By Fleur Connick, ABC Science

Aotearoa New Zealand's smallest bird may be a "missing link" in the evolution of vocal mimicry in songbirds.

Scientists used to think birds were divided into two groups: those that learn or copied sounds from their environment, such as parrots and songbirds, and those whose tweets and cheeps were simply innate.

But the titipounamu or rifleman (Acanthisitti chloris), a New Zealand wren that weighs no more than two teaspoons of sugar, bridges the two.

According to a study published in the journal Communications Biology, the tiny wren can mimic its nestmates' chittering, but only on a rudimentary level.

Waipapa Taumata Rau University of Auckland wildlife biologist Ines Moran, who led the study, said the work added to a growing body of evidence that suggests the ability to imitate and learn sounds from the environment - known as "vocal learning" - is not all or nothing in birds, but exists on a spectrum.

New Zealand wrens are the closest living relatives of the earliest Passeriformes, commonly known as songbirds, and share a common ancestor with parrots.

Parrots can learn to imitate human speech with great accuracy, while songbirds such as lyrebirds can imitate "all types of random sounds", Moran said.

"But what about all these other species that we haven't explored yet that may be vocal learners, at least rudimentary learners, or have some type of ability but it's more subtle?"

Prior to her study, biologists didn't know whether the rifleman had any type of vocal learning ability, she added.

"In fact, in this field, these birds are not really well studied at all.

"It's a little sad because they are a key lineage in order to understand vocal learning evolution."

'Looking for a needle in a haystack'

To find out if riflemen did indeed have the ability to copy calls, Moran and her colleagues had to find the birds and record their calls.

It was a task easier said than done.

The diminutive green wrens typically live high up in the canopy of native forests, making them hard to distinguish among the leaves.

"It felt like we were looking for a needle in a haystack," Moran said.

"Their nesting grounds are very diverse; they can nest in tree holes, high up in the trees, or sometimes underneath the leaves on the ground."

But their spotting skills and patience paid off. The team closely monitored rifleman nests over three summers near Te Ika-a-Māui/North Island's Hawke Bay, recording more than 6800 calls routinely made by adult birds bringing food to the young nestlings.

They also analysed the birds' DNA to establish which adults were parents of the nestlings, and which were unrelated "helpers" that also delivered meals to the chicks.

The researchers wanted to know if the wrens' calls were innate and already written in their genes, or a learned behaviour influenced by their social environment.

Similar to humans and other animals, each rifleman has a unique vocal signature - a distinct voice.

By analysing riflemen "voiceprints", researchers found the birds changed their vocalisations to sound more like the birds they associated with.

In other words, they shared a "nest vocal signature".

Moran said the helper birds' voiceprints were more similar to those of their unrelated nestmates compared to their close relatives living far away.

This behaviour is known as vocal convergence.

"It's a vocal modulation, where it's not an exact copy of the sound, but some features are more similar to one another altogether," she said.

This means New Zealand wren calls are strongly shaped by their social environment, rather than genetics, similar to other vocal learners such as zebra finches.

In human linguistics, this is called vocal accommodation, where people tend to adjust their ways of speaking in different social settings, or adopt accents from those around them.

A 'missing link'

These observations provide a "missing link" in the evolution of vocal learning in songbirds, according to Simon Griffith, an evolutionary biologist at Macquarie University who was not involved in the study.

"What [Moran's team has] shown in their study is that one of the prerequisites for learning complex songs is actually the ability to listen to a [call] and to slightly change it, but also slightly copy some elements of it," he said.

While the rifleman does not have the same vocal learning ability as parrots or songbirds, it shares some prerequisite skills for song learning, which "lead to really complex songs evolving in later ancestors," Griffith said.

This makes New Zealand wrens an important group of birds in vocal learning evolution research.

"They take us back in time, to the root of that family tree, and gives us a good idea of what these common ancestors would have been like," he said.

This story was originally published by ABC News.