Out on patrol (Image: Plainpicture)

Birds cannot read road signs, but they know that some roads have higher speed limits than others. They will take off further away from an approaching car on a faster road than on a slower road – regardless of the speed of the car.

When Pierre Legagneux of the University of Quebec at Rimouski and Simon Ducatez of McGill University in Montreal, both in Canada, were working together in France in 2006, they began studying the birds they encountered on the drive home from the lab.

They found that where there was a 50-kilometre-per-hour speed limit, birds on the road typically took off when the car was about 15 metres away, whereas on a 110-km-per-hour road, they took off when a car was nearer 75 metres away. They did this even when faced with a car travelling faster on the slow road or slower on the fast road.


Know your limits

“What was really cool is that birds did not respond to the speed of the car but rather to the speed limit of the road section,” says Legagneux. “It was like they were able to read road signs – although they obviously do not.”

The researchers think the birds treat cars as predators, and realise that in some parts of their environment the predators are more dangerous than in others.

The two biologists also discovered that the distance at which the birds took off varied according to season. They let cars get closer in the spring, and behaved more cautiously in autumn. Legagneux and Ducatez think that this is either because birds are more active in the spring feeding their children, or that juvenile birds are first learning about roads then and have less experience with cars.

“Birds are able to associate environments, like forests or roads, with risk,” says Christopher Lepczyk, an ornithologist at the University of Hawaii, Manoa. He thinks the work could prompt follow-up studies comparing birds in urban and rural areas, and perhaps encourage more innovative methods. “I just think it’s really cool,” he says. “We don’t do enough of this kind of work.”

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2013.0417