More than just a pretty sound, bird song serves important social functions. It plays a central role in attracting mates and defending territory, and may have a crucial function in the evolution of new bird species.
Bird song is relatively well studied in the northern hemisphere (where only male birds sing) as the result of centuries of studies by naturalists. In contrast, in the southern hemisphere (where both sexes sing) relatively little is known about many aspects of our native bird songs, despite the interesting song-based male and female social interactions that are observed here.
Professor Dianne Brunton, head of Massey’s Institute of Natural and Mathematical Sciences, is working on a Marsden-funded project looking at the role of song in social interactions in our native bellbird. The bellbird is a honeyeater, one of only two species of this type endemic to New Zealand. Bellbirds are prolific singers year round, and social interactions are important to them given that they gather in flocks outside the breeding season to feed on nectar and fruit, generally in concentrated areas.
‘If you remove predators from locations where bellbirds live, then their numbers get really high, so social interactions are very important,’ explains Professor Brunton. ‘A lot of singing is used aggressively between individuals, with females tending to be more aggressive with other females, similar to male–male interactions.
‘We began our research by simply describing the song,’ she says. ‘We found that males and females have separate sets of syllables, with separate repertoires. There are a few shared syllables, but the songs of males and females are put together differently.’
Next, Professor Brunton, together with PhD student Michelle Roper, examined how bellbird song develops in each sex in their natural habitats, from the time birds are chicks until they set up their territories. ‘Studies in the wild are harder than in captive birds,’ she says. ‘However, wild studies are worth the effort because what animals do in captivity often is not a realistic look at how they behave in their natural habitat.’ Monitoring hundreds of nests over a number of years showed that the development of song occurred on similar timelines for males and females.
We found that males and females have separate sets of syllables, with separate repertoires.
PROFESSOR DIANNE BRUNTON
Another PhD student, Wesley Webb, was interested in whether bellbird populations have different dialects when populations are geographically separated. Studies of singing in bellbirds across 10 islands in the Hauraki Gulf, as well as in two mainland areas, showed that each population have a different song dialect, just like different dialects in human language. ‘There are distinct song repertoires of bellbirds between islands,’ says Professor Brunton, ‘so if you know enough about them, you can tell where a bird is from.’
Professor Brunton has also worked on the cultural evolution of song in the New Zealand saddleback, in an earlier Marsden-funded project. Saddleback are a unique bird to study because the entire species was restricted to a single island population until multiple translocations to conservation islands were carried out over the last thirty years in order to save the species. Saddleback are highly vulnerable to invasive predators and require intensive predator control to thrive; they are also unable to fly to different locations. Hence, the history of the many island populations is well documented and provides a unique opportunity to study how rapidly songs can change in isolated bird populations.
PhD student Kevin Parker undertook the intensive fieldwork required to sample the island populations. ‘We found that the extent of changes could be predicted by geography and time,’ Professor Brunton says. ‘We also did some experiments to show that birds recognise the local dialect more than ancestral dialects. In a final experiment, we undertook several new translocations to set up new ‘mixed’ populations comprised of birds from different areas, to see whether it made a difference in mate selection. If a female did not recognise a particular dialect because she had not previously been exposed to it, would that affect her choice of mate? So far, it does look as though they may select a mate based on whether they are from the same island location.’
The results show that mate selection on the basis of song dialect could potentially be a pathway for new species to evolve. New species can form when groups within a species stop interbreeding with each other. The lack of genetic mixing means differences start to accumulate between the groups until eventually they are genetically distinct enough to become separate species. ‘If these islands are isolated, and there is a rapid change in dialects which means that birds from different islands don’t recognise each other or that they don’t react in the same way when a stranger comes in, this could eventually lead to the formation of new species.’