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Flight Risk

The collision course we create for birds

All Animals

  • Birds might not be able to tell the difference between trees and their deadly reflection on a building. iStockphoto.com

by Angela Moxley

Wherever they fly—above cities, through suburban backyards, over roads through country fields—birds face an indiscriminate threat.

Hurtling toward tall buildings, houses, shopping malls, even automobiles, they're unable to recognize clues like sills or framework that might warn them to change course before they collide with a window.

To birds, the transparency and reflectivity of glass indicates open, unobstructed space.

“Birds can respond to just a tiny spot of light, and what's more, they're flying,” says ornithologist Daniel Klem, who has studied the issue for three decades. “So their ability to fly makes them vulnerable because they can instantly build up enough momentum to kill themselves.

They could leave a perch on a twig of a tree or a bush just slightly over three feet away and … strike the glass. Unless your window’s totally dirty or you've put something on it, these animals are just deceived.”

Major losses

While most people have probably seen or heard a bird hitting a window on occasion, few realize how much the numbers add up. Even back in the 1970s using field data he had collected, Klem estimated that 1 to 10 birds fatally strike each building in the U.S. every year. Based on census data, that’s 100 million to 1 billion birds annually, or 274,000 to 2.74 million per day.

Klem, a professor at Pennsylvania’s Muhlenberg College, says these estimates have drawn skepticism not just from architects, engineers, and other building industry professionals but also from colleagues in the birding and conservation fields.

They don’t believe the numbers because they don’t see the carnage—fallen birds can be snatched by predators, hidden in vegetation, squashed by pedestrians or street sweepers, or discarded by cleaning crews. Or they might fly away and succumb to their injuries.

Long cautious about using the 1 billion figure for fear of being ridiculed or, worse, ignored, Klem now says the estimate is conservative.

The presence of glass, he says, takes a toll that is “exponentially higher” than that of many other threats, such as pesticides, cats, and wind turbines. Short of habitat destruction, he believes glass is the greatest human-related cause of mortality to birds.

Fatal attraction

Glass is equally deceiving whether it’s transparent (birds see a clear flight path ahead) or reflective (they think they’re flying into trees or sky). Birds face an additional challenge as they fly over cities at night, relying on the stars and moon to navigate.

Lights shining from the buildings below attract them, says Michael Mesure, executive director of Toronto's Fatal Light Awareness Program (FLAP). “They will either collide outright with those structures, or they circle in that lit-up source until they drop with exhaustion.”

Once on the ground, the worn-out birds often fly into lit lobby windows in an attempt to reach the light source. As day breaks, they try to fly into the virtual habitat reflected in the windows or to the indoor plants and trees visible from outside.

Successful collision prevention

Since 1993, FLAP has pioneered efforts to rescue birds and persuade building managers to turn off lights at night and add window and landscaping features that deter birds by day. 

Groups in other hot spots on birds’ migratory pathways have followed suit—first in New York and later in places like Minnesota’s Twin Cities and Wisconsin’s Green Bay. In 2004, Chicago became the first U.S. city to “go dark” at night during the spring and fall migrations, says Annette Prince, director of the Chicago Bird Collision Monitors.

In each of these cities, volunteers scour downtown areas for injured birds and triage them on-site or take them to wildlife rehabilitators. They collect data to show building managers just how deadly their structures can be. They help the managers make the buildings less dangerous by rearranging indoor and outdoor plants and installing glass treatments, shielded lighting, and motion sensor lights.

The strategies work: “The reduced lighting has definitely ended having a [single] night where one particular building can have just hundreds and hundreds of birds impacted,” Prince says. And government partnerships have proved promising. Working with FLAP, Toronto's city council has adopted bird-friendly guidelines that encourage the use of less lethal features in building designs.

(New York City's Audubon Society, which has its own window strike program, has developed similar recommendations.) FLAP members hope the guidelines will someday be included in North American green building standards.

A question of conservation

While these urban rescuers are motivated by the chance to save individual birds, the scale of their efforts makes it clear that something bigger is at stake.

In Toronto, FLAP has picked up more than 40,000 birds since 1993, or about 2,900 a year—“a fraction of the birds who are colliding,” says Mesure. With 940,000 structures in the city, he believes 1 million to 10 million birds might strike each year.

In the spring of 2007 alone, Prince's group found 1,500 birds in the 1 square mile of downtown Chicago it patrols.

What’s more, Prince says, it’s a common misperception that only “city birds” such as pigeons, English house sparrows, or European starlings strike glass.

More than 220 species in the U.S. and Canada have been known to fly into windows, and Prince notes that Chicago volunteers find endangered birds and members of species that some birders wait their whole lives to glimpse.

When faced with “compensatory” factors like starvation or predation, the strongest animals can survive and help a given population rebound from year to year.

But glass is what scientists call an “additive” factor—it affects the fit and unfit members of a species alike, and populations must struggle to recover from the damage it causes on top of a host of other natural stresses.

“[Glass is] taking out perhaps the best hope of a species,” Prince says. “It’s also taking out birds whose numbers are already highly threatened by other causes, and they can ill afford additional losses.”

A chance to help

Although glass presents a huge threat, it’s one that people may be able to manage—by modifying windows on their homes, educating other homeowners and property managers, and working with glass manufacturers to develop a bird-friendly product.

The efforts in Chicago, New York, and Toronto are serving as templates for grassroots programs in other places that make cities safer for birds.

What you can do

  • Evaluate your own home for its potential for bird strikes and take preventative steps.
  • Talk to your local birding club to see if they have, or should, organize a volunteer program to rescue birds that have hit structures and are in need of rehabilitative care.
  • Talk to local wildlife rehabilitators to get a sense of the issue from their perspective.
  • Organize an awareness and prevention program in your city, or join if one already exists.

For more information about the contact Fatal Light Awareness Program (FLAP).

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