By Ella Davies
Earth News reporter
Stress may play a crucial role in determining whether some birds mimic the sounds of others, say researchers.
Scientists studied the vocal repertoire of bowerbirds. Best known for their elaborate nests or "bowers", the birds can also copy up to fifteen sounds.
Bowerbirds were previously thought to mimic predators as a form of defence.
But recordings reveal they prefer to copy a variety of alarm calls made by bird species that are either bullying each other, or which feel threatened.
That suggests that the birds learn and reproduce calls only in stressful situations, say the researchers.
Spotted bowerbirds (
Ptilonorhynchus maculatus) are found in Australia and New Guinea and are best known for their elaborate "bowers", created by males seeking to impress a mate.
Reports from egg collectors led scientists to believe the birds mimicked the calls of predators as a way of defending their territory.
However, researchers from the University of St Andrews, UK and Deakin University, Geelong, Victoria, Australia found different results in their study.
They have published details in the journal Naturwissenschaften.
Recording male spotted bowerbirds in central Queensland, Australia, scientists found that predator calls only accounted for 20% of the birds' repertoire.
"We looked at what sounds the bowerbirds chose to mimic from their sound environment, and found that males mimicked "bully" species rather than predators," said lead researcher Dr Laura Kelley.
"We were surprised to find that spotted bowerbirds weren't only mimicking predators given that mimicry in this species has been previously described as an anti-predator behaviour."
To add further mystery, as well as the "bullying" calls of aggressive bird species, scientists found that the bowerbirds mimicked the alarm calls of threatened species and even human speech.
Dr Kelley suggested that the circumstances that trigger the calls played a key role in explaining the behaviour.
The birds mostly mimicked the alarm, mobbing and aggressive calls of neighbouring species: sounds mainly heard during tense situations such as when territories are violated.
"Stress has been shown to enhance learning and memory in many tasks," said Dr Kelley.
"We suggest that bowerbirds learn these alarm sounds when stressed and then later reproduce them when in another stressful situation."
"This is the first study to suggest that stress may play a role in vocal mimicry, and may explain why mimicry of predators and alarm calls are observed in so many species."