A study shows on roads with higher speed limits, birds are quicker to take to the air to avoid oncoming traffic, but where limits are lower, they wait longer.
The behaviour is related to speed restrictions and not the result of birds assessing the speed of approaching vehicles, researchers found.
Scientists admitted breaking speed limits to carry out the study in western France.
Motoring through the French countryside in a white hatchback, they recorded the activity of birds standing at the edges of roads with speed limits of 20, 50, 90 and 110kph.
The car was driven at under or over the speed limit and a timer used to calculate “flight initiation distance” (FID), the closest the car came before the birds flew out of danger. A total of 134 FIDs were measured for 21 species.
The researchers, led by Pierre Legagneux, from Laval University in Canada, wrote in the Royal Society journal Biology Letters: “Birds had significantly higher FID on road sections with higher speed limits.
“By contrast, car speed had no significant effect on FID, both when considering absolute car speed or the difference between car speed and speed limit.”
Reacting to speed limits on roads is the result of birds adapting to what they consider a “habitat characteristic”, the scientists suggest: “As road traffic directly causes mortality in birds, we expect birds to respond to road traffic in a similar way as they would respond to predation.
“As road mortality probably increases with speed limits we would expect individuals to adjust their anti-predator behaviour to vehicle speed and/or to the speed limit. Species with longer FID are known to have smaller risk of getting killed by cars, suggesting that adjusting FID might be an adaptive way to respond to road traffic.”
The study did not address bird behaviour on pelican (pedestrian) crossings.