Great tits have personalities. That means each bird consistently behaves in certain ways, while others consistently behave differently. For example, some great tits are bolder than others when it comes to exploring new places.
Andrea Grunst at the University of Antwerp, Belgium, and her colleagues examined whether exposure to heavy metals can alter these behavioural traits.
They studied five populations of great tits living outside Antwerp, near a smelter and metal refinery known for emitting toxic metals like cadmium and lead. The five sites were at varying distances from the plant, ranging from zero to 8.5 kilometres.
Birds with Attitude
The team captured a total of 249 great tits and brought them to a lab. There the birds were placed in a new environment with artificial trees, to see how boldly they explored it. The closer the birds lived to the smelter, the slower they were to explore the novel surrounding.
The team also examined two other behaviours -- aggression and nest defence -- in the wild.
They placed taxidermically stuffed decoy birds into nests, accompanied by the song of an adult male great tit -- making it look like there was an intruder. Male birds responded aggressively, for example by singing alarm calls or diving at the decoy.
In other tests, the decoy mimicked a predator: the great spotted woodpecker. In response, female great tits defended their nests by hissing at the "predator".
Unlike exploratory behaviour, aggression and nest defence did not vary with distance to the smelter, suggesting these aspects of personality were not affected.
"Individual variation in [exploratory behaviour] could have fitness effects in the wild, because it affects how they interact with their environment," says Grunst.
Grunst suggests that heavy metals could be damaging the birds' brains or changing their hormone levels, leading to the altered personalities. But she says the study only establishes a correlation. The team is now examining whether levels of toxic metals in individual birds can be linked to changes in behaviour.
If metals really are changing the birds' behaviour, it could have knock-on effects for other species, especially predators, says John Swaddle at the College of William and Mary in Williamsburg, Virginia.
"If metal exposure leads to general lethargy, then the individuals with the heaviest metal burden in their body might be the most lethargic and the easiest prey item to be eaten," he says. "This would then pass on a larger concentration of the metal to the predator -- leading to stronger bio-magnification."
Science of the Total Environment