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How you count birds can make all the difference in airport design


How you count birds can make all the difference in airport design

When they conducted a network analysis to track bird movement, researchers discovered a better way to evaluate the impacts of big infrastructure projects
April 12, 2023

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In much of the world, construction of large infrastructure such as freeways, airports or pipelines must be scrutinized to see if it will harm wildlife.

Government agencies often rule the work will have a negligible effect, touching a small fraction of habitat. Conservation groups sometimes object that officials are downplaying the damage from the project, or the cumulative toll of similar work dotting the landscape.

Now, a group of British and Portuguese scientists have developed a tool that could give a more complete view of the potential harm such work does to wide-ranging animals. In the case of a migratory shorebird and an airport planned for the outskirts of Portugal’s capital city, Lisbon, the resulting picture is not pretty.

While the airport’s environmental assessment concluded less than 6% of the area’s black-tailed godwit population would be affected, “we found that more than 68% of godwits …would in fact be exposed to disturbance,” said Josh Nightingale, a PhD student with the University of East Anglia in the United Kingdom and Portugal’s University of Aveiro.

The difference comes down largely to how you count the birds.


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The original environmental study reportedly gauged the effect by estimating the number of birds found in places around the proposed airport where low-flying airplanes would produce regular bursts of noise above 65 decibels—roughly the equivalent of the chatter in an office.

That’s a relatively small fraction of the total number of godwits in the Tagus Estuary. This vast network of marshes and mudflats serves as a vital migratory destination for birds ranging from flamingos to the brown-and-white godwits that patrol the shore, prodding the mud with their long, slender bills.

But the godwits’ penchant for moving from one marsh to another raises the question of whether counting birds in one place really provides an accurate count of how many birds touch down there throughout the winter.

To get a more complete picture, Nightingale and colleagues used an approach called “network analysis” to discover how birds moved between spots within the estuary.

They turned to a 20-year-record that tracked sightings of individual godwits marked with distinctive bands. Banded godwits have been monitored in 30 different locations within the estuary. The same bird was often spotted at more than one of these places in a single season. The scientists used repeat sightings of 693 individual birds to better understand the connections between different hotspots.

The result was a map resembling the ones showing airplane routes crisscrossing a continent from one airport to another. This revealed that a sizeable fraction of the godwits spent at least some of their time in areas that would be most affected by airport noise. In early winter, 40% of the godwits there would be touching down in an area exposed to 65 decibels, the researchers reported in the journal Animal Conservation. At the peak of the winter migration, the larger number of birds flooding the area pushed down the percentage inside the noisiest regions to 23%.

The numbers grew when scientists lowered the noise threshold to 55 decibels—a point at which previous research had seen effects on godwits. Then more than two-thirds of birds in the early winter spent time in the noisy zone, as did nearly 40% during the height of the winter season.

The analysis also revealed that three out of the five most heavily used nodes—places where bird visits showed the greatest number of connections to other parts of the estuary—would be exposed to the 55 decibel noise levels. Much like some airports have an outsized role in maintaining the air travel network (think Chicago’s O’Hare), disruptions in those places could have bigger effect on the overall population.

Network analyses like this could be used to better understand environment impacts of projects in other places as well, “particularly those affecting waterbirds and coastal habitats where tracking data is available,” said José Alves, a researcher at the University of Aveiro who was involved in the work.

It could also have repercussions for the fate of the proposed Lisbon airport. Environmental groups have challenged the development’s approval in court. “We hope our findings will help strengthen the case by showing the magnitude of the impacts, which substantially surpass those quantified in the developer’s environmental impact assessment,” said Alves.

Nightingale, et. al. “Conservation beyond Boundaries: using animal movement networks in Protected Area assessment.” Animal Conservation. April 7, 2023.

Photo: ©José Alves

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