The researchers studied three categories of zebra finches—'laid back', 'normal' and 'stressed'—that had been selectively bred as such, on the basis of their levels of stress hormone. 'Laid-back' birds had lower levels of hormone than the 'stressed' birds.
They found that the 'stressed' birds were bolder and took more risks in a new environment than the group that was usually more 'laid-back'.
Writing about their findings in the journal Hormones and Behaviour, the researchers said that just like animals and humans, birds respond to stress, created by the appearance of a predator or a change in their environment, by producing a hormone.
In birds, this hormone is called 'corticosterone' and some individuals have higher levels of the hormone than others.
During the study, the researchers put the birds into a new environment, which housed several unfamiliar objects, including new feeders. They found that the 'stressed' birds were the first to visit the new feeders, which they also returned to more quickly than the other birds after being startled.
In all, the 'stressed' birds approached more objects than their normally more relaxed peers, showing greater risk-taking behaviour and arguably handling the situation better, the researchers added. "It initially seems counter-intuitive that birds with higher levels of the stress hormone showed bolder behaviour, normally associated with confidence.
However, corticosterone is released to help tackle stress by encouraging the animal to adopt key survival behaviours like seeking food. So on reflection, perhaps it is not surprising that these birds are more likely to explore the environment and look for food," said Dr Thais Martins of the University of Exeter.
In previous studies, animals were found to show consistent individual differences in their behaviour when faced with certain challenges. Traditionally, birds would be separated into two groups—namely, 'bold' and 'shy' or 'active' and 'passive'—based on observations of their behavioural strategies, and then they would be studied for physiological differences.
However, the researchers adopted the opposite approach in the current study, separating the birds into groups by physiology based on their corticosterone production levels, and then looking for their behavioural differences.