A clever experiment reveals how fake traffic noise drives songbirds out of forests and harms the ones that stay behind.
In the autumns
of 2012 and 2013, any hikers walking along a particular ridge in Idaho’s
Lucky Peak would have become very confused. One minute they’d be
enjoying the tranquil chirps and rustles of a temperate woodland. The
next, they’d be immersed in the unnervingly realistic sounds of a
roadway: metal rushing past at speed, tires rolling over asphalt,
honking horns. The nearest road, however, was several miles away. These
noises were coming from 15 pairs of large bullhorn speakers that had
been lashed to the trees. The hikers were walking along a phantom road.
This half-mile corridor of disembodied sound,
this road in noise only, was the work of Jesse Barber and a team of
scientists and volunteers from Boise State University. Their goal was to
answer a simple question: How does the bustle of traffic affect the
presence and health of birds?
Others studies have found that noisy man-made structures, from roads to natural-gas plants,
can drive away wildlife and drown out their calls. But in all of these
studies, noise was accompanied by other problems—pollution, the
potential for collisions, and predators patrolling the roadsides. “No
one had done the obvious thing: use speakers to simulate the noise
component of a road and create a phantom road,” Barber said.
Once everything
was in place, the team turned the road on and off for four-day
stretches throughout the autumn months, just as migrating songbirds were
flocking to the ridge. During this time, they hung large nets between
the trees to capture the passing birds, and to count their numbers and
analyze their health. The team also compared the birds at the phantom
road to those at the neighboring Intermountain Bird Observatory, a site
just a kilometer away, where migratory birds have been monitored for
more than 20 years.
They found
the phantom road drove a third of the local birds away from the ridge.
And those that stayed didn’t have it easy. At least five species,
including MacGillivray’s warblers, western tanagers, and Cassin’s finches,
were substantially lighter for their size if they arrived when the road
was in effect compared with their counterparts who arrived when the
road noise was turned off.
That’s
a problem. The area where the team summoned their phantom road is a
critical staging ground for migrating songbirds. They stop there for
just under a week, gorging on the blankets of berries to prepare
themselves for the arduous journey ahead. Beyond the ridge, they must
fly south over the Snake River Plain, which has little food, few trees
to hide within, and one of the highest densities of birds of prey in
North America. Imagine, then, a flock of weak and ill-prepared
songbirds, running this gauntlet of arid climate and hooked talons.
Why were the birds failing to pack on the
pounds, despite the rich glut of berries around them? Barber’s team
unpicked one possible reason through laboratory experiments. They played
road noises to captive white-crowned sparrows and saw that the birds
spent less time looking for food with their heads down, and more time
scanning for danger with their heads up. Traffic noise, by drowning out
the sounds of impending threats or alarm calls, forces birds to look for
danger instead. This distracts them from the business of finding food,
and leaves them physically weaker.
Just look at the sparrow in these two clips.
First, surrounded only by ambient noise, it bounds over the ground,
searching for morsels with a carefree air. Later, bombarded by a phantom
road, it stays rooted to the spot, and seems wary and tentative. Same
bird, more noise, totally different behavior.
“How many
critical stopover habitats do we have around the world that are
influenced by anthropogenic noise?” Francis adds. “Lots of migratory
birds are declining around the world and we haven’t nailed down what’s
driving these declines. There are a lot of known threats, but the
acoustic environment may be another really important one to focus our
attention on.”
The same might apply to other groups of animals, says Hans Slabbekoorn from Leiden University, who studies how man-made noise affects birdsong. He
notes that scientists have often looked at the impact of man-made noise
on fish and marine mammals by mapping their whereabouts. “That may not
tell the whole story,” he says. “Moving away may not even be the worst
thing that can happen to an animal that is faced with a choice between
noisy and more quiet areas.”
Barber agrees. He is also testing ways of
minimizing the effects of noisy roads, including lowering the speed
limits, erecting noise-cancelling walls, or using quiet pavements whose
textures and materials are designed to produce less noise. Over four
years, he will test these measures to see if reducing road noise in
national parks benefits both human visitors and local wildlife.
And as Barber explains this to me, a fire truck
drives past outside his office, and his words disappear amid its
wailing siren. After the din dies down, he says, “And speaking of noise
pollution…”
Barber is also expanding his research beyond
songbirds. He is now setting up a phantom natural-gas field, where huge
stadium speakers broadcast the ruckus of gas compressors. “We’re trying
to see if birds and bats are pushed out by noise,” he says, “how that
impacts the insects, and how that cascades down to plants.” How, in
other words, does a force we cannot see affect organisms that cannot
hear?
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