Trilly love songs

Could the pre-dawn song of the male superb fairy wren clinch the courtship deal? New research points to intriguing possibilities for bird studies.

The gardens are shrouded in darkness. A lone cyclist glides up to the gate, tucks the bike aside, and searches in her pocket for a key. Once inside, she walks with purpose up the hill, clutching what looks like a thin wand. The vegetation grows thicker as she climbs the slope. She stops, suddenly, by a bush. Something squeaks. She quietly walks a little way off, drops to her haunches, and points the microphone in the direction of the bush. She waits. First light is still half an hour away. Without fanfare, a rich song emerges from the branches. The singer is joined by another, and another, until the air is thick with a tapestry of voices and the swishing of wings. She sits, and listens. 

Anastasia Dalziell has spent many early mornings monitoring the dawn chorus in the Australian National Botanic Gardens in Canberra. The half-hour before sunrise is when singing birds are usually most vocal; but one species holds forth with remarkable vigour. The superb fairy wren population at the gardens, which numbers in the hundreds, has been studied by scholars from ANU for years.

But now Dalziell, who recently graduated with honours from the School of Botany and Zoology, has opened the way for a new line of enquiry into how the male wrens use song to attract mates.

“The dawn chorus is very conspicuous,” Dalziell explains.

“You’ve got this little 10 gram bird that is singing for half an hour, and each song lasts for three seconds. He fits nine into a minute. It’s very energetic and he’s doing nothing else. The first question is, ‘Why does he do something that could attract predators? Why use up so much time and energy; both valuable commodities for a small bird?’

“We thought it might have to do with mate choice, because during the dawn chorus is also when females seek extra-pair mates. It’s at this time that a female will leave her own territory, and mate with the male of her dreams.”

It is also thought that the more attractive male wrens might also be better at other things, like foraging. By mating with the most attractive male she can find, the female might be looking to improve other characteristics in her offspring.

This promiscuity is only part of the complex social relations among superb fairy wrens. Professor Andrew Cockburn, who began studying the colony at the gardens 18 years ago, explains that each wren territory includes a social pair and ‘helper’ males, who help to protect the nest and feed the young.

“So far as we are aware, the fairy wren social system is unique among all birds,” he says. “It’s most interesting because it throws some of the most substantial problems in evolutionary biology into sharp relief.

“One of those problems is the evolution of cooperative behaviour, where animals contribute to further the survival of other individuals instead of behaving for self-interest. Evolutionary biology predicts individual selfishness, so cooperative behaviour is always problematic wherever it is observed.”

While evolutionary biologists ponder the problem of cooperative behaviour, one might think the female wrens would have no problem finding an extra-pair mate, given the plethora of males in each territory. But the females are actually quite picky about which extra-pair mates they choose, so Dalziell believes song could be the key to ensuring the female finds her desired mate.

When it comes to song, Dalziell is well qualified. Her biological studies were undertaken concurrently with a degree in musicology and she is also a singer. She says learning to hear the technical aspects of music paid off when it came to breaking down the fairy wren dawn chorus into its constituent parts, which alternate between a territorial chatter song and a trilling song.

It’s this latter song that Dalziell believes could be operating as a musical identifier, as it could help to identify the older males in the area who are also more likely to be those with the most desirable characteristics. Dalziell found that older males tend to sing longer trills than their younger counterparts.

“Maybe the female can go to the area, listen to all the males singing around her, and then say, ‘Ah, you’re the one I want.’”

One important aspect of Dalziell’s study was the discovery of competitive behaviour between male wrens from a common region. It was already know that males from adjoining territories will perform the dawn chorus in clumps at the boundaries of their territories. But Dalziell discovered that these clumped males will share a common way of singing.

“This suggests that perhaps the males are trying to confuse the females. When she comes to mate with her choice of male, she’s confronted with all these males in a cluster singing the same dialect.”

“Anastasia has really taken the things that we’ve been doing a long way forward. She has been able to dissect the courtship that all males do to win these fertilisations outside the pairbond by singing during the dawn chorus. Fairy wrens are fanatical singers, and they sing with more complexity that other birds known for their pre-dawn chorus.

“What she has done is redirect the questions we ask and pointed us in quite a new direction into how the wren society is organised and how the mate choice is consummated. I think many of the avenues we’re now taking will be shaped by her work.”

Dalziell, meanwhile, says she has not missed rising for the early morning starts since finishing her Honours project late last year. Currently working as a research assistant for Cockburn, she says the possibility of continuing her work into fairy wrens is intriguing.

“In the case of bird song, a lot of the current thinking is based on studies of a handful of species from North America and Western Europe where the academics are concentrated. A lot of the near-tropical birds and Australian birds haven’t been studied much at all. A few of the findings that have come out of my work suggest that some of those assumptions are incorrect, as the superb fairy wrens have a completely different lifestyle, and song in their society is likely to play a very different role.”

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ANU Reporter
Autumn 2006