ROBERT SIEGEL, HOST:
Right now we're in the peak of the fall migration season. Billions of birds are making their way south. Many species travel at night, and a new study shows how artificial light can affect their journey through darkness. NPR's Nell Greenfieldboyce reports.
NELL GREENFIELDBOYCE, BYLINE: This time of year the night sky is full of birds.
KYLE HORTON: Sparrows, warblers, thrushes, grosbeaks, tanagers.
GREENFIELDBOYCE: Kyle Horton works at the Cornell Lab of Ornithology. He says flying at night makes a lot of sense.
HORTON: These birds are effectively running or flying marathons through the air space. And it's cooler at night. The air space is calmer.
GREENFIELDBOYCE: And there's fewer predators. The night travelers navigate using the stars and landmarks like rivers and coastlines. As they look down at the world below, they also see bright lights. Horton says there's evidence that birds can sometimes be drawn toward them.
HORTON: Things like a low cloud ceiling or fog paired with light, that really disorients migrants.
GREENFIELDBOYCE: But this hasn't been studied much, so he decided to do a kind of opportunistic experiment using some really powerful, unique lights. Each year on September 11, New York City has something called the Tribute in Light. To remember the lives lost in the Twin Towers, two vertical columns of light are created by searchlights.
HORTON: The lights that you would see maybe one or two, let's say, for a grand opening of something. And we're talking about 88 of those pointed skyward. The magnitude of the light is in many ways overwhelming.
GREENFIELDBOYCE: It's been known for years that this draws in migrating birds. They circle around in the beams. If volunteers on the ground see more than a thousand, the tribute gets turned off for about 20 minutes. But Horton and his colleagues used radar data and found many, many more birds than the volunteers could see.
HORTON: What we're detecting on the radar are estimates of hundreds of thousands of birds.
GREENFIELDBOYCE: More than a million birds were affected over the seven nights in the study. The results are described in the Proceedings of the National Academy of Sciences. Jeffrey Buler is a researcher at the University of Delaware who also uses radar to study bird migrations. He says the sheer number of birds affected by this light display is surprising. So was the way the birds quickly scattered into the night whenever the lights got turned off.
JEFFREY BULER: The figures from the paper are just amazing where they show when the lights are on and when the lights are off and how the numbers of the birds just oscillate up and down.
GREENFIELDBOYCE: He says this light was so extreme it makes the birds' response easy to see. But his recent research suggests this is happening on a much larger scale. Migrating birds seem to be drawn towards the ordinary nighttime glow of big cities.
BULER: We're concerned that we are drawing birds into more urbanized landscapes where there are less suitable habitats for them to stop over.
GREENFIELDBOYCE: They could have trouble finding food or be delayed, and that would make their long migrations that much harder. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.