Motorway Speed Limits Give Birds Survival Clues

A new study suggests that birds use highway speed limits to gauge when to safely take off to avoid being hit by an oncoming car.

While the researchers are not suggesting that birds can read the signs, the are suggesting that the winged creatures might be paying more attention to the posted speed limits than some motorists.

Researchers found that birds on European motorways will take to the air further way from an approaching car on a faster road compared to a road with a low speed limit -- regardless of the speed of the oncoming car.

"We tested whether common European birds changed their flight initiation distances (FIDs) in response to vehicles according to road speed limit (a known factor affecting killing rates on roads) and vehicle speed. We found that FID increased with speed limit, although vehicle speed had no effect.," the researchers wrote in the abstract to their study, which is published in the journal Biology Letters.

The idea to see whether there was correlation between FID and speed limit arose like many good ideas -- while in the middle of a boring drive. In the case of study co-author Pierre Legagneux, a biologist at Canada's University of Quebec in Rimouski, boredom set in as he commuted from home to his lab in France while he was working there in 2006.

"I found [the commute] very boring so I had to do something while driving, so I started to record birds flying away," Legagneux said, according to National Geographic.

Legagneux and his colleague Simon Ducatez of McGill University in Montreal began to measure when the birds took before an approaching car by using a stopwatch, the car's speedometer and a keen eye for birds on the side of the road.

"When the birds flew away, I started my timer and I fixed the point where the birds [were standing]. And when I passed over this point, I stopped my timer," Legagneux explained. "So I had the time elapsed, and because I also recorded our vehicle speed, I also had the distance."

According to New Scientist, the researchers observed that when there was a 50 kilometer-per-hour speed limit the birds took off when the car was about 15 meters away, whereas when the road's speed limit was 110 kph the birds took off when the oncoming vehicle was about 75 meters away. This was true even when when faced with a fast car on a slow road or a slow car on a fast road.

"What was really cool is that birds did not respond to the speed of the car but rather to the speed limit of the road section," said Legagneux. "It was like they were able to read road signs -- although they obviously do not."

The researchers suspect that the birds think of the cars as predators and know them to be more dangerous in some areas than others.

"If the truck is moving fast, the bird is going to get knocked around by the vortices coming off that truck," Daniel Blumstein, a biologist and bird behaviorist at the University of California, Los Angeles, told National Geographic. Blumstein was not involved in the study.

"So the bird, if it survives, is going to learn very quickly that the truck produced a very adverse experience ... One or a few trials of getting knocked around may be sufficient for the bird to learn that cars are approaching faster on certain roads than other roads."

Legagneux told National Geographic that the finding could have future implications for making roads safer for wildlife.

"If you have different speed limits for similar roads in similar landscapes, it could be dangerous for birds because they hardly have any cues of those changes," he said.