Bellbird ballads

Wesley Webb is exploring how culture evolves by studying male and female songs in a virtuosic New Zealand songbird.

 
 Illustration by Leigh Douglas

Illustration by Leigh Douglas

 
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Ethograms is a monthly column published in collaboration with the Australasian Society for the Study of Animal Behaviour (ASSAB), showcasing the work of early-career researchers. Dominique Potvin is a lecturer at the University of the Sunshine Coast, and an outreach officer for ASSAB.

At dawn on Tawhiti Rahi, an island northeast of Whangarei, New Zealand, the New Zealand bellbirds are singing their chorus in full force. Among the cacophony stands Wesley Webb, a PhD student at Massey University, armed with a microphone, a recorder and binoculars. Webb and his accompanying team of recordists aren’t here just to enjoy the sounds of nature — they’re here to document them. The microphones are amassing data that will form part of an incredible collection of bellbird sounds across 12 islands and peninsulas. The plan for this data is to inform our understanding of how culture evolves: the selective pressures, rate of change, and role of genetics in why populations of the same species can sound so different from each other.

Akin to genetic evolution, cultural evolution involves the flow of memes (no, not those memes—units of information) between individuals and populations. In true songbirds, songs and song syllables are examples of memes that are passed from one individual to another, leading to their spread throughout the population.

Events and processes such as colonisation, copying errors, selection and innovation can all play a part in how songs change over time, both between populations and locations. Webb likens this to the diversification of human languages, where the spread of culture over large geographic areas leads to changes in the syllable pool (something Kiwis are not strangers to), eventually causing individual populations to sound different.

 
  Wesley Webb, hard at work studying New Zealand bellbirds. © Wesley Webb

Wesley Webb, hard at work studying New Zealand bellbirds.© Wesley Webb

 

Especially intriguing is the divergence of female song. It was long thought that males were the only singers in songbird species, so most of the work on cultural evolution has focussed on males. But research from the Southern Hemisphere has shown this is not necessarily the case. “Virtually nothing is known about how female birdsong culturally evolves,” says Webb, “so it will be fascinating to see whether the pattern and drivers of song diversification are different between the sexes in my species.”

So how does one use a collection of song recordings from all over northeastern New Zealand to answer this kind of question? Careful classification and comparison. But this process is possibly the biggest challenge Webb faces. “For each population, we record thousands of song syllables which need to be extracted, measured, categorised and databased,” he says. Unfortunately, most of this work must be done manually. Identifying the need for more automated approaches has led Webb to work with computer programmers and statisticians to create tools to help future research in this area.

 
  A female (left) and male New Zealand bellbird  (Anthornis melanura) .  © Aaron Harmer; © Michelle Roper

A female (left) and male New Zealand bellbird (Anthornis melanura)© Aaron Harmer; © Michelle Roper

 

At the time of writing, results are slowly trickling in. To Webb, the differences in each population’s songs are striking. “Each island has its own distinctive set of syllables and ways of stringing them together to form songs,” he says. “For example, on Burgess Island, ‘Happy Birthday to You’ is top of the bellbird pop charts; on Hauturu, [chiming like] Big Ben is all the rage.” On Tawharanui Peninsula, bellbirds sing a unique ‘flying saucer’ syllable, so named by Webb because of its science-fiction sounding tone.

 
 

Understanding the differences in population culture is extremely important for getting at population divergence and evolution as a whole. Culture can evolve and spread through a population much more rapidly than genetic mutations or adaptations, meaning it can often act as a precursor or indicator of genetic divergence — an initial step towards speciation. “So it’s important to understand how it works. Birdsong is a great tool for the job!” says Webb. 

The next steps in this project will be to include historical recordings, so that populations can be compared not only through space but also through time. Before this can happen, however, long hours in the computer laboratory must be spent classifying, categorising and then analysing different birdsong repertoires.

  Tawhiti Rahi, the larger of the two Poor Knights Islands, off New Zealand's North Island.  © Aaron Harmer 

Tawhiti Rahi, the larger of the two Poor Knights Islands, off New Zealand's North Island. © Aaron Harmer 

It may seem a long way from recording birdsong in the field, but, as with most scientific endeavours, the processes of data collection and data analysis each have their pros and cons. Webb enjoys the many adventures that come with fieldwork, complete with excitable tuataras, rough seas and aggressive seabirds. The lab appears calm by comparison, but the analyses undertaken within build a different kind of excitement — for results.

“Do song differences between populations have genetic underpinnings, or are they purely cultural?” asks Webb. “What ecological and social selective pressures drive song diversity? How rapidly can song syllables and syntax change in a population?” The answers to such grand evolutionary questions have been sung by the bellbirds at full volume for centuries. We just needed to listen.  

Edited by Andrew Katsis and Sara Paradowski