruit dispersal, but generally in New Zealand bird gapes (the interior of a bird’s open mouth) are smaller. Seeds need to be swallowed whole, so that constrains the size of the fruit,” Janice says.

Interestingly the absence of endemic land mammals in New Zealand, aside from bats, has also influenced the colour and flavour of berries in many of our native plant varieties. That’s largely because most of our fruit-eating native birds have little or no sense of smell, and look for food based on colour, generally purples, reds and black.

“The colour distribution of fruit is about the same as you’d find in Europe and North America but they don’t have the same aromas and flavours because they are not catering to mammals like bears,” Janice says. “Our berries don’t appeal to a mammalian palate.”

Tangled tales

Another way the evolutionary relationship between our native flora and fauna is demonstrated is through our high number of divaricating native trees and shrubs.

Divaricating plants are those that branch at wide angles, often creating a twiggy tangle, and, in the case of New Zealand natives, often with small leaves and a marked difference in their appearance between their juvenile and mature forms.

Interestingly, the divarication has evolved at multiple different times in unrelated, diverse groups of plants.

There are a number of theories around why our native plants may have evolved these features, Janice says, from it being a way of adapting to New Zealand’s changeable climate to protecting the plant from frost, or even helping it manage light.

One idea that’s caught popular imagination is that plants like the toothed lancewood evolved their spiky, interlaced juvenile forms to ward off hungry moa. Once the lancewood reaches about 3m high, placing its leaves out of reach of even the lanky moa, it changes shape, eventually developing a tall straight trunk and upward pointing leaves.

While everyone loves a good moa story, Janice says the theory is still fairly contentious. “Though the moa hypothesis does seem to explain all the different divaricated forms we see, theories around climate and light only account for some forms.”

And while the moa/lancewood relationship might still be up for debate, Janice says New Zealand’s large, ground birds would have created disturbances for young native plants which may explain how many have evolved.

“There would quite probably have been herds of moa moving through the undergrowth, eating things and generally making a mess of the place, and it’s not just moa. There were other large ground birds as well.”

She points to Australia where ground-dwelling lyrebirds scrabble in the understorey of local plants, disturbing and sometimes eating the plants as well as piling up mounds for nests.

“Those kinds of things were probably happening here too. The plant response to that is to not look too tasty when you’re young, small and vulnerable.

“Lots of our native plants have cryptic colours or very jagged leaves when they’re young, compared with when they’re adult,” Janice says, citing New Zealand jasmine ( Parsonsia heterophylla) and mataī ( Prumnopitys taxifolia) as natives with a very “scruffy-looking juvenile phase”.

“Even South Island kōwhai is tangled when young, then it sorts itself out into a property tree when it’s bigger.”

In his new book The Story of New Zealand’s Unique Birds (from Adzebill to Yellow-Eyed Penguin), author

and bird expert Alan Froggatt talks about New Zealand’s geographical isolation, and how being the last major landmass to be inhabited by humans turned our collection of islands into the perfect biological science lab.

Our birds evolved to “fill the ecological niches normally filled by mammals elsewhere on the planet”, becoming bigger, heavier and stronger than birds found anywhere else, Alan says in his book. “All were descendants of birds that could fly but many lost the ability to do so. All they needed to do to escape aerial predators was hide on the ground or blend into the background.”

This same isolated, pre-human, science lab also impacted the plants those birds used for camouflage.

“That isolation has meant we have some really unusual growth forms. We have tree daisies, for example, which are actually not common elsewhere in the world, though we kind of take them for granted. Our tree fuchsia is another one; the largest in the world,” Janice says, citing it also as an example of a plant with an unusual response to changing seasons.

While most of our natives are evergreen, the tree fuchsia ( Fuchsia excorticata) is one of our few deciduous trees. And while it does lose leaves in winter, that is also when it flowers, with nectar-rich flowers perfect for pollinating by tūī, bellbirds and silvereyes (tauhou).

