Bird-window Collisions FAQ

How many birds are killed each year in collisions with windows?
The question seems like an easy one, but we just don’t know. By monitoring local buildings, we have learned much about the numbers and species affected here. Internationally, estimates vary widely and are likely in the hundreds of millions. Bird collisions are a source of potentially significant and indiscriminate mortality for birds, many of which are already in serious decline due to other pressures. Learn more by reading Bird–building collisions in the United States: Estimates of annual mortality and species vulnerability. This reasearch states that "Building collisions, and particularly collisions with windows, are a major anthropogenic threat to birds, with rough estimates of between 100 million and 1 billion birds killed annually in the United States."Back to top of page

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. Many charasmatic birds are included on this list, including Ruby-throated Hummingbirds, Indigo Buntings, Black-capped Chickadees and many others including a number of warblers.

Over 250 species migrate through Minnesota, many of them small songbirds that migrate at night. Some of these are threatened species whose populations already show steep declines. Ironically, common city birds such as rock pigeons and house sparrows are infrequent collision victims. This may be due to these species’ adaptations for living among buildings. Back to top of page

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 circle it repeatedly until falling to the ground in exhaustion. Tall, lighted buildings are especially lethal in fog, low clouds and rain.

As the human population grows, we build more and more structures, and the areas that are attractive to humans—riverbanks, coasts, shorelines—are birds’ traditional migratory pathways. Our built environment is proving to be a big challenge to migrating birds. Back to top of page

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 some place 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. Back to top of page

How many birds are killed each year in St. Paul and Minneapolis?
We monitored routes in the Twin Cities for ten years in order to estimate the size of the problem. Trained volunteers surveyed routes in both downtown Minneapolis and St. Paul during spring and fall migration to collect fallen birds. Live birds can often be released or taken to the Wildlife Rehabilitation Center if injured. Dead birds are taken to Bell Museum of Natural History where information is recorded about their age, sex, species, and the location where they were found. To date we have found over 100 species of birds at just a handful of buildings being monitored throughout the state. Back to top of page

Why should we care if some birds are lost on migration in this way?
As stewards of the environment everyone should be concerned about 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 and their fall departure precedes the changeover to winter. Their songs and daily activity bring joy to nearly everyone. We can save millions of birds by turning off the lights in tall buildings at night. Back to top of page

BirdSafe Homes: You can help right now by making your home more bird-safe.