Birds Count on Drivers to Obey Speed Limits

Birds in Flight

Story at-a-glance -

  • A new study concludes that birds on roadways make decisions about when to lift off to safety based not on how fast an oncoming car is approaching, but on the average traffic speeds of a given road.
  • Two researchers in Canada recorded dozens of instances of birds taking flight as they approached in a car. Based on their measurements of flight initiation distances and other factors, the pair concluded that birds accumulate traffic speed information over a period of days, weeks, months or longer, and use that information to decide when to fly to safety in the face of oncoming traffic.
  • This study suggests bird-vehicle collisions are primarily the result of people driving over the speed limit, and birds being taken by surprise by the excessive speed of a vehicle. These results are another reminder of the importance of obeying speed limits.

By Dr. Becker

If you’ve come upon birds in the roadway while you’re out driving (and who hasn’t), as you slowed down to give them time to get out of the way, you might have wondered exactly what it is that finally causes them to scatter or take flight. Do they suddenly see your vehicle? Do they hear it? And why is it that something as huge and threatening as an oncoming automobile doesn’t scare them off long before you get close enough to notice them?

A new study published August 21 in Biology Letters1 may provide at least a partial answer.

Birds on Roadways Assess Average Speeds Traveled

According to Pierre Legagneux, behavioral ecologist at the University of Quebec and one of the study authors, birds estimate highway speed limits to determine how much time they need to get safely airborne in the face of oncoming traffic. This comes as a bit of a surprise, since it has been generally assumed birds in roadways gauge the speed of each vehicle as it heads in their direction.

In the new study, Legagneux and Simon Ducatez of McGill University in Montreal conclude that birds observe the speed at which cars travel over a certain road for many days, weeks, months or longer, and build a memory map based on the average speed. As cars approach, they access their memory maps and make decisions about when it’s time to lift off.

According to the researchers, armed with a stopwatch, drove the roadways and timed how long it took birds ahead of them in the roadway to take flight. When a bird flew in front of them, they timed the seconds it took to drive at a constant speed to the point of flight. They called this measure the Flight Initiation Distance (FID). Then they stopped to measure the distance traveled. Next, they varied the speed at which they drove, sometimes going under the speed limit, sometimes over it, and sometimes moving at the speed limit.

They also tested birds on different roads with different speed limits, from 20 kilometers per hour (12 mph) to 110 kilometers per hour (68 mph). This is how they discovered the birds weren’t using the speed of individual cars to make decisions, but rather the average speeds driven on different roads.

They also discovered that birds hanging out in the middle of the road took flight sooner than those standing closer to the side of the road.

The researchers recorded a total of 134 instances of birds taking flight. Over 20 species were involved in the study, with over half from just three species: carrion crows, house sparrows and Eurasian blackbirds. Bigger, heavier birds tended to have longer FIDs than smaller birds. And FIDs grew longer for all birds as the speed limit increased.

The study suggests most vehicle-bird accidents on roadways are the result of drivers exceeding the speed limit. Since a great many birds are killed by cars, this study gives us all another good reason not to drive over the speed limit.