One idea that’s caught popular imagination is that plants like the toothed lancewood evolved their spiky, interlaced juvenile forms to ward off hungry moa.

“So while the fuchsia loses its leaves, it’s not dormant like other deciduous trees. There’s been a bit of work looking at how when it loses its leaves, those flowers become a lot more visible against that lovely orange bark,” Janice says.

And the reason so many of our native trees are evergreen? Simply our changeable climate, she says. “Because we’re an island with an oceanic climate sometimes, for example, the summers aren’t great. A plant that habitually loses its leaves in winter can miss out on growing opportunities.

“With an unpredictable climate, it’s far better to just be evergreen. Being deciduous only serves the plant when there are definite seasons with really different temperatures.”

Welcoming immigrants

While our native flora and fauna have largely had a nicely compatible history, particularly when they had New Zealand to themselves, much of our birdlife has adapted to the arrival of introduced plant species too.

Kererū, for example, are famous for their love of a good plum as many frustrated growers can testify.

But, Janice says, they’re also fond of tree lucerne, to the point where the introduced species are used in some conservation areas to help build up the kererū population. “They’ll eat the fresh flowers and the leaves just as they would with kōwhai because they’re both from the pea family.

“I’ve also seen tūī and bellbirds on red hot poker which is fascinating. The plant is bird-pollinated where it comes from in South Africa but it is very different to a lot of our native flora, with very bright colourful flowers. So it’s really interesting that our birds have learned to visit those flowers.”

Of course while our birds might have adapted to visit the exotic plants; the plants have not always adapted to our native birds. Last month NZ Gardener reported about the flocks of kākā devastating exotic trees in Wellington, including historic northern hemisphere conifers in the Wellington Botanic Gardens.

Theories have abounded about the cause of the birds’ behaviour and subsequent destruction of the trees, but Janice says it could just come down to kākā being kākā, especially given the surge in numbers – ironically, thanks to the success of Zealandia Ecosanctuary.

“The parrot family worldwide are super intelligent, highly curious birds and with beaks like that, the normal

way they feed is by getting into things and breaking things open; like kea and windscreen wipers,” she says. “I think those Wellington kākā are basically hordes of teenagers exploring the world without parental supervision.”

Janice says it’s not clear whether native species have evolved to be more resilient to kākā and some of their destructive habits, but the Wellington problem could boil down to that fact that a lot of introduced species are deciduous, combined with the rise in kākā numbers. “I have definitely seen native trees that have been damaged by kākā as well. With deciduous trees, there is more sap and fresher leaves at different times of the year which could make some of them more attractive to the birds at certain times.

“I think having so many kākā that they’ve become a nuisance in some areas, is quite a good problem to have.”

In your garden

While you might not want kākā tearing away at your trees, there are plenty of ways to entice more native birds safely into your backyard, Janice says.

The obvious one is planting the shrubs and trees that they love. For those that have the space, kōwhai and pōhutukawa (if you have the space) are a good choice, as are “any of the lancewoods”, she says. “They don’t all have those odd juvenile leaves, but they all have berries that will attract the birds.”

Places for birds to safely sit and nest are also key, she says. “Leaving small messy or wild areas in your garden will help bring in the birds. Hedges are also really important and plants like muehlenbeckia – which I know a lot of people don’t like – offer good protection for birds. And don’t cut down big trees!”

In smaller garden, she suggests growing nectar feeders as a simple way to attract birds, or smaller species such as Marguerite daisies which also appeal to native bees and hoverflies.

When it comes to planting natives, Janice encourages gardeners to look beyond well known, readily available species to try new things.

“There are a lot more natives out there than you might realise that will work in home gardens. If you go to a nursery that specialises in natives, you’ll be amazed at the diversity of foliage shapes and forms.

“They’re easy to grow. It’s just a matter of the right plant in the right place. And they will help bring native birds into the garden, as well as bring in insects which will then encourage insectivores like fantails.”