Big birds' rumble in the jungle
Cassowaries living in the dense jungles of Papua New Guinea emit ultra-low booming sounds - deeper than almost all other bird species - so low it can hardly be heard by humans, new research has found.
Cassowaries are colourful flightless birds that can weigh as much as 57 kilograms. They are very important to the diet and culture of people in Papua New Guinea (PNG), and also play a key ecological role in seed dispersal and forest regeneration.
Mack said he noticed the unusual vocalisations while involved in other cassowary conservation-related research. He recorded the sounds and analysed the sound spectrum, discovering that some of the frequencies they were producing were unusually low for birds.
His research team found the birds produced a series of harmonics from as low as 23 Hertz up to 300 Hertz. While these frequencies were generated simultaneously, they were not always heard together.
"The lower the frequency, the further it carries, so if you are far enough a way from a bird, the upper parts of the harmonics might get attenuated by leaves and tree trunks while the low are still carrying," Mack told ABC Science Online by phone from his field base in Goroka, PNG.
When the researchers were far enough away from the cassowaries, they would hear only a very low, indistinct rumbling: "Sometimes you'd think it's like a plane or a vehicle rumbling far away in the distance," said Mack. "With this frequency you can't tell where it's coming from, it's just a presence."
Mack said he thought the booming sounds had probably evolved to help the birds communicate over large distances in dense forest. He said they ranged over hundreds of hectares in very low densities and higher pitched sounds were not likely to ever reach other birds: "It's a big rainforest and this would be the best way for one to advertise to another - this is where I am, this is my territory, or to advertise and find a mate," he said.
Mack said the aim of the research was to develop ideas on how to manage cassowary hunting as they were being hunted out in some places. The introduction of the shotgun, wire for snares, population pressure and habitat destruction from logging and oil palm plantations all added to the pressure on bird populations.
He said one problem in studying cassowaries is that they are very secretive.
"People only get glimpses of them and we don't know much about them," he said. "So I'm excited about the vocalisation because we can potentially use this to census birds with a listening device."
The researchers are continuing to research the physics of the low frequency sound production and reception.