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.