Birds avoid the sounds of roads

In the United States, you’re rarely far from a road. And as you get closer to one, or other bits of human infrastructure, bird populations decline. But are the birds avoiding our cars or the noises produced by them? Noise might be a big factor, scientists have reasoned, because they’ve seen declines in bird populations near noisy natural gas compressor sites.

It turns out that the sound of cars driving down a road is enough to deter many bird species from an area.

Researchers from Boise State University in Idaho created a “phantom road” at a site in the Boise Foothills that is a stopover for migratory birds in the fall. They put up 15 speakers in Douglas fir trees and played recorded sounds of a road at intervals of four days — four days on, four days off. They then counted birds at three locations along their phantom road and three locations nearby where the road noises couldn’t be heard.

The scientists spotted lots of birds during their study — more than 8,000 detections and 59 species. The birds they saw changed as the fall progressed, which was natural because the various species of migrating birds hit the stopover point at different times. But all that variation was good for the experiment, the researchers say, because it helped even out any fluctuations they might have seen from site to site and from noise-on to noise-off intervals, letting the researchers tease out the effects of the road noise.

When the researchers looked at all their data, they found strong trends. “When the noise was on,” they write, “fewer birds were present near the phantom road.” Bird abundance declined by more than a quarter near the make-believe road, and two species — the cedar waxwing and yellow warbler — almost completely avoided it. Only one species, the Cassin’s finch , was more likely to be seen near the phantom road than in the quiet areas. The results appear November 6 in the Proceedings of the Royal Society B .

Noisy areas can be bad for birds in a couple of ways. If the sounds are of certain frequencies, they can interfere with bird communication; the birds won’t be able to hear each other. The noise can also drown out the sounds of predators and prey. So if a migrating bird stops near a noisy road, it might have to spend more time trying to detect predators and less time finding the food it needs to fuel its long journey.

Migrating birds, though, can avoid a noisy road and camp out somewhere quieter. Birds that are territorial breeders or simply not as mobile may be less able or less willing to avoid these areas; they might be more affected by the road noise. But even the movers might not have too many places left to find quiet, the researchers note. “Because 83% of the USA is within 1 km of a road, it is likely that noise-sensitive species such as the Yellow Warbler avoid substantial areas of otherwise suitable habitat simply because they are too loud,” the team concludes.

Perhaps one day road noise won’t be a problem for wildlife. Electric cars are so quiet that some have called them a safety hazard and called for rules, now under consideration, that would require they make noise. But maybe quiet is good, and we’d just have to learn to be more vigilant. Then animals wouldn’t have to worry about our vehicles drowning them out, and instead only have to worry about our cars hitting them.