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Frequently Asked Questions

Kenneth Herdy 2009
Kenneth Herdy 2009

How many birds are killed each year in collisions with windows?
The question seems like an easy one, but we don’t really know. It is estimated that 100 million to 1 billion birds die in the U.S. alone every year as a result of window strikes. This is a mortality rate second only to habitat destruction. Bird collisions are a source of both significant and indiscriminate mortality for birds, many of which are already in serious decline due to other pressures. Unlike some of the other major threats to birds, such as habitat loss and pesticide use, this is something that citizens can do something about in their daily lives.

What kinds of birds collide with buildings?
Bird mortality from window strikes has been recorded in more than half the bird species in the United States. Here in Portland, we have recorded golden-crowned kinglet, rufous hummingbird, Lincoln 's sparrow, song sparrow, Swainson's thrush, varied thrush, black-throated gray warbler,  black-capped chickadee, red-breasted nuthatch, and Cooper’s hawk as window strike casualties.

Over 209 species of birds migrate through Oregon, many of them small diurnal songbirds that migrate at night. Some of these are threatened species whose populations already show steep declines. Ironically, common city birds such as rock doves and English house sparrows are infrequent collision victims. This may be due to these species’ adaptation to living among buildings.

Birds have been migrating for millennia — why don’t they learn to avoid buildings?
Birds collide with windows in the daytime when they see the outdoors reflected in the glass and think they have a clear flight path. Most migrants fly at night, and the artificial lights in tall buildings confuse them and cause them either to crash into the structure or to circle it repeatedly until falling to the ground in exhaustion. Tall lighted buildings are especially lethal in fog, low clouds and rain.

As human populations grow, our cities grow with them: we build more and more structures, and areas that are attractive to humans — riverbanks, coasts and shorelines — are birds’ traditional migratory pathways. Our built environment is proving to be more and more of a challenge to migrating birds.

If buildings are so dangerous, why don’t we see piles of dead birds on the sidewalk each day?
Many of the birds that strike windows are killed outright and fall to the ground where predators — such as gulls, crows, cats and other scavengers — quickly carry them off, or they are cleaned up by maintenance workers. Those that survive an impact may be too injured to fly and find someplace to hide as the city wakes up. Those that retain flight ability may be so frightened by the noise and activity of a city in daytime that they fly up and collide with a building again.

How many birds are killed each year in Portland?
We don't know yet! We do know that our Wildlife Care Center admits, on average, 200-300 window strike cases per year. We have only been monitoring seasonal routes in Portland since September 2009 in order to estimate the magnitude of strikes. Trained volunteers survey our routes during spring and fall migration to collect fallen birds. Live birds can be released or taken to the Audubon Wildlife Care Center for treatment. Dead birds are collected and examined at the care center to determine if the injuries are consistent with window strike mortality.

Why should we care if some birds die this way?
As stewards of the environment, everyone can be involved in conserving birds. Birds perform many useful functions, from pollination to seed dispersal to insect control. In spring, their return signals the renewal of the seasons just as their fall departure precedes the changeover to winter. Across the globe, we can save millions of birds by extinguishing unnecessary overnight lighting in buildings during fall and spring migration. Migration is such an arduous business for birds, let’s do our part to make it easier for them.

Reducing this overnight lighting also helps to minimize ecological light pollution, a phenomenon of increasing study as we realize that our manipulation of natural light-dark cycles has a tremendous and unmeasured impact on the circadian rhythms of plants, wildlife and humans. Disruption of these cycles impacts migration, reproduction, feeding and predator/prey relationships, and it is only beginning to be quantified.

Audubon Minnesota generously provided some of the above information.

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