On roads with higher speed limits, birds are quicker to take to the air to avoid on-coming 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.
Smaller, more agile species waited longer before taking flight.
Scientists admitted breaking speed limits to carry out the study. Motoring through the French countryside, they recorded the activity of birds standing on or at the edges of roads with speed limits of 20, 50, 90 and 110kph.
A timer was used to calculate "flight initiation distance" (FID) - the closest distance the car came before the birds flew out of danger.
The researchers, led by Pierre Legagneux, from Laval University in Canada, said: "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.
"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."