In the winter of 2006, Pierre Legagneux started measuring when birds would fly away from him, as he sped towards them in his white Peugeot. This wasn’t an official part of his research; he was just bored. After a recent move, his mornings of cycling past bucolic villages and forests had been replaced by long, tedious drives. “I found it very boring so I found something to do while driving,“ he says. “I started recording birds flying away in front of my car.”
Legagneux drove down roads with speed limits of 20, 50, 90 or 110 kilometres per hour, and either stuck exactly to those speeds or deliberately drove under or over them. Every time he saw a bird on the road—usually a crow, blackbird or pigeon*—he started a stopwatch when it flew away and stopped it when the car passed over the point where the bird had been. By multiplying the car’s speed by the time on the stopwatch, he could work out how far away he was from the bird when it took off—the “flight initiation distance”.
One year and 134 measurements later, Legagneux clearly showed that birds flee from incoming traffic at greater distances on roads with higher speed limits. On a 110 km/h road, they’d be airborne when the Peugeot was 75 metres away. On a 20 km/h road, they’d wait till the car was less than 10 metres away before taking off.
That’s fairly predictable. But more surprisingly, Legagneux also found that the birds didn’t react to the actual speed of his car. Their flight initiation distance depended on the speed limit of the road, but not on the speed of the incoming traffic.
Other factors were important too. The birds allowed the car to get closer if they were smaller (and presumably more agile) or during the autumn and winter (when newborn chicks have had a few seasons to get accustomed to roadside perils). But regardless, the main results still held even after Legagneux accounted for the season, the weight of the birds, their location on the road, and their evolutionary relationships.
We know that birds can inherit risk-taking behaviour from their peers, so faster roads could select for more cautious birds. Indeed, urban birds eventually evolve longer flight initiation distances than rural ones, and some swallows that nest near roads have evolved shorter wings and tighter turns. But Legagneux doesn’t think that the birds in his study were actually adapting to the different roads, since they’re likely to experience a variety of speed limits.
Instead, they’ve probably come to associate different stretches of road with different risks of collision, and adjusted their behaviour accordingly. This may explain why birds with larger territories are more vulnerable to cars—they might not be able to tailor their activities to local roads.
* No chickens…
Reference: Legagneux & Ducatez. 2013. European birds adjust their flight initiation distance to road speed limits. Biology Letters http://dx.doi.org/10.1098/rsbl.2013.0417