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In daytime, birds collide with buildings when they mistake reflections of sky, trees, or shrubs for the real thing.
Research to help builders protect birds
By Jennifer Amie
July 8, 2008
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 Bell Museum curator of ornithology Bob Zink, who is leading a study on bird/building collisions and their effect on bird populations. The problem seems to be heightened when highly reflective buildings are set amid greenspaces, amplifying the illusion of trees and bushes that are inviting to birds.
After dark, the hazards are different. Many migrating birds travel at night, navigating by the stars. When these natural signposts are obscured by cloud cover, 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.
Fast Facts from Project
- 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
In an attempt to mitigate these hazards, owners, tenants, and management companies from 32 buildings in Minneapolis, St. Paul, Bloomington, and Rochester are participating 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.
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 are a threat to bird populations, but just how much mortality they cause is unknown. That's a question Project BirdSafe aims to answer. 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."
Bob Zink with specimens from the Bell Museum's bird collection. Photo by Patrick O'Leary
Among the questions Zink hopes to answer is 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 collide with windows only rarely.
Perhaps the most startling finding, at this point in the study, 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.