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University of Minnesota
University of Minnesota

Collision course

December 5, 2008

Hand holding a little yellow bird.

Songbirds like this Nashville warbler account for many of the deaths when birds collide with buildings.

Photo: Joanna Eckles

An effort is under way to keep migrating birds from colliding with buildings

By Jennifer Amie

A bird's eye view of downtown Minneapolis or St. Paul is, to say the least, befuddling. Architectural designs that are pleasing to humans are often deceptive to birds, and sometimes with fatal consequences. Imagine an office tower featuring a 20-foot glass atrium filled with ornamental trees. Now imagine that you're flying, looking for a place to perch, and you can't see the glass.

Collisions between birds and buildings can happen any time, but especially during migration seasons. Bob Zink, curator of ornithology at the University's Bell Museum of Natural History, is leading a study of bird/building collisions and their effects on bird populations and will discuss the topic on Tuesday, December 9, at the museum's next CafŽ Scientifique. The informal talk starts at 7 p.m. in Nomad World Pub, 501 Cedar Ave., on Minneapolis's West Bank.

In daytime, birds collide with buildings because they mistakenly perceive a clear flight path where there is none. They also are deceived by the highly reflective glass commonly used in modern architecture, slamming into buildings when they attempt to fly to a tree or a patch of sky that turns out to be a mirage.

"Birds are apparently unable to distinguish a reflection from the object itself," says Zink. The problem seems to be heightened when highly reflective buildings are set amid green spaces, amplifying the illusion of trees and bushes that are inviting to birds.

Fast facts from Project BirdSafe
* In spring 2007, volunteers collected 174 birds of 52 species that had collided with buildings in Minneapolis, St. Paul, Bloomington, and Rochester.
* In fall 2007, 363 birds of 49 species were collected.
* Most common birds found in spring 2007: ovenbird, Tennessee warbler, white-throated sparrow, dark-eyed junco, Nashville warbler.
* Most common birds found in fall 2007: Nashville warbler, white-throated sparrow, black-capped chickadee, Tennessee warbler, dark-eyed junco

After dark, the hazards are different. Many migrating birds travel at night, navigating by the stars. When these natural signposts are obscured by clouds, birds fly lower and, for unknown reasons, are attracted to city lights.

"They become disoriented and fly around and around lighted buildings," says Zink. They may strike buildings, or even drop from exhaustion.

In an attempt to mitigate these hazards, owners, tenants, and management companies from 32 buildings in Minneapolis, St. Paul, Bloomington, and Rochester are taking part in a lights out program coordinated by Project BirdSafe, a collaboration between the Bell Museum, Audubon Minnesota, the Minnesota Department of Natural Resources' Nongame Wildlife Program, the Wildlife Rehabilitation Center, and Bird Conservation Minnesota. During spring and fall migration, interior and exterior lights at participating buildings are switched off or reduced from midnight till dawn.

"Buildings weren't built to kill birds, but some of them happen to kill a few, and some of them happen to kill a lot."

Similar programs are in place in Toronto, New York, and Chicago. The Twin Cities, says Zink, are an important addition to these efforts because of their location along the Mississippi River flyway, a major thoroughfare for migrating birds.

"I'd like to see lights out as just a way of doing business," says project coordinator Joanna Eckles. "There's no reason for lights to be on all the time. Turning them off saves energy and money and saves birds."

It's well known that building collisions pose a threat to bird populations, and Project BirdSafe is out to discover just how much mortality they cause.

Volunteers collect data by walking a prescribed route through downtown, recording the number of dead or injured birds they find at various building sites--or the lack of birds found. Injured birds are taken to the Wildlife Rehabilitation Center. Dead birds are identified and tagged, with wing and tissue samples entered into the Bell Museum's scientific collections.

"The goal," says Zink, "is to figure out where building collisions rank in the list of threats to birds that we know about."

Zink hopes to learn whether the bird species that collide with buildings most often are declining at breeding sites. Preliminary results suggest that they are not. The Tennessee warbler, white-throated sparrow, and Nashville warbler, for example, account for a high percentage of the birds collected around buildings, but breeding bird surveys indicate their populations are stable to the north. Also, many of the most common birds that migrate through the Twin Cities--such as the red-eyed vireo, ovenbird, American redstart, yellow-rumped warbler, hermit thrush, and least flycatcher--appear to rarely collide with windows.

Perhaps the most startling finding so far is that relatively few buildings account for a vast majority of bird deaths. In St. Paul, 44 percent of all bird deaths were caused by two buildings along the sampled route. In Minneapolis, 67 percent of birds were killed at two buildings.

"Buildings weren't built to kill birds," says Eckles, "but some of them happen to kill a few, and some of them happen to kill a lot." This is good news, notes Zink, because it means that mitigation efforts can be concentrated on the buildings that cause the most harm.

By studying the characteristics of the deadliest buildings, architects and city planners can better understand what design elements contribute to bird collisions. In fact, says Zink, "the designers of the Bell Museum's new building are already focusing on economically feasible ways to prevent it from becoming a source of new specimens for the collections."

Eckles and Zink hope to continue the Project BirdSafe research for three to five years, amassing enough data to truly shed light on the problem of bird/building collisions and what can be done to prevent them.

This story is taken fromImprint, the magazine of the Bell Museum of Natural History.