Minneapolis poses serious problem for migrating birds

MINNEAPOLIS -- Scientists estimate that about 600 million birds die each year in the U.S. from striking buildings, communications towers and other structures, although not always because of artificial light.

Now, the Twin Cities has been named one of the worst urban areas in the country for migrating birds, a result of its bright nighttime lights and location in a flyway with a huge volume of birds pouring through, according to Jennifer Bjorhus , writing for the Star Tribune.

Chicago, Houston and Dallas topped the list.

According to the Star Tribune report, citing a study by the Cornell Lab of Ornithology, Minneapolis sits at the top of the Mississippi Flyway, the primary navigation corridor for an estimated 60 percent of North American migrating bird species. That helps explain why it finished near the top of 125 cities analyzed in a new study The Twin Cities ranked No. 6 in the spring migration and No 7 in the fall. Boston, by contrast, is not near the flyway and ranked 36 and 24.

The Star Tribune says that although researchers have written extensively about light pollution and disoriented birds crashing into buildings, the Cornell research breaks new ground, the authors say, by taking light radiance data from satellites and overlaying it with bird counts from weather surveillance radar.

The data on migrating birds from more than 140 weather stations was the new element, said Kyle Horton, a Cornell researcher and lead author of the study, which will be published in the May issue of "Frontiers in Ecology and the Environment."

"It's probably one of the only tools for detecting the broad passage of birds," Horton said. "These are some of the first U.S.-wide depictions of migration that we've had access to."

Climate change and cats still pose greater threats to rapidly declining bird populations, Horton said, but light pollution poses a notable threat.

That's because most birds migrate, and most favor nighttime. They tend to be safer from predators in the dark, for example, and weather tends to be cooler and calmer.

As a result, birds often use the stars and polarized light from the setting sun to set their compasses and navigate. But night flight leaves them vulnerable to light "pollution" from streetlights, office buildings, homes, car dealerships, stadiums and other buildings.

Intense glow at night attract birds, who sometimes fixate on lights, much like moths on a porch light, Horton said. Some birds become entranced — circling and circling, sometimes all night. In extreme cases, the birds grow exhausted and collide with buildings. The risk of mortality for young birds probably increases in the fall because they aren't experienced fliers.