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Birds sing rainforest's history

David Westcott recording birdsong

Dr David Westcott, Rainforest CRC

Scientists working in the tropical rainforests of North Queensland, Australia, have found that birds of the same species don't all sing the same song -- and the differences give clues to the history of the forest.

Dr David Westcott of the Rainforest Cooperative Research Centre at Atherton said that the songs of bird populations in the cool patches of forest on different mountaintops were not just slightly, but dramatically different from each other. This suggested the populations had been isolated for some time.

"The golden bower birds of Mt Edith on the Atherton Tablelands, for example, sing a very different song from the birds of Mt Baldy just 20 kilometres away," said Dr Westcott. "Birdsong is a learned trait -- a cultural attribute which the birds learn from their own community. When we played them the song from a different place, they just ignored it -- as if it had been the song of another bird species."

Dr Westcott and his colleague, Dr Frederieke Kroon, spent weeks in the upland forests in a 500-km strip between Townsville and Cooktown, getting up before dawn and recording the songs of a variety of species. They then analysed the time, frequency and volume of their songs. "The sonagrams or sound graphs are quite distinct," said Dr Westcott. "Anybody could tell the difference." (click here to listen -- get Real Audio)

Golden bower birds, he said, gave a graphic example of the regional "dialects" created by geographic isolation. About half the species in the area are restricted to mountaintop "islands" -- patches of cool forest surrounded by a sea of warm forest. The songs of these upland populations showed significant geographical variation, while the songs of more widely distributed species tended not to vary from place to place.

A few thousand years ago, the cool forest types on the mountaintops were probably more widely distributed, and scientists studying forest evolution have been keen to find new ways of working out how different fragments are related. "There are patches left, but we don't know if they were ever connected," said Dr Westcott. Populations with more similar songs suggest their patches may have been connected more recently.

For the birds, the findings suggest that the different populations may be evolving into different races, and this is partly backed up by genetic studies. No research has been done to assess whether the different populations are interbreeding, but since birdsongs are used by golden bowerbirds to attract a mate, it is possible that breeding is restricted within populations.

Although upland rainforest is partly protected within World Heritage areas, some of it is still broken up in freehold and leasehold areas. "At the very least we have to be aware that we're dealing with populations that are significantly different from one another," said Dr Westcott. "If we think that birds are in trouble it might not be appropriate to manage them as one species."