Study documents effects of road noise on migrating birds

What effect do roads have on migrating birds? Scientists have a pretty good feeling that roads impact birds. Indeed, studies have documented fewer birds near busy roads than in other areas but that doesn't really tell us why. There is a problem of confounding factors. While doing an experiment, scientists want to only test one variable at a time. Real roads are full of variables. Are their fewer birds because of the physical presence of cars or the road or is something is missing in the habitat because of the road? Is it just a lack of trees? Do the birds dislike linear open spaces? Are roads naturally placed in locations that coincidentally are also lower in bird diversity? Are there fewer birds because predators use the roads as hunting grounds? Does the traffic noise drive them away?

These are all possibilities and experiments need to be carefully set up to test each factor. Researchers at Boise State University have conducted a new study designed to eliminate all factors except traffic noise to see what effect it has.

Researchers created a virtual road to simulate traffic noise in a roadless area.

Researchers set up a virtual road along Lucky Peak which is near the Idaho Bird Observatory's field site. The virtual road was created with speakers hung on trees to simulate only one variable of roads, traffic noise.

Post doctorial research associate Christopher J.W. McClure said, "We present the first study to experimentally apply traffic noise to a roadless area at a landscape scale, thus avoiding the other confounding aspects of roads present in past studies."

The researchers alternated periods of playing and not playing noise during fall migration and used bird surveys to document differences in bird presence during those times.

“We documented more than a one-quarter decline in bird abundance and almost complete avoidance by some species between noise-on and noise-off periods along the phantom road,” said Jesse R. Barber, assistant professor of biological sciences.

Could this have been a fluke or coincidence? As birders know, migration can be hit or miss. Luckily, the researchers also included control sites in their experiment. Barber states, "There were no such effects at control sites. This suggests that traffic noise is a major driver of the effects of roads on populations of animals.”

In science we need to be careful about drawing conclusions. The researchers say traffic noise is a major driver but this does not mean we can exclude other factors. We need more experiments like this, that only test one variable at a time, to truly understand the big picture of how roads impact birds. What we can say, based on this data, is that traffic noise should be strongly considered by land use managers as they think about bird populations.

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“An experimental investigation into the effects of traffic noise on distributions of birds: avoiding the phantom road,” 10.1098/rspb.2013.2290  published in the Proceedings of the Royal Society

Source:
This article was based on information in a press release from Boise State University.
Photos courtesy of Boise State University.