Birds know road speed limits

Crows, house sparrows and other species judge when to flee the asphalt by average traffic rates rather than an oncoming car's speed

SLOW DOWN  Birds that hang out on roads may rely on the speed  limit to judge when to flee instead of checking how fast an oncoming car is moving, a new study finds.


Highway-savvy birds don’t read road signs but they may pay more attention to speed limits than some human drivers do.

As a car roars toward birds standing on the asphalt, they don’t check the driver’s exact speed when judging how soon to flap out of the way, says behavioral ecologist Pierre Legagneux of the University of Quebec in Rimouski.

Instead, the speed limit on the road, rather than the speed of the approaching vehicle, is a better predictor of how close a car gets before a bird startles into the air, Legagneux and Simon Ducatez of McGill University in Montreal report August 21 in Biology Letters.

Substantial numbers of birds get hit by cars, Legagneux says, so the  new paper gives drivers another reason not to speed. Birds may not expect over-the-limit traffic and may fail to dodge away soon enough.

The project also opens up evolutionary questions, he says. He looks forward to untangling how much of the birds’ reactions comes from learning an average speed along a particular road and from the evolutionary force of selection as cars kill off the birds that fail to get out of the way.

Other researchers have studied how close an alarming human can get before animals flee, called the flight initiation distance. Yet Legagneux says that as far as he knows, this is the first study to analyze the distance between road birds and approaching cars.

He started collecting the data to enliven his drive home from the lab when he worked in western France. To figure out car-to-bird distances, he kept a timer handy. When a bird flew in front of him, he timed the seconds his car took at a constant speed to reach the point of flight. His total of 134 measurements included 21 different species, but more than half came from three species: carrion crows, house sparrows and Eurasian blackbirds.

Whether Legagneux drove faster than the speed limit or slower proved less important than the speed limit itself, which birds probably perceived as the average traffic speed. On stretches of road with a speed limit of 20 kilometers per hour, birds waited to fly until the car was about 10 meters away. That distance increased to roughly 25 meters on roads with a 90-km/h limit and around 75 meters at a 110-km/h limit.

Animals can become quite sensitive to the quirks of human activity, says behavioral ecologist Ted Stankowich of California State University, Long Beach. When he studied what spooks black-tailed deer in California, he could walk as close as six feet to animals familiar with humans, as long as he stayed on a road. At even greater distances though, “if you take one step off the road, they’ll run away,” he says. “It’s all about context.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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