New study reveals why some New Zealand birds are disappearing faster than others
Monday, 5 July 2021
New research has uncovered why New Zealand’s oldest forest bird families are dying out faster than new arrivals, and it could change how the country approaches conservation work.
The study, by Manaaki Whenua – Landcare Research scientists, was published in the New Zealand Journal of Ecology on Tuesday.
They compared data collected by thousands of keen birdwatchers for the Ornithological Society of New Zealand’s two bird atlases, published in the 1970s and early 2000s, with factors like different species traits.
Author Susan Walker said how long a bird’s ancestors had lived in the country played a big part in how fast they were disappearing.
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They found three traits – birds nesting in tree cavities or burrows, having a large body size, and having less ability to travel – were more common in species that had been evolving in New Zealand for a longer time.
But those traits also made them more vulnerable to the two biggest threats facing New Zealand birds: introduced predators, and the loss of their native forest homes.
Species whose ancestors arrived a long time ago were referred to as endemic, and the earliest arrivals were called ‘deep endemics’, Walker said.
Examples of deep endemics included kiwi, kōkako, parrots like kea and kākāpō, wrens like the rock wren and rifleman, and whiteheads, yellowheads, and brown creepers.
Next came endemics like tūī, bellbirds, kererū, and blue ducks. Then came the newest native bird species, including silvereyes, fantails, and grey warblers.
While close relatives of silvereyes and grey warblers could be found in Australia, Walker said the older endemic birds were a different story.
“[Deep endemics] evolved in isolation in New Zealand, [they] evolved these special characteristics that only occur in New Zealand,” she said.
“Endemic birds are not only our taonga, they're the world’s taonga. If we lose kiwis here, the world loses kiwis.
“Moa were an endemic order as well, and this whole branch of the tree of life was lost when we lost the moa.”
The study could have some unexpected implications for New Zealand’s conservation and predator control work.
Firstly, forest loss would limit the population recovery of some forest birds, particularly the larger, less-mobile endemics, and large tracts of native bush would be needed to help them bounce back, Walker said.
“With Predator Free 2050, the assumption is that we’ll get more birds overall, [but] some of our birds will not survive the deforestation well.
“It won’t have the same impact everywhere.”
Large parts of the North Island and almost all the South Island’s east coast have been nearly cleared of native bush, making it harder for cavity-nesting species and ones that could not get around easily to regenerate.
Secondly, Walker said large-bodied birds also needed more food to recover, but a lot of endemic birds had retreated to New Zealand’s colder, less bountiful forests.
“They retreat to the places their predators like least, but they're suboptimal for restoration.”
Pest control work needed to be prioritised in warmer forests with abundant food sources too, rather than just the colder, last refuges where endemic birds hid out, she said.
Finally, New Zealanders needed to adjust their expectations as to what a predator-free country would really look like.
Walker said they predicted the bird community would change, rather than all species’ populations increasing.
“As predators are removed, there’s likely to be an increase in endemic species … but silvereye numbers, for example, may fall.”
She said because endemic birds had evolved to live here, they may actually be better adapted to compete.
“You might walk into a forest and hear a lot of grey warblers and think, ‘this is great’, but you may not be looking at a healthy forest.